Hydration forces between silica surfaces: Experimental data and predictions from different theories

Measuring Colloidal Forces With Atomic Force Microscopy 1: Salt Influence on Hydrophobic and Hydrophilic Interactions

Colloidal forces are essential for maintaining the stability and functionality of colloidal systems, affecting various industrial, biological, and environmental processes. They play an important role in determining the behavior of particles in suspensions, including stability, aggregation, and surface interactions. In this primer, we present basic concepts and protocols for studying colloidal interactions at different salt concentrations using atomic force microscopy (AFM). Following this methodology, hydrophilic substrates (i.e., silica) were easily functionalized with a hydrophobic fluorocarbon (1H,1H,2H,2H-Perfluorooctyltrimethoxysilane, FOTS) via chemical vapor deposition (CVD) and characterized by the sessile drop method, electrophoretic light scattering, AFM imaging, and scanning electron microscopy (SEM) to determine parameters such as contact angle, zeta potential, and surface roughness, respectively. Thus, after the preparation and characterization of a well-defined colloidal system, force-distance experiments using AFM allowed for the measurement of hydrophobic and hydrophilic interactions in salt solutions. Furthermore, we describe in detail the processing and fitting of the experimental data with an extended DLVO model.

Ion Correlations Decrease Particle Aggregation Rate by Increasing Hydration Forces at Interfaces

The connection between solution structure, particle forces, and emergent phenomena at solid-liquid interfaces remains ambiguous. In this case study on boehmite aggregation, we established a connection between interfacial solution structure, emerging hydration forces between two approaching particles, and the resulting structure and kinetics of particle aggregation. In contrast to expectations from continuum-based theories, we observed a nonmonotonic dependence of the aggregation rate on the concentration of sodium chloride, nitrate, or nitrite, decreasing by 15-fold in 4 molal compared to 1 molal solutions. These results are accompanied by an increase in repulsive hydration forces and interfacial oscillatory features from 0.27-0.31 nm in 0.01 molal to 0.38-0.52 nm in 2 molal. Moreover, molecular dynamics (MD) simulations indicated that these changes correspond to enhanced ion correlations near the interface and produced loosely bound aggregates that retain electrolyte between the particles. We anticipate that these results will enable the prediction of particle aggregation, attachment, and assembly, with broad relevance to interfacial phenomena.

Effect of Solvent Composition on Non-DLVO Forces and Oriented Attachment of Zinc Oxide Nanoparticles

Oriented attachment (OA) occurs when nanoparticles in solution align their crystallographic axes prior to colliding and subsequently fuse into single crystals. Traditional colloidal theories such as DLVO provide a framework for evaluating OA but fail to capture key particle interactions due to the atomistic details of both the crystal structure and the interfacial solution structure. Using zinc oxide as a model system, we investigated the effect of the solvent on short-ranged and long-ranged particle interactions and the resulting OA mechanism. In situ TEM imaging showed that ZnO nanocrystals in toluene undergo long-range attraction comparable to 1kT at separations of 10 nm and 3kT near particle contact. These observations were rationalized by considering non-DLVO interactions, namely, dipole-dipole forces and torques between the polar ZnO nanocrystals. Langevin dynamics simulations showed stronger interactions in toluene compared to methanol solvents, consistent with the experimental results. Concurrently, we performed atomic force microscopy measurements using ZnO-coated probes for the short-ranged interaction. Our data are relevant to another type of non-DLVO interaction, namely, the repulsive solvation force. Specifically, the solvation force was stronger in water compared to ethanol and methanol, due to the stronger hydrogen bonding and denser packing of water molecules at the interface. Our results highlight the importance of non-DLVO forces in a general framework for understanding and predicting particle aggregation and attachment.

Dual roles of goethite coating on the transport of plastic nanoparticles in heterogeneous porous media: The significance of collector surface roughness

This study systematically examines the roles of positive goethite on the retention and release of negative plastic nanoparticles (PSNPs) with different surface functional groups (Blank, -COOH, and -NH2). It provides the first evidence for the dual roles of goethite coatings on colloid transport; e.g., increased transport caused by surface morphology modification or decreased transport due to increased surface roughness and charge heterogeneity. Although previous work has shown that goethite-coated sand increases the retention of negative colloids, this work demonstrates that collector surface roughness can also reduce the retention of PSNPs due to increased interaction energy profiles. Nonmonotonic retention of all the different functionalized PSNPs was observed in goethite-coated rough sand, and the magnitude of variations was contingent on the PSNP functionalization, the solution ionic strength (IS), and the goethite coating. The release of PSNPs with IS decrease (phase I) and pH increase (phase II) varied significantly due to differences in energy barriers to detachment, e.g., release in phase I was inhibited in both goethite-coated sands, whereas release in phase II was enhanced in coated smooth sand but completely inhibited in rough sand. The findings of this study provide innovative insight into transport mechanisms for colloidal and colloid-associated contaminants.

The multivariate interaction between Au and TiO2 colloids: the role of surface potential, concentration, and defects

The established DLVO theory explains colloidal stability by the electrostatic repulsion between electrical double layers. While the routinely measured zeta potential can estimate the charges of double layers, it is only an average surface property which might deviate from the local environment. Moreover, other factors such as the ionic strength and the presence of defects should also be considered. To investigate this multivariate problem, here we model the interaction between a negatively charged Au particle and a negatively charged TiO2 surface containing positive/neutral defects (e.g. surface hydroxyls) based on the finite element method, over 6000 conditions of these 6 parameters: VPart (particle potential), VSurf (surface potential), VDef (defect potential), DD (defect density), Conc (salt concentration), and R (particle radius). Using logistic regression, the relative importance of these factors is determined: VSurf > VPart > DD > Conc > R > VDef, which agrees with the conventional wisdom that the surface (and zeta) potential is indeed the most decisive descriptor for colloidal interactions, and the salt concentration is also important for charge screening. However, when defects are present, it appears that their density is more influential than their potential. To predict the fate of interactions more confidently with all the factors, we train a support vector machine (SVM) with the simulation data, which achieves 97% accuracy in determining whether adsorption is favorable on the support. The trained SVM including a graphical user interface for querying the prediction is freely available online for comparing with other materials and models. We anticipate that our model can stimulate further colloidal studies examining the importance of the local environment, while simultaneously considering multiple factors.

