Solutes Can Transfer and Partition

Solutes Can Transfer and Partition from One Medium to Another 

In many processes, including chromatographic separations, the adsorption of drugs to surfaces, the equihbration of solutes between oh and water phases, and diffusion across polymeric or biological membranes, molecules start in one environment and end in another. Nucleation, aggregate or droplet formation, membrane and micelle formation, and protein folding have been modeled in terms of such processes. 

So far we ve concentrated on colligative processes that involve the transfer of solvent molecules from one place to another. Now we focus on the transfer of solutes, so we will use the solute convention (see page 284). 

Solvents A and B are immiscible or otherwise isolated from each other in some way. A solute species s dissolves in both solvents A and B, and can exchange between them. If, at equilibrium, the concentration of 5 in A is Xsa, and the concentration of 5 in 5 is Xsb, then the partition coefficient is defined as = XsbIXsa at equilibrium. (Partition coefficients can also be expressed in other concentration units.) 

The degree of freedom is the number of solute molecules in each medium. The total number of solute molecules is fixed. The chemical potential for s in A is Pi (A) and the chemical potential for 5 in B is Pi(il). To determine the partition coefficient, use the equilibrium condition that the chemical potential of the solute must be the same in both phases. 

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