Molecular dynamics in confined monomolecular layers. A field-cycling nuclear magnetic resonance relaxometry study of liquids in porous glass

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Abstract
Liquids filled in porous media such as porous glass do not freeze at the bulk freezing temperatures. Two phases must be distinguished. A one to at most two monolayer thick film adsorbed on the inner surfaces does not freeze at all, whereas free liquid within the pores freezes at reduced temperatures relative to the bulk values as predicted by the Gibbs/Thompson equation. The fraction of non-freezing liquid can be evaluated from the reduction factor of the low-frequency spin-lattice relaxation time upon freezing of the free liquid. A method for the determination of the pore size may be established on this basis. Water and tetradecane, i.e., a polar and a nonpolar adsorbate, filled in porous glass have been studied with the aid of field-cycling nuclear magnetic resonance (NMR) relaxometry. Above the freezing range the frequency dependences of the spin-lattice relaxation time T1 of the two liquids strongly deviate from each other owing to the different adsorption properties. On the other hand, with frozen samples the same frequency dependence of the liquid phase, that is essentially T1∝ν0.67, was found with both adsorbates. This proves that a nonpolar liquid confined to a thin layer on a polar surface underlies an equivalent relaxation mechanism as a strongly adsorbed polar liquid. As the dominating process, reorientations mediated by translational displacements along the curved and rugged surface are considered.