Measurement of the surface osmotic pressure

At present time, the most sensitive technique for measuring surface osmotic pressures is the Wilhelmy technique (1968) 4 it consists in measuring a lowering of surface tension. Let r0 be the surface tension of a pure liquid, and r0+, the surface tension of a liquid whose surface has been spread with a defined quantity of solute. It will be shown that the surface osmotic pressure is simply 

Let us consider a mobile barrier of length L, vertically separating the thin layer from the pure liquid surface. It will be assumed that this barrier is wetted on both sides (see Fig. 5.5). The superficial tension r0 of the bearing liquid exerts, on the barrier, a horizontal force 

In the same way, the surface tension r0+1 of the liquid covered with the thin layer exerts a horizontal force on the barrier 

By definition, the surface osmotic pressure is the force which has to be exerted on the barrier to ensure equilibrium, and therefore 

The surface tensions, r0 of the pure liquid, and r0+1 of the liquid covered with the adsorbed solute, are measured separately. 

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