Predicting the α-relaxation time of glycerol confined in 1.16 nm pores of zeolitic imidazolate frameworks

Abstract

Uhl et al. [J. Chem. Phys., 2019, 150, 024504] studied the molecular dynamics of glycerol confined in a microporous zeolitic imidazolate framework (ZIF-8) with well-defined pore diameters of 1.16 nm by broadband dielectric spectroscopy. Of interest is a fast process in the central part of the pores identified as the α-relaxation of the confined supercooled glycerol with relaxation times τ_α,conf(T) reduced from τ_α,bulk(T) of bulk glycerol and having a temperature dependence different from the super-Arrhenius temperature of the latter. The focus of Uhl et al. was relating the confined molecular dynamics to the cooperativity length scales L_corr(T) of molecular motion above the glass transition, and deducing the limiting high-temperature value of the correlation length of about 1.22 nm. Not yet considered by anyone are the observed values of τ_α,conf(T) and temperature dependence. Since the cooperativity length scales L_corr(T) were found to be larger than the pore size of ZIF-8 over the temperature range studied and the density of the glycerol in the pore is possibly lower than the bulk, the cooperativity of the α-relaxation of glycerol confined in ZIF-8 is drastically reduced. Thus, within the framework of the Coupling Model (CM), τ_α,conf(T) should be nearly the same as the primitive relaxation time τ₀(T) for glycerol when devoid of intermolecular coupling and cooperativity. Consistent with the absence of cooperativity of the glycerol confined in ZIF-8, we find the calculated τ_α,conf(T) are either the same or slightly longer than the calculated values of τ₀(T). The quantitative prediction of the CM is verified. At this time we know of no other theory that can make such a quantitative prediction.