Diffusiophoretic Transport of Charged Colloids in Ionic Surfactant Gradients Entirely below versus Entirely above the Critical Micelle Concentration

When placed in an ionic surfactant gradient, charged colloids will undergo diffusiophoresis at a velocity, uDP = MDP∇ ln S, where MDP is the diffusiophoretic mobility and S is the surfactant concentration. The diffusiophoretic mobility depends in part on the charges and diffusivities of the surfactants and their counterions. Since micellization decreases surfactant diffusivity and alters charge distributions in a surfactant solution, MDP of charged colloids in ionic surfactant gradients may differ significantly when surfactant concentrations are above or below the critical micelle concentration (CMC). The role of micelles in driving diffusiophoresis is unclear, and a previously published model that accounts for micellization suggests the possibility of a change in the sign of MDP above the CMC [Warren, P. B.; . Soft Matter 2019, 15, 278-288]. In the current study, microfluidic channels were used to measure the transport of negatively charged polystyrene colloids in sodium dodecyl sulfate (SDS) surfactant gradients established at SDS concentrations that are either fully above or fully below the CMC. Interpretation of diffusiophoresis was aided by measurements of the colloid electrophoretic mobility as a function of SDS concentration. A numerical transport model incorporating the prior diffusiophoretic mobility model for ionic surfactant gradients was implemented to elucidate signatures of positive and negative diffusiophoretic mobilities and compare with experiments. The theoretically predicted sign of the diffusiophoretic mobility below the CMC was determined to be particularly sensitive to uncertainty in colloid and surfactant properties, while above the CMC, the mobility was consistently predicted to be positive in the SDS concentration range considered in the experiments conducted here. In contrast, experiments only showed signatures of a negative diffusiophoretic mobility for these negatively charged colloids with no change of sign. Colloid diffusiophoretic transport measured in micellar solutions was more extensive than that below the CMC with the same ∇ ln S.

Interaction of impinging marangoni fields

HYPOTHESIS

Surface tension gradient driven Marangoni flows originating from multiple sources are important to many industrial and medical applications, but the theoretical literature focuses on single surfactant sources. Understanding how two spreading surfactant sources interact allows insights from single source experiments to be applied to multi-source applications. Two key features of multi-source spreading - source translation and source deformation - can be explained by transport modeling of a two-source system.

MODELING

Numerical simulations of two oleic acid disks placed at varying initial separation distances on a glycerol subphase were performed using COMSOL Multiphysics and compared to spreading of a single surfactant source.

FINDINGS

Interaction of two spreading sources can be split into three regimes: the independent regime - where each source is unaffected by the other, the interaction regime - where the presence of a second source alters one or more features of the spreading dynamics, and the quasi-one disk regime - where the two sources merge together. The translation of the sources, manifested as increasing separation distance between disk centers of mass, is driven by the flow fields within the subphase and the resultant surface deformation, while deformation of the sources occurs only once the surfactant fronts of the two sources meet.

Effect of polymer/surfactant complexation on diffusiophoresis of colloids in surfactant concentration gradients

HYPOTHESIS

A concentration gradient of surfactants in the presence of polymers that non-covalently associate with surfactants will exhibit a continually varying distribution of complexes with different composition, charge, and size. Since diffusiophoresis of colloids suspended in a solute concentration gradient depends on the relaxation of the gradient and on the interactions between solutes and particles, polymer/surfactant complexation will alter the rate of diffusiophoresis driven by surfactant gradients relative to that observed in the same concentration gradient in the absence of polymers.

EXPERIMENTS

A microfluidic device was used to measure diffusiophoresis of colloids suspended in solutions containing a gradient of sodium dodecylsulfate (SDS) in the presence or absence of a uniform concentration of Pluronic P123 poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) nonionic triblock copolymers. To interpret the effect of P123 on the rate of colloid diffusiophoresis, electrophoretic mobility and dynamic light scattering measurements of the colloid/solute systems were performed, and a numerical model was constructed to account for the effects of complexation on diffusiophoresis.

FINDINGS

Polymer/surfactant complexation in solute gradients significantly enhanced diffusiophoretic transport of colloids. Large P123/SDS complexes formed at low SDS concentrations yielded low collective solute diffusion coefficients that prolonged the existence of strong concentration gradients relative to those without P123 to drive diffusiophoresis.

Charge, Aspect Ratio, and Plant Species Affect Uptake Efficiency and Translocation of Polymeric Agrochemical Nanocarriers

An incomplete understanding of how agrochemical nanocarrier properties affect their uptake and translocation in plants limits their application for promoting sustainable agriculture. Herein, we investigated how the nanocarrier aspect ratio and charge affect uptake and translocation in monocot wheat (Triticum aestivum) and dicot tomato (Solanum lycopersicum) after foliar application. Leaf uptake and distribution to plant organs were quantified for polymer nanocarriers with the same diameter (∼10 nm) but different aspect ratios (low (L), medium (M), and high (H), 10-300 nm long) and charges (-50 to +15 mV). In tomato, anionic nanocarrier translocation (20.7 ± 6.7 wt %) was higher than for cationic nanocarriers (13.3 ± 4.1 wt %). In wheat, only anionic nanocarriers were transported (8.7 ± 3.8 wt %). Both low and high aspect ratio polymers translocated in tomato, but the longest nanocarrier did not translocate in wheat, suggesting a phloem transport size cutoff. Differences in translocation correlated with leaf uptake and interactions with mesophyll cells. The positive charge decreases nanocarrier penetration through the leaf epidermis and promotes uptake into mesophyll cells, decreasing apoplastic transport and phloem loading. These results suggest design parameters to provide agrochemical nanocarriers with rapid and complete leaf uptake and an ability to target agrochemicals to specific plant organs, with the potential to lower agrochemical use and the associated environmental impacts.

Temperature-Responsive Bottlebrush Polymers Deliver a Stress-Regulating Agent In Vivo for Prolonged Plant Heat Stress Mitigation

Anticipated increases in the frequency and intensity of extreme temperatures will damage crops. Methods that efficiently deliver stress-regulating agents to crops can mitigate these effects. Here, we describe high aspect ratio polymer bottlebrushes for temperature-controlled agent delivery in plants. The foliar-applied bottlebrush polymers had near complete uptake into the leaf and resided in both the apoplastic regions of the leaf mesophyll and in cells surrounding the vasculature. Elevated temperature enhanced the in vivo release of spermidine (a stress-regulating agent) from the bottlebrushes, promoting tomato plant (Solanum lycopersicum) photosynthesis under heat and light stress. The bottlebrushes continued to provide protection against heat stress for at least 15 days after foliar application, whereas free spermidine did not. About 30% of the ∼80 nm short and ∼300 nm long bottlebrushes entered the phloem and moved to other plant organs, enabling heat-activated release of plant protection agents in phloem. These results indicate the ability of the polymer bottlebrushes to release encapsulated stress relief agents when triggered by heat to provide long-term protection to plants and the potential to manage plant phloem pathogens. Overall, this temperature-responsive delivery platform provides a new tool for protecting plants against climate-induced damage and yield loss.

