Adsorption of Ions at Uncharged Insoluble Monolayers.

A method is proposed for the experimental determination of the adsorption of inorganic electrolytes at a surface covered with insoluble surfactant monolayer. This task is complicated by the fact that the change of the salt concentration alters both chemical potentials of the electrolyte and the surfactant. Our method resolves the question by combining data for the surface pressure versus area of the monolayer at several salt concentrations with data for the equilibrium spreading pressure of crystals of the surfactant (used to fix a standard state). We applied the method to alcohols spread at the surface of concentrated halide solutions. The measured salt adsorption is positive and has nonmonotonic dependence on the area per surfactant molecule. For the liquid expanded film, depending on the concentration, there is one couple of ions adsorbed per each 3-30 surfactant molecules. We analyzed which ion, the positive or the negative, stands closer to the surface, by measuring the effect of NaCl on the Volta potential of the monolayer. The potentiometric data suggest that Na(+) is specifically adsorbed, while Cl(-) remains in the diffuse layer, i.e., the surface is positively charged. The observed reverse Hofmeister series of the adsorptions of NaF, NaCl, and NaBr suggests the same conclusion holds for all these salts. The force that causes the adsorption of Na(+) seems to be the interaction of the ion with the dipole moment of the monolayer.

Silica Gels Doped with Gold Nanoparticles: Preparation, Structure and Optical Properties.

A novel, one-pot sol-gel preparation scheme leading to reproducible incorporation of 20-40 nm sized gold nanoparticles (AuNPs) in SiO2 gels is developed based on in situ reduction during gelation using chloroauric acid and ascorbic acid. Variation in the preparation conditions affects the chemical composition, optical properties and size distribution of the AuNPs incorporated in the silica gels. Different organic dopants, i.e., oleic acid, acetic acid or dodecanethiol, are applied to modify the final composite material and to control the rate of reduction and growth of the AuNPs in the gels. The synthesized samples are characterized by UV/Vis/NIR spectroscopy, X-ray diffraction, transmission electron microscopy, thermal conductivity measurements and DTA/TG measurements. The optical properties of the obtained composites are explained using Mie theory. The incorporation of AuNPs leads to an increase in the thermal conductivity of the silica gels. The best process method in this contribution is the use of NaOH as a gelation catalyst and oleic acid as an organic modifier, leading to 20 nm AuNPs dispersed in the silica matrix.

Electrostatic interaction of neutral semi-permeable membranes.

We consider an osmotic equilibrium between bulk solutions of polyelectrolyte bounded by semi-permeable membranes and separated by a thin film of salt-free liquid. Although the membranes are neutral, the counter-ions of the polyelectrolyte molecules permeate into the gap and lead to a steric charge separation. This gives rise to a distance-dependent membrane potential, which translates into a repulsive electrostatic disjoining pressure. From the solution of the nonlinear Poisson-Boltzmann equation, we obtain the distribution of the potential and of ions. We then derive an explicit formula for the pressure exerted on the membranes and show that it deviates from the classical van't Hoff expression for the osmotic pressure. This difference is interpreted in terms of a repulsive electrostatic disjoining pressure originating from the overlap of counterion clouds inside the gap. We also develop a simplified theory based on a linearized Poisson-Boltzmann approach. A comparison with simulation of a primitive model for the electrolyte is provided and does confirm the validity of the theoretical predictions. Beyond the fundamental result that the neutral surfaces can repel, this mechanism not only helps to control the adhesion and long-range interactions of living cells, bacteria, and vesicles, but also allows us to argue that electrostatic interactions should play enormous role in determining behavior and functions of systems bounded by semi-permeable membranes.

Electro-Marangoni effect in thin liquid films.

