ABSTRACT
Recent measurements have suggested that the antiplasticizing effect of glycerol on trehalose can significantly increase the preservation times of proteins stored in this type of preservative formulation. In order to better understand the physical origin of this phenomenon, we examine the nature of antiplasticization in trehalose-glycerol mixtures by dielectric spectroscopy. These measurements cover a broad frequency range between 40 Hz to 18 GHz (covering the secondary relaxation range of the fragile glass-former trehalose and the primary relaxation range of the strong glass-former glycerol) and a temperature (T) range bracketing room temperature (220 K to 350 K). The Havriliak-Negami function precisely fits our relaxation data and allows us to determine the temperature and composition dependence of the relaxation time tau describing a relative fast dielectric relaxation process appropriate to the characterization of antiplasticization. We observe that increasing the glycerol concentration at fixed T increases tau (i.e., the extent of antiplasticization) until a temperature dependent critical "plasticization concentration" xwp is reached. At a fixed concentration, we find a temperature at which antiplasticization first occurs upon cooling and we designate this as the "antiplasticization temperature," Tant. The ratio of the tau values for the mixture and pure trehalose is found to provide a useful measure of the extent of antiplasticization, and we explore other potential measures of antiplasticization relating to the dielectric strength.
ABSTRACT
Dielectric spectroscopy was employed to study the effects of water on the primary alpha -relaxation and the secondary beta -relaxation of xylitol. The measurements were made on anhydrous xylitol and mixtures of xylitol with water with three different water concentrations over a temperature range from 173 K to 293 K. The alpha -relaxation speeds up with increasing concentration of water in xylitol, whereas the rate of the beta -relaxation is essentially unchanged. Some systematic differences in the behavior of alpha -relaxation for anhydrous xylitol and the mixtures were observed. Our findings confirm all the observations of Nozaki et al. [J. Non-Cryst. Solids 307, 349 (2002)]] in sorbitol/water mixtures. Effects of water on both the alpha - and beta -relaxation dynamics in xylitol and sorbitol are explained by using the coupling model.
ABSTRACT
From temperature studies at ambient pressure, it was pointed out for several glass-forming liquids that the alpha-relaxation time (tau) can be related to the dc-ionic conductivity (sigma) through the phenomenological fractional Debye-Stokes-Einstein (DSE) equation. In the present paper we test the validity of fractional DSE equation for relaxation data obtained from pressure variable experiments. To this end we carried out broadband dielectric measurements (10 mHz-10 MHz) in a wide range of pressures (0.1-300 MPa). The material under study were N,N-diglycidyl-4-glycidyloxyaniline and N,N-diglycidylaniline. As a result we found that the fractional DSE equation is also obeyed for pressure pathways.