Ion adsorption and hydration forces: a comparison of crystalline mica vs. amorphous silica surfaces

Hydration forces are ubiquitous in nature and technology. Yet, the characterization of interfacial hydration structures and their dependence on the nature of the substrate and the presence of ions have remained challenging and controversial. We present a systematic study using dynamic Atomic Force Microscopy of hydration forces on mica surfaces and amorphous silica surfaces in aqueous electrolytes containing chloride salts of various alkali and earth alkaline cations of variable concentrations at pH values between 3 and 9. Our measurements with ultra-sharp AFM tips demonstrate the presence of both oscillatory and monotonically decaying hydration forces of very similar strength on both atomically smooth mica and amorphous silica surfaces with a roughness comparable to the size of a water molecule. The characteristic range of the forces is approximately 1 nm, independent of the fluid composition. Force oscillations are consistent with the size of water molecules for all conditions investigated. Weakly hydrated Cs+ ions are the only exception: they disrupt the oscillatory hydration structure and induce attractive monotonic hydration forces. On silica, force oscillations are also smeared out if the size of the AFM tip exceeds the characteristic lateral scale of the surface roughness. The observation of attractive monotonic hydration forces for asymmetric systems suggests opportunities to probe water polarization.

A model for water sorption isotherms and hydration forces in sugar surfactants

HYPOTHESIS

Hydration forces between surfactant bilayers can be assessed using water sorption isotherms of surfactants. For a quantitative description, a water sorption model that relates water activity to water content in surfactant-based systems should be proposed.

THEORY AND SIMULATIONS

A water sorption model for nonionic surfactant systems based on the idea on partial solvent accessibility is proposed. The model contains only two parameters: one describes the strength of interactions, the other describes the fraction of surfactant available for water. For comparison, molecular dynamics simulations of bilayers of n-octyl β-d-glucoside with different water contents are presented.

FINDINGS

The model provides an excellent fit of experimental data on water sorption isotherms of two sugar surfactants. The results of the fitting are compared with molecular dynamics simulations and show a good correlation between simulations and the theory proposed. Analysis of interaction energies shows weakly endothermic hydration both in the simulations and in the sorption model, which agrees with calorimetric data on hydration. The model also shows a non-exponential decay of hydration forces with respect to the distance between bilayers; an expression for the decay length is derived.

Correlation between Electrostatic and Hydration Forces on Silica and Gibbsite Surfaces: An Atomic Force Microscopy Study

The balance between hydration and Derjaguin-Landau-Verwey-Overbeek (DLVO) forces at solid-liquid interfaces controls many processes, such as colloidal stability, wetting, electrochemistry, biomolecular self-assembly, and ion adsorption. Yet, the origin of molecular scale hydration forces and their relation to the surface charge density that controls the continuum scale electrostatic forces is poorly understood. We argue that these two types of forces are largely independent of each other. To support this hypothesis, we performed atomic force microscopy experiments using intermediate-sized tips that enable the simultaneous detection of DLVO and molecular scale oscillatory hydration forces at the interface between composite gibbsite:silica-aqueous electrolyte interfaces. We extract surface charge densities from forces measured at tip-sample separations of 1.5 nm and beyond using DLVO theory in combination with charge regulation boundary conditions for various pH values and salt concentrations. We simultaneously observe both colloidal scale DLVO forces and oscillatory hydration forces for an individual crystalline gibbsite particle and the underlying amorphous silica substrate for all fluid compositions investigated. While the diffuse layer charge varies with pH as expected, the oscillatory hydration forces are found to be largely independent of pH and salt concentration, supporting our hypothesis that both forces indeed have a very different origin. Oscillatory hydration forces are found to be distinctly more pronounced on gibbsite than on silica. We rationalize this observation based on the distribution of hydroxyl groups available for H bonding on the two distinct surfaces.

Formation of Supported Lipid Bilayers (SLBs) from Buffers Containing Low Concentrations of Group I Chloride Salts

Supported lipid bilayers (SLBs) are a useful tool for studying the interactions between lipids and other biomolecules that make up a cell membrane. SLBs are typically formed by the adsorption and rupture of vesicles from solution. Although it is known that many experimental factors can affect whether SLB formation is successful, there is no comprehensive understanding of the mechanism. In this work, we have used a quartz crystal microbalance (QCM) to investigate the role of the salt in the buffer on the formation of phosphatidylcholine SLBs on a silicon dioxide (SiO₂) surface. We varied the concentration of sodium chloride in the buffer, from 5 to 150 mM, to find the minimum concentration of NaCl that was required for the successful formation of an SLB. We then repeated the experiments with other group I chloride salts (LiCl, KCl, and CsCl) and found that at higher salt concentrations (150 mM) SLB formation was successful for all of the salts used, and the degree of deformation of the adsorbed vesicles at the critical vesicle coverage was cation-dependent. The results showed that at an intermediate salt concentration (50 mM) the critical vesicle coverage was cation-dependent and at low salt concentrations (12.5 mM) the cation used determined whether SLB formation was successful. We found that the successful formation of SLBs could occur at lower electrolyte concentrations for KCl and CsCl than it did for NaCl. To understand these results, we calculated the magnitude of the vesicle-surface interaction energy using the Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended-DLVO theory. We managed to explain the results obtained at higher salt concentrations by including cation-dependent surface potentials in the calculations and at lower salt concentrations by the addition of a cation-dependent hydration force. These results showed that the way that different cations in solution affect the 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)-SiO₂ surface interaction energy depends on the ionic strength of the solution.

