Equilibrium particle aggregates in attractive colloidal suspensions

The formation of equilibrium clusters in colloidal particle suspensions, due to the balance of a short-range attraction and a long-range repulsion interaction, has been recently widely discussed, considering its important role in many areas of material science and soft-matter physics. In this work, we describe a simple phenomenological model in which particles interact with a screened electrostatic repulsive interaction modelled by a Yukawa potential and with a short-ranged attractive potential ('charge patch' attraction) due to the strong lateral correlations among the adsorbed polyions at the surface of the approaching particles. This 'correlated adsorption' generates a more or less ordered 'patchwork-like' surface structure with excess negative charge domains (adsorbed polyelectrolyte domains) and excess positive charge domains (polyelectrolyte free domains), which give rise to a short-range attractive potential. In low-density colloidal liposome suspensions, particles form stable equilibrium clusters. We investigated the cluster formation and the growth in the cluster size in cationic liposomal particle suspensions (built up by DOTAP and DOTAP/DOPE mixtures), by means of a dynamic light scattering technique, in several experimental conditions (adsorbing polyions of different molecular weights, liposomes of different surface charge densities, presence of added salt), and a generally good agreement with the model predictions has been found.