The present paper reports a new drainage model accounting for the electro-Marangoni effect in thin liquid films stabilized by ionic surfactants. It was shown that the liquid outflow during the film drainage drifts charges from the diffuse part of the electrical double layer toward the film rim and thus generates a streaming potential along the film plane. This creates reverse fluxes near the film surfaces due to the requirement for zero electric current in the film. In a previous paper on this model (Tsekov et al. Langmuir, 2010, 26, 4703), the immobile surfaces were assumed. Here, the film surfaces were considered mobile, and surface velocity is controlled by an electro-Marangoni number. It was also shown that the motion of the charges makes the film surfaces more mobile, and they flow in reverse direction to the overall liquid outflow. The theory was validated by experimental data on drainage of planar foam films stabilized by cationic (tetrapentyl ammonium bromide) and anionic (sodium dodecyl sulfate) surfactants. A good agreement between the theoretical prediction and experimental data was found.

Streaming potential effect on the drainage of thin liquid films stabilized by ionic surfactants.

Dynamic effects originating from the electric double layers (EDL) are studied in thin liquid films (TLF) containing ionic and nonionic surfactants. To account for such effects, the EDL are to be incorporated into the differential equations describing the TLF drainage. Numerical simulations in the literature have shown that foam films containing ionic surfactants can drain at a slower rate than that predicted by the Reynolds equation (V(Re)) which postulates rigid planar film surfaces. However, the physical reason of the trend has remained unclarified, and the numerical results have not been validated by any experimental data. In the present study, experiments on the drainage of planar foam films were conducted with the anionic surfactant sodium dodecylsulfate (SDS) in the presence of additional electrolyte (0.02 M NaCl) and with the cationic tetrapentylammonium bromide (TPAB). The obtained results are in accord with the numerical simulations from the literature (V/V(Re) < 1). Such behavior was observed already in our preceding experiments on planar TLF with SDS without added electrolyte. These results were compared to the data of the experiments with TLF containing nonionic surfactant, and differences in the drainage pattern between ionics and nonionics were established. A new theoretical model was developed to account for the dynamic effects arising from EDL. According to the present model, the liquid outflow drags the bulk charges of EDL toward the film border, thus generating streaming potential (as in capillary tubes), which in turn brings the charges back toward the center to maintain the state of zero total electrical current. This creates reverse convection of the liquid near the surfaces, resulting in a velocity of film drainage smaller than V(Re). The present theory predicts kinetic dependence closer to the experiment than the Reynolds equation. The limitations of this new model are specified: it is valid for high ionic strength or low value of the surface potential.

Quantum hydrodynamics of electron gases.

Electron gases in metals are described as quantum charged Newtonian viscous fluids experiencing Ohmic Darcy friction on the solid lattice ions as well. The dispersion relation of the electron acoustic waves is derived, which shows the existence of new quantum diffusion processes. The electric double layer near a metal surface is studied, which exhibits a new quantum oscillatory-decaying behavior different from the Friedel oscillations.

Osmotic pressure acting on a semipermeable shell immersed in a solution of polyions.

We study theoretically the osmotic equilibria for a shell immersed in a suspension of polyions (e.g., colloids, polyelectrolytes, etc.). The shell is treated as impermeable for polyions, but allowing free diffusion of counterions that permeate inside the shell. From the solution of linearized Poisson-Boltzmann equation, we obtain the distribution of a potential and concentration profiles for polyions and counterions. We then obtain an explicit formula for the excess osmotic pressure of a polyion solution exerted on the shell, which includes a quadratic term in order to provide a self-consistency of a linear theory. As a result this pressure is larger than given by a concentration of polyions at the outer shell boundary obtained within linearized theory. It is, however, always smaller than or equal to the bulk osmotic pressure. This difference is attributed to a repulsive electrostatic disjoining pressure due to an overlap of counterion clouds inside the shell. A comparison with molecular dynamics simulations is provided and demonstrates that although the concentration profiles obtained within a linear theory deviate from simulation data at large potential, the theoretical and simulation pressures are in surprisingly good harmony.

Ripples in a wetting film formed by a moving meniscus.