Heterogeneous assembly of water from the vapor phase-Physical experiments and simulations with binding trifunctional organosilanes at the vapor/solid interface

A trace amount of interfacial water is required to initiate hydrosilation reactions of trifunctional organosilanes to form surface assemblies. In recent studies, we have learned that water also has a critical role in directing molecular placement on surfaces because water can react with silicon to provide oxygenated sites for surface binding. Consequently, the wettability nature of substrates influences the placement and density of organosilane films formed by vapor-phase reactions. Nanopatterning protocols were designed using vapor-phase organosilanes and colloidal lithography to compare the wettability differences of hydrophilic mica(0001) compared to relatively hydrophobic Si(100) as a strategy for tracking the location of water on surfaces. The competition between hydrophobic and hydrophilic domains for the adsorption and coalescence of water condensed from vapor can be mapped indirectly by mapping the organosilanes, which bind to water at the solid interface, using atomic force microscopy. Trifunctional octadecyltrichlorosilane (OTS) was used as a marker molecule to map out the areas of the surface where water was deposited. The effect of systematic changes in film thickness and surface coverage of OTS was evaluated at the vapor/solid interface by adding an incremental amount of water to sealed reaction vessels to wet the surface and assessing the outcome after reaction with vapor-phase trichlorosilane. Reactive molecular dynamics simulations of the silicon-water vapor interface combined with electronic structure calculations of oxygenated silicon clusters with methyltrichlorosilane provided insight of the mechanism for surface binding, toward understanding the nature of the interface and wettability factors, which influence the association and placement of silane molecules on surfaces.

Interactions of nanoscale plastics with natural organic matter and silica surfaces using a quartz crystal microbalance

Interactions of nanoscale plastics with natural organic matter (NOM) and silica surfaces were investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). Polyethylene and polystyrene are the most used plastic polymers and most likely to accumulate in the environment, and thus their nano-scale interactions were investigated in this study. Deposition and release of polyethylene and polystyrene nanoscale plastics were investigated on silica and NOM-coated surfaces in the presence of different salt types (NaCl, CaCl₂, MgCl₂) and ionic strengths (IS). Polyethylene nanoscale plastics showed negligible deposition on silica surface, while significant deposition of polystyrene nanoscale plastics was observed on silica surface. However, both polyethylene and polystyrene nanoscale plastics showed significant deposition on NOM-coated surfaces, with polystyrene showing higher deposition. Increased IS resulted in greater deposition of both polyethylene and polystyrene nanoscale plastics on NOM-coated surfaces due to the functional groups, following DLVO theory. Deposited polyethylene nanoscale plastics on NOM-coated surfaces can be remobilized whereas deposition of polystyrene nanoscale plastics was irreversible on both silica and NOM-coated surfaces. Overall, higher deposition of nanoscale plastics on NOM-coated surfaces indicates that fate and mobility of nanoscale plastics in the environment will be significantly governed by their interactions with NOM.

Study of soil expansion characteristics in rainfall-induced red-bed shallow landslides: Microscopic and macroscopic perspectives

The main cause of rainfall-induced red-bed shallow landslides is the tendency of red-bed weathered soil to expand when it meets water. However, studies on the expansion mechanism of expansive soil have not considered the effects of hydration and particle orientation. In this study, the hydration force of soil was determined according to the electric double-layer theory, the particle direction of soil was determined by analyzing images of soil obtained by scanning electron microscopy, and, finally, a microscopic model of the electrical double layer of red-bed weathered expansive soil was established in which the hydration force and soil-particle orientation were taken into account. The results showed that the expansion of red-bed weathered expansive soil is the result of hydration forces and repulsive forces in the electric double layer. The grain orientation of the soil strongly influenced the microscopic model. The unloading expansion rate of red-bed weathered expansive soil decreased with an increase in cation concentration and a decrease in pH value. It increased with an increase in the hydration cation radius. These experiments indicate the reliability of the microscopic model and provide a theoretical basis for the prevention and control of rainfall-induced red-bed shallow landslides.

Mesoporous Silica Nanoparticles with Dispersibility in Organic Solvents and Their Versatile Surface Modification

Recently, colloidal mesoporous silica nanoparticles (MSNs) have attracted keen interest in scientific and technological fields. A significant issue regarding the effective use of colloidal MSNs is their dispersibility in various solvents, which is essential for their applications through surface modification. However, the dispersion media for colloidal MSNs have been extremely limited. Here, we report a new method for obtaining stable colloidal MSNs dispersed in various organic solvents through a gradual solvent exchange of colloidal MSNs from acidic water to an organic solvent by dialysis. This allows the colloidal MSNs to be dispersed as primary nanoparticles in organic solvents such as 1-butanol, 1-dodecanol, and tetrahydrofuran (THF), which are capable of hydrogen bonding with surface silanol groups. In addition, MSNs dispersed in THF can be modified with chlorosilanes while maintaining colloidal stability. Various organosilyl groups, such as trimethylsilyl and dimethylsilyl groups, can be densely grafted on the surfaces of MSNs. After trimethylsilylation, MSNs become dispersible even in a nonpolar and hydrophobic solvent like octane through the solvent exchange due to the preferential evaporation of THF. This method will offer a versatile approach to functionalizing colloidal MSNs toward a wide range of applications.

Probing Anisotropic Surface Properties of Illite by Atomic Force Microscopy

For the purpose of understanding the colloidal behaviors of illite in mineral processing, probing the surface charging property of illite is of great significance. This research explored the edge and basal surfaces of illite using an atomic force microscope (AFM). The interaction forces between Si/Si₃N₄ probes and illite edge/basal surfaces were measured, respectively, at different pH values in 10 mM KCl solutions. Theoretical Derjaguin-Landau-Verwey-Overbeek forces were matched up with the measured forces to derive the surface potentials of the two surfaces. On the illite basal surface, an attractive force occurred at pH 3.0, while repulsive forces dominated from pH 5.0 to 10.0. On the illite edge surface, a slight attractive force was also obtained at pH 3.0. However, the interaction changed into repulsion at pH 5.0, and this repulsive force increased gradually from pH 6.0 to 10.0. Illite basal and edge surfaces were both negatively charged, but the basal surface exhibited more negative charges than the edge surface from pH 3.0 to 10.0. Increasing solution pH from 3.0 to 10.0, there was no detection of the point of zero charge (PZC) of the illite basal surface; however, the PZC of the illite edge surface should have occurred at a pH slightly lower than 3.0. This is the first time that surface potentials of illite edge and basal surfaces were attained separately by direct force measurements. These findings provide insights into the colloidal behaviors of illite in mineral processing and oil sands extraction.