Effect of a Surfactant Additive on Drug Transport and Distribution Uniformity After Aerosol Delivery to Ex Vivo Lungs

Background: Inhaled drug delivery can be limited by heterogeneous dose distribution. An additive that would disperse drug over the internal surfaces of the lung after aerosol deposition could improve dosing uniformity and increase the treated area. Our previous studies demonstrated that surfactant additives can produce surface tension-driven (Marangoni) flows that effectively dispersed aerosol-delivered drugs over mucus surfaces. Here we sought to determine whether the addition of a surfactant would increase transport of an aerosol between lung regions and also improve dosing uniformity in human lungs. Methods: We compared the deposition and postdeposition dispersion of surfactant (10 mg/mL dipalmitoylphosphatidylcholine; DPPC) and saline-based liquid aerosols, admixed with Technetium 99m (Tc99m) diethylenetriaminepentaacetic acid, using gamma scintigraphy. Deposition images were obtained ex vivo in eight pairs of ventilated human lungs. The trachea was intubated and the mainstem bronchi were alternately clamped so that saline was delivered to one lung and then DPPC to the other (sides alternated). The lungs were continually imaged for 15 minutes during delivery. We assessed transport of the deposited aerosol by quantifying the percentage of Tc99m in each of four lung quadrants over time. We quantified dose uniformity within each lung quadrant by measuring the coefficient of variation (CV = standard deviation of the pixel associated radioactive counts/mean of the counts within each quadrant). Results: There was no change in the percentage of Tc99m in each quadrant over time, indicating no improvement in transport with the addition of the surfactant. The addition of surfactant was associated with a statistically significant decrease in CV in the lower inner lung quadrant at each of the three time points, indicating an improvement in dosing uniformity. Conclusion: These preliminary results indicate the possible utility of adding surfactant to aerosols to improve drug distribution uniformity to lower inner lung regions.

Star Polymers with Designed Reactive Oxygen Species Scavenging and Agent Delivery Functionality Promote Plant Stress Tolerance

Plant abiotic stress induces reactive oxygen species (ROS) accumulation in leaves that can decrease photosynthetic performance and crop yield. Materials that scavenge ROS and simultaneously provide nutrients in vivo are needed to manage this stress. Here, we incorporated both ROS scavenging and ROS triggered agent release functionality into an ∼20 nm ROS responsive star polymer (RSP) poly(acrylic acid)-block-poly((2-(methylsulfinyl)ethyl acrylate)-co-(2-(methylthio)ethyl acrylate)) (PAA-b-P(MSEA-co-MTEA)) that alleviated plant stress by simultaneous ROS scavenging and nutrient agent release. Hyperspectral imaging indicates that all of the RSP penetrates through the tomato leaf epidermis, and 32.7% of the applied RSP associates with chloroplasts in mesophyll. RSP scavenged up to 10 μmol mg^-1 ROS in vitro and suppressed ROS in vivo in stressed tomato (Solanum lycopersicum) leaves. Reaction of the RSP with H₂O₂in vitro enhanced the release of nutrient agent (Mg²⁺) from star polymers. Foliar applied RSP increased photosynthesis in plants under heat and light stress compared to untreated controls, enhancing the carbon assimilation, quantum yield of CO₂ assimilation, Rubisco carboxylation rate, and photosystem II quantum yield. Mg loaded RSP improved photosynthesis in Mg deficient plants, mainly by promoting Rubisco activity. These results indicate the potential of ROS scavenging nanocarriers like RSP to alleviate abiotic stress in crop plants, allowing crop plants to be more resilient to heat stress, and potentially other climate change induced abiotic stressors.

Tuning chemotactic and diffusiophoretic spreading via hydrodynamic flows

The transport of microorganisms by chemotaxis is described by the same "log-sensing" response as colloids undergoing diffusiophoresis, despite their different mechanistic origins. We employ a recently-developed macrotransport theory to analyze the advective-diffusive transport of a chemotactic or diffusiophoretic colloidal species (both referred to as "colloids") in a circular tube under a steady pressure-driven flow (referred to as hydrodynamic flow) and transient solute gradient. First, we derive an exact solution to the log-sensing chemotactic/diffusiophoretic macrotransport equation. We demonstrate that a strong hydrodynamic flow can reduce spreading of solute-repelled colloids, by eliminating super-diffusion which occurs in an otherwise quiescent system. In contrast, hydrodynamic flows always enhance spreading of solute-attracted colloids. Second, we generalize the exact solution to show that the above tunable spreading phenomena by hydrodynamic flows persist quantitatively for decaying colloids, as may occur with cell death, for example. Third, we examine the spreading of chemotactic colloids by employing a more general model that captures a hallmark of chemotaxis, that log-sensing occurs only over a finite range of solute concentration. Apart from demonstrating for the first time the generality of the macrotransport theory to incorporate an arbitrary chemotactic flow model, we reveal via numerical solutions new regimes of anomalous spreading, which match qualitatively with experiments and are tunable by hydrodynamic flows. The results presented here could be employed to tailor chemotactic/diffusiophoretic colloid transport using hydrodynamic flows, which are central to applications such as oil recovery and bioremediation of aquifers.

Surfactant spreading on a deep subphase: Coupling of Marangoni flow and capillary waves

HYPOTHESIS

Surfactant-driven Marangoni spreading generates a fluid flow characterized by an outwardly moving "Marangoni ridge". Spreading on thin and/or high viscosity subphases, as most of the prior literature emphasizes, does not allow the formation of capillary waves. On deep, low viscosity subphases, Marangoni stresses may launch capillary waves coupled with the Marangoni ridge, and new dependencies emerge for key spreading characteristics on surfactant thermodynamic and kinetic properties.

EXPERIMENTS AND MODELING

Computational and physical experiments were performed using a broad range of surfactants to report the post-deposition motion of the surfactant front and the deformation of the subphase surface. Modeling coupled the Navier-Stokes and advective diffusion equations with an adsorption model. Separate experiments employed tracer particles or an optical density method to track surfactant front motion or surface deformation, respectively.

FINDINGS

Marangoni stresses on thick subphases induce capillary waves, the slowest of which is co-mingled with the Marangoni ridge. Changing Marangoni stresses by varying the surfactant system alters the surfactant front velocity and the amplitude - but not the velocity - of the slowest capillary wave. As spreading progresses, the surfactant front and its associated surface deformation separate from the slowest moving capillary wave.

pH-Dependent Interfacial Tension and Dilatational Modulus Synergism of Oil-Soluble Fatty Acid and Water-Soluble Cationic Surfactants at the Oil/Water Interface

While the concept of interfacial tension synergism in surfactant mixtures is well established, little attention has been paid to the possibility of synergistic effects on the interfacial rheology of mixed surfactant systems. Furthermore, interfacial tension synergism is most often investigated for mixtures of surfactants residing in a single phase. Here, we define dilatational modulus synergism and report a study of interfacial tension isotherms and complex dilatational moduli for a binary surfactant system with the two surfactants accessing the oil/water interface from opposite sides. Using an oil-soluble fatty acid surfactant (palmitic acid, PA) that may be ionized at the oil/water interface and a quaternary ammonium water-soluble cationic surfactant (tetradecyltrimethylammonium bromide, TTAB), the binary interfacial interaction was tuned by the aqueous phase pH. Interfacial tensions and dilatational moduli were measured by the pendant drop method for the binary surfactant system as well as the corresponding single-surfactant systems to identify synergistic effects. The possible occurrence of dilatational modulus synergism was probed from two perspectives: one for a fixed total surfactant concentration and the other for a fixed interfacial tension. The aqueous pH was found to have a controlling effect on both interfacial tension synergism and the dilatational modulus synergism. The conditions for interfacial tension synergism coincided with those for the storage modulus synergism: both tension and storage modulus synergisms were observed under all conditions tested at pH 7 where PA was mostly deprotonated, for both perspectives examined, but not for any conditions tested at pH 3 where PA is mostly protonated. The loss modulus synergism exhibited more complex behaviors, such as frequency and interfacial tension dependences, but again was only observed at pH 7. The tension and modulus synergism at pH 7 were attributed to the increased attraction between ionized PA and cationic TTAB and the formation of catanionic complexes at the oil/water interface.

