The effect of counterions on surfactant-hydrophobized surfaces

A common method for creating hydrophobic monolayers on charged surfaces is by self-assembly of ionic surfactants from solution. Several factors are important in controlling the structure and properties of such layers: the hydrophobic interactions between adjacent chains, the electrostatic interactions between adjacent headgroups, and electrostatic interactions between the headgroups and the surface charges. We have discovered that the surfactant counterions can have a remarkable effect on the hydrophobicity and hydrophobic interactions of a self-assembled layer. The experimental system was stearoyl(C18)trimethylammonium surfactant with iodide, bromide or chloride counterion (STAI, STABr, and STACl respectively) self-assembled onto mica substrates. Changing the surfactant counterions alters the wetting properties of hydrophobic monolayers on mica. Using a surface force balance we have carried out direct measurements of the interaction force between two surfactant-coated surfaces across water, revealing a strong effect of counterion on the normal interactions. Paradoxically, STAI-coated mica has both the highest water contact angle (is 'most hydrophobic') at the same time as having the highest surface charge relative to STABr and STACl. We use measurements of interfacial tension, asymmetric force measurements, and XPS to lead us towards an interpretation of these results and an understanding of the effect of counterion on the structure of self-assembled monolayers.