Nonsemipermeable or Leaky Membranes

With nonsemipermeable membranes, both the solute and the solvent can pass through the membrane. Let be the volume flow and Jd be the flow due to diffusion (permeation). Both these flows can result from either a hydrostatic pressure difference Ap or an osmotic pressure IT  

and Lp are called the phenomenological coefficients. They have the following meaning  

With solutions of equal concentration (for example, even pure solvent) on both sides of the membrane, the osmotic pressure is zero. If a hydrostatic pressure Ap is now applied, a volume flow occurs. is the permeability coefficient for equal concentration solutions. Thus, it follows from equation (9-16) for n = 0 that 

The volume flow J. can. however, also be produced by an osmotic pressure O, i.e., in the case of two solutions at different concentrations with Ap = 0. We can therefore write 

If the hydrostatic pressure is Ap = 0, and if there are two solutions of different concentration, then permeation will take place as a result of 

9 Determination of Molar Mass and Molar Mass Distributions 

The pressure difference at zero volume flow (Jv= 0) is, of course, measured by dynamic osmometry. Thus, with Equation (9-16), we have 

The negative ratio of what are known as the two phenomenological coefficients, Lpo and Lp, is called the selectivity, reflection, or Staverman coefficient. 

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