Hydration Layer Structure of Biofouling-Resistant Nanoparticles

Hydrophilic surface chemistries can strongly bind water to produce surfaces that are highly resistant to protein adsorption and fouling. The interfacial bound water and its distinct properties have intrigued researchers for decades, yet the relationship between the water three-dimensional structure and function in antifouling coatings remains elusive. Here, we use hydrophilic, epoxy organosilane modified silica nanoparticles to demonstrate cheap, robust, and practically applied coatings that we discover have broad-ranging, ultralow fouling properties when challenged by various proteins, bacteria, and fungal spores. To understand their excellent antifouling properties, frequency modulation-atomic force microscopy is used to directly observe the interfacial water structure at subatomic resolution, which we validate using all-atom molecular dynamic simulations that strikingly predict similar structures of water layers on the original and ultralow fouling surfaces. The convergence of experimental and modeling data reveals that suitably spaced, flexible chains with hydrophilic groups interact with water molecules to produce a connective, quasi-stable layer, consisting of dynamic interfacial water, that provides a basis for antifouling performance of ultrathin, hydrophilic surface chemistries.

Measuring Inner Layer Capacitance with the Colloidal Probe Technique

The colloidal probe technique was used to measure the inner layer capacitance of an electrical double layer. In particular, the forces were measured between silica surfaces and sulfate latex surfaces in solutions of monovalent salts of different alkali metals. The force profiles were interpreted with Poisson-Boltzmann theory with charge regulation, whereby the diffuse layer potential and the regulation properties of the interface were obtained. While the diffuse layer potential was measured in this fashion in the past, we are able to extract the regulation properties of the inner layer, in particular, its capacitance. We find systematic trends with the type of alkali metal ion and the salt concentration. The observed trends could be caused by difference in ion hydration, variation of the binding capacitance, and changes of the effective dielectric constant within the Stern layer. Our results are in agreement with recent experiments involving the water-silica interface based on a completely independent method using X-ray photoelectron spectroscopy in a liquid microjet. This agreement confirms the validity of our approach, which further provides a means to probe other types of interfaces than silica.

Dilatant effect enhancers for silica dispersions in poly(propylene glycols)

Shear thickening fluids have found many applications in energy damping materials such as sports guards and liquid body armors. Therefore, an additive which could tailor the dilatant properties of such fluids without significantly affecting other properties, especially zero shear viscosity, could significantly increase the versatility of protective materials based on shear thickening fluids. In this paper, poly(propylene glycols) (PPGs) diacetates are investigated as dilatant effect enhancers for nano-silica dispersions in poly(propylene glycols). The influence of the modifiers on rheological properties of the dispersion is studied and discussed. Additionally, FTIR and rheological properties measurements are conducted in order to determine relative interactions strength between hydroxyl groups of PPGs and silica and carbonyl groups of PPG diacetates. Our findings suggest that the relative attractive interaction strength in studied systems can be arranged in the following order: COCO < COOH < OHOH. Therefore, the addition of PPG diacetate hinders the attractive interactions between liquid and solid. We report that the addition of diacetates can lead both to enhancement and deterioration of dilatant effect depending on the concentration of the modifier and its chain length. Based on conducted measurements and literature data, mechanism explaining that phenomenon is suggested. As a result, we propose an easy to make and cheap dilatant effect enhancer for widely used shear thickening fluids which, when used in small amounts (1-2.5%), raises the viscosity jump drastically. Additionally, the presence of the modifier does not significantly affect the zero shear viscosity of the shear thickening fluid.

Interactions of Silver Nanoparticles Formed in Situ on AFM Tips with Supported Lipid Bilayers

A facile approach for functionalizing atomic force microscopy (AFM) tips with nanoparticles (NPs) will provide exciting opportunities in the field of tip-enhanced vibrational spectroscopy and in probing the interactions between NPs and biological systems. In this study, through successive exposure to polydopamine and AgNO₃ solutions, the apex of AFM tips was functionalized with silver nanoparticles (AgNPs). The AgNP-modified AFM tips were used to measure the interaction forces between AgNPs and supported lipid bilayers (SLBs) formed on mica, as well as to probe the penetration of SLBs by AgNPs, with an emphasis on the effect of human serum albumin (HSA) proteins. AgNPs experienced predominantly repulsive forces when approaching SLBs. The presence of HSA resulted in an enhancement in the repulsive interactions between AgNPs and SLBs, likely through steric repulsion. Finally, the forces required for AgNPs to penetrate SLBs were higher in the presence of HSA probably due to the increase in the effective size of the nanoscale protuberances on the AFM tip stemming from the formation of protein coronas around the AgNPs.

Stability, Composition, and Core-Shell Particle Structure of Uranium(IV)-Silicate Colloids

Uranium is typically the most abundant radionuclide by mass in radioactive wastes and is a significant component of effluent streams at nuclear facilities. Actinide(IV) (An(IV)) colloids formed via various pathways, including corrosion of spent nuclear fuel, have the potential to greatly enhance the mobility of poorly soluble An(IV) forms, including uranium. This is particularly important in conditions relevant to decommissioning of nuclear facilities and the geological disposal of radioactive waste. Previous studies have suggested that silicate could stabilize U(IV) colloids. Here the formation, composition, and structure of U(IV)-silicate colloids under the alkaline conditions relevant to spent nuclear fuel storage and disposal were investigated using a range of state of the art techniques. The colloids are formed across a range of pH conditions (9-10.5) and silicate concentrations (2-4 mM) and have a primary particle size 1-10 nm, also forming suspended aggregates <220 nm. X-ray absorption spectroscopy, ultrafiltration, and scanning transmission electron microscopy confirm the particles are U(IV)-silicates. Additional evidence from X-ray diffraction and pair distribution function data suggests the primary particles are composed of a UO₂-rich core and a U-silicate shell. U(IV)-silicate colloids formation correlates with the formation of U(OH)₃(H₃SiO₄)₃^2- complexes in solution indicating they are likely particle precursors. Finally, these colloids form under a range of conditions relevant to nuclear fuel storage and geological disposal of radioactive waste and represent a potential pathway for U mobility in these systems.

