Effects Of Surface Confinement Research Articles

Effects of Surface Adsorption and Confinement on the Photochemical Selectivity of Previtamin D3Adsorbed within Porous Sol−Gel Derived Alumina

It has been demonstrated that porous sol−gel derived (SGD) alumina can function as an effective microorganized medium in which to perform size- or shape-selective organic transformations. For the photolysis of 7-dehydrocholesterol (7D) encapsulated within SGD-alumina, it was found that the adsorption characteristics of 7D and the pore size of the SGD-alumina greatly influenced the photochemical selectivity of the cis−trans isomerization of previtamin D3 to tachysterol3. An increase in the ratio of previtamin D3 (P3) to tachysterol3 (T3) was obtained on photolysis of 7D adsorbed within SGD-alumina with a pore size of 10−20 A. This result is attributed to a confinement effect that inhibits the cis−trans isomerization of P3 to T3. In addition to the confinement effect on the photochemical selectivity, it was found that the adsorption characteristics of 7D to the surface of SGD-alumina also play a critical role in the photochemical selectivity. The results indicate that confinement in addition to chemisorptio...

Effects of surface and physical confinement on the phase transitions of cyclohexane in porous silica.

We have measured the specific heat (${\mathit{C}}_{\mathit{p}}$) of cyclohexane at 120T300 K when cyclohexane was physically restricted in porous Spherosil (silica) samples of pore radii 4, 7.5, 15, 30, and 62.5 nm. The behaviors of the monoclinic-to-cubic structural transition and of the melting transition of cyclohexane were determined. As expected, both transition temperatures, i.e., solid-solid and melting, inversely scaled with the pore radius (${\mathit{R}}_{\mathit{p}}$). It is argued that the surface heterogeneity, the presence of hydroxyl groups, and the radius of curvature (especially for smaller pores) induce considerable disorder in the adsorbed layers of cyclohexane, thus, resulting in the nucleation of crystalline grains of various sizes rather than in a single crystalline plug of cyclohexane in the porous silica samples. The distribution of the grain boundaries of crystalline cyclohexane produces a specific-heat peak for the melting transition, which almost resembles the \ensuremath{\lambda} shape for ${\mathit{R}}_{\mathit{p}}$\ensuremath{\le}30 nm. Unlike the monoclinic-to-cubic transition, the \ensuremath{\lambda} anomaly of the melting transition does not reflect a logarithmic dependence near the transition temperature. It is also argued, from the comparative observed ${\mathit{C}}_{\mathit{p}}$ behavior of the bulk cyclohexane with the physically restricted cyclohexane, that the cyclohexane liquid is more viscous than the bulk when it is confined in pores of 4 and 7.5 nm.