Visualizing self-assembly: Force microscopy of ionic surfactant aggregates at solid-liquid interfaces

The adsorption of surfactants from micellar solutions onto solid surfaces plays a crucial role in applications such as surface wetting, particulate detergency, and colloidal stabilization. Recent work (see below) has shown that the shape, size, and lateral organization of ionic surfactant aggregates at solid-liquid interfaces can be determined directly by atomic force microscopy (AFM). Imaging is performed using repulsive stabilization forces between surfactant layers adsorbed to both the tip and sample, and aggregate structures are determined by comparing AFM images with previous adsorption measurements. The observed interfacial structures for ionic surfactants result from a tradeoff between intermolecular interactions (i.e. geometric packing considerations) and molecule-surface interactions (i.e. the density, type, and crystalline order of adsorption sites). The hydrophobic, crystalline cleavage planes of graphite and MoS₂ adsorb and orient single-tail surfactants along the substrate symmetry axes, and this horizontal adsorption serves as a template for half-cylindrical aggregates. The hydrophilic, anionic surfaces of mica and silica interact with cationic headgroups, giving rise to spherical, cylindrical, or planar aggregates depending on the surfactants geometry and density of electrostatic binding sites.