Compositional relaxation on the approach to the glass transition in a model trehalose solution

SDW Hannam and PJ Daivis and G Bryant, PHYSICAL REVIEW E, 99, 032602 (2019).

DOI: 10.1103/PhysRevE.99.032602

Molecular dynamics simulation was used to study the temperature dependence of the mutual diffusion coefficient D-m and the intermediate scattering function of equilibrium and metastable aqueous solutions of the cryoprotectant molecule trehalose at very low (2.2 and 9 wt.%) and very high (80 and 95 wt.%) concentrations. The simulations were conducted over a range of temperatures approaching the glass transition temperature T-g for each concentration Similar to a recent observation made on a glass-forming model polydisperse colloidal suspension Hannam et al., Phys. Rev. E 96, 022609 (2017), we confirmed by a set of independent computations that D-m is responsible for the long-time decay of the intermediate scattering function. We observed that D-m decreased on the approach to the glass transition temperature, resulting in an extremely slow long-time decay in the intermediate scattering function that culminated in the arrest of compositional fluctuations and a plateau in the intermediate scattering function at T-g. In both cases, crystallization requires a change in the composition of the solution, which is a process controlled by D-m. This transport coefficient can either increase or decrease as solidification is approached, because it depends on a product of thermodynamic and mobility factors. Our observations show that in both cases, for the glass-forming liquids, it is observed to decrease, while for a previously studied monodisperse colloidal suspension which crystallizes easily, it increases. The similarity in the behavior of these two very different glass-forming systems (the polydisperse colloidal suspension and the sugar solution) shows the importance of the mutual diffusion coefficient to our understanding of vitrification and suggests a possible distinction between between glass-forming and crystallizing solutions.

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