Phosphate Polymer Nanogel for Selective and Efficient Rare Earth Element Recovery

Demand for rare earth elements (REEs) is increasing, and REE production from ores is energy-intensive. Recovering REEs from waste streams can provide a more sustainable approach to help meet REE demand but requires materials with high selectivity and capacity for REEs due to the low concentration of REEs and high competing ion concentrations. Here, we developed a phosphate polymer nanogel (PPN) to selectively recover REEs from low REE content waste streams, including leached fly ash. A high phosphorus content (16.2 wt % P as phosphate groups) in the PPN provides an abundance of coordination sites for REE binding. In model solutions, the distribution coefficient (K_d) for all REEs ranged from 1.3 × 10⁵ to 3.1 × 10⁵ mL g^-1 at pH = 7, and the sorption capacity (q_m) for Nd, Gd, and Ho were ∼300 mg g^-1. The PPN was selective toward REEs, outcompeting cations (Ca, Mg, Fe, Al) at up to 1000-fold excess concentration. The PPN had a K_d of ∼10⁵-10⁶ mL g^-1 for lanthanides in coal fly ash leachate (pH = 5), orders of magnitude higher than the K_d of major competing ions (∼10³-10⁴ mL g^-1). REEs were recovered from the PPN using 3.5% HNO₃, and the material remained effective over three sorption-elution cycles. The high REE capacity and selectivity and good durability in a real waste stream matrix suggest its potential to recover REEs from a broad range of secondary REE stocks.

Star Polymer Size, Charge Content, and Hydrophobicity Affect their Leaf Uptake and Translocation in Plants

Determination of how the properties of nanocarriers of agrochemicals affect their uptake and translocation in plants would enable more efficient agent delivery. Here, we synthesized star polymer nanocarriers poly(acrylic acid)-block-poly(2-(methylsulfinyl)ethyl acrylate) (PAA-b-PMSEA) and poly(acrylic acid)-block-poly((2-(methylsulfinyl)ethyl acrylate)-co-(2-(methylthio)ethyl acrylate)) (PAA-b-P(MSEA-co-MTEA)) with well-controlled sizes (from 6 to 35 nm), negative charge content (from 17% to 83% PAA), and hydrophobicity and quantified their leaf uptake, phloem loading, and distribution in tomato (Solanum lycopersicum) plants 3 days after foliar application of 20 μL of a 1g L^-1 star polymer solution. In spite of their property differences, ∼30% of the applied star polymers translocated to other plant organs, higher than uptake of conventional foliar applied agrochemicals (<5%). The property differences affected their distribution in the plant. The ∼6 nm star polymers exhibited 3 times higher transport to younger leaves than larger ones, while the ∼35 nm star polymer had over 2 times higher transport to roots than smaller ones, suggesting small star polymers favor symplastic unloading in young leaves, while larger polymers favor apoplastic unloading in roots. For the same sized star polymer, a smaller negative charge content (yielding ζ ∼ -12 mV) enhanced translocation to young leaves and roots, whereas a larger negative charge (ζ < -26 mV) had lower mobility. Hydrophobicity only affected leaf uptake pathways, but not translocation. This study can help design agrochemical nanocarriers for efficient foliar uptake and targeting to desired plant organs, which may decrease agrochemical use and environmental impacts of agriculture.

Surfactant Driven Marangoni Spreading in the Presence of Predeposited Insoluble Surfactant Monolayers

When an insoluble surfactant is deposited on the surface of a thin fluid film, stresses induced by surface tension gradients drive Marangoni spreading across the subphase surface. The presence of a predeposited layer of an insoluble surfactant alters that spreading. In this study, the fluid film was aqueous, the predeposited insoluble surfactant was dipalmitoylphosphatidylcholine (DPPC), and the deposited insoluble surfactant was oleic acid. An optical density-based method was used to measure subphase surface distortion, called the Marangoni ridge, associated with propagation of the spreading front. The movement of the Marangoni ridge was correlated with movement of surface tracer particles that indicated both the boundary between the two surfactant layers and the surface fluid velocities. As the deposited oleic acid monolayer spread, it compressed the predeposited DPPC monolayer. During spreading, the surface tension gradient extended into the predeposited monolayer, which was compressed nonuniformly, from the deposited monolayer. The spreading was so rapid that the compressed predeposited surfactant could not have been in quasi-equilibrium states during the spreading. As the initial concentrations of the predeposited surfactant were increased, the shape of the Marangoni ridge deformed. When the initial concentration of the predeposited surfactant reached about 70 A²/molecule, there was no longer a Marangoni ridge but rather a broadly distributed excess of fluid above the initial fluid height. The nonuniform compression of the annulus of the predeposited monolayer also caused tangential motion ahead of both the Marangoni ridge and the boundary between the two monolayers. Spreading ceased when the two monolayers reached the same final surface tension. The final area per molecule of the DPPC monolayer matched that expected from the equilibrium DPPC isotherm at the same final surface tension. Thus, at the end of spreading, there was a simple surface tension balance between the two distinct monolayers.

Amphiphilic Thiol Polymer Nanogel Removes Environmentally Relevant Mercury Species from Both Produced Water and Hydrocarbons

Technologies for removal of mercury from produced water and hydrocarbon phases are desired by oil and gas production facilities, oil refineries, and petrochemical plants. Herein, we synthesize and demonstrate the efficacy of an amphiphilic, thiol-abundant (11.8 wt % S, as thiol) polymer nanogel that can remove environmentally relevant mercury species from both produced water and the liquid hydrocarbon. The nanogel disperses in both aqueous and hydrocarbon phases. It has a high sorption affinity for dissolved Hg(II) complexes and Hg-dissolved organic matter complexes found in produced water and elemental (Hg⁰) and soluble Hg-alkyl thiol species found in hydrocarbons. X-ray absorption spectroscopy analysis indicates that the sorbed mercury is transformed to a surface-bound Hg(SR)₂ species in both water and hydrocarbon regardless of its initial speciation. The nanogel had high affinity to native mercury species present in real produced water (>99.5% removal) and in natural gas condensate (>85% removal) samples, removing majority of the mercury species using only a 50 mg L^-1 applied dose. This thiolated amphiphilic polymeric nanogel has significant potential to remove environmentally relevant mercury species from both water and hydrocarbon at low applied doses, outperforming reported sorbents like sulfur-impregnated activated carbons because of the mass of accessible thiol groups in the nanogel.

Temperature- and pH-Responsive Star Polymers as Nanocarriers with Potential for in Vivo Agrochemical Delivery

Climate change is increasing the severity and length of heat waves. Heat stress limits crop productivity and can make plants more sensitive to other biotic and abiotic stresses. New methods for managing heat stress are needed. Herein, we have developed ∼30 nm diameter poly(acrylic acid)-block-poly(N-isopropylacrylamide) (PAA-b-PNIPAm) star polymers with varying block ratios for temperature-programmed release of a model antimicrobial agent (crystal violet, CV) at plant-relevant pH. Hyperspectral-Enhanced Dark field Microscopy was used to investigate star polymer-leaf interactions and route of entrance. The majority of loaded star polymers entered plant leaves through cuticular and epidermis penetration when applied with the adjuvant Silwet L-77. Up to 43 wt % of star polymers (20 μL at 200 mg L^-1 polymer concentration) applied onto tomato (Solanum lycopersicum) leaves translocated to other plant compartments (younger and older shoots, stem, and root) over 3 days. Without Silwet L-77, the star polymers penetrated the cuticle, but mainly accumulated at the epidermis cell layer. The degree of the star polymer temperature responsiveness for CV release in vitro in the range of 20 to 40 °C depends on pH and the ratio of the PAA to PNIPAm blocks. Temperature-responsive release of CV was also observed in vivo in tomato leaves. These results underline the potential for PAA-b-PNIPAm star polymers to provide efficient and temperature-programmed delivery of cationic agrochemicals into plants for protection against heat stress.

