Relative role of surface interactions and topological effects in nuclear magnetic resonance of confined liquids

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Abstract
The relative roles of surface and topological effects on the nuclear relaxation rates T−11, T−12, and T−11ρ of polar or nonpolar liquids in porous sol‐gel silica glasses are identified via their very different pore size and frequency dependences. On the basis of theory, experimental relaxation rates, and molecular dynamics simulations for the modeled porous systems, the 1/Ti’s are interpreted in terms of a linear combination of bulk, confinement, and surface effects: 1/Ti = 1/Tibulk + ai/R2+ bi/R, where R is the average pore size and ai and bi are given in terms of the usual relaxation parameters of the studied molecular species. This simple expression which allows the determination of the relative roles of surface and topological effects has been used to fit the observed 1H NMR relaxation rates as a function of pore size and frequency for methylcyclohexane, nitrobenzene, pyridine, and toluene both for nonmodified and surface modified porous silica glasses. Using this method, the surface (∝1/R) and pure geometrical (∝1/R2) relaxation contributions are evaluated and the surface and translational correlation times are calculated. More generally, the experimental data allow us to explain the following seemingly paradoxical results obtained for confined liquids: (i) The pure confinement effect is independent of the polarities of the liquid molecules in pores and is very sensitive to the frequency. (ii) The finding of the frequency variation of T−11 and T−11ρ both for polar or nonpolar liquids confined to small pores, shows that the geometrical confinement effects dominate over the surface interaction effects at low frequency and for small pores.