Solvent drag, reflection coefficient and the pore concept

Viscous flow of water through a porous membrane will affect the unidirectional fluxes to small polar solutes as well as tracer water passing through these pores. For example, the unidirectional fluxes of small polar solutes or tracer water in the same direction as bulk water flow will be enhanced while that in the opposite direction will he hindered. Solvent drag refers to the effect of bulk water movement on the unidirectional fluxes of either tracer, water or small polar solutes. The term (1 — a)C J in Eqn. 10 represents the component of the overall unidirectional flux of the solute which is due to solvent drag (it is positive when and J are in the same direction and negative otherwise). 

In order to discuss the concept of solvent drag in quantitative terms let us consider a membrane barrier separating two dilute solutions of uncharged solute S. 

As shown in Fig. 1, the first compartment will be denoted by O and the second compartment will be denoted by Ax. Let us assume that a small amount of the isotope of this substance is added to the first compartment (O) to give a final concentration of Q.. The unidirectional flux of this substance from o Ax in the absence of a volume flow of water is given by — u RTAC, . On the other 

Using the same analysis, if the isotope was added to the second compartment (Ax) instead of the first one (0) and bulk water flow was still going in the same direction as in the previous case (0 - Ax) then the unidirectional flux of the substance will be given by 

If the concentration of the isotope added to the two sides are the same, i.e., Cf. = eft then the flux ratio is given by 

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