The Influence of Temperature and Glass Surface Chemistry on the Wetting Behavior of Binary-Liquid Mixtures.

The wetting behavior of phase-separated binary -liquid mixtures in contact with a glass substrate was systematically studied as a function of two parameters: glass surface chemistry and temperature. That wetting is sensitive to surface chemistry is well known and is often exploited for applications in which a liquid wets a solid substrate. For binary-liquid mixtures, temperature can also be an important factor in the wetting of a liquid/substrate interface by the coexisting liquid phase. This is, in part, because the composition of the two phases, and hence their relative affinities for the substrate, vary directly with the distance away from the critical mixing temperature T_ {c}. Glass capillary tubes reacted with the vapor of hexamethyldisilazane for different periods of time to produce substrates with uniform and controllable degrees of silylation. The wetting behavior of a liquid mixture against these substrates was characterized by contact angle via a capillary rise technique. For substrates of low coverage, both mixtures studied, CS_2 plus CH_3 NO_2, and C_6 H_{12} plus (CH _3CO)_2O, exhibited a series of first-order transitions to complete wetting upon approach to the critical temperature. The temperature dependence of the contact angle was consistent with scaling arguments based on short-range substrate/liquid interactions. For substrates of high coverage, the mixtures exhibited different systematic deviations from short-range force scaling. In particular, evidence was found for both the "continuous" and "partial" drying transitions expected for the case of opposing short- and long-range forces. An alternative silylation procedure in which alkyltrichlorisilanes are chemically adsorbed from dilute solution is demonstrated. The thickness of a CH_3NO _2-rich wetting layer intruding between the bottom of a clean glass sample cell and a CS _2-rich phase was measured by reflectivity. It was observed to have a large non-equilibrium response to small temperature perturbations. A model is developed which provides, for the first time, a direct means of measuring the forces responsible for wetting and their effect on the dynamics of diffusion-limited interfacial motion.