Composition in the Interface between Clay Mineral Surfaces and Divalent Cation Electrolytes

The interfacial free energy of a solid, which determines its adsorption properties, depends on interactions between the surface and the fluid. A change in surface composition can completely change the behavior of the solid. Decades of work have explored adsorption and its effects at solid-fluid interfaces from the macroscopic perspective and using molecular modeling, so the concept of the electric double layer (EDL) is well established in the community. However, direct, molecular level, experimental observations of the composition within the interface region, and its change with time and conditions, are not abundant. We used cryogenic X-ray photoelectron spectroscopy (cryoXPS) to observe the composition in the clay mineral-solution interface region as a function of bulk solution composition, on illite and chlorite in MgCl₂ and CaCl₂ electrolytes, over a range of concentrations (1-125 mM), in situ, on vitrified samples. These samples were prepared from very thin smears of centrifuged wet paste that were instantaneously chilled to liquid N₂ temperature. They preserved the adsorbed solution in its amorphous state, maintaining the location of the ions and water with respect to the solid, without the disruption that occurs during drying or the rearrangement that results as water crystallizes during freezing. With decreasing ionic strength, we could directly monitor the loss of negative charge in the interface region, producing an anion deficiency, as predicted by theory. The Cl^-/Me²⁺ ratio dropped below 1 for chlorite at 12-25 mM MeCl₂ and for illite at 75-100 mM. In addition to better understanding of clay mineral behavior in solution, this work demonstrates that only those clay minerals where surface charge density is the same or lower than that for chlorite contribute to a low salinity enhanced oil recovery response (LS EOR). This explains many of the contradictory results from studies about the role of clay minerals in LS EOR.

Ionic Specificity in Rapid Coagulation of Silica Nanoparticles

The Smoluchowski theory has been widely accepted as the basic theory to estimate the rapid coagulation rate of colloidal particles in electrolyte solutions. However, because the size and specificity of molecules and ions are not taken into account, the theory is applicable only if the particle size is large enough to neglect the effects caused by the structured layers composed of water molecules, ions, and hydrated ions adsorbed on the colloidal surface. In the present study, the rapid coagulation rates of silica nanoparticles in concentrated chloride and potassium solutions were measured by using a low-angle light-scattering apparatus, and the dependence of the experimental value of rapid coagulation rate, K_E^R, on the particle diameter, D_p, and also on the Gibbs free energy of hydration of ions, ΔG_hyd, was investigated extensively. The following were found. (1) When the particle size was small enough, the value of K_E^R reduced exponentially not only with the decreasing particle size but also with the increasing value of (-ΔG_hyd) of cations (counterions) in the case of chloride solutions and with that of anions (coions) in the case of potassium solutions. (2) Silica nanoparticles of D_p ≲ 70 nm in 1 M KNO₃ and KSCN solutions did not coagulate at all, although they coagulated at D_p ≳ 100 nm as in the other potassium solutions. These unexpected phenomena were explained by the proposed mechanisms.

Thin film modeling of crystal dissolution and growth in confinement

We present a continuum model describing dissolution and growth of a crystal contact confined against a substrate. Diffusion and hydrodynamics in the liquid film separating the crystal and the substrate are modeled within the lubrication approximation. The model also accounts for the disjoining pressure and surface tension. Within this framework, we obtain evolution equations which govern the nonequilibrium dynamics of the crystal interface. Based on this model, we explore the problem of dissolution under an external load, known as pressure solution. We find that in steady state, diverging (power-law) crystal-surface repulsions lead to flat contacts with a monotonic increase of the dissolution rate as a function of the load. Forces induced by viscous dissipation then surpass those due to disjoining pressure at large enough loads. In contrast, finite repulsions (exponential) lead to sharp pointy contacts with a dissolution rate independent of the load and the liquid viscosity. Ultimately, in steady state, the crystal never touches the substrate when pressed against it. This result is independent from the nature of the crystal-surface interaction due to the combined effects of viscosity and surface tension.

Incorporation of ion and solvent structure into mean-field modeling of the electric double layer

An electric double layer forms when the small mobile ions of an electrolyte interact with an extended charged object, a macroion. The competition between electrostatic attraction and translational entropy loss of the small ions results in a diffuse layer of partially immobilized ions in the vicinity of the macroion. Modeling structure and energy of the electric double layer has a long history that has lead to the classical Poisson-Boltzmann theory and numerous extensions that account for ion-ion correlations and structural ion and solvent properties. The present review focuses on approaches that instead of going beyond the mean-field character of Poisson-Boltzmann theory introduce structural details of the ions and the solvent into the Poisson-Boltzmann modeling framework. The former include not only excluded volume effects but also the presence of charge distributions on individual ions, spatially extended ions, and internal ionic degrees of freedom. The latter treat the solvent either explicitly as interacting Langevin dipoles or in the form of effective non-electrostatic interactions, in particular Yukawa interactions, that are added to the Coulomb potential. We discuss how various theoretical models predict structural properties of the electric double layer such as the differential capacitance and compare some of these predictions with computer simulations.

