The effect of isotopic substitution on self-diffusion in methanol under pressure

Abstract

Data have been obtained for self-diffusion in methanol to approximately 250 MPa pressure and methan(D)ol to about 304 MPa at 283, 298 and 313 K and at atmospheric pressure at 328 K. The average value of the ratio of the diffusion coefficients at atmospheric pressure is given by

              D_R=D(CH₃OH)/D(CH₃OD)=1.09±0.03

At each temperature there is an increase in D_R with pressure to a maximum at 250 MPa which is c. 6 % greater than the value at atmospheric pressure. The value of D_R at atmospheric pressure is much greater than would be expected from the classical (smooth hard spheres) behaviour. Analysis of these values of DR in conjunction with moments of inertia data suggests that the major influence on diffusion is rotation of the methanol molecule about the principal axis.