Depletion Forces Induced by Mixed Micelles of Nonionic Block Copolymers and Anionic Surfactants

Depletion forces were measured between a silica sphere and a silica plate in solutions containing nonionic Pluronic P123 poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) triblock copolymers and anionic sodium dodecyl sulfate (SDS) surfactants using colloidal probe atomic force microscopy. Prior research established synergistic depletion force enhancement in solutions containing SDS and unimeric Pluronic F108 block copolymers via formation of large pseudo-polyelectrolyte complexes. The current work addresses a more complex system where the polymer is above its critical micelle concentration, and surfactant binding alters not only the size and charge of the micelles but also the number of polymers per micelle. Force profiles were measured in 10 000 ppm P123 (1 wt %, corresponding to 1.72 mM based on average molar mass) solutions containing SDS at concentrations up to 64 mM and compared to micellar P123 solutions and to P123-free SDS solutions. Whereas force profiles in the SDS-free micellar P123 solutions were purely repulsive, P123/SDS complexation produced synergistic depletion force enhancement for SDS concentrations between 2 and 32 mM. The synergism that occurred within a finite SDS concentration range was explained by comparing the hydrodynamic size, molar mass, charge, and concentration of depletants in P123/SDS mixtures and their respective single-component solutions obtained with the aid of dynamic light scattering, static light scattering, and dodecyl sulfate ion-selective electrode measurements. These measurements showed that complexation produced effects that would be mutually counteracting with respect to depletion forces: decreasing the mixed micelle hydrodynamic diameter relative to SDS-free P123 micelles would tend to weaken depletion forces, while adding charge and decreasing the aggregation number of polymers per micelle (thereby increasing the number concentration of micellar depletants) would tend to strengthen depletion forces.

Advective-diffusive spreading of diffusiophoretic colloids under transient solute gradients

We analytically calculate the one-dimensional advective-diffusive spreading of a point source of diffusiophoretic (DP) colloids, driven by the simultaneous diffusion of a Gaussian solute patch. The spreading of the DP colloids depends critically on the ratio of the DP mobility, M (which can be positive or negative), to the solute diffusivity, D_s. For instance, we demonstrate, for the first time, that solute-repelling colloids (M < 0) undergo long-time super-diffusive transport for M/D_s < -1. In contrast, the spreading of strongly solute-attracting colloids (M/D_s≫ 1) can be spatially arrested over long periods on the solute diffusion timescale, due to a balance between colloid diffusion and DP under the evolving solute gradient. Further, a patch of the translating solute acts as a "shuttle" that rapidly transports the colloids relative to their diffusive timescale. Finally, we use numerical computations to show that the above behaviors persist for three-dimensional, radially symmetric DP spreading. The results presented here could guide the use of DP colloids for microscale particle sorting, deposition, and delivery.

Colloidal Depletion and Structural Force Synergism or Antagonism in Solutions of Mutually Repelling Polyelectrolytes and Ionic Surfactants

Depletion and structural forces were measured between a silica sphere and plate in solutions containing sodium polyacrylate (Na-PAA) anionic polyelectrolyte and sodium dodecyl sulfate (SDS) anionic surfactant using colloidal probe atomic force microscopy, at high pH where the two species are electrostatically repelling from each other and from the silica surfaces. Measurements were performed for a range of SDS and Na-PAA concentrations to span conditions where only one of the species or both of the species would exert a detectable depletion or structural force when present in a single-component solution. In mixed solutions, conditions were identified (i) where depletion attraction was synergistically enhanced or antagonistically weakened relative to single component solutions; (ii) where the range of the depletion attraction was significantly extended and the repulsive structural force barrier was eliminated, due to simultaneous depletion of both species over different length scales; and (iii) where one species was the dominant depletant and forces in mixtures were indistinguishable from those in a single component solution of the dominant depletant. Force measurements were interpreted with the aid of pyrene solubilization assays of SDS micellization and dynamic light scattering investigation of the state of assembly of the polyelectrolyte or surfactant. The variety of colloidal force effects were attributed to ionic strength and excluded volume effects of Na-PAA on SDS micellization, ionic strength effects of SDS on Na-PAA chain clustering in solution, and ionic strength effects on the counterion contribution to polyelectrolyte osmotic pressure. While prior studies have shown that depletion force synergism occurs when polymers and surfactants form mixed complexes, this work shows that it can occur in noncomplexing mixtures as well, and it indicates the variety of effects that should be taken into account when attempting to predict forces in such mixtures.

Flow regime transitions and effects on solute transport in surfactant-driven Marangoni flows

HYPOTHESIS

Surfactant-driven Marangoni flow on liquid films is predicted to depend on subphase depth and initial surface tension difference between the subphase and deposited surfactant solution drop. Changes in flow behavior will impact transport of soluble species entrained in the Marangoni flow along the surface. In extreme cases, the subphase film may rupture, limiting transport. Understanding this behavior is important for applications in drug delivery, coatings, and oil spill remediation.

EXPERIMENTS

A trans-illumination optical technique measured the subphase height profiles and drop content transport after drop deposition when varying initial subphase depth, surfactant concentration, and subphase viscosity.

FINDINGS

Three distinct flow regimes were identified depending on the subphase depth and surfactant concentration and mapped onto an operating diagram. These are characterized as a "central depression" bounded by an outwardly traveling ridge, an "annular depression" bounded by a central dome and the traveling ridge, and an "annular dewetting" when the subphase ruptures. Well above the critical micelle concentration, transitions between regimes occur at characteristic ratios of gravitational and initial surface tension gradient stresses; transitions shift when surfactant dilution during spreading weakens the stress before the completion of the spreading event. Drop contents travel with the ridge, but dewetting hinders transport.

Responsive behavior of a branched-chain polymer network: a molecular dynamics study

Smart polymer hydrogels, which can undergo structural and volume phase transitions in response to external stimuli, have gained much attention for their widespread technological applications. Compared to linear polymers, branched chains offer more extensive opportunities to rationally design functional materials, since they permit more extensive structural tunability-for instance by adjusting the balance between hydrophobic and hydrophilic units, the grafting fraction of backbone monomers, or the side chain length, topology, and solubility. Here we conduct coarse-grained molecular dynamics simulations to assess how well generic physical principles capture this complex interplay of tuning parameters, specifically when building networks from complex branched chains with a hydrophobic backbone. Swollen chains collapse upon reducing side chain solubility, length, and grafting density, but neither the sharpness of this transition nor its dynamic range, if measured via chain extension, depends monotonically on these parameters. Networks comprising such chains are more swollen and exhibit even sharper transitions, but their higher responsiveness goes along with a swelling ratio that falls behind that of single chains.

Surfactant-induced Marangoni transport of lipids and therapeutics within the lung

Understanding the fundamentals of surface transport on thin viscous films has important application in pulmonary drug delivery. The human lung contains a large-area interface between its complex fluid lining and inhaled air. Marangoni flows driven by surface tension gradients along this interface would promote enhanced distribution of inhaled therapeutics by carrying them from where they are deposited in the upper airways, along the fluid interface to deeper regions of the lung. Motivated by the potential to improve therapies for acute and chronic lung diseases, we review recent progress in modeling and experimental studies of Marangoni transport induced by the deposition of surfactant-containing microliter drops and liquid aerosols (picoliter drops) onto a fluid interface. The roles of key system variables are identified, including surfactant solubility, drop miscibility with the subphase, and the thickness, composition and surface properties of the subphase liquid. Of particular interest is the unanticipated but crucial role of aerosol processing to achieve Marangoni transport via phospholipid vesicle dispersions, which are likely candidates for a biocompatible delivery system. Progress in this field has the potential to not only improve outcomes in patients with chronic and acute lung diseases, but also to further our understanding of surface transport in complex systems.

Moringa oleifera Seed Protein Adsorption to Silica: Effects of Water Hardness, Fractionation, and Fatty Acid Extraction

Motivated by the proposed use of cationic protein-modified sand for water filtration in developing nations, this study concerns the adsorption of Moringa oleifera seed proteins to silica surfaces. These proteins were prepared in model waters of varying hardness and underwent different levels of fractionation, including fatty acid extraction and cation exchange chromatography. Adsorption isotherms were measured by ellipsometry, and the zeta potentials of the resulting protein-decorated surfaces were measured by the rotating disk streaming potential method. The results indicate that the presence of fatty acids has little effect on the M. oleifera cationic protein adsorption isotherm. Adsorption from the unfractionated extract was indistinguishable from that of the cationic protein isolates at low concentrations but yielded significantly greater extents of adsorption at high concentrations. Adsorption isotherms for samples prepared in model hard and soft fresh waters were indistinguishable from each other over the measured bulk solution concentration range, but adsorption from hard or soft water was more extensive than adsorption from deionized water at moderate protein concentrations. Streaming potential measurements showed that adsorption reversed the net sign of the zeta potential of silica from negative to positive for all protein fractions and water hardness conditions at protein bulk concentrations as low as 0.03 μg/mL. This suggests that sands can be effectively modified with M. oleifera proteins using small amounts of seed extract under various local water hardness conditions. Finally, ellipsometry indicated that M. oleifera proteins adsorb irreversibly with respect to rinsing in these model fresh waters, suggesting that the modified sand would be stable on repeated use for water filtration. These studies may aid in the design of a simple, effective, and sustainable water purification device for developing nations.

