Molecular Organization of Surfactants at Solid-Liquid Interfaces

TLDR

Surfactant–solid surface interactions are critical for processes such as colloidal stabilization, ore flotation, and soil removal, yet the interfacial aggregation of surfactants remains poorly understood. Atomic force microscopy revealed that quaternary ammonium surfactants form half‑cylinders on crystalline hydrophobic substrates, full cylinders on mica, and spheres on amorphous silica, indicating that these interfacial aggregates could be exploited to pattern surfaces at nanometer scales.

Abstract

Interactions between surfactant solutions and solid surfaces play a key role in technologically important processes such as colloidal stabilization, ore flotation, and soil removal; however, the interfacial aggregation of surfactant molecules is not yet well understood. Direct images of surfactant aggregates at solid surfaces in aqueous solutions were obtained with atomic force microscopy. The resulting structures for quaternary ammonium surfactants (above the critical micelle concentration) are consistent with half-cylinders on crystalline hydrophobic substrates, full cylinders on mica, and spheres on amorphous silica. These structures-surprisingly different from earlier models-appear to result from a compromise between the natural free curvature as defined by intermolecular interactions and the constraints imposed by specific surfactant-surface interactions. Such interfacial aggregates can potentially be used to pattern surfaces at nanometer-length scales.