A Spin Probe ESR Study of Sugar Water Mixtures in the Liquid and Glassy State

Abstract

Conventional and saturation transfer ESR spectroscopy are used to study the rotational behaviour of two different nitroxide spin probes: 4-hydroxy, 2,2,6,6-tetra-methylpiperidino-oxyl (TEMPOL) and 3-maleimido-2,2,5,5,tetra-methyl-1pyrrolodinyloxyl in glycerol water, sucrose water and dextrin water mixtures as a function of temperature. In the liquid state the results are discussed in terms of slip and stickiness of the spin probes, which depend on the strength of the hydrogen bonds between the spin probe and the solvent. Except for the malemeimido spin probe in anhydrous glycerol, which can form an extended solvation shell, the slip increases as the water content of the mixture decreases. This is explained by a decrease of hydrogen bonds between the spin probe and the solvent. In sucrose water mixtures above 70% by weight sucrose and in sucrose water mixtures in the glassy state the spin probes are presumably present in cavities. At the temperature at which the transition from glassy state to solution takes place an extreme increase in rotational mobility of the spin probes can be observed. The temperature at which this increase is observed agrees reasonably well with the glass transition temperature as determined by differential scanning calorimetry experiments. The apparent activation energy for rotation in glassy maltodextrin increases as the molecular weight becomes larger, presumably due to a decrease in the free volume of the sample. No influence of the water content of the glassy samples at temperatures above the freezing point of water on the apparent activation energy is observed.