Friction and adhesion control between adsorbed layers of polyelectrolyte brush-grafted nanoparticles via pH-triggered bridging interactions

HYPOTHESIS

Adsorption of polyelectrolyte brush-grafted nanoparticles (BGNPs) produces a heterogeneous interface with sub-monolayer surface coverage resulting from lateral electrostatic repulsions that limit packing. As a result, the interaction forces between opposing BGNP layers include an adhesive cross-surface BGNP-substrate bridging force that depends on the interparticle spacing, particle size, and strength of electrostatic interactions. We hypothesize that BGNPs with pH-responsive, annealed polyelectrolyte brushes can undergo controlled changes in surface area coverage through post-adsorption swelling or de-swelling into non-equilibrium layer conformations and that such changes in surface coverage can switch off or switch on particle intercalation, bridging attractions, and enhanced energy dissipation upon sliding. This work aims to characterize the nature of surface forces in heterogeneous BGNP adsorbed layers and to utilize pH-sensitive bridging forces as a mechanism to tune friction and adhesion.

EXPERIMENTS

Colloidal probe atomic force microscopy (CP-AFM) is used to measure normal and lateral forces between negatively charged silica surfaces with adsorbed pH-responsive cationic BGNPs. The BGNPs are poly(2-dimethylaminoethyl methacrylate) brush-grafted silica nanoparticles. Adhesion force and friction analysis is complemented by simultaneous quartz-crystal microbalance and ellipsometry measurements under conditions that render the particles strongly charged and swollen (acidic) or weakly charged and de-swollen (basic).

FINDINGS

Adsorbed BGNPs can be swollen or de-swollen via pH rinses, enabling direct control of surface coverage and bridging interactions. Transitions from adhesive bridging contacts with high friction to non-adhesive contacts with low friction forces occur when adsorbed BGNP layers are switched from a de-swollen/weakly charged state to a swollen/highly charged state. The ability to controllably shift the character of normal and lateral forces via coverage-mediated bridging interactions is a unique feature of adsorbed nanoparticulate brush constructs and highlights their potential to condition surfaces with additional functionality compared to dense, planar homopolymer brushes.

Evolution and Disappearance of Solvent Drops on Miscible Polymer Subphases

Traditionally, an interface is defined as a boundary between immiscible phases. However, previous work has shown that even when two fluids are completely miscible, they maintain a detectable "effective interface" for long times. Miscible interfaces have been studied in various systems of two fluids with a single boundary between them. However, this work has not extended to the three-phase system of a fluid droplet placed on top of a miscible pool. We show that these three-phase systems obey the same wetting conditions as immiscible systems, and that their drop shapes obey the Augmented Young-Laplace Equation. Over time, the miscible interface diffuses and the shape of the drop evolves. We place 2-microliter drops of water atop miscible poly(acrylamide) solutions. The drop is completely wetted by the subphase, and then remains detectable beneath the surface for many minutes. An initial effective interfacial tension can be approximated to be on the order of 0.5 mN/m using the capillary number. Water and poly(acrylamide) are completely miscible in all concentrations, and yet, when viewed from the side, the drop maintains a capillary shape. Study of this behavior is important to the understanding of effective interfaces between miscible polymer phases, which are pervasive in nature.

Adsorbed poly(aspartate) coating limits the adverse effects of dissolved groundwater solutes on Fe0 nanoparticle reactivity with trichloroethylene

For in situ groundwater remediation, polyelectrolyte-modified nanoscale zerovalent iron particles (NZVIs) have to be delivered into the subsurface, where they degrade pollutants such as trichloroethylene (TCE). The effect of groundwater organic and ionic solutes on TCE dechlorination using polyelectrolyte-modified NZVIs is unexplored, but is required for an effective remediation design. This study evaluates the TCE dechlorination rate and reaction by-products using poly(aspartate) (PAP)-modified and bare NZVIs in groundwater samples from actual TCE-contaminated sites in Florida, South Carolina, and Michigan. The effects of groundwater solutes on short- and intermediate-term dechlorination rates were evaluated. An adsorbed PAP layer on the NZVIs appeared to limit the adverse effect of groundwater solutes on the TCE dechlorination rate in the first TCE dechlorination cycle (short-term effect). Presumably, the pre-adsorption of PAP "trains" and the Donnan potential in the adsorbed PAP layer prevented groundwater solutes from further blocking NZVI reactive sites, which appeared to substantially decrease the TCE dechlorination rate of bare NZVIs. In the second and third TCE dechlorination cycles (intermediate-term effect), TCE dechlorination rates using PAP-modified NZVIs increased substantially (~100 and 200%, respectively, from the rate of the first spike). The desorption of PAP from the surface of NZVIs over time due to salt-induced desorption is hypothesized to restore NZVI reactivity with TCE. This study suggests that NZVI surface modification with small, charged macromolecules, such as PAP, helps to restore NZVI reactivity due to gradual PAP desorption in groundwater.

Aerosolizing Lipid Dispersions Enables Antibiotic Transport Across Mimics of the Lung Airway Surface Even in the Presence of Pre-existing Lipid Monolayers

BACKGROUND

Secondary lung infections are the primary cause of morbidity associated with cystic fibrosis lung disease. Aerosolized antibiotic inhalation is potentially advantageous but has limited effectiveness due to altered airway aerodynamics and deposition patterns that limit drug access to infected regions. One potential strategy to better reach infected areas is to formulate aerosols with surfactants that induce surface tension gradients and drive postdeposition drug dispersal via Marangoni transport along the airway surface liquid (ASL). Since this relies on surfactant-induced surface tension reduction, the presence of endogenous lipid monolayers may hinder drug dispersal performance.

METHODS

Tobramycin solutions were formulated with dipalmitoylphosphatidylcholine (DPPC), a major component of endogenous pulmonary surfactant, to drive postdeposition aerosol dispersal across a model ASL based on a liquid layer or "subphase" of aqueous porcine gastric mucin (PGM) solution with predeposited DPPC monolayers to mimic the endogenous surfactant. In vitro subphase samples were collected from regions outside the aerosol deposition zone and assayed for tobramycin concentration using a closed enzyme donor immunoassay. The motion of a tracking bead across the subphase surface and the corresponding decrease in surface tension on aerosol deposition were tracked both with and without a predeposited DPPC monolayer. The surface tension/area isotherm for DPPC on PGM solution subphase was measured to aid in the interpretation of the tobramycin dispersal behavior.

RESULTS AND CONCLUSIONS

Transport of tobramycin away from the deposition region occurs in aerosols formulated with DPPC whether or not predeposited lipid is present, and tobramycin concentrations are similar in both cases across biologically relevant length scales (∼8 cm). When DPPC is deposited from an aerosol, it induces ultralow surface tensions (<5 mN/m), which drive Marangoni flows, even in the presence of a dense background layer of DPPC. Therefore, aerosolized phospholipids, such as DPPC, will likely be effective spreading agents in the human lung.

