Dielectric relaxation of liquids at the surface of a porous glass

We have measured the dielectric relaxation of the glass-forming liquid N-methyl-$\varepsilon$-caprolactam (as a representative of nonassociating liquids of low molecular weight) confined to the mesopores of a controlled porous glass with 102 Å pore diameter. Three distinct relaxation peaks within ${\varepsilon }^{*}\left(\omega \right)$ are found for the confined liquid: a broadened $\alpha$ process with slightly modified temperature dependence compared to the bulk relaxation, an intermediate peak which originates from Maxwell-Wagner polarization of the heterogeneous system, and an extremely slow process ascribed to the layer associated with the pore surface. Our assignment of the three features is supported by the results of surface modification using trimethylchlorosilane, Maxwell-Wagner-Sillars calculations, and supplementary experiments where the pores are only partially filled with the liquid under study. By analyzing ${[-\frac{\partial {log}_{10}\left({\tau }_m\right)}{\partial T}]}^{-\frac{1}{2}}$, where ${\tau }_m$ is the peak relaxation time, we find that the variations of ${\tau }_m$ with $T$ are virtually identical for the three processes.