Molecular mobility and dynamic site heterogeneity in amorphous lactose and lactitol from erythrosin B phosphorescence

Abstract

We have used phosphorescence from erythrosin B to characterize the molecular mobility and dynamic heterogeneity in dry films of amorphous lactose and lactitol from − 25 to 120 °C. The phosphorescence emission spectra red-shifted and broadened with temperature in both sugars, indicating that both the rate of dipolar relaxation and the extent of inhomogeneous broadening increased dramatically at higher temperature. Phosphorescence intensity decays were well fit using a stretched exponential decay model; the rate constant for non-radiative quenching due to collisions with the matrix was calculated from the lifetimes. Arrhenius plots of this rate were non-linear, increasing very gradually at low and dramatically at high temperatures in both sugars. The rate of quenching was significantly lower in a 1:1 (wt/wt) mixture of lactose/lactitol in both the glass and the melt, providing strong evidence that specific interactions within the mixture lowered the matrix mobility. The lifetimes varied systematically with emission wavelength in both matrixes; analysis of the temperature dependence indicated that the activation energy for non-radiative quenching of the triplet state varied somewhat with emission wavelength. Time-resolved emission spectra collected as a function of delay time following pulsed excitation exhibited significant shifts to higher energy as a function of time. These data support a photophysical model in which erythrosin B molecules are distributed among matrix sites that vary such that blue-emitting sites with slower rates of matrix dipolar relaxation also have slower rates of molecular collisions. The amorphous matrixes of lactose and lactitol in both the glass and the melt state are thus characterized by dynamic site heterogeneity in which different sites vary in terms of their overall molecular mobility.