The Osmotic Pressure Point of View

It was pointed out at the beginning of this section that ir could be viewed as arising from an osmotic pressure difference between a surface region comprising an adsorbed film and that of the pure solvent. It is instructive to develop 

To review briefly, the osmotic pressure in a three-dimensional situation is that pressure required to raise the vapor pressure of solvent in a solution to that of pure solvent. Thus, remembering Eq. Ill-16, 

Let us now suppose that the surface region can be regarded as having a depth T and an area and hence volume V. A volume V of surface region, if made up of pure solvent, will be 

nfV /T is just the surface area, and, moreover, V /t and Vi/t have the dimensions of molar area, (/ the surface region is considered to be just one molecule thick, V /t and V /t becomes A and A2, the actual molar areas, so that Eq. III-115 takes on the form 

If further. A is neglected in comparison with A , then Eq. III-117 becomes the same as the nonideal gas law, Eq. III-103. 

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