Ellipsometric Study of Monodisperse Silica Particles at an Oil-Water Interface

Measurement of the Optical Constants of Sand Samples Using Ellipsometry on Sand-Adhesive Composites

In order to model the propagation of light through a sand cloud, it is critical to have accurate data for the optical constants of the sand particles that comprise it. The same holds true for modeling propagation through particles of any type suspended in a medium. Few methods exist, however, to measure these quantities with high accuracy. In this paper, a characterization method based on spectroscopic ellipsometry (SE) that can be applied to a particulate material is presented. In this method, a polished disc of an adhesive compound is prepared, and its optical constants are measured. Next, a mixture of the adhesive and a sand sample is prepared and processed into a polished disc, and SE is performed. By treating the mixture as a Bruggeman effective medium, the optical constants of the particulate material are extracted. For verification of the proposed method, it is first applied to pure silica powder, demonstrating good agreement between measured optical constants and literature values. It is then applied to Arizona road dust, a standard reference material, as well as real desert sand samples. The resulting optical constant data is input into a rigorous scattering model to predict extinction coefficients for various types of sand. Modeling results are compared to spectroscopic measurements on static sand samples, demonstrating good agreement between predicted and measured spectral properties including the presence of a Christiansen feature near a wavelength of 8 µm.

Bare and Sterically Stabilized PLGA Nanoparticles for the Stabilization of Pickering Emulsions

Pickering emulsions were formulated using biodegradable and biocompatible poly(lactic- co-glycolic acid) (PLGA) nanoparticles (NPs) prepared without surfactants or any other polymer than PLGA. A pharmaceutical and cosmetic oil (Miglyol) was chosen as the oil phase at a ratio of 10% w/w. These emulsions were then compared with emulsions using the same oil but formulated with well-described PLGA-poly(vinyl alcohol) (PVA) NPs, i.e., with PVA as NP stabilizers. Strikingly, the emulsions demonstrated very different structures at macroscopic, microscopic, and interfacial scales, depending on the type of NPs used. Indeed, the emulsion layer was significantly thicker when using PLGA NPs rather than PLGA-PVA NPs. This was attributed to the formation and coexistence of multiple water-in-oil-in-water (W/O/W) and simple oil-in-water (O/W) droplets, using a single step of emulsification, whereas simple O/W emulsions were obtained with PLGA-PVA NPs. The latter NPs were more hydrophilic than bare PLGA NPs because of the presence of PVA at their surface. Moreover, PLGA NPs only slightly lowered the oil/water interfacial tension whereas the decrease was more pronounced with PLGA-PVA NPs. The PVA chains at the PLGA-PVA NP surface could probably partially desorb from the NPs and adsorb at the interface, inducing the interfacial tension decrease. Finally, independent of their composition, NPs were adsorbed at the oil/water interface without influencing its rheological behavior, possibly due to their mobility at their interface. This work has direct implications in the formulation of Pickering emulsions and stresses the paramount influence of the physicochemical nature of the NP surface into the stabilization of these systems.

Silica-Encapsulated DNA-Based Tracers for Aquifer Characterization

Environmental tracing is a direct way to characterize aquifers, evaluate the solute transfer parameter in underground reservoirs, and track contamination. By performing multitracer tests, and translating the tracer breakthrough times into tomographic maps, key parameters such as a reservoir's effective porosity and permeability field may be obtained. DNA, with its modular design, allows the generation of a virtually unlimited number of distinguishable tracers. To overcome the insufficient DNA stability due to microbial activity, heat, and chemical stress, we present a method to encapsulated DNA into silica with control over the particle size. The reliability of DNA quantification is improved by the sample preservation with NaN₃ and particle redispersion strategies. In both sand column and unconsolidated aquifer experiments, DNA-based particle tracers exhibited slightly earlier and sharper breakthrough than the traditional solute tracer uranine. The reason behind this observation is the size exclusion effect, whereby larger tracer particles are excluded from small pores, and are therefore transported with higher average velocity, which is pore size-dependent. Identical surface properties, and thus flow behavior, makes the new material an attractive tracer to characterize sandy groundwater reservoirs or to track multiple sources of contaminants with high spatial resolution.

