Structural Absorption of Ultrasonic Waves in Methanol

Hall's theory of structural relaxation is applied to explain the excess ultrasonic absorption in methyl alcohol in the pressure range 1-2000 kg/${\mathrm{cm}}^2$. Two states, one characterized by a higher volume (icelike structure) and the other by a lower volume (close-packed structure), are assumed to be available to each molecule of methanol, just as in water. It is found that Hall's theory for this case must be modified in order to be in agreement with the experimental results. The modification consists in assuming that the variation of the volume difference $\Delta V$ and the free-energy difference $\Delta F$ is nonlinear with pressure, in contrast with water. This leads to excellent agreement with theory, and also to the conclusion that the variation of $\Delta F$ with pressure is similar to that of ultrasonic velocity in methanol. Assumed values of the relaxational compressibility ${\beta }_r$ and the structural relaxation time $\tau$ (at $P=1000\mathrm{}$ kg/${\mathrm{cm}}^2$) are also justified because the other parameters calculated are as expected.