This paper features the interfacial behavior of nonionic surfactants and surfactant-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 studied by ellipsometry and direct force measurements using a bimorph surface force apparatus. The ellipsometric measurements of the adsorbed layer properties provided evidence of an initial lateral swelling of adsorbed bilayer 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 covered by a continuous bilayer with a thickness of 42 Angstrom, a refractive 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 surfactant-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 coverage 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 height. The steric force onset distance, however, was always found to be proportional to the thickness of the adsorbed layers. Hence, the adsorbed layer properties measured by ellipsometry and the interaction curves measured by direct force measurements were found to correlate well. Variations were sometimes seen in force profiles measured on different glass surfaces. In most cases, the force onset distance correlated well with the thickness of two adsorbed bilayers. However, in some cases, it agreed closely with the thickness of one bilayer. These variations were not easy to predict with regard to the pretreatment and measured properties of the glass surface. Our interpretation is that this difference is caused either by very small changes in the interaction strength between adsorbed surfactant headgroups and the glass surface or by defects of the adsorbed layer resulting from the "topochemical" heterogeneity of the glass surface.