The triolein/aqueous interface and lipase activity studied by spectroscopic ellipsometry and coarse grained simulations

In spite of the importance of the triglyceride aqueous interface for processes like emulsification, surfactant interactions and lipase activity, relatively little is known about this interface compared to that between alkanes and water. Here, the contact between triolein and water was investigated in terms of water inclusion in the oil phase and orientation of the molecules at the interface. Coarse grained models of triglycerides in contact with water were constructed and correlated with experimental results of the changes in thickness and refractive index, obtained using spectroscopic ellipsometry of spin-coated triolein films. The topography of the layer was revealed by atomic force microscopy. Dry triolein and a triolein sample after equilibration with water were also compared structurally using small-angle X-ray scattering. Additionally, the kinetics of adsorption/activity of three different variants of the Thermomyces lanuginosus lipase (TLL) were investigated. The results show that uptake of water in the triolein phase leads to increase in thickness of the layer. The observed increase of thickness was further enhanced by an active lipase but reduced when an inactive mutant of the enzyme was applied.

Particle detachment from fluid interfaces: theory vs. experiments

Microparticle adsorption and self-assembly at fluid interfaces are strongly affected by the particle three-phase contact angle θ. On the single-particle level, θ can be determined by several techniques, including colloidal-probe AFM, the gel-trapping technique (GTT) and the freeze-fracture shadow-casting (FreSCa) method. While GTT and FreSCa provide contact angle distributions measured over many particles, colloidal-probe AFM measures the wettability of an individual (specified) particle attached onto an AFM cantilever. In this paper, we extract θ for smooth microparticles through the analysis of force-distance curves upon particle approach and retraction from the fluid interface. From each retraction curve, we determine: (i) the maximal force, Fmax; (ii) the detachment distance, Dmax; and (iii) the work for quasistatic detachment, W. To relate Fmax, Dmax and W to θ, we developed a detailed theoretical model based on the capillary theory of flotation. The model was validated in three different ways. First, the contact angles, evaluated from Fmax, Dmax and W, are all close in value and were used to calculate the entire force-distance curves upon particle retraction without any adjustable parameters. Second, the model was successfully applied to predict the experimental force-distance curve of a truncated sphere, whose cut is positioned below the point of particle detachment from the interface. Third, our theory was confirmed by the excellent agreement between the particle contact angles obtained from the colloidal-probe AFM data and the ensemble-average contact angles measured by both GTT and FreSCa. Additionally, we devised a very accurate closed-form expression for W (representing the energy barrier for particle detachment), thus extending previous results in the literature.

Pickering emulsions stabilized by charged nanoparticles

The stabilization of o/w Pickering emulsions in cases of weak adsorption of solid particles at the surface of oil droplets is addressed. Though the adsorption is usually very strong and irreversible when partial wetting conditions are fulfilled, electrostatic repulsions between charged solid particles act against the adsorption. The regime of weak adsorption was reached using charged silica nanoparticles at high pH and low ionic strength. O/w Pickering emulsions of the diisopropyl adipate oil were stabilized by colloidal nanoparticles of Ludox® AS40 consisting of non-aggregated particles of bare silica (hydrophilic). The combination of stability assessment, droplet size and electrokinetic potential measurements at various pH values, adsorption isotherms and cryo-SEM observations of the adsorbed layers disclosed the specificities of the stabilization of Pickering emulsions by adsorption of solid nanoparticles against strong electrostatic repulsions. Not only the long-term stability of emulsions was poor under strong electrostatic repulsions at high pH, but emulsification failed since full dispersion of oil could not be achieved. Emulsion stability was ensured by decreasing electrostatic repulsions by lowering the pH from 9 to 3. Stable emulsions were stabilized by a monolayer of silica particles at 54% coverage of the oil droplet surface at low silica content and an adsorption regime as multilayers was reached at higher concentrations of silica although there was no aggregation of silica in the bulk aqueous phase.

