High-resolution calorimetric studies have been made of the liquid crystal phase transitions for several dispersions of 70-Å-diam silica spheres ͑aerosil͒ in octylcyanobiphenyl ͑8CB͒ as a function of silica density S. The excess specific heat peaks associated with the nematic-isotropic (N-I) and the nematic-smectic-A (N-SmA) transitions both exhibit shifts to lower temperatures, decreases in the specific heat maximum values, and decreases in the transition enthalpies as S is increased. Two distinct regimes of S-dependent behaviors are observed with a crossover between them at S Х0.1 g cm Ϫ3. For lower silica densities, sharp second-order C p peaks are observed at the N-SmA transitions, characterized by effective critical exponents that decrease monotonically with S from the pure 8CB value toward the three-dimensional XY value, and two closely spaced but distinct first-order C p features are observed at the N-I transition. For higher silica densities, both the N-SmA and the N-I transitions exhibit a single rounded C p peak, shifting in temperature and decreasing in total enthalpy in a manner similar to that observed in 8CBϩaerogel systems. Small angle x-ray scattering data are qualitatively aerogel-like and yield temperature-independent mass-fractal dimensionalities for aerosil aggregates that differ for samples with silica densities above and below the crossover density.

The infrared spectrum of CO chemisorbed on rhodium surfaces has been investigated in the region from 1700-4000 cm.-1. The rhodium is supported on a high area alumina and the nature of this deposit can be varied by controlling the per cent, metal and the heat treatment. On a series of rhodium surfaces, spectra have been studied as a function of coverage for adsorption and desorption. Reaction of the chemisorbed CO with oxygen, hydrogen and water have been investigated spectroscopically. The results are interpreted in terms of several structures for chemisorbed CO on rhodium.

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Comprehensive x-ray scattering studies have characterized the smectic ordering of octylcyanobiphenyl (8CB) confined in the hydrogen-bonded silica gels formed by aerosil dispersions. For all densities of aerosil and all measurement temperatures, the correlations remain short range, demonstrating that the disorder imposed by the gels destroys the nematic (N) to smectic-A (SmA) transition. The smectic correlation function contains two distinct contributions. The first has a form identical to that describing the critical thermal fluctuations in pure 8CB near the N-SmA transition, and this term displays a temperature dependence at high temperatures similar to that of the pure liquid crystal. The second term, which is negligible at high temperatures but dominates at low temperatures, has a shape given by the thermal term squared and describes the static fluctuations due to random fields induced by confinement in the gel. The correlation lengths appearing in the thermal and disorder terms are the same and show a strong variation with gel density at low temperatures. The temperature dependence of the amplitude of the static fluctuations further suggests that nematic susceptibility becomes suppressed with increasing quenched disorder. The results overall are well described by a mapping of the liquid-crystal-aerosil system onto a three-dimensional XY model in a random field with disorder strength varying linearly with the aerosil density.

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