No abstract

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.

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.

Critical binary liquid mixtures have proven to be extremely useful for quantitatively studying wetting and adsorption phenomena that occur at the surfaces of these systems. In this paper recent experimental developments in our understanding of adsorption gained via the study of critical mixtures are reviewed. At the noncritical liquid/vapor surface the component possessing the smallest surface tension completely saturates this surface for sufficiently large surface tension differences σ between the two components. This complete (or strong) adsorption is described by a universal scaling function P which depends upon the dimensionless depth z/ξ where ξ is the correlation length. Weakly polar and nonpolar binary liquid mixtures have been used to define the scaling function P. If the surface energy difference σ between the two components is small, then the surface is no longer completely saturated by one component. In this weak adsorption regime the adsorption behavior is now described by a universal function G of both z/ξ and σ . The functional dependence of G is elucidated by examining a homologous series of critical binary AB liquid mixtures where component A is an n-alkane while component B (methyl formate) is fixed. For this system σ changes sign with increasing n-alkane chain length. In addition to adsorption, dipole surface orientational order at the liquid/vapor surface of highly polar + nonpolar mixtures is also discussed. This orientational order is induced by the dipoleimage dipole interaction in the vicinity of a surface.