MOLECULAR-DYNAMICS IN CONFINED MONOMOLECULAR LAYERS - A FIELD-CYCLINGNUCLEAR-MAGNETIC-RESONANCE RELAXOMETRY STUDY OF LIQUIDS IN POROUS-GLASS

Liquids filled in porous media such as porous glass do not freeze at t he bulk freezing temperatures. Two phases must be distinguished. A one to at most two monolayer thick film adsorbed on the inner surfaces do es not freeze at all, whereas free liquid within the pores freezes at reduced temperatures relative to the bulk values as predicted by the G ibbs/Thompson equation. The fraction of non-freezing liquid can be eva luated from the reduction factor of the low-frequency spin-lattice rel axation time upon freezing of the free liquid. A method for the determ ination of the pore size may be established on this basis. Water and t etradecane, i.e., a polar and a nonpolar adsorbate, filled in porous g lass have been studied with the aid of field-cycling nuclear magnetic resonance (NMR) relaxometry. Above the freezing range the frequency de pendences of the spin-lattice relaxation time T-1 of the two liquids s trongly deviate from each other owing to the different adsorption prop erties. On the other hand, with frozen samples the same frequency depe ndence of the liquid phase, that is essentially T-1 alpha v(0.67), was found with both adsorbates. This proves that a nonpolar liquid confin ed to a thin layer on a polar surface underlies an equivalent relaxati on mechanism as a strongly adsorbed polar liquid. As the dominating pr ocess, reorientations mediated by translational displacements along th e curved and rugged surface are considered. (C) 1995 American Institut e of Physics.