Ion-Specific Aggregation of Hydrophobic Particles

This work shows that colloidal stability and aggregation kinetics of hydrophobic polystyrene (PS) nanospheres are extremely sensitive to the nature of the salt used to coagulate them. Three PS latices and four aggregating electrolytes, which all share the same cation (Na+) but have various anions located at different positions in the classical Hofmeister series depending on their kosmotropic or chaotropic character, are used. The present study focuses on analyzing different aggregating parameters, such as critical coagulation concentrations (CCC), cluster size distributions (CSD), initial kinetic constants K11, and fractal dimensions of the aggregates df. While aggregation induced by SO42- and Cl- behaved according to the predictions of the classical DerjaguinLandauVerweyOverbeek theory, important discrepancies are found with NO3-, which become dramatic when using SCN-. These discrepancies among the anions were far more significant when they acted as counterions rather than as co-ions. While SO42- and Cl- trigger fast diffusion-limited aggregation, SCN- gives rise to a stationary cluster size distribution in a few aggregation times when working with cationic PS particles. Clear differences are found among all analyzed parameters (CCC, CSD, K11, and df), and the experimental findings show that particles aggregate in potential wells whose depth is controlled by the chaotropic character of the anion. This paper presents new experimental evidence that may help to understand the microscopic origin of Hofmeister effects, as the observations are consistent with appealing theoretical models developed in the last few years.