Mutual diffusion coefficient of concentrated trehalose aqueous solutions including supercooled regions measured by the Soret forced Rayleigh scattering method

Abstract

We measured the binary mutual diffusion (interdiffusion) coefficient of trehalose aqueous solutions D in the concentration range from 0.1 to 0.8 mass fraction and the temperature range from 25 to 50 °C including supercooled regions under atmospheric pressure. The experimental apparatus was based on the Soret forced Rayleigh scattering (SFRS) technique, which uses the Soret effect to create a periodic spatial concentration modulation (Δw <10−5) of micrometer-order fringe spacing in a disaccharide aqueous solution due to the absorption of an optical interference grating generated by two crossing pulsed excitation laser beams (ΔT <10−2 °C). The decay of the mass diffusion process is monitored by the diffraction intensity of a probing laser beam within a few milliseconds. These characteristics of SFRS enabled us to measure highly viscous disaccharide aqueous solutions under supercooled states. The uncertainties of the experimental results were estimated to be within ±3% for low concentration and ±9% for high concentration (w > 0.55). The experimental results demonstrated that the rate of decrease in the mutual diffusion coefficient with the increasing concentration is more pronounced in supercooled regions than in dilute normal regions. We also measured the mutual diffusion coefficient of sucrose aqueous solutions at 30 °C in the supercooled regions to compare the results with those obtained from trehalose. We observed that the mutual diffusion coefficient of the trehalose aqueous solutions were smaller than those of sucrose with the increasing concentration, the ratio of Dtrehalose to Dsucrose showed an almost linear concentration-dependence towards the values that were less than half. Our findings suggest that trehalose can be more effective than sucrose in terms of the relaxation of the composition near their glass transitions.