Practical Use of Concentration-Dependent Contact Angles as a Measure of Solid-Liquid Adsorption. 1. Theoretical Aspects

Theoretical aspects of interpreting concentration-dependent contact angles are discussed. A thermodynamic analysis was applied to deduce relative magnitudes of solid-vapor (sv) and solid-liquid (sl) adsorption for surfaces exhibiting a full range of wettability. It was concluded that the surface excess parameter [Г_(sl) -Г_(sv)], which simultaneously measures si and sv adsorption, can be interpreted in terms of si adsorption for nonwettable surfaces under experimental conditions that avoided solute deposition at sv interfaces. Practical computational techniques are described and applied to an experimental system consisting of glass cover slips, with and without a hydrophobic silane layer, and polystyrene plaques with different levels of surface wettability imparted by plasma oxidation. The nonionic surfactant Tween-80 (in saline) and the cationic surfactant cetyl bromide (in water) were applied as test surfactants. A relatively smooth decrease in [Г_(sl) -Г_(sv)] with increasing solid surface wettability was observed for the Tween/saline system. These observations were interpreted as a monotonic decrease in si adsorption with increasing wettability. By contrast, [Г_(sl) -Г_(sv)] values for cetyl bromide/water exhibited sharp changes as a function of surface wettability which were attributed to a rapid transition from a “Tween-like” hydrophobic adsorption mechanism to an ion-pairing adsorption mechanism involving the cationic head group and putative anionic functionalities on oxidized surface.