Particle contact angles at fluid interfaces: pushing the boundary beyond hard uniform spherical colloids

Micro and nanoparticles at fluid interfaces have been attracting increasing interest in the last few decades as building blocks for materials, as mechanical and structural probes for complex interfaces and as models for two-dimensional systems. The three-phase contact angle enters practically all aspects of the particle behavior at the interface: its thermodynamics (binding energy to the interface), dynamics (motion and drag at the interface) and interactions with the interface (adsorption and wetting). Moreover, many interactions among particles at the interface also strongly depend on the contact angle. These concepts have been extensively discussed for non-deformable, homogeneous and mostly spherical particles, but recent progress in particle synthesis and fabrication has instead moved in the direction of producing more complex micro and nanoscale objects, which can be responsive, deformable, heterogenous and/or anisotropic in shape, surface chemistry and material properties. These new particles have a much greater potential for applications and new science, and the study of their behavior at interfaces has only very recently started. In this paper, we critically review the current state of the art of the experimental methods available to measure the contact angle of micro and nanoparticles at fluid interfaces, indicating their strengths and limitations. We then comment on new particle systems that are currently attracting increasing interest in relation to their adsorption and assembly at fluid interfaces and discuss if and which ones of the current techniques are suited to investigate their properties at interfaces. Based on this discussion, we will finally try to indicate a direction in which new experimental methods should develop in the future to tackle the new challenges posed by the novel types of particles that more and more often are used at interfaces.

Pickering emulsions: what are the main parameters determining the emulsion type and interfacial properties?

We synthesized surface-active lipophilic core-hydrophilic shell latex particles, and we probed their efficiency as emulsion stabilizers. The relative weight percentage of the shell, RS/P, was varied to trigger the balance between lipophilicity and hydrophilicity of the particles. Particle wettability could concomitantly be tuned by the pH of the aqueous phase determining the surface charge. Emulsions covering a wide range of RS/P and pH values were fabricated, and their type, oil-in-water (O/W) or water-in-oil (W/O), and kinetic stability were systematically assessed. By adapting the particle gel trapping technique to pH-variable systems and by exploiting the limited coalescence process, we were able to determine the proportion of oil/water interfacial area, C, covered by the particles as well as their contact angle, θ. All of these data were gathered into a single generic diagram showing good correlation between the emulsion type and the particle contact angle (O/W for θ < 90° and W/O for θ > 90°) in agreement with the empirical Finkle rule. Interestingly, no stable emulsion could be obtained when the wettability was nearly balanced and a "bipolar"-like behavior was observed, with the particles adopting two different contact angles whose average value was close to 90°. For particles such that θ < 90°, O/W emulsions were obtained, and, depending on the pH of the continuous phase, the same type of particles and the same emulsification process led to emulsions characterized either by large drops densely covered by the particles or by small droplets that were weakly covered. The two metastable states were also accessible to emulsions stabilized by particles of variable origins and morphologies, thus proving the generality of our findings.

Proteins, polysaccharides, and their complexes used as stabilizers for emulsions: alternatives to synthetic surfactants in the pharmaceutical field?

Emulsions are widely used in pharmaceutics for the encapsulation, solubilization, entrapment, and controlled delivery of active ingredients. In order to answer the increasing demand for clean label excipients, natural polymers can replace the potentially irritative synthetic surfactants used in emulsion formulation. Indeed, biopolymers are currently used in the food industry to stabilize emulsions, and they appear as promising candidates in the pharmaceutical field too. All proteins and some polysaccharides are able to adsorb at a globule surface, thus decreasing the interfacial tension and enhancing the interfacial elasticity. However, most polysaccharides stabilize emulsions simply by increasing the viscosity of the continuous phase. Proteins and polysaccharides may also be associated either through covalent bonding or electrostatic interactions. The combination of the properties of these biopolymers under appropriate conditions leads to increased emulsion stability. Alternative layers of oppositely charged biopolymers can also be formed around the globules to obtain multi-layered "membranes". These layers can provide electrostatic and steric stabilization thus improving thermal stability and resistance to external treatment. The novel biopolymer-stabilized emulsions have a great potential in the pharmaceutical field for encapsulation, controlled digestion, and targeted release although several challenging issues such as storage and bacteriological concerns still need to be addressed.

