Composition profile of a wetting film in a binary mixture

The composition profile at a solid surface in the one-phase region of a binary liquid mixture in the complete wetting regime has been studied with neutron reflectometry. A hexane-rich film adsorbs to an octadecyl-coated silicon surface from a n-hexane-d(14)/perfluoro-n-hexane mixture. Above the critical temperature, T-c, of 14.4 degrees C, there is a comparatively thin (5-20 nm) film with an excess n-hexane content that decays exponentially toward the bulk, with the n-hexane content and decay length increasing as the temperature approaches T-c. Below T-c, as the coexistence temperature T-0 of 8.9 degrees C is approached, a very long-range effect on the density profile becomes apparent and the n-hexane content of the film remains greater than that of bulk out to distances z > 100 nm. Although the data are consistent with a z(-3) decay of the composition profile, as expected for van der Waals forces, we cannot unambiguously assign a decay to the long-range 80-120 nm region of the film. However, as for van der Waals forces, the n-hexane surface excess, Gamma, increases as (T - T-0)(-1/3) near coexistence, but more steeply closer to T-c.