Silver Sink Effect of Humic Acid on Bacterial Surface Colonization in the Presence of Silver Ions and Nanoparticles

Silver nanoparticles (AgNPs) released from consumer products may enter the environment and possibly harm microbial communities. Prior research showed that surface-adherent AgNPs inhibit bacterial surface colonization, a precursor to biofilm formation, only when planktonic bacterial inoculum concentrations are less than a threshold level ( Wirth and co-workers, J. Colloid Interface Sci. 2016 , 467 , 17 - 27 ). This inoculum effect is due to a decrease in free silver ion concentration associated with sublethal binding to bacteria. Natural organic matter can be an additional silver sink in environmental systems. Using Pseudomonas fluorescens as a model biofilm-forming bacterium, we find significant increases in minimum bactericidal concentrations for AgNP suspensions and Ag⁺ in solution when adding humic acid (HA) to bacterial suspensions. When HA is present, planktonic bacteria survive and colonize AgNP-laden glass surfaces at lower bacterial inoculum concentrations than were needed for survival and colonization in its absence. This occurs despite the observed tendency of HA to inhibit colonization on bare glass surfaces when silver is absent. Results are interpreted through equilibrium Ag⁺ binding isotherms to HA and suspended bacteria. These results indicate that silver ion sinks may lessen AgNP impacts on natural microbial ecology relative to the disruption observed in pristine laboratory conditions.

Effect of emplaced nZVI mass and groundwater velocity on PCE dechlorination and hydrogen evolution in water-saturated sand

The effect of nZVI mass loading and groundwater velocity on the tetrachloroethylene (PCE) dechlorination rate and the hydrogen evolution rate for poly(maleic acid-co-olefin) (MW=12K) coated nZVI was examined. In batch reactors, the PCE reaction rate constant (3.7×10^-4Lhr^-1m^-2) and hydrogen evolution rate constant (1.4 nanomolLhr^-1m^-2) were independent of nZVI concentration above 10g/L, but the PCE dechlorination rate decreased and the hydrogen evolution rate increased for nZVI concentration below 10g/L. The nonlinearity between nZVI mass loading and PCE dechlorination and H₂ evolution was explained by differences in pH and E_h at each nZVI mass loading; PCE reactivity increased when solution E_h decreased, and the H₂ evolution rate increased with decreasing pH. Thus, nZVI mass loading of <5g/L yields lower reactivity with PCE and lower efficiency of Fe° utilization than for higher nZVI mass loading. The PCE dechlorination rate increased with increasing pore-water velocity, suggesting that mass transfer limits the reaction at low porewater velocity. Overall, this work suggests that design of nZVI-based reactive barriers for groundwater treatment should consider the non-linear effects of both mass loading and flow velocity on performance and expected reactive lifetime.

Enhanced interfacial activity of multi-arm poly(ethylene oxide) star polymers relative to linear poly(ethylene oxide) at fluid interfaces

Interfacial tension reduction, dynamic dilatational elasticity and extent of adsorption were investigated for linear poly(ethylene oxide) (PEO) chains of varying molecular weight and for PEO star polymers with an average of 64 arms per star at air/water, xylene/water, and cyclohexane/water interfaces.

Sequential Adsorption of Nanoparticulate Polymer Brushes as a Strategy To Control Adhesion and Friction

This work investigates surface forces that result from adsorbed layers of silica nanoparticles with grafted pH-responsive, cationic poly(2-(dimethylamino)ethyl methacrylate) brushes (SiO₂-g-PDMAEMA) and how adhesive bridging forces can be suppressed and friction forces reduced by "backfilling" these heterogeneous adsorbed layers with nonionic poly(ethylene oxide) star copolymers (Star PEO₄₅MA). Adsorption of SiO₂-g-PDMAEMA and Star PEO₄₅MA to silica is measured as a function of pH by quartz crystal microbalance with dissipation (QCM-D) in order to evaluate the electrostatically driven adsorption of SiO₂-g-PDMAEMA and hydrogen-bonding-driven adsorption of Star PEO₄₅MA. Force measurements performed using colloidal probe force microscopy show the strong role that limited surface coverage plays in adhesive bridging forces between silica with adsorbed SiO₂-g-PDMAEMA, while Star PEO₄₅MA adsorption produces purely repulsive steric interactions. Bridging between SiO₂-g-PDMAEMA-coated surfaces produces frictional forces that tend to be larger than those acting between bare surfaces at similar normal loads, while friction is consistently decreased by Star PEO₄₅MA adsorption. Sequential adsorption of SiO₂-g-PDMAEMA and Star PEO₄₅MA generates high-coverage mixed nanoparticulate brush layers with uniformly repulsive normal forces and reduced friction forces. Adsorption and force measurements reveal that Star PEO₄₅MA not only adsorbs to silica but also binds to SiO₂-g-PDMAEMA. The latter allows sequential adsorption of the two components to produce mixed multilayers. The mixed SiO₂-g-PDMAEMA/Star PEO₄₅MA multilayers exhibit larger layer thicknesses, no bridging, and sustained smooth friction, highlighting their potential usefulness as aqueous boundary lubricant layers.

Enabling Marangoni flow at air-liquid interfaces through deposition of aerosolized lipid dispersions

It has long been known that deposited drops of surfactant solution induce Marangoni flows at air-liquid interfaces. These surfactant drops create a surface tension gradient, which causes an outward flow at the fluid interface. We show that aqueous phospholipid dispersions may be used for this same purpose. In aqueous dispersions, phospholipids aggregate into vesicles that are not surface-active; therefore, drops of these dispersions do not initiate Marangoni flow. However, aerosolization of these dispersions disrupts the vesicles, allowing access to the surface-active monomers within. These lipid monomers do have the ability to induce Marangoni flow. We hypothesize that monomers released from broken vesicles adsorb on the surfaces of individual aerosol droplets and then create localized surface tension reduction upon droplet deposition. Deposition of lipid monomers via aerosolization produces surface tensions as low as 1mN/m on water. In addition, aerosolized lipid deposition also drives Marangoni flow on entangled polymer solution subphases with low initial surface tensions (∼34mN/m). The fact that aerosolization of phospholipids naturally found within pulmonary surfactant can drive Marangoni flows on low surface tension liquids suggests that aerosolized lipids may be used to promote uniform pulmonary drug delivery without the need for exogenous spreading agents.

Transport of a partially wetted particle at the liquid/vapor interface under the influence of an externally imposed surfactant generated Marangoni stress

Marangoni flows offer an interesting and useful means to transport particles at fluid interfaces with potential applications such as dry powder pulmonary drug delivery. In this article, we investigate the transport of partially wetted particles at a liquid/vapor interface under the influence of Marangoni flows driven by gradients in the surface excess concentration of surfactants. We deposit a microliter drop of soluble (sodium dodecyl sulfate aqueous solution) surfactant solution or pure insoluble liquid (oleic acid) surfactant on a water subphase and observe the transport of a pre-deposited particle. Following the previous observation by Wang et al. [1] that a surfactant front rapidly advances ahead of the deposited drop contact line initiates particle motion but then moves beyond the particle, we now characterize the two dominant, time- and position-dependent forces acting on the moving particle: 1) a surface tension force acting on the three-phase contact line around the particle periphery due to the surface tension gradient at the liquid/vapor interface which always accelerates the particle and 2) a viscous force acting on the immersed surface area of the particle which accelerates or decelerates the particle depending on the difference in the velocities of the liquid and particle. We find that the particle velocity evolves over time in two regimes. In the acceleration regime, the net force on the particle acts in the direction of particle motion, and the particle quickly accelerates and reaches a maximum velocity. In the deceleration regime, the net force on the particle reverses and the particle decelerates gradually and stops. We identify the parameters that affect the two forces acting on the particle, including the initial particle position relative to the surfactant drop, particle diameter, particle wettability, subphase thickness, and surfactant solubility. We systematically vary these parameters and probe the spatial and temporal evolution of the two forces acting on the particle as it moves along its trajectory in both regimes. We find that a larger particle always lags behind the smaller particle when placed at an equal initial distance from the drop. Similarly, particles more deeply engulfed in the subphase lag behind those less deeply engulfed. Further, the extent of particle transport is reduced as the subphase thickness decreases, due to the larger velocity gradients in the subphase recirculation flows.

