Does the hydrophobic attraction contribute to the interaction between colloidal silica spheres coagulated by an adsorbing cationic surfactant?

Abstract

The character of the attractive interaction between hydrophobic surfaces, which is beyond the realm of the DLVO theory, is still under debate. The strength and extension of this non-DLVO, hydrophobic attraction (HA) between fixed macroscopic surfaces have been found to vary depending on the substrate type and, especially, the procedure adopted to render its surface hydrophobic. A related question is how HA manifests between hydrophobic colloidal particles in disperse systems. Here, the way of the particles’ hydrophobization is expected to be of utmost importance again. In the present article we compare experimentally determined absolute aggregation rate constants of uniform silica spheres at once hydrophobized and aggregated in water by a cationic surfactant (hexadecyltrimethylammonium bromide, CTAB) with their theoretical counterparts calculated from the DLVO theory in combination with hydrodynamic interactions and without any adjustable parameters. It has been found that both constants become identical when approaching a limiting magnitude in the specific situation of CTA+ cations adsorbed on the surface of silica colloids to such an extent that they compensate the negative charge and at the same time maximally hydrophobize the surface. Moreover, the limiting magnitude of the rate constants is comparable with that measured and predicted for silica colloids coagulated by K+ cations (KCl). It is concluded that the aggregation of hydrophobic silica particles initiated by the surfactant adsorption can be considered as the electrolytic coagulation in the fast (diffusion-limited) regime with no energy barrier due to the electrostatic repulsion, driven solely by the van der Walls attraction. Consequently, HA does not seem to influence the coagulation process at least in this colloidal system. Possible reasons of the disagreement of this finding with previous results of direct force experiments on similar surfactant-hydrophobized macroscopic systems are discussed.