Stationary and transient acoustically induced birefringence of methyl acetate molecules dissolved in ethanol

Abstract

A detailed study of the acoustically induced birefringence has been performed in methyl acetate molecules in solutions with ethanol to evaluate the relative variations of the acoustic intensity. Static and dynamic birefringence signals are ascribed to the orientation of the molecules along the direction of the applied ultrasonic field. The birefringence in dilute and concentrated solutions was investigated as a function of frequency, ultrasonic intensity and concentration. The transient behavior of the birefringence is indicative of a single exponential function implying a single relaxation mechanism. Systematic analysis of the experimental results is performed in the context of the presence of two distinct types of MA molecules in the solutions, namely the molecules that are similar to those existing in bulk material and the “solution”-type molecules that are distorted after the interaction with the ethanol/solvent molecules. The estimated relatively slow relaxation times, obtained from the transient birefringence measurements, imply that the acoustically induced birefringence is affected by the collective motion over the short-to-medium range order. Relaxation times exhibit a characteristic change below and above ~ 0.6 volume fraction of MA, which is related to the presence of the two discrete types of methyl acetate molecules in the solutions.