Interactions between silica particles in the presence of multivalent coions

Forces between charged silica particles in solutions of multivalent coions are measured with colloidal probe technique based on atomic force microscopy. The concentration of 1 : z electrolytes is systematically varied to understand the behavior of electrostatic interactions and double-layer properties in these systems. Although the coions are multivalent the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory perfectly describes the measured force profiles. The diffuse-layer potentials and regulation properties are extracted from the forces profiles by using the DLVO theory. The dependencies of the diffuse-layer potential and regulation parameter shift to lower concentration with increasing coion valence when plotted as a function of concentration of 1 : z salt. Interestingly, these profiles collapse to a master curve if plotted as a function of monovalent counterion concentration.

Extending the limits of direct force measurements: colloidal probes from sub-micron particles

Direct force measurements by atomic force microscopy (AFM) in combination with the colloidal probe technique are widely used to determine interaction forces in colloidal systems. However, a number of limitations are still preventing a more universal applicability of this technique. Currently, one of the most significant limitations is that only particles with diameters of several micrometers can be used as probe particles. Here, we present a novel approach, based on the combination of nanofluidics and AFM (also referred to as FluidFM-technique), that allows to overcome this size limit and extend the size of suitable probe particles below diameters of 500 nanometers. Moreover, by aspiration of colloidal particles with a hollow AFM-cantilever, the immobilization process is independent of the particle's surface chemistry. Furthermore, the probe particles can be exchanged in situ. The applicability of the FluidFM-technique is demonstrated with silica particles, which are also the types of particles most often used for the preparation of colloidal probes. By comparing 'classical' colloidal probes, i.e. probes from particles irreversibly attached with glue, and various particle sizes aspirated by the FluidFM-technique, we can quantitatively evaluate the instrumental limits. Evaluation of the force profiles demonstrate that even for 500 nm silica particles the diffuse layer properties can be evaluated quantitatively. Therefore, direct force measurements on the level of particle sizes used in industrial formulations will become available in the future.

Direct Observation of Interactions between Nanoparticles and Nanoparticle Self-Assembly in Solution

Hierarchically organized nanoparticles (NPs) possess unique properties and are relevant to various technological applications. An important "bottom-up" strategy for building such hierarchical nanostructures is to guide the individual NPs into ordered nanoarchitectures using intermolecular interactions and external forces. However, our current understanding of the nanoscale interactions that govern such self-assembly processes usually relies on post-synthesis/assembly or indirect characterization. Theoretical models that can derive these interactions are presently constrained to systems with only a few particles or on short time scales. Hence, except for a number of special cases, a description that captures the detailed mechanisms of NP self-assembly still eludes us. By imaging the assembly of NPs in solution with subnanometer resolution and in real-time, in situ liquid cell transmission electron microscopy (LC-TEM) can identify previously unknown intermediate stages and improve our understanding of such processes. Here, we review recent studies where we explored NP self-assembly at different organization length scales using LC-TEM: (1) we followed the transformation of atoms into crystalline NPs in solution, (2) we highlighted the role of solvation forces on interaction dynamics between NPs, and (3) we described the assembly dynamics of NPs in solution. In the case of nanocrystal nucleation, we identified the existence of three distinct steps that lead to the formation of crystalline nuclei in solution. These steps are spinodal decomposition of the precursor solution into solute-rich and solute-poor liquid phases, nucleation of amorphous clusters within the solute-rich liquid phase, followed by crystallization of these amorphous clusters into crystalline NPs. The next question we ask is how NPs interact in solution once they form. It turns out that the hydration layer surrounding each NP acts as a repulsive barrier that prevents NPs from readily attaching to each other due to attractive vdW forces. Consequently, two interacting NPs form a metastable pair separated by their one water molecule thick hydration shell and they undergo attachment only when this water between them is drained. Next, we explore the self-assembly of many NP systems where the formation of linear chains from spherical NPs or nanorods (NRs) is mediated by linker molecules. At low linker concentration, both spherical NPs and NRs tend to form linear chains because of the need to reduce electrostatic repulsion between NP building blocks. When the concentration of linkers is increased, the attachment of NPs is no longer linear. For example, we find that two NRs undergo side-to-side assembly due to decreased electrostatic repulsion and the anisotropic distribution of linkers on NR surfaces at high linker concentration. Lastly, we look at the formation of NP nanorings directed by ethylenediaminetetraacetic acid (EDTA) nanodroplets in water. Our study shows that nanoring assemblies form via sequential attachment of NPs to binding sites located along the circumference of the EDTA nanodroplet, followed by rearrangement and reorientation of the attached NPs. Our approach based on real-time visualization of nanoscale processes not only reveals all the intermediate steps of NP assembly, but also provides quantitative description on the interactions between nanoscale objects in solution.

Orders of Magnitude Reduction of Rapid Coagulation Rate with Decreasing Size of Silica Nanoparticles

The modification of the classical Smoluchowski theory for the rapid coagulation rate of colloidal particles, which takes account of the effect of the squeezing flow between colliding particles, has been widely accepted because it predicts experimental results adequately. However, it is not clear whether the modified theory, in which the coagulation rate is independent of the particle size, is applicable even to nanoparticles in solutions. In the present study, the rapid coagulation rates of silica particles in various 2 M chloride and 1 M potassium solutions were measured by using a low-angle light-scattering apparatus, and the dependence of rapid coagulation rate on the particle diameter, D_p, was investigated extensively. It was clearly shown that the rapid coagulation rate of spherical silica particles reduces by the orders of magnitude with decreasing particle size at D_p ≤ 300 nm, whereas it coincides with the value predicted by the modified theory at D_p ≥ 300 nm. A possible mechanism is proposed, and an analytical equation, which predicts the dramatic reduction in the rapid coagulation rate with decreasing particle size, is derived.