We carry out a theoretical investigation of the evolution of a wetting film formed by pressing a bubble against a solid substrate. Our model incorporates the effects of capillarity and Derjaguin-Landau-Verwey-Overbeek (DLVO) (van der Waals and electrostatic) components of the disjoining pressure. Rapid changes in the relative position of the bubble and the substrate are shown to result in surprisingly rich dynamics of wetting film deformations. Even for stable films, we find transient rippled deformations with several points of local maximum of wetting film thickness and discuss how their evolution depends on changes in the meniscus position relative to the substrate and the disjoining pressure parameters. A connection is made to the recently reported experimental observations of one such rippled deformation, the so-called wimple, characterized by a local minimum of the thickness in the center, surrounded by a ring of greater film thickness and bounded at the outer edge by the barrier rim. Guidelines are provided for experimental detection of more complex rippled deformations in stable wetting films. Development of instability is studied in a situation when the electrostatic component of disjoining pressure is destabilizing, with particular emphasis on the nonlinear evolution and rupture of the film. Potential applications of our findings to small-scale mixing and deposition of nanoparticles are discussed.

Effect of ionic surfactants on drainage and equilibrium thickness of emulsion films.

This paper presents new theoretical and experimental results that quantify the role of surfactant adsorption and the related interfacial tension changes and interfacial forces in the emulsion film drainage and equilibrium. The experimental results were obtained with plane-parallel microscopic films from aqueous sodium dodecyl sulphate solutions formed between two toluene droplets using an improved micro-interferometric technique. The comparison between the theory and the experimental data show that the emulsion film drainage and equilibrium are controlled by the DLVO interfacial forces. The effect of interfacial viscosity and interfacial tension gradient (the Marangoni number) on the film drainage is also significant.

Resonant diffusion on modulated surfaces.

Some theoretical methods for description of the diffusion on modulated surfaces are reviewed. A general formula for calculation of the diffusion coefficient of a particle moving in the field of a periodic potential is developed, which takes into account both the potential barrier effect and the dependence of the friction coefficient on the potential. The application of the theory to the diffusion of dimers on solid surfaces reveals a non-monotonous dependence of the diffusion coefficient on the ratio between the dimer and solid spatial parameters. This resonant dependence sometimes can be suppressed by the rotations and vibrations on the dimer. In the present theory the latter are described via an adiabatic approach.

Tribology of thin wetting films between bubble and moving solid surface.

This work shows a successful example of coupling of theory and experiment to study the tribology of bubble rubbing on solid surface. Such kind of investigation is reported for the first time in the literature. A theory about wetting film intercalated between bubble and moving solid surface was developed, thus deriving the non-linear evolution differential equation which accounted for the friction slip coefficient at the solid surface. The stationary 3D film thickness profile, which appears to be a solution of the differential equation, for each particular speed of motion of the solid surface was derived by means of special procedure and unique interferometric experimental setup. This allowed us to determine the 3D map of the lift pressure within the wetting film, the friction force per unit area and the friction coefficient of rubbing at different speeds of motion of the solid surface. Thus, we observed interesting tribological details about the rubbing of the bubble on the solid surface like for example: 1. A regime of mixed friction between dry and lubricated friction exists in the range of 6-170 µm/s, beyond which the rubbing between the bubble and solid becomes completely lubricated and passes through the maximum; 2. The friction coefficient of rubbing has high values at very small speeds of solid's motion and reduces substantially with the increase of the speed of the solid motion until reaching small values, which change insignificantly with the further increase of the speed of the solid. Despite the numerous studies on the motion of bubble/droplet in close proximity to solid wall in the literature, the present investigation appears to be a step ahead in this area as far as we were able to derive 3D maps of the bubble close to the solid surface, which makes the investigation more profound.

Bubble rubbing on solid surface: experimental study.

This is the first interferometric study in the literature on wetting film entrapped between bubble and moving solid substrate. Unique experimental setup was specially designed for monitoring the thickness profiles of wetting film, intercalated between the bubble and moving solid surface. For this reason, special procedure developed for this study was applied for determination of 3D film thickness profiles. This allowed us to determine 3D profiles of the disjoining, the lift pressures as well as the viscous stress tensor as a function of the velocity of the solid surface. Thus, one can see that a strong linear dependence between the average film thickness and the speed of motion of the solid surface exists until a certain critical speed of motion, beyond which the dependence becomes weaker but keeps its linear trend. Similar is the propensity with the average lift pressure. Moreover, one can observe how the inhomogeinity of the film surfaces changes upon increasing the speed of motion of the solid surface. The proposed technique reveals new possibilities for investigation of bubbles and solid surfaces on deeper level when they are in relative motion towards each other. Thus, one can conduct detailed tribological studies on bubbles moving in close proximity to solid surfaces.

