Adsolubilization-induced structural change in adsorbed surfactant aggregates: Equilibrium and kinetics monitored by AFM and QCM-D

Abstract

Organic solutes with low water-solubility are solubilized into surfactant aggregates formed at solid/aqueous solution interfaces by the process of adsolubilization. In this study, we have characterized the adsolubilization of 1-hexanol (C6OH) into hexadecyltrimethylammonium bromide (HTAB) aggregates adsorbed on a silica surface, by means of atomic force microscopy (AFM) and quartz crystal microbalance with dissipation monitoring (QCM-D) techniques. The adsolubilization reduces electrostatic head-to-head repulsion between HTAB molecules forming the surface aggregates, along with additional adsorption of HTAB molecules at the solid/aqueous solution interface. At an HTAB concentration above the critical micelle concentration (cmc), the adsolubilization led to (i) a greater magnitude of repulsive interactions in the AFM force-distance data, (ii) a morphological change of the surface aggregates from a spherical or partially wormlike assembly into a flat bilayer, and (iii) an increased energy dissipation, reflecting an increased elasticity of the adsorbed aggregates. The kinetics of the adsolubilization-induced structural change was also studied at an HTAB concentration below the cmc; the increased C6OH concentration resulted in increased initial responses in frequency and dissipation. It seems likely that the kinetics is primarily driven by the diffusion of HTAB and C6OH molecules toward the silica surface, and the molecular reorganization within the adsorbed aggregates may also contribute to the kinetics especially at a low C6OH concentration.