Measuring single-nanoparticle wetting properties by freeze-fracture shadow-casting cryo-scanning electron microscopy

Nanoparticles at fluid interfaces are central to a rapidly increasing range of cutting-edge applications, including drug delivery, uptake through biological membranes, emulsion stabilization and the fabrication of nanocomposites. Understanding nanoscale wetting is a challenging issue, still unresolved for individual nanoparticles, and is essential in designing nanoparticle-building blocks with controlled surface properties. The core information about the structural and thermodynamic properties of particles at fluid interfaces is enclosed in the three-phase contact angle θ. Here we present a novel in situ method, on the basis of freeze-fracture shadow-casting cryo-scanning electron microscopy, that allows the measurement of contact angles of individual nanoparticles with 10 nm diameter, and thus greatly surpasses the current state of the art. We study hydrophilic and hydrophobic, organic and inorganic nanoparticles, demonstrating general applicability to systems of fundamental and applied nanotechnological interest. Significant heterogeneity in the wetting of nanoparticles is observed.

Theoretical evaluation of nano- or microparticulate contact angle at fluid/fluid interfaces: analysis of the excluded area behavior upon compression

A novel method for the determination of the particle contact angle at the liquid/gas or liquid/liquid interface based on the excluded area concept revealed, in spite of its simplicity, some serious difficulties connected with the exact quantitative particle deposition at the interface and with changes in the particulate contact angle upon binary monolayer compression. The comprehensive theoretical consideration of the contact angle behavior made for such a system allowed considerable improvements: firstly, the prediction of direction of the particles' displacement at surface pressure increase is now possible and hence an unambiguous identification of particle hydrophobicity can be done. Secondly, the analytical relation describing the dependence of the particulate contact angle on the surface tension (surface pressure) was derived, allowing the prediction of whether or not particles of a given hydrophobicity will be expelled from the monolayer at certain surface pressure and of the area relinquished by the displaced particles. Thirdly, the transformation of this relation upon taking into consideration the initial conditions led to a linear dependence between excluded area and normalized surface tension allowing the determination of the particle contact angle and the exact number of deposited particles simultaneously and independently of each other. Finally, the application of the improved approach to the previously collected experimental data yielded reasonable values for both particle contact angle and number of deposited particles.

Polyelectrolyte/magnetite nanoparticle multilayers: preparation and structure characterization

Polyelectrolyte composite planar films containing a different number of iron oxide (Fe3O4) nanoparticle layers have been prepared using the layer-by-layer adsorption technique. The nanocomposite assemblies were characterized by ellipsometry, UV-vis spectroscopy, and AFM. Linear growth of the multilayer thickness with the increase of the layer number, N, up to 12 reflects an extensive character of this parameter in this range. A more complicated behavior of the refractive index is caused by changes in the multilayer structure, especially for the thicker nanocomposites. A quantitative analysis of the nanocomposite structure is provided comparing a classical and a modified effective medium approach taking into account the influence of light absorption by the Fe3O4 nanoparticles on the complex refractive index of the nanocomposite and contributions of all components to film thickness. Dominant influence of co-adsorbed water on their properties was found to be another interesting peculiarity of the nanocomposite film. This effect, as well as possible film property modulation by light, is discussed.

