Fluctuations in the number of irreversibly adsorbed particles

Fluctuations in the number of colloid particles adsorbed irreversibly under pure diffusion transport conditions were determined as a function of surfa ce density and ionic strength of the suspension. The experiments were carri ed out for monodisperse polystyrene latex particles of micrometer size rang e adsorbing irreversibly at mica surface. The surface density of adsorbed p articles at various areas was determined using the direct microscope observ ation method. A new experimental cell was used enabling in situ observation s of particles adsorption under conditions of negligible gravity effects. I t was found that the particle density fluctuations for high ionic strength were in a good agreement with the theoretical results derived from the rand om sequential adsorption (RSA) model. Also, the theoretical results stemmin g from the equilibrium scaled particle theory reflected the experimental da ta satisfactorily. For lower ionic strength a deviation from the hard spher e behavior was experimentally demonstrated. This effect due to the repulsiv e electrostatic interactions was interpreted in terms of the effective hard particle concept. The universal dependence of variance on particle density obtained in this way was found in a good agreement with the RSA model for all ionic strength. These results proved that fluctuations in particle dens ity of monolayer formed under diffusional conditions differ fundamentally f rom these obtained under ballistic transport conditions. (C) 2000 American Institute of Physics. [S0021-9606(00)71547-9].