Mean Free Path and Ultrasonic Vibrational Relaxation in Liquids and Dense Gases

Abstract

Measurements of ultrasonic absorption and velocity in CO2 yielded values of the vibrational relaxation times over a range of densities from 400 to 610 amagats at 25°C and 50.2°C. On the basis that only binary collisions are important it is found that the Enskog theory for determining collision times in the dense fluid failed to account for the results. Application of the Eyring-Hirschfelder cell model of the liquid accounted well for the density dependence of the relaxation frequency but gave values of mean free path which were too large. It is proposed that for mean free path calculations a realistic cell model must not use fixed walls as the Eyring and Hirschfelder model does but rather moving walls. When this modification is included excellent agreement is obtained for both CO2 and CS2. It is further shown that attractive forces do not play the role in determining the collision efficiencies in a liquid or dense gas that they do in the dilute gas. After correcting for the reduced effect of attractive forces, it is found that the collision efficiencies for the dilute gas agree with values obtained for the liquid and dense gas.