Compression and structure of monolayers of charged latex particles at air/water and octane/water interfaces

We have studied the compression and structure of compressed monolayers of s ulfate polystyrene latex particles on air/water and octane/water interfaces . If compressed sufficiently (on a Langmuir trough) the monolayers at air/w ater surfaces give rafts of hexagonally packed particles, while those at oi l/water interfaces undergo a transition from the originally hexagonal to a rhombohedral structure. We have found that beyond collapse the particle mon olayers on both air/water and octane/water interfaces fold and corrugate, a nd there is no expulsion of individual particles or particle aggregates fro m the interface. In the case of air/water interfaces, the structuring of pa rticle monolayers (below collapse) was found to be very sensitive to the el ectrolyte concentration in the aqueous phase. At low electrolyte concentrat ion, a fairly ordered structure resulting from the interparticle repulsion was observed, while at high electrolyte concentration, the particles form 2 D clusters. In marked contrast, particle monolayers at octane/water interfa ces remain highly ordered as a result of long-range repulsion, even on conc entrated 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 ads orbed particles and includes the effect of image forces. From this we have derived an analytic formula for the electrostatic surface pressure vs troug h area, which agrees well with the experimental data over a wide range of s urface pressure.