Search

Few foam systems stabilized by solid particles have been reported, and usually the particles have been used in combination with surfactants. We report how foams can be stabilized solely with a new class of anisotropic particles, hydrophobic polymer microrods of diameter less than 1 μm and length of a few tens of micrometers. The obtained foams were extraordinary stable, retaining a constant volume over many days and even surviving drying of most of the free liquid. The bubbles in these foams were sterically stabilized by dense thick “hairy” layers. The rigid intertwined protective shells around the bubbles did not allow the formation of thin films between them. The lifetime of these foams was orders of magnitude longer than the ones stabilized with typical foaming surfactants such as sodium dodecyl sulfate. The addition of sodium dodecyl sulfate led to hydrophilization of the microrods and suppressed the superstabilization effect. Thus, common foaming agents effectively act as defoamers for the ultrastable foams stabilized by microrods.

We have used the recently developed gel trapping technique (GTT) to determine the three-phase contact angles of submicrometer silica particles partially coated with octadecyl groups. The particles were spread at air−water and decane−water surfaces, and the aqueous phase was subsequently gelled with a nonadsorbing polysaccharide. The particles trapped at the surface of the aqueous gel were lifted by molding with curable poly(dimethylsiloxane) and imaged with scanning electron microscopy (SEM) to determine the particle contact line diameter which allows their contact angle at the original air−water or oil−water interface to be estimated. We report for the first time the use of the GTT for characterizing the contact angle of individual submicrometer particles adsorbed at liquid interfaces. The SEM images also reveal the structure of the particle monolayer at the interface and the structure of adsorbed particle aggregates. We have also determined the contact angles of agglomerated gold powder microparticles at the air−water and the decane−water interfaces. It was found that agglomerated gold particles demonstrate considerably higher contact angles than those on flat gold-coated surfaces.

A conceptually novel method has been developed for determining the three-phase contact angle of solid colloid particles adsorbed at the air−water or oil−water interface. The method is applicable for particle diameters ranging from several hundred nanometers to several hundred micrometers. This new “gel trapping technique” (GTT) is based on spreading of the particles on an air−water or oil−water surface and subsequent gelling of the water phase with a nonadsorbing polysaccharide. The particle monolayer trapped on the surface of the gel is then replicated and lifted up with poly(dimethylsiloxane) (PDMS) elastomer, which allows the particles embedded within the PDMS surface to be imaged with high resolution by using a scanning electron microscope (SEM). The position of the particles with respect to the PDMS surface has been determined from the SEM images, which gives information on the particle contact angle at the air−water or the oil−water interface. Three samples of latex and silica particles of different size and surface chemistry have been examined by using the GTT. It has been found that the contact angles of sulfate polystyrene latex particles at the air−water and decane−water interface determined by the GTT are slightly lower than the contact angles of water drops on flat polystyrene substrates under air or decane.

A general method for calculating the spontaneous curvature and the bending modulus of surfactant monolayers or grafted polymer brushes is presented which is based on a density functional theory, where the bending parameters and the spontaneous curvature are analytically represented in terms of integral moments of the solution for the flat geometry. The novelty here is that the problem is solved for an arbitrary form of the free energy density function with the only requirement being that the energy functional is local. The difference compared to other approaches is that for the case of local density functional theories; the lateral pressure and its derivatives with respect of curvature could be expressed analytically in terms of the density profiles of the flat geometry. This general approach enables a variety of problems involving polymer brushes, surfactants, and amphiphilic diblock copolymers to be treated directly and has direct application in modeling the phase behavior of microemulsions and surfactant solutions. As particular examples we consider the cases of low-density arbitrary solvent, arbitrary density in ideal solvent and Θ solvent of a polymer brush and an amphiphilic diblock copolymer.

In this paper we present a thermodynamic model for the adsorption of charged colloidal particles to air−water and oil−water interfaces from water-based suspensions and derive the equations for the respective particle adsorption isotherms. The effects of electrolyte concentration, surface charge density of both the particles and the air−water (oil−water) interface, particle contact angle, and particle volume fraction are studied in detail. The model is based on a detailed account of the interfacial free energy of particles and electric double layer formation, as well as collective effects related to the free energy of mixing of the particles with the liquid phases (water and oil) and surface charge displacement. The effect of particle size on the adsorption at the air−water and the oil−water interfaces is discussed. An analytical expression for the distribution coefficient of particles between oil and water has been derived and discussed in terms of the effects of electrolyte, contact angle, surface charge, and oil−water interfacial tension. The predictions of the model are in accord with relevant experimental data. The results are directly relevant for understanding the behavior of solid particles as emulsifier agents and foam stabilizers and in the process of antifoaming and liquid−liquid extraction of minerals.

A simple analytical model for the spontaneous curvature and the bending constants of nonionic surfactant monolayers at an oil/water interface is developed with the goal of allowing correlation and prediction of phase behavior. The surfactant molecules are treated as diblock copolymers grafted to the interface. The change in the free energy of the surfactant monolayer due to the bending of the interface is calculated as a sum of the contributions of the hydrophobic and the hydrophilic blocks. The equilibrium thickness of a spherical and cylindrical surfactant monolayer is found by minimizing the sum of the free energy of mixing and the stretching energy of the hydrophilic and the hydrophobic blocks. Comparison with the Helfrich model gives expressions for the spontaneous curvature H0, the bending elasticity κ, and the saddle splay modulus κ, of the surfactant monolayer. An analytical expression is obtained for the phase inversion temperature, which correlates well with experimental data for a wide series of nonionic surfactants CiEj and oils (n-alkanes). The model gives the temperature dependence of the spontaneous curvature and the bending parameters of nonionic surfactant monolayers. Phase boundaries in C12E5/octane/water microemulsions are calculated without use of any adjustable parameters and are in a good agreement with experimental data.

