Carbon-13 NMR study of the effect of confinement on the molecular dynamics of 2-ethylhexyl benzoate

Spin-lattice relaxation times of individual carbons in 2-ethylhexyl benzoate (EHB) in bulk and confined to porous silica glasses of pore radii of 33, 52, and 102 Angstrom were measured as a function of temperature from +40 to -51 degrees C. The two-state fast exchange model was successfully applied, in the wide temperature range including beyond the motional narrowing regime, to determine the spin-lattice relaxation times of the molecules in the surface layer (T-IS) as a function of temperature. The Cole-Davidson distribution model was used to analyze the relaxation data for both the bulk (T-1B) and surface layer (T-1S) of the confined liquid. The line broadening of confined EHB at lower temperatures below -21 degrees C inherently limited the temperature range over which T-1S could be estimated and prevented the quantitative analysis of T-1S by the model. Through the phenomenological comparison of T-1S with T-1B in terms of the parameters of the Cole-Davidson model, it was found that the motional behavior of the phenyl ring part of the EHB molecule was significantly affected by the surface interaction and the effect is smaller for the flexible alkyl chain. Small temperature dependence of the relaxation rate of T-1S at motional narrowing regime showed a characteristic feature of liquid diffusing on a two-dimensional surface. In addition, the results of the quantitative analysis of T-1B were compared with a previous experiment performed as a function of pressure.