Practical Use of Concentration-Dependent Contact Angles as a Measure of Solid-Liquid Adsorption. 2. Experimental Aspects

Experimental aspects of using either contact angle goniometry or Wilhelmy balance tensiometry in the measurement of surfactant adsorption through concentration-dependent contact angles is discussed. A test system consisting of nonwettable, silane-treated glass slides and the nonionic detergent Tween-80 (polyoxyethylene sorbitan monooleate) was used to illustrate differences in adsorption results obtained by these two distinct methods. Wetting data were interpreted by application of Gibbs' adsorption isotherm which quantifies adsorption through a surface excess parameter, [Г_(sl) -Г_(sv)], that simultaneously measures surfactant adsorption at both solid-liquid (sl) and solid-vapor (sv) interfaces. It was shown that Wilhelmy balance tensiometry consistently gave lower values for [Г_(sl) -Г_(sv)], which was attributed to unavoidable solute deposition at the sv interface caused by liquid-front vibrations and solvent (water) evaporation at the moving solid-liquid-vapor (slv) three-phase line. By contrast, the slv line was stationary in the goniometry method and [Г_(sl)-Г_(sv)] ≈ Г_(sl), so that surface excess could be unambiguously interpreted in terms of sl adsorption at these hydrophobic surfaces. Adsorption results were interpreted in terms of the molecular configuration of Tween in the adsorbed state at liquid and solid interfaces. Experimental methods were extended to human serum albumin to explore the potential utility of concentration-dependent contact angle measurements in the study of protein adsorption to solid surfaces.