Spatial variation of molecular dynamics in the nanoconfined glass-former methanol

We report filling-fraction-dependent dielectric measurements on methanol confined in an array of parallel-aligned channels of 8 nm diameter and 100 μm length in a monolithic mesoporous silica membrane. For channel fillings up to 43%, the channel walls are covered with films of average thickness from 0 to 1 nm. Upon filling, the average relaxation time decreases by three orders of magnitude, and the apparent glass-transition temperature T decreases from 99 to 90 K. The local relaxation times τ of the molecules added in each adsorption step even change by 3.5 orders of magnitude. Analyzing the distribution of relaxation times, we show that added molecules alter the dynamics of those previously deposited. The speeding up of molecular dynamics upon filling hints to spatial heterogeneity and is in accordance with two scenarios, which are experimentally undistinguishable: (i) heterogeneity of the inner channel surface, where molecules are at first attached to strong adsorption sites and only subsequently to weaker sites or on top of already deposited molecules, and (ii) an exponential decay of molecular relaxation times with increasing average distance z from the channel walls, that is, In(τ/τ) exp(-z/ζ) holds. Following a theoretical model, we interpret ζ as a dynamical correlation length of the supercooled liquid. We find ζ ≈ 0.22 nm at T = 125 K, a value close to the molecular radius of methanol. The latter result corroborates molecular dynamics simulations reported in the literature. © 2014 American Chemical Society.