Some hydrodynamic boundary layer influences on mass transfer coefficients

As an initial approach to the quantitative understanding of the importance of various components of intestinal' contents as factors in mass transfer resistance, studies have been performed in diffusion cells containing polydimethylsiloxane membranes under circumstances sensitive to the boundary layers. (1) The effect of temperature change on permeability was measured. Under the conditions of the study, demonstration of a clear transition from membrane control to boundary control with rising temperature proved elusive. (2) When membrane thickness was systematically decreased, a gradual transition from membrane to boundary control was demonstrated, yet practical restrictions of the experimental conditions and system limited the degree to which either membrane or diffusion layers could be manipulated into a rate controlling role. (3) The effect of solution viscosity on mass transfer was also studied. Methylcellulose and sucrose were used as polymeric and low molecular weight viscosity inducing agents respectively. It was readily shown that viscosity has a much greater influence on the flux of a permeant through decreased diffusion coefficients than through increased boundary layer thicknesses, as is predicted by diffusional theories.