Neutron reflectivity experiments have been performed to investigate the adsorption behavior of aqueous solutions of n-2-butoxyethanol (n-C4E1) and i-2-butoxyethanol (i-C4E1) in D2O against a hydrophilic silica substrate. The measurements were made in the one-phase region near the lower critical solution point of the aqueous systems. At temperatures removed from the lower critical solution temperature, TLCS, evidence for the existence of a microscopically thick (∼40 Å) adsorbed D2O-rich layer is presented along with data suggesting that the amphiphile is strongly depleted from the boundary layer. Experiments using a thick “tuning” layer of silica reinforce these observations. Analysis of the data as T→TLCS reveals that the region beneath the adsorbed D2O-rich layer becomes increasingly enriched with D2O, and the nature of the composition profile normal to the interface has been considered. Analysis identifies the exponent describing the power-law decay part of the critical adsorption profile appended to the D2O-rich region and associates a temperature-dependent correlation length with the critical adsorption profile. Ellipsometric data are presented which complement the neutron reflectivity results. The adsorption behavior of the n-C4E1 and i-C4E1 systems are found to be very similar with the only discernible difference being the magnitude of the adsorption preference of water for the n-C4E1 system in the long-range profile.

We report the results of a preliminary investigation by specular neutron reflectivity of adsorption and wetting of alkane + perfluoroalkane binary liquid mixtures in the vicinity of the liquid/liquid critical point against a chemically modified (alkane-like) silicon wall. The results suggest the existence of a surprisingly long-ranged profile , even some degrees Kelvin above the upper critical point, and enable us to detect a wetting layer and its liquid/liquid interface below the critical point. The technique is capable of determining the composition, uniformity, and thickness of the wetting layer.

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For interfacial order parameter profiles which decay as Az(-mu), such as the composition profile of the noncritical interface of a binary liquid mixture at a critical end point, there is a reported one-to-one correspondence between the profile and the reflectivity which can be described by an analytical theory [S. Dietrich and R. Schack, Phys. Rev. Lett. 58, 140 (1987)]. Evidence for mu=0.53+/-0.02 for adsorption at the hydrophilic silicon/liquid surface of the mixture (2-butoxyethanol+deuterium oxide) near its lower critical end point, determined from neutron reflectivity measurements, is presented. This value is in good agreement with the theoretical prediction of mu=0.516+/-0.004. Further examination of the data permits the determination of the asymptotic surface enrichment scaling factor amplitude P0 approximately 0.11, which is not in agreement with the theoretical value P(0) = 0.94+/-0.05 and values determined by other experimental methods.

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