Nuclear Magnetic Resonance and Broadband Dielectric Spectroscopy Studies on the Dynamics of Ethylene Glycol in Mesoporous Silica

We use nuclear magnetic resonance and broadband dielectric spectroscopy to study rotational and translational dynamics of ethylene glycol confined to mesoporous silica over wide temperature ranges. We adjust the diameters of the silica pores in the range 2.4-9.2 nm to systematically ascertain effects of partial crystallization on the glass transition of confined ethylene glycol. In the weakly cooled temperature range, where all molecules are in the liquid state, it is found that the correlation times increase and the diffusion coefficients decrease when the pore size is reduced. Despite this slowdown, the Stokes-Einstein-Debye relation is obeyed in the silica pores. In the deeply cooled temperature range, partial crystallization can lead to coexisting liquid and crystalline phases with complex dynamical behaviors inside the pores, depending on the confinement size. Under such circumstances, the volume accessible to the liquid is further restricted by the crystal, interfering with a systematic dependence of the dynamical behavior on the nominal pore diameter. It is observed that the structural a relaxation of the confined liquid is strongly affected by this severe confinement; in particular, it can evade dielectric detection when cooling toward the reduced glass transition temperature of T-g similar to 147 K in narrow silica pores.