Interfacial properties of nonionic surfactants and decane-surfactant microemulsions at the silica-water interface. An ellipsometry and surface force study

This paper features the interfacial behavior of nonionic surfactants and su rfactant-decane microemulsions at the silica-water interface from micellar solutions and water-rich tricomponent CnEm-decane-water microemulsions. The adsorption of a nonionic surfactant (pentaethyleneglycol n-dodecyl ether, C12E5) and its decane microemulsions to silica and borosilicate glass was s tudied by ellipsometry and direct force measurements using a bimorph surfac e force apparatus. The ellipsometric measurements of the adsorbed layer pro perties provided evidence of an initial lateral swelling of adsorbed bilaye r segments with increasing bulk oil fraction. At a weight fraction of about 0.12 w/w decane-to-surfactant + decane, the surface appeared to be fully c overed by a continuous bilayer with a thickness of 42 Angstrom, a refractiv e index of 1.448, and an mean area per surfactant of about 49 Angstrom (2). Further increase of the oil content results in the swelling of the bilayer in the direction normal to the surface plane. Force measurements between s urfactant-covered surfaces showed a subtle dependence on the properties of the glass substrate. The height of the force barrier prior to jumping into hard-wall contact was found to increase with increasing lateral Surface cov erage up to a decane content of 0.12 w/w. However, further increase in the fraction of decane resulted in a marked decrease of the force barrier heigh t. The steric force onset distance, however, was always found to be proport ional to the thickness of the adsorbed layers. Hence, the adsorbed layer pr operties measured by ellipsometry and the interaction curves measured by di rect force measurements were found to correlate well. Variations were somet imes seen in force profiles measured on different glass surfaces. In most c ases, the force onset distance correlated well with the thickness of two ad sorbed bilayers. However, in some cases, it agreed closely with the thickne ss of one bilayer. These variations were not easy to predict with regard to the pretreatment and measured properties of the glass surface. Our interpr etation is that this difference is caused either by very small changes in t he interaction strength between adsorbed surfactant headgroups and the glas s surface or by defects of the adsorbed layer resulting from the "topochemi cal" heterogeneity of the glass surface.