Studies of methylene blue monomer-dimer reaction in super-cooled glycerol

Abstract

In 1979, Barkatt and AngeU [1] followed the change in the spectra of two different carboycanine dyestuffs in supercooled polyalcohol sorbitol as the solution was cooled and heated with specific cooling-heating rates. The results were modeled using the Tool- Narayanaswamy-Moynihan model (TNM). This study duplicates these experiments with methylene blue dissolved in glycerol. In addition to continuous cooling-heating experiments, kinetic and equilibrium experiments were carried out to determine the validity of the TNM model in fitting the data. The TNM model is designed to model a liquid near the glass transition point. It also proved to be a good model for fitting the change in the spectrum of dyestuff in solution with an optical glass transition point well above the calorimetric glass transition point. In these studies a hysteresis was observed between the cooling and heating curves. Of the four parameters determined in the TNM model, â and x are the most important. In these studies â was determined to be near one, indicating the relaxation is exponential, and x was less than one. A value of x< 1 implies an asymmetry in the relaxation times for heating and cooling. The observed hysteresis formed in the temperature range of 200-220K. Kinetic studies were conducted in this temperature range in 2K rapid jumps. After each jump the solution was allowed to come to equilibrium. The changes in monomer and dimer peaks were recorded. A characteristic relaxation time was determined from each temperature jump. The relaxation curves were exponential confirming the findings of the TNM model. However, when relaxing to a set temperature the characteristic relaxation tune for heating was within experimental error not much different than that of the cooling which contradicts the predictions of the TNM model. Equilibrium studies were conducted in the temperature range of 200-260K in l0K jumps. The concentration was decreased until no dimer peak was seen at 200K. With equilibrium constant data, as a function of temperature, relaxation times obtained from temperature jump studies were combined to obtain a rate constant for the formation of the dimer. This rate constant is consistent with diffusion-controlled reactions.