Delta-comb potential in modeling three-phase contact line (TPCL) on periodically patterned surfaces.

This work is a study of wetting of small water droplets on smooth glass surfaces with periodic patterns in the form of imprinted net with hydrophilic cells and hydrophobic bars. Microcover slides consisted of soda lime glass were used. The imprinted images of the net were with cell sizes in the range 40-200 µm, which corresponds to a quite narrow scope of hydrophilic surface fractions f(1)(30-36%) due to the relative increase in the size of the hydrophobic bars. The receding contact angles θ(R) of small water droplets, positioned on the patterned surfaces, were measured. The experiment showed significantly lower receding contact angles as compared to the theoretical expectations by the Cassie formula, which accounts for the contribution to the contact angle of the surface fraction of the imprinted hydrophobic/hydrophilic net. For this reason, we developed new theory accounting for the periodicity of the surface and the contribution of the three-phase contact line on the contact angle. This new theory considered delta-comb potential energy Δ(x,y) of the surface, effective line tension κ, and the lattice parameter a. The restriction of theory was discussed as well. It was pointed out that the theory is not valid for very small and very large lattice parameters.

Wetting films on chemically patterned surfaces.

The behavior of thin wetting films on chemically patterned surfaces was investigated. The patterning was performed by means of imprinting of micro-grid on methylated glass surface with UV-light (λ=184.8 nm). Thus imprinted image of the grid contained hydrophilic cells and hydrophobic bars on the glass surface. For this aim three different patterns of grids were utilized with small, medium and large size of cells. The experiment showed that the drainage of the wetting aqueous films was not affected by the type of surface patterning. However, after film rupturing in the cases of small and medium cells of the patterned grid the liquid from the wetting film underwent fast self-organization in form of regularly ordered droplets covering completely the cells of the grid. The droplets reduced significantly their size upon time due to evaporation. In the cases of the largest cell grid, a wet spot on the place of the imprinted grid was formed after film rupturing. This wet spot disassembled slowly in time. In addition, formation of a periodical zigzag three-phase contact line (TPCL) was observed. This is a first study from the planned series of studies on this topic.

Anomalous ion effects on rupture and lifetime of aqueous foam films formed from monovalent salt solutions up to saturation concentration.

We report the effects of ions on rupture and lifetime of aqueous foam films formed from sodium chloride (NaCl), lithium chloride (LiCl), sodium acetate (NaAc), and sodium chlorate (NaClO 3) using microinterferometry. In the case of NaCl and LiCl, the foam films prepared from the salt solutions below 0.1 M were unstable they thinned until rupturing. The film lifetime measured from the first interferogram (appearing at a film thickness on the order of 500 nm) until the film rupture was only a second or so. However, relatively long lasting and nondraining films prepared from salt solutions above 0.1 M were observed. The film lifetime was significantly longer by 1 to 2 orders of magnitude, i.e., from 10 to 100 s. Importantly, both the film lifetime and the (average) thickness of the nondraining films increased with increasing salt concentration. This effect has not been observed with foam films stabilized by surfactants. The film lifetime and thickness also increased with increasing film radius. The films exhibited significant surface corrugations. The films with large radii often contained standing dimples. There was a critical film radius below which the films thinned until rupturing. In the cases of NaAc and NaClO 3, the films were unstable at all radii and salt concentrations they thinned until rupturing, ruling out the effect of solution viscosity on stabilizing the films.

Ferroelectric phase transitions near ionic liquid/vacuum interfaces.

