Abstract
In investigations of ultrasonic relaxations caused by two state processes—such as monomer-dimer association or the interconversion of rotational isomers—the energy difference between states is usually calculated from experimental results on the assumption that the difference in molecular volume is zero, meaning that only the temperature variation in a sound wave affects the molecular equilibrium. However the pressure variation in a sound wave in a liquid is so large with respect to the temperature variation (∼50 atmos/°C) that significant errors or even castatrophes can be introduced by the rather small changes in molecular volume (⩽5%) that are likely for these processes. The additional experimental variable needed to separate these effects may be provided by measuring solutions of the relaxing substance in inert solvents which possess different ratios of pressure to temperature variation for sound waves. This separation is illustrated by measurements of the ultrasonic absorption of methyl cyclohexane in xylene and nitrobenzene. The difference in free energy and enthalpy between the two chair isomers of methyl cyclohexane in these solutions is found to be 2.2±0.4 and 3.5±0.8 kcal/mole, respectively, and a difference in molecular volume of less than 1% is indicated.
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