Emulsification of partially miscible liquids using colloidal particles: nonspherical and extended domain structures

We present microscopy studies of particle-stabilized emulsions with unconventional morphologies. The emulsions comprise pairs of partially miscible fluids and are stabilized by colloids. Alcohol-oil mixtures are employed; silica colloids are chemically modified so that they have partial wettability. We create our morphologies by two distinct routes: starting with a conventional colloid-stabilized emulsion or starting in the single-fluid phase with the colloids dispersed. In the first case temperature cycling leads to the creation of extended fluid domains built around some of the initial fluid droplets. In the second case quenching into the demixed region leads to the formation of domains which reflect the demixing kinetics. The structures are stable due to a jammed, semisolid, multilayer of colloids on the liquid-liquid interface. The differing morphologies reflect the roles in formation of the arrested state of heterogeneous and homogeneous nucleation and spinodal decomposition. The latter results in metastable, bicontinuous emulsions with frozen interfaces, at least for the thin-slab samples, investigated here.

Determination of Contact Angles of Nanosized Silica Particles by Multi-Angle Single-Wavelength Ellipsometry

The determination of nanoparticle wettability is an area of great practical importance to materials science and engineering fields. There has been some recent interest in using spectroscopic analysis to indirectly categorize behaviour of monolayers, using the reflective and refractive properties of immersed particles. A method is developed here to calculate specific contact information for nanoparticles at an air–water interface, using single wavelength ellipsometry. A two-layer model is used that considers the refractive index of the immersed and non-immersed particle portions. Pressure–area isotherms and Brewster angle images were used to confirm and categorize the nature of a packed monolayer, and ψ and Δ measurements taken of the interface. Immersion thickness was calculated and related to an equilibrium contact angle, and combined thickness data for the two layers was successfully related to the diameter of the particles.

Quasiwetting on spherical solid surfaces by oil-water-amphiphile mixtures

We study the wetting behavior on spherical walls by ternary mixtures of oil, water, and an amphiphile. We use the Ginzburg-Landau free energy with a single order parameter and find that there are different stable structures of the interface and that a quasiwetting transition is the mechanism involved in the transition among them. We calculate these wetting transitions for two sets of parameters in the bulk free energy which are known to show microemulsion behavior. The surface transitions are thin-thick first-order transitions (continuous transitions are absent), and the phase diagram in surface parameter space is constructed. For the first set of bulk parameters water, oil, and a microemulsion coexist, and we study the first-order transition where the oil phase wets the wall-microemulsion interface and its behavior as the radius of the wall becomes large. Therefore, we recover the known wetting transitions on a planar wall. In the second set of bulk parameters only water and oil coexist, and for some sizes of the solid wall, the oil phase wets the wall-water interface, and the phase behavior is extremely rich. We obtain a coexistence of four surface phases or two triple points followed by three lines of first-order transitions which end at three critical points depending on the radius of the surface. When there are micellar metastable solutions in bulk, the behavior of the thickness of the wetting layer of the oil phase as the radius of the spherical wall gets larger is nonmonotonic. We associate this behavior with the intrinsic micelle structure due to the spontaneous curvature of the model.

Silica particle-stabilized emulsions of silicone oil and water: aspects of emulsification

A study of the emulsification of silicone oil and water in the presence of partially hydrophobic, monodisperse silica nanoparticles is described. Emulsification involves the fragmentation of bulk liquids and the resulting large drops and the coalescence of some of those drops. The influence of particle concentration, oil/water ratio, and emulsification time on the relative extents of fragmentation and coalescence during the formation of emulsions, prepared using either batch or continuous methods, has been investigated. For batch emulsions, the average drop diameter decreases with increasing particle concentration as the extent of limited coalescence is reduced. Increasing the oil volume fraction in the emulsion at fixed aqueous particle concentration results in an increase in the average drop diameter together with a dramatic lowering of the uniformity of the drop size distribution as coalescence becomes increasingly significant until catastrophic phase inversion occurs. For low oil volume fractions (phi(o)), fragmentation dominates during emulsification since the mean drop size decreases with emulsification time. For higher phi(o) close to conditions of phase inversion, coalescence becomes more prevalent and the drop size increases with time with stable multiple emulsions forming as a result.