A simple theoretical model is developed describing ionic liquids as regular solutions. The separation of these ionic mixtures is studied on the base of the Cahn-Hilliard theory coupled with electrostatics. It is shown that the ionic liquids decompose to thin layers of oppositely charged liquids at low temperatures. At larger temperatures the separation occurs only near the ionic liquid/vacuum surface, thus explaining the oscillatory-decaying structure of the electric double layer observed via computer simulations. In contrast to noncharged liquids the ionic ones exhibit two critical temperatures, where the temperature coefficients of all characteristic lengths possess singularities. These second order ferroelectric phase transitions are possible explanations of the experimentally measured via light scattering peculiar temperature dependence of the interfacial dipole moment density on several ionic liquid/vacuum interfaces.

Electro-osmotic equilibria for a semipermeable shell filled with a solution of polyions.

The authors study theoretically the electrostatic equilibria for a shell filled with a suspension of polyions (e.g., colloids, polyelectrolytes, etc.) and immersed in an infinite salt-free reservoir. The shell is treated as impermeable for polyions, but allowing free diffusion of counterions. From the solution of the linearized Poisson-Boltzmann equation we obtain the distribution of the potential and concentration profiles for polyions. The authors then derive explicit formulas for the excess electro-osmotic pressure of a polyion solution exerted by the shell. This is shown to be due to a concentration of polyions at the inner shell boundary and can be very different from the pressure of a corresponding bulk polyion solution.

Dynamics and stability of dispersions of polyelectrolyte-filled multilayer microcapsules.

The authors report dynamic and coagulation properties of a dispersion of polyelectrolyte multilayer microcapsules filled with solutions of a strong polyelectrolyte. Microcapsules are shown to take a charge of the sign of encapsulated polyions and are characterized by a nonuniform distribution of inner polyions, which indicates a semipermeability of the shell and a leakage of counterions. The capsule self-diffusion coefficient in the vicinity of the similarly charged wall is measured using a particle tracking procedure from confocal images of the dispersion. The diffusion of capsules in the force field suggests that the effective interaction potential contains an electrostatic barrier, so that we deal with the same types of interaction forces as for solid particles. The theoretical estimates of the authors show that when microcapsules are in close proximity, their interaction should even be quantitatively the same as that of colloids with the same surface potential. However, due to the mobility of inner polyions they might repel stronger at large distances. The authors thus conclude that the encapsulation of charged polymers is an important factor in determining the adhesion and interaction properties of multilayer microcapsules.

A qualitative theory of wimples in wetting films.

It has long been known that hydrodynamic pressures in a thin draining liquid film can cause inversion of the curvature of a drop surface as it approaches another surface, creating a so-called dimple. However, it was recently found that a different shape, dubbed a wimple, can be formed if a fluid drop, which is already in the field of repulsive surface forces, is abruptly pushed toward the wall. The drop shape might include a central region in which the film remains thin, surrounded by a ring of greater film thickness bounded at the outer edge by a barrier rim. Here we present a qualitative theory of the wimple formation. It is shown that this is mainly driven by the film hydrodynamics, and a qualitative criterion for the wimple/dimple transition is derived.

Size Dependence of Protein-Induced Flocculation of Phosphatidylcholine Liposomes.

The adsorption of lysozyme and cytochrome C on phosphatidylcholine liposomes essentially changes the physical properties of the phospholipid membranes and under certain circumstances greatly affects the stability of the colloid dispersion by inducing bridging liposome flocculation. This study was designed to examine experimentally the influence of liposome size on two kinetic parameters of the flocculation, its rate constant and activation energy. As the liposome radius increased in the range 50-500 nm, the activation energy tended to decrease, resulting in an increased flocculation rate, except for the flocculation of 400-nm liposomes, which was greatly impeded. The pronounced influence of the liposome size on the flocculation rate constant was evident, since a well-defined minimum in the kinetic rate of flocculation of 400-nm liposomes was detected experimentally. The obtained nonlinear radius dependencies of the flocculation rates and activation energies are interpreted in terms of the bridging mechanism of the protein-induced liposome flocculation and the supplementary concept of the stability of thin liquid films formed between approaching protein-adsorbed liposomes. Copyright 2000 Academic Press.