Effect of polyelectrolyte-surfactant complexation on Marangoni transport at a liquid-liquid interface

Complexation of surfactants and oppositely charged polyelectrolytes is expected to alter Marangoni transport at a fluid interface compared to either single component system due to altered interfacial tension isotherms and mass transfer rates as well as adsorption irreversibility effects. We investigate Marangoni transport at the oil/water interface by passing mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) and cationic polyelectrolyte poly(3-(2-methylpropionamide)propyl) trimethylammonium chloride-acrylamide (poly[AM-MAPTAC]), or rinsing solutions, over an oil/water interface in a radial, stagnation point flow. The displacements of adsorbed tracer particles are recorded through optical microscopy. The net displacement, defined as the sum of the displacements occurring during the adsorption and desorption stages of one application and rinsing cycle, is up to 10 times greater for complexing surfactant/polymer mixtures compared to either single component system. The enhanced net displacement is largely determined by the enhanced transport upon adsorption, while the reverse displacement that would normally occur upon rinsing is partially suppressed by partially irreversible polymer adsorption at the oil/water interface. In addition to effects of complexation on interfacial tension gradient induced flow, complexation effects on the bulk, and possibly interfacial, viscosity also influence the interfacial transport.

Ionic Surfactant Binding to pH-Responsive Polyelectrolyte Brush-Grafted Nanoparticles in Suspension and on Charged Surfaces

The interactions between silica nanoparticles grafted with a brush of cationic poly(2-(dimethylamino) ethyl methacrylate) (SiO2-g-PDMAEMA) and anionic surfactant sodium dodecyl sulfate (SDS) is investigated by dynamic light scattering, electrophoretic mobility, quartz crystal microbalance with dissipation, ellipsometry, and atomic force microscopy. SiO2-g-PDMAEMA exhibits pH-dependent charge and size properties which enable the SDS binding to be probed over a range of electrostatic conditions and brush conformations. SDS monomers bind irreversibly to SiO2-g-PDMAEMA at low surfactant concentrations (∼10(-4) M) while exhibiting a pH-dependent threshold above which cooperative, partially reversible SDS binding occurs. At pH 5, SDS binding induces collapse of the highly charged and swollen brush as observed in the bulk by DLS and on surfaces by QCM-D. Similar experiments at pH 9 suggest that SDS binds to the periphery of the weakly charged and deswollen brush and produces SiO2-g-PDMAEMA/SDS complexes with a net negative charge. SiO2-g-PDMAEMA brush collapse and charge neutralization is further confirmed by colloidal probe AFM measurements, where reduced electrosteric repulsions and bridging adhesion are attributed to effects of the bound SDS. Additionally, sequential adsorption schemes with SDS and SiO2-g-PDMAEMA are used to enhance deposition relative to SiO2-g-PDMAEMA direct adsorption on silica. This work shows that the polyelectrolyte brush configuration responds in a more dramatic fashion to SDS than to pH-induced changes in ionization, and this can be exploited to manipulate the structure of adsorbed layers and the corresponding forces of compression and friction between opposing surfaces.

Surfactant Driven Post-Deposition Spreading of Aerosols on Complex Aqueous Subphases. 2: Low Deposition Flux Representative of Aerosol Delivery to Small Airways

BACKGROUND

Cystic fibrosis (CF) is associated with the accumulation of dehydrated mucus in the pulmonary airways. This alters ventilation and aerosol deposition patterns in ways that limit drug delivery to peripheral lung regions. We investigated the use of surfactant-based, self-dispersing aerosol carriers that produce surface tension gradients to drive two-dimensional transport of aerosolized medications via Marangoni flows after deposition on the airway surface liquid (ASL). We considered the post-deposition spreading of individual aerosol droplets and two-dimensional expansion of a field of aerosol droplets, when deposited at low fluxes that are representative of aerosol deposition in the small airways.

METHODS

We used physically entangled aqueous solutions of poly(acrylamide) or porcine gastric mucin as simple ASL mimics that adequately capture the full miscibility but slow penetration of entangled macromolecular chains of the ASL into the deposited drop. Surfactant formulations were prepared with aqueous solutions of nonionic tyloxapol or FS-3100 fluorosurfactant. Fluorescein dye served as a model "drug" tracer and to visualize the extent of post-deposition spreading.

RESULTS

The surfactants not only enhanced post-deposition spreading of individual aerosol droplets due to localized Marangoni stresses, as previously observed with macroscopic drops, but they also produced large-scale Marangoni stresses that caused the deposited aerosol fields to expand into initially unexposed regions of the subphase. We show that the latter is the main mechanism for spreading drug over large distances when aerosol is deposited at low fluxes representative of the small airways. The large scale convective expansion of the aerosol field drives the tracer (drug mimic) over areas that would cover an entire airway generation or more, in peripheral airways, where sub-monolayer droplet deposition is expected during aerosol inhalation.

CONCLUSIONS

The results suggest that aerosolized surfactant formulations may provide the means to maximize deposited drug uniformity in and access to small airways.

Transient Marangoni transport of colloidal particles at the liquid/liquid interface caused by surfactant convective-diffusion under radial flow

HYPOTHESIS

Interfacial tension gradients at a liquid/liquid interface drive Marangoni flows. When colloidal particles are adsorbed to an interface in systems with spatial and temporal gradients of surfactant concentration, these interfacial flows can be potentially significant contributors to the direction and rate of particle transport.

EXPERIMENTS

In this work, we use optical microscopy to measure the interfacial velocities of 5μm diameter polystyrene latex particles adsorbed at an oil/water interface, using olive oil to represent polar oils often encountered in cleaning applications.

FINDINGS

On surfactant adsorption the maximum interfacial velocity scales linearly with bulk surfactant concentration, even for concentrations exceeding the critical micelle concentration (CMC). The maximum interfacial velocity weakly decreases with increasing flow rate, but it varies non-monotonically with the radial distance from the inlet. Upon surfactant desorption into a rinse solution, the maximum velocity increases with increasing concentration of the original surfactant solution, but only up to a plateau near the CMC. These experimental trends are well-described by a convective-diffusion model for surfactant transport to or from the liquid/liquid interface coupled with Langmuir-type adsorption, using a constitutive relation between the interfacial tension gradient and interfacial velocity based on the interfacial tangential stress jump.

Surfactant Driven Post-Deposition Spreading of Aerosols on Complex Aqueous Subphases. 1: High Deposition Flux Representative of Aerosol Delivery to Large Airways

BACKGROUND

Aerosol drug delivery is a viable option for treating diseased airways, but airway obstructions associated with diseases such as cystic fibrosis cause non-uniform drug distribution and limit efficacy. Marangoni stresses produced by surfactant addition to aerosol formulations may enhance delivery uniformity by post-deposition spreading of medications over the airway surface, improving access to poorly ventilated regions. We examine the roles of different variables affecting the maximum post-deposition spreading of a dye (drug mimic).

METHODS

Entangled aqueous solutions of either poly(acrylamide) (PA) or porcine gastric mucin (PGM) serve as airway surface liquid (ASL) mimicking subphases for in vitro models of aerosol deposition. Measured aerosol deposition fluxes indicate that the experimental delivery conditions are representative of aerosol delivery to the conducting airways. Post-deposition spreading beyond the locale of direct aerosol deposition is tracked by fluorescence microscopy. Aqueous aerosols formulated with either nonionic surfactant (tyloxapol) or fluorosurfactant (FS-3100) are compared with surfactant-free control aerosols.

RESULTS

Significant enhancement of post-deposition spreading is observed with surfactant solutions relative to surfactant-free control solutions, provided the surfactant solution surface tension is less than that of the subphase. Amongst the variables considered--surfactant concentration, aerosol flow-rate, total deposited volume, time of delivery, and total deposited surfactant mass--surfactant mass is the primary predictor of maximum spread distance. This dependence is also observed for solutions deposited as a single, microliter-scale drop with a volume comparable to the total volume of deposited aerosol.

