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.

Using a laser tweezers method, we have determined the long-range repulsive force as a function of separation between two charged, spherical polystyrene particles (2.7 microm diameter) present at a nonpolar oil-water interface. At large separations (6 to 12 microm between particle centers) the force is found to decay with distance to the power -4 and is insensitive to the ionic strength of the aqueous phase. The results are consistent with a model in which the repulsion arises primarily from the presence of a very small residual electric charge at the particle-oil interface. This charge corresponds to a fractional dissociation of the total ionizable (sulfate) groups present at the particle-oil surface of approximately 3 x 10(-4).

We report a study of the compression of monolayers of monodisperse spherical polystyrene particles at the interface between aqueous surfactant solutions and octane. The particle size (2.6 µm diameter) was selected so that direct in situ microscopic observation of the monolayer structure could be made during lateral compression and "collapse". Monolayers have been formed on a miniature Langmuir trough placed on a microscope stage. Our study has focused on (a) the relationship between the monolayer collapse pressure, Πcol, and the interfacial tension, γ*, of the oil/water interface in the absence of a particle monolayer and (b) the mode of monolayer "collapse" at high surface pressure. Interfacial tensions γ* have been adjusted (in the range 50-4 mN m -1 ) by addition of surfactants over a range of concentration. We find that the monolayer collapses by buckling (folding) when the surface pressure is equal to the surface tension of the oil/water interface. Particle promotion out of the interface is not observed experimentally.