Competitive Adsorption of Polyelectrolytes onto and into Pellicle-Coated Hydroxyapatite Investigated by QCM-D and Force Spectroscopy

A current effort in preventive dentistry is to inhibit surface attachment of bacteria using antibacterial polymer coatings on the tooth surface. For the antibacterial coatings, the physisorption of anionic and cationic polymers directly onto hydroxyapatite (HA) and saliva-treated HA surfaces was studied using quartz crystal microbalance, force spectroscopy, and atomic force microscopy. First, single species adsorption is shown to be stronger on HA surfaces than on silicon oxide surfaces for all polymers (i.e., Gantrez, sodium hyaluronate (NaHa), and poly(allylamine-co-allylguanidinium) (PAA-G75)). It is observed through pH dependence of Gantrez, NaHa, and PAA-G75 adsorption on HA surfaces that anionic polymers swell at high pH and collapse at low pH, whereas cationic polymers behave in the opposite fashion. Thicknesses of Gantrez, NaHa, and PAA-G75 are 52 nm (46 nm), 35 nm (11 nm), and 6 nm (54 nm) at pH 7 (3.5), respectively. Second, absorption of charged polymer is followed by absorption of the oppositely charged polymer. Upon exposure of the anionic polymer layers, Gantrez and NaHa, to the cationic polymer, PAA-G75, films collapse from 52 to 8 nm and 35 to 11 nm, respectively. This decrease in film thickness is attributed to the electrostatic cross-linking between anionic and cationic polymers. Third, for HA surfaces pretreated with artificial saliva (AS), the total thickness decreases from 25 to 16 nm upon exposure to PAA-G75. Force spectroscopy is used to further investigate the PAA-G75/AS coating. The results show that the interaction between a negatively charged colloidal bead and the AS surface is strongly repulsive, whereas PAA-G75/AS is attractive but varies across the surface. Additionally, AFM studies show that AS/HA is smooth with a RMS roughness of 1.7 nm, and PAA-G75-treated AS/HA is rough (RMS roughness of 5.4 nm) with patches of polymer distributed across the surface with an underlying coating. The high roughness of PAA-G75 treated AS/HA is attributed to the strong adsorption of the relatively small PAA-G75 onto the heterogeneously distributed negatively charged AS surface. In addition, uptake of PAA-G75 by pellicle layer (saliva-treated HA surface) is observed, and the adsorbed amount of PAA-G75 on/into pellicle layer is ∼2 times more than that on/into AS layer. These studies show that polymer adsorption onto HA and saliva-coated HA depends strongly on the polymer type and size and that there is an electrostatic interaction between polymer and saliva and/or oppositely charged polymers that stabilizes the coatings on HA. Lastly, assessing the viability of the adherent bacteria collected from the PAA-G75-coated surfaces showed a significant reduction (∼93%) in bacterial viability when compared to bacteria collected from untreated and Gantrez-coated HA. These results suggest the potential antimicrobial activity of PAA-G75.

Hydrogel-colloid interfacial interactions: a study of tailored adhesion using optical tweezers

Dynamics of colloidal particles adhering to soft, deformable substrates, such as tissues, biofilms, and hydrogels play a key role in many biological and biomimetic processes. These processes, including, but not limited to colloid-based delivery, stitching, and sorting, involve microspheres exploring the vicinity of soft, sticky materials in which the colloidal dynamics are affected by the fluid environment (e.g., viscous coupling), inter-molecular interactions between the colloids and substrates (e.g., Derjaguin-Landau-Verwey-Overbeek (DLVO) theory), and the viscoelastic properties of contact region. To better understand colloidal dynamics at soft interfaces, an optical tweezers back-focal-plane interferometry apparatus was developed to register the transverse Brownian motion of a silica microsphere in the vicinity of polyacrylamide (PA) hydrogel films. The time-dependent mean-squared displacements are well described by a single exponential relaxation, furnishing measures of the transverse interfacial diffusion coefficient and binding stiffness. Substrates with different elasticities were prepared by changing the PA crosslinking density, and the inter-molecular interactions were adjusted by coating the microspheres with fluid membranes. Stiffer PA hydrogels (with bulk Young's moduli ≈1-10 kPa) immobilize the microspheres more firmly (lower diffusion coefficient and position variance), and coating the particles with zwitterionic lipid bilayers (DOPC) completely eliminates adhesion, possibly by repulsive dispersion forces. Remarkably, embedding polyethylene glycol-grafted lipid bilayers (DSPE-PEG2k-Amine) in the zwitterionic fluid membranes produces stronger adhesion, possibly because of polymer-hydrogel attraction and entanglement. This study provides new insights to guide the design of nanoparticles and substrates with tunable adhesion, leading to smarter delivery, sorting, and screening of micro- and nano-systems.

Ice-like water supports hydration forces and eases sliding friction

The nature of interfacial water is critical in several natural processes, including the aggregation of lipids into the bilayer, protein folding, lubrication of synovial joints, and underwater gecko adhesion. The nanometer-thin water layer trapped between two surfaces has been identified to have properties that are very different from those of bulk water, but the molecular cause of such discrepancy is often undetermined. Using surface-sensitive sum frequency generation (SFG) spectroscopy, we discover a strongly coordinated water layer confined between two charged surfaces, formed by the adsorption of a cationic surfactant on the hydrophobic surfaces. By varying the adsorbed surfactant coverage and hence the surface charge density, we observe a progressively evolving water structure that minimizes the sliding friction only beyond the surfactant concentration needed for monolayer formation. At complete surfactant coverage, the strongly coordinated confined water results in hydration forces, sustains confinement and sliding pressures, and reduces dynamic friction. Observing SFG signals requires breakdown in centrosymmetry, and the SFG signal from two oppositely oriented surfactant monolayers cancels out due to symmetry. Surprisingly, we observe the SFG signal for the water confined between the two charged surfactant monolayers, suggesting that this interfacial water layer is noncentrosymmetric. The structure of molecules under confinement and its macroscopic manifestation on adhesion and friction have significance in many complicated interfacial processes prevalent in biology, chemistry, and engineering.

