Lateral structure of a surfactant layer adsorbed at a hydrophilic solid/liquid interface

The lateral structure of the adsorbed layer of a nonionic surfactant at a hydrophilic silicon substrate against water was studied over a wide range of surface concentrations 1 of the surfactant by grazing-incidence small-angle neutron scattering (GISANS). For the chosen surfactant, tetraoxyethylene-mono-n- octyl-ether (C₈E₄), these changes in surface concentration were brought about by varying the temperature at a constant bulk concentration somewhat below the CMC of the surfactant. The GISANS measurements indicate the existence of transient surfactant aggregates without a preferred lateral structure at half coverage of the surface, and a correlation length of similar magnitude as the diameter of the surfactant micelles. This study shows that the picture of distinct surface aggregates of a characteristic size and separation that emerges from AFM studies of saturated surfactant layers on such substrates cannot be generalized to the situation below complete surface coverage.