Effect of xanthan on the molecular mobility of amorphous sucrose detected by erythrosin B phosphorescence.

Molecular mobility in amorphous solids is modulated by composition and environmental conditions such as temperature. Phosphorescence of erythrosin B was used to generate a mobility map of amorphous sucrose film doped with xanthan gum at weight ratios of xanthan/sucrose ranging from 0.0001 to 0.01. On the basis of analysis of the emission energy and lifetime of erythrosin B in pure sucrose and sucrose-xanthan films over the temperature range from 5 to 100 degrees C, we conclude that xanthan influences the molecular mobility as well as the dynamic site heterogeneity of amorphous sucrose in a dose-dependent fashion. At xanthan/sucrose weight ratios below approximately 0.0005, both emission energy and lifetime decreased and k(TS0) (the nonradiative decay rate of the triplet state) increased, indicating that xanthan increased the matrix molecular mobility. At weight ratios above 0.001, both emission energy and lifetime increased and k(TS0) decreased, indicating that xanthan decreased matrix mobility, reaching a plateau at weight ratios between 0.005 and 0.01. The concentration at which the effect of xanthan switched from increasing to decreasing mobility was similar to the concentration at which polymer chains overlapped in solution, suggesting that the dynamic changeover reflected the onset of chain overlap in the amorphous solid. Systematic trends in the emission bandwidth and lifetime heterogeneity and variations in the emission lifetime vs wavelength indicated that xanthan reduced the matrix dynamic site heterogeneity except at a weight ratio of 0.01. These data illustrate the complex effects of a polymer with a rigid structure and large side chains on the mobility of an amorphous, hydrogen-bonded sugar matrix.