The effect of pressure on the diffusion coefficient D of p-aminoazobenzene in cellulose diacetate swollen with water, benzene and n-alcohols of structure CnH2n+1OH was investigated at 80°C under high hydrostatic pressures up to 1500 bar. From the pressure dependence of D, the activation volume, ΔV_??_, for the diffusion was found to be 20.9 cm3/mol for the water swollen and 39.5 cm3/mol for the benzene swollen system, while, for n-alcohols, ΔV_??_ was found to increase linearly with the increase in the carbon number n, from 17.7 cm3/mol for n-propanol to 42.9 cm3/mol for n-heptanol, resulting in a ΔV_??_ increment of 6 cm3/mol per one methylene unit. On the other hand, the degree of swelling, S, of cellulose diacetate was obtained to be approximately the same in benzene and in all the alcohols, 19-20%. This value of S was more than twice as large as the value found in water (S=7.5%).
The above results were apparently inconsistent with our previous argument that ΔV_??_ should decrease with increasing degree of swelling which supplies the larger free volume and hence reduced ΔV_??_. A diffusion mechanism was proposed for the increased occupied volume of the diffusant due to the solvation by a specific solvent. Thus the volume of the Eyring's hole for accommodation of the activated diffusant increases with increasing number of solvation as well as increasing intrinsic volume of the solvent molecule. During these processes, it is to be noticed that the free volume effect due to the swelling, which has been proposed in a previous study, is superseded by the effect of solvation.