Comparison of critical adsorption scaling functions obtained from neutron reflectometry and ellipsometry.

Carpenter et al. [Phys. Rev. E 59, 5655 (1999); 61, 532 (2000)] managed to explain ellipsometric critical adsorption data collected from the liquid-vapor interface of four different critical binary liquid mixtures near their demixing critical temperature using a single model. This was the first time a single universal function had been found which could quantitatively describe the surface critical behavior of many different mixtures. There have also been various attempts to investigate this surface critical behavior using neutron and x-ray reflectometries. Results have been mixed and have often been at variance with Carpenter et al. In this paper, the authors show that neutron reflectometry data collected from a crystalline quartz-critical mixture interface, specifically deuterated water plus 3-methylpyridine, can be quantitatively explained using the model of Carpenter et al. derived from ellipsometric data.

Ellipsometric search for vapor layers at liquid-hydrophobic solid surfaces.

We use precision ellipsometry to evaluate the existence of nanometer thick vapor films at the surface between a liquid and a hydrophobic alkylsilane coated Si wafer. We find no evidence for such vapor films. All of our fluid-solid ellipsometry measurements can be explained using a double layer model consisting of an oxide plus silane layer between the fluid and bulk Si substrate. We have carefully checked our ellipsometer for residual phase shifts which might, under certain circumstances, cause a mis-interpretation of the experimental results. We find that the most reliable ellipsometric results for thin films (which are relatively immune to the presence of small residual phase shifts) are collected at the Brewster angle. The dielectric constant of the native oxide coating is also compared with similar measurements for two thick (approximately 100-300 nm) thermally grown oxide coatings on a Si wafer.

Critical adsorption at silicon surfaces in binary liquid mixtures.

In critical binary liquid mixtures the preferential adsorption that occurs at liquid-vapor or liquid-solid surfaces is expected to be described by a universal surface scaling function. In this paper, we show that aniline strongly adsorbs at an oxide-coated Si wafer surface from a critical mixture of aniline + cyclohexane where this solid-liquid adsorption can be described by the same universal function found at liquid-vapor surfaces. For a tetrabromoethane + n-dodecane critical mixture the n-dodecane adsorption on an alkylsilane coated Si wafer cannot be described by previously determined adsorption functions. We speculate that this discrepancy is caused by chemical heterogeneities at the alkylsilane surface due to differing surface distributions of -CH3 and -CH2- groups within the silane layer.