Dispersion forces acting between silica particles across water: influence of nanoscale roughness

Force profiles between pairs of silica particles in concentrated aqueous solutions of a monovalent salt are measured using atomic force microscopy (AFM). Under such conditions, the double layer forces are negligible, and the profiles are dominated by van der Waals dispersion forces at larger distances. Heat treatment of the particles strongly influences the strength of dispersion forces. The dispersion force between the particles heated at 1200 °C was strongly attractive, and was characterized by a Hamaker constant of 2.4 × 10^-21 J. This value is in good agreement with the current best theoretical estimate of the Hamaker constant for silica across water. For untreated particles, however, the dispersion force is much weaker and the Hamaker constant is 7 × 10^-23 J. The Hamaker constant can be continuously tuned by adjusting the heating temperature between 1000 and 1200 °C. Such substantial variations of the Hamaker constant are caused by moderate differences in surface roughness on the nanoscale. The root mean square (RMS) of the roughness correlates inversely with the Hamaker constant, whereby the particles treated at 1200 °C have an RMS value of 0.63 nm, while the untreated particles have an RMS value of 2.5 nm. Other effects that could influence the Hamaker constant, such as changes in the degree of crystallinity, porosity, and shape of the particles, could be excluded.

Oxyhydroxy Silicate Colloids: A New Type of Waterborne Actinide(IV) Colloids

At the near-neutral and reducing aquatic conditions expected in undisturbed ore deposits or in closed nuclear waste repositories, the actinides Th, U, Np, and Pu are primarily tetravalent. These tetravalent actinides (AnIV) are sparingly soluble in aquatic systems and, hence, are often assumed to be immobile. However, AnIV could become mobile if they occur as colloids. This review focuses on a new type of AnIV colloids, oxyhydroxy silicate colloids. We herein discuss the chemical characteristics of these colloids and the potential implication for their environmental behavior. The binary oxyhydroxy silicate colloids of AnIV could be potentially more mobile as a waterborne species than the well-known mono-component oxyhydroxide colloids.

Kinetically-controlled synthesis of ultra-small silica nanoparticles and ultra-thin coatings

The understanding of silica as a polymer-like globule allows us to synthesize ultra-small silica nanoparticles (NPs) via a kinetic controlled process. The synthetic system is quite simple with Tetraethyl orthosilicate (TESO) as the precursor and H2O as the solvent and reactant. The reaction conditions are gentle with a temperature of around 35 to 60 °C with an incubation time of 7-12 hours. The final product of the silica NPs is very uniform and could be as small as 10 nm. The silica NPs can further grow up to 18 nm under the controlled addition of the precursors. Also, these silica NPs can be used as seeds to generate larger silica NPs with sizes ranging from 20 to 100 nm, which can be a useful supplement to the size range made by the traditional Stöber method. Moreover, these ultra-small Au NPs can be used as a depletion reagent or as building blocks for an ultrathin silica coating, which has significant applications in fine-tuning the plasmons of AuNPs and thin spacers for surface enhanced spectroscopies.

Hydration Layer-Mediated Pairwise Interaction of Nanoparticles

When any two surfaces in a solution come within a distance the size of a few solvent molecules, they experience a solvation force or a hydration force when the solvent is water. Although the range and magnitude of hydration forces are easy to characterize, the effects of these forces on the transient steps of interaction dynamics between nanoscale bodies in solution are poorly understood. Here, using in situ transmission electron microscopy, we show that when two gold nanoparticles in water approach each other at a distance within two water molecules (∼5 Å), which is the combined thickness of the hydration shell of each nanoparticle, they form a sterically stabilized transient nanoparticle dimer. The interacting surfaces of the nanoparticles come in contact and undergo coalescence only after these surfaces are fully dehydrated. Our observations of transient steps in nanoparticle interactions, which reveal the formation of hydration layer mediated metastable nanoparticle pairs in solution, have significant implications for many natural and industrial processes.

Probing Anisotropic Surface Properties of Molybdenite by Direct Force Measurements

Probing anisotropic surface properties of layer-type mineral is fundamentally important in understanding its surface charge and wettability for a variety of applications. In this study, the surface properties of the face and the edge surfaces of natural molybdenite (MoS2) were investigated by direct surface force measurements using atomic force microscope (AFM). The interaction forces between the AFM tip (Si3N4) and face or edge surface of molybdenite were measured in 10 mM NaCl solutions at various pHs. The force profiles were well-fitted with classical DLVO (Derjaguin-Landau-Verwey-Overbeek) theory to determine the surface potentials of the face and the edge surfaces of molybdenite. The surface potentials of both the face and edge surfaces become more negative with increasing pH. At neutral and alkaline conditions, the edge surface exhibits more negative surface potential than the face surface, which is possibly due to molybdate and hydromolybdate ions on the edge surface. The point of zero charge (PZC) of the edge surface was determined around pH 3 while PZC of the face surface was not observed in the range of pH 3-11. The interaction forces between octadecyltrichlorosilane-treated AFM tip (OTS-tip) and face or edge surface of molybdenite were also measured at various pHs to study the wettability of molybdenite surfaces. An attractive force between the OTS-tip and the face surface was detected. The force profiles were well-fitted by considering DLVO forces and additional hydrophobic force. Our results suggest the hydrophobic feature of the face surface of molybdenite. In contrast, no attractive force between the OTS-tip and the edge surface was detected. This is the first study in directly measuring surface charge and wettability of the pristine face and edge surfaces of molybdenite through surface force measurements.

Centrifugation-assisted Assembly of Colloidal Silica into Crack-Free and Transferrable Films with Tunable Crystalline Structures

Self-assembly of colloidal particles into colloidal films has many actual and potential applications. While various strategies have been developed to direct the assembly of colloidal particles, fabrication of crack-free and transferrable colloidal film with controllable crystal structures still remains a major challenge. Here we show a centrifugation-assisted assembly of colloidal silica spheres into free-standing colloidal film by using the liquid/liquid interfaces of three immiscible phases. Through independent control of centrifugal force and interparticle electrostatic repulsion, polycrystalline, single-crystalline and quasi-amorphous structures can be readily obtained. More importantly, by dehydration of silica particles during centrifugation, the spontaneous formation of capillary water bridges between particles enables the binding and pre-shrinkage of the assembled array at the fluid interface. Thus the assembled colloidal films are not only crack-free, but also robust and flexible enough to be easily transferred on various planar and curved substrates.