Sorption and diffusion in ethyl cellulose. Part II. Quantitative examination of settled isotherms and permeation rates

A quantitative analysis has been made of sorption isotherms and of diffusion data obtained with benzene, acetone, and methanol in ethyl cellulose. The concentration range covered was for volume fractions of penetrant not above 0.1. In this range reproducible measurements were possible. The sorption data show significant departures from the lattice theory of solution. In particular, isotherms tend to curve in the same sense as the Langmuir isotherm; heats of dilution are exothermal; and entropies of dilution are negative. The sign of heats of dilution and the isotherm shapes can be explained if gaps in the polymer provide sorption sites, but the entropy values cannot. An additional ordering or immobilization of the structure around the site, or of penetrant molecules on it, is required. In all these effects it is the nonpolar molecule benzene which shows them most clearly. Diffusion coefficients, D , Arrhenius energies of activation, E a , and the coefficients D 0 in the equation D = D 0 exp { − E a / RT } all depend upon the molecular dimensions of the penetrant. The permeability and D change as the concentration of penetrant increases, except for methanol. These and other aspects of diffusion have been discussed, and compared with results obtained in other polymers.