CONCLUSIONS

Marangoni stress-assisted spreading after surfactant-laden aerosol deposition at high fluxes on a complex fluid subphase is capable of driving aerosol contents over significantly greater distances compared to surfactant-free controls. Total delivered surfactant mass is the primary determinant of the extent of spreading, suggesting a great potential to extend the reach of aerosolized medication in partially obstructed airways via a purely physical mechanism.

Correlation of the physicochemical properties of natural organic matter samples from different sources to their effects on gold nanoparticle aggregation in monovalent electrolyte

Engineered nanoparticles (NPs) released into natural environments will interact with natural organic matter (NOM) or humic substances, which will change their fate and transport behavior. Quantitative predictions of the effects of NOM are difficult because of its heterogeneity and variability. Here, the effects of six types of NOM and molecular weight fractions of each on the aggregation of citrate-stabilized gold NPs are investigated. Correlations of NP aggregation rates with electrophoretic mobility and the molecular weight distribution and chemical attributes of NOM (including UV absorptivity or aromaticity, functional group content, and fluorescence) are assessed. In general, the >100 kg/mol components provide better stability than lower molecular weight components for each type of NOM, and they contribute to the stabilizing effect of the unfractionated NOM even in small proportions. In many cases, unfractionated NOM provided better stability than its separated components, indicating a synergistic effect between the high and low molecular weight fractions for NP stabilization. Weight-averaged molecular weight was the best single explanatory variable for NP aggregation rates across all NOM types and molecular weight fractions. NP aggregation showed poorer correlation with UV absorptivity, but the exponential slope of the UV-vis absorbance spectrum was a better surrogate for molecular weight. Functional group data (including reduced sulfur and total nitrogen content) were explored as possible secondary parameters to explain the strong stabilizing effect of a low molecular weight Pony Lake fulvic acid sample to the gold NPs. These results can inform future correlations and measurement requirements to predict NP attachment in the presence of NOM.

Electrostatically controlled swelling and adsorption of polyelectrolyte brush-grafted nanoparticles to the solid/liquid interface

Adsorption of 20 nm diameter silica nanoparticles grafted with a high density brush of the weak polymeric base poly(2-(dimethylamino)ethyl methacrylate) (SiO2-g-PDMAEMA) to the silica/aqueous interface was investigated using ellipsometry and streaming potential measurements. We measured SiO2-g-PDMAEMA adsorption to negatively charged silica surfaces in 1-100 mM sodium chloride solutions in the pH range 5-10 to investigate the role of electrostatics in the adsorption mechanism. In this system pH and ionic strength determine not only the charge density of the silica adsorption substrate but also the degree of ionization and swelling of the PDMAEMA brushes on the nanoparticles, resulting in nonmonotonic dependences of the extent of adsorption on pH and ionic strength. SiO2-g-PDMAEMA displays significantly different adsorption behavior from the linear PDMAEMA analogue, most notably in terms of a strongly hysteretic adsorption response to altered pH and a greater tendency to adsorb under weak surface attraction conditions that prevail at high pH.

Quasi-immiscible spreading of aqueous surfactant solutions on entangled aqueous polymer solution subphases

Motivated by the possibility of enhancing aerosol drug delivery to mucus-obstructed lungs, the spreading of a drop of aqueous surfactant solution on a physically entangled aqueous poly(acrylamide) solution subphase that mimics lung airway surface liquid was investigated. Sodium dodecyl sulfate was used as the surfactant. To visualize spreading of the drop and mimic the inclusion of a drug substance, fluorescein, a hydrophilic and non-surface-active dye, was added to the surfactant solution. The spreading progresses through a series of events. Marangoni stresses initiate the convective spreading of the drop. Simultaneously, surfactant escapes across the drop's contact line within a second of deposition and causes a change in subphase surface tension outside the drop on the order of 1 mN/m. Convective spreading of the drop ends within 2-3 s of drop deposition, when a new interfacial tension balance is achieved. Surfactant escape depletes the drop of surfactant, and the residual drop takes the form of a static lens of nonzero contact angle. On longer time scales, the surfactant dissolves into the subphase. The lens formed by the water in the deposited drop persists for as long as 3 min after the convective spreading process ends due to the long diffusional time scales associated with the underlying entangled polymer solution. The persistence of the lens suggests that the drop phase behaves as if it were immiscible with the subphase during this time period. Whereas surfactant escapes the spreading drop and advances on the subphase/vapor interface, hydrophilic dye molecules in the drop do not escape but remain with the drop throughout the convective spreading. The quasi-immiscible nature of the spreading event suggests that the chemical properties of the surfactant and subphase are much less important than their physical properties, consistent with prior qualitative studies of spreading of different types of surfactants on entangled polymer subphases: the selection of surfactant for pulmonary delivery applications may be limited only by physical and toxicological considerations. Further, the escape of surfactant from individual drops may provide an additional spreading mechanism in the lung, as hydrodynamic and/or surface pressure repulsions may drive individual droplets apart after deposition.

Effects of molecular weight distribution and chemical properties of natural organic matter on gold nanoparticle aggregation

The complexity of natural organic matter (NOM) motivates determination of how specific components in a NOM mixture interact with and affect nanoparticle (NP) behavior. The effects of two Suwannee River NOM fractions (separated by a 100,000 g/mol ultrafiltration membrane) on gold NP aggregation are compared. The weight-average molecular weight, Mw, for the unfractionated NOM was 23,300 g/mol, determined by size exclusion chromatography with multiangle light scattering. The NOM was comprised of ~1.8 wt % of >100,000 g/mol retentate (NOMr, Mw = 691,000 g/mol) and 98 wt % of filtrate (NOMf, Mw = 12,800 g/mol). Ten ppm of NOMr provided significantly better NP stability against aggregation than 10 ppm of NOMf in 100 mM NaCl due to steric effects. In the unfractionated NOM, the relative importance of the two components was concentration-dependent. For a low concentration of unfractionated NOM (10 ppm), both fractions contributed to the NOM effects; for a high concentration (560 ppm), NP stability was controlled by the small amount (10 ppm) of NOMr present, rather than the higher amount (550 ppm) of NOMf. Therefore, large humic aggregates in a heterogeneous NOM sample can have disproportionately strong effects, and characterization of Mw distributions (rather than average Mw) may be required to explain NOM effects on NP behavior.

Poly(ethylene oxide) star polymer adsorption at the silica/aqueous interface and displacement by linear poly(ethylene oxide)

Multiarm star copolymers with approximately 460 poly(ethylene oxide) (PEO) arms that have a degree of polymerization N = 45 were synthesized via atom transfer radical polymerization (ATRP) of PEO-methacrylate macromonomers in the presence of divinyl benzene cross-linkers. These are an example of molecular or nanoparticulate brushes that are of interest as steric stabilizers or boundary lubrication agents when adsorbed from solution to a solid/aqueous interface. We use ellipsometry to measure adsorption isotherms at the silica/aqueous interface for PEO star polymers and linear PEO chains having molecular weights comparable either to the star polymer or to the individual arms. The compactness of the PEO star polymers (molecular weight 1.2 × 10(6)) yields a saturation surface excess concentration that is approximately 3.5 times greater than that of the high molecular weight (1 × 10(6)) linear PEO. Adsorption of low molecular weight (6000) linear PEO was below the detection limit. Competitive adsorption experiments were conducted with ellipsometry, complemented by independent quartz crystal microbalance with dissipation (QCM-D) measurements. Linear PEO (high molecular weight) displaced preadsorbed PEO star polymers over the course of approximately 1.5 h, to form a mixed adsorbed layer having not only a significantly lower overall polymer surface excess concentration, but also a significantly greater amount of hydrodynamically entrapped water. Challenging a preadsorbed linear PEO (high molecular weight) layer with PEO star polymers produced no measurable change in the overall polymer surface excess concentration, but changes in the QCM-D energy dissipation and resonance frequency suggested that the introduction of PEO star polymers caused a slight swelling of the layer with a correspondingly small increase in entrapped water content.