We have studied the compression and structure of compressed monolayers of sulfate polystyrene latex particles on air/water and octane/water interfaces. If compressed sufficiently (on a Langmuir trough) the monolayers at air/water surfaces give rafts of hexagonally packed particles, while those at oil/water interfaces undergo a transition from the originally hexagonal to a rhombohedral structure. We have found that beyond collapse the particle monolayers on both air/water and octane/water interfaces fold and corrugate, and there is no expulsion of individual particles or particle aggregates from the interface. In the case of air/water interfaces, the structuring of particle monolayers (below collapse) was found to be very sensitive to the electrolyte concentration in the aqueous phase. At low electrolyte concentration, a fairly ordered structure resulting from the interparticle repulsion was observed, while at high electrolyte concentration, the particles form 2D clusters. In marked contrast, particle monolayers at octane/water interfaces remain highly ordered as a result of long-range repulsion, even on concentrated electrolyte solution. We attribute the enhanced lateral repulsion between the latex particles at the octane/water interface to the existence of residual surface charges at the particle/octane interface. We propose a simple model, which describes the electrostatic interaction between the adsorbed particles and includes the effect of image forces. From this we have derived an analytic formula for the electrostatic surface pressure vs trough area, which agrees well with the experimental data over a wide range of surface pressure.

In this work we illustrate how the excluded volume of the hydrated electrolyte ions can be accounted for in the theory of the electric double layer and in the DLVO theory. It is shown that the finite size of the ions affects their distributions and the electric potential not only in the close vicinity of the charged interface but also over all of the electric double layer. We derive the correction to the electrostatic disjoining pressure due to the excluded volume of the ions in the film for the regime of weak overlap of the electric double layers. It is repulsive for large separations and can be accounted for by introducing an effective surface potential. The repulsion for large separations increases with an increase in the volume of the hydrated counterions. We demonstrate that this effect contributes to an increase in the critical coagulation concentration of electrolyte for a charged colloid. By contrast, the small separation asymptotic of the electrostatic disjoining pressure at constant surface potential, which takes into account the excluded volume, predicts a reduced repulsion compared to DLVO theory.

Theoretical expressions are derived for calculating the capillary meniscus interactions between two vertical cylinders or two spheres partially immersed in a liquid layer. In general, the radii and the contact angles of the two particles can be different. Two alternative approaches are followed. The first one consists in calculation of capillary interaction energy, which is then differentiated to get the force. In the second approach the force is determined directly by integrating the pressure and the interfacial tension. A very good coincidence between the numerical results of these two independent approaches is established. The results can contribute to a better understanding of phenomena like surface aggregation, surface coagulation, and formation of two-dimensional arrays at interfaces. Copyright 1993, 1999 Academic Press

A formal analogy between the linear theories of the lateral capillary interaction between solid bodies at a liquid−fluid interface and the electrostatic interaction between charged surfaces in electrolyte solution (linear DLVO theory) has been recognized. Thanks to that analogy, the well-developed methods for calculation of electrostatic interactions in DLVO theory can be utilized to explore the less well-studied lateral capillary interactions between particles adsorbed at a liquid−fluid interface. A few examples of the usefulness of such approach to capillary interactions in different geometries are given. A complete electrostatic analogy is found for the lateral capillary interaction between plates or rods, but not in the case of spherical particles. It is demonstrated how the approximations used in the nonlinear theory of electrostatic interaction work for the case of lateral capillary forces in colloid systems. The analogy between electrostatic image forces and capillary image forces is also discussed. The author believes that this study will be helpful for better understanding of the complex phenomena accompanying the capillary interactions between colloid particles attached to a liquid−fluid interface, the formation two-dimensional ordered aggregates of colloid particles in wetting films, etc.

The motion of the front between thick and thin foam film formed in a vertical frame is studied both experimentally and theoretically. In the experiments we used sodium dodecyl sulfate solutions with added NaCl. The rate of motion of the front turns out to be constant for a given electrolyte concentration. Other important quantities that are measured are the thickness of the black film and the jump of the film tension, when the front reaches the bottom meniscus. The rate of the front turns out to be proportional to the jump of the film tension irrespective of the amount of added electrolyte. The theoretical model we developed allows quantitative interpretation of the experimental data. It is demonstrated that the motion of the front “thick−thin” film is accompanied with the formation of microscopic steady capillary waves in the transition zone. We show that the energy dissipation in the film is concentrated mainly into the latter part of the transition zone. The study can be helpful for the understanding of the mechanism of the foam films drainage as related to the stability of foams.

Particle−particle and particle−wall capillary interactions were measured as a function of the separation distance. The “particles” were vertical thin glass cylinders and/or small glass spheres, protruding from an air/liquid interface. The particles attract each other due to the overlapping of the menisci formed around each of them. The force of interaction is detected by a sensitive torsion microbalance. It is based on counterbalancing the moment of a couple of forces, acting between two pairs of particles, by the torsion moment of a thin platinum wire. By varying the wire diameter, we accessed forces differing by several orders of magnitude, from about 5 dyn at small separation between the particles down to 0.001 dyn at large separation. The smallest force was measured with two cylinders of diameters about 300 μm. For two spheres (diameters 1.2 mm) we obtained difference in the forces corresponding to different heights of protrusion from the liquid surface. For interacting sphere and glass cylinder the force follows similar trends as the forces between two spheres or two cylinders. In the case of sphere and glass wall, however, the force first increases with decreasing the distance and then decreases close to the wall passing through a maximum. The predictions of the theory of capillary immersion forces are in quantitative agreement with the experimental results.