The solvent dynamics at pore surfaces in molecular gels studied by field-cycling magnetic resonance relaxometry.

Abstract

The molecular dynamics of the solvent molecules at liquid-solid interfaces in low molecular mass gels and in bulk solvents have been identified and characterized with the aid of field-cycling NMR relaxometry. The gels are formed using ethylene glycol (EG) and 1,3-propanediol (PG) with different concentrations of 4,6,4',6'-O-terephthalylidene-bis(methyl α-D-glucopyranoside) (gelator 1). The spin-lattice relaxation times of bulk solvents measured in the function of Larmor frequency were analyzed assuming the intramolecular and intermolecular dipole-dipole interactions. For analysis of the relaxation data for confined solvents the two-phase fast-exchange model was assumed. It was found that in a low-frequency range a dominating NMR relaxation mechanism of solvent interacting with internal surfaces of pores in studied molecular gels is reorientation mediated by translational displacements (RMTD). This dynamic process allows us to explain a very long correlation time of the order of 10(-5) s calculated for confined EG molecules and an even longer one for PG. The RMTD contribution to the relaxation is described by power-law frequency dependence. In the 1/EG gels the exponent is equal to 0.5 for all gelator concentrations suggesting the equipartition of the diffusion modes with different wavelengths. In this gel the relaxation dispersion data were transformed to a susceptibility representation and a "master-like" curve was constructed. In the 1/PG gel the exponent varies in the function of gelator concentration. Different behavior of the relaxation dispersion shape is due to the relative sizes of the ordered (at surface) and bulk-like phase. In the 1/EG gel the surface layer of the ordered molecules is always much smaller than the dimensions of the gel cavities whereas it differs in the 1/PG gel as a consequence of the disruption of the PG aggregates due to the solvent-gelator interaction.