Semi-permeable membranes and barriers for monolayers

The measurements of the volume osmotic pressure (in three dimensions) are reliable if the semi-permeable membrane works well, and only in this case. In principle, this membrane is permeable to the solvent species and impermeable to the solute species. In the case of polymer solutes, this effect results from a size difference between the molecules under consideration. The membrane is made of a porous material and the diameter of the pores must be larger than the size of the solvent molecules and smaller than the size of the solvent molecules. 

A more sensitive method consists in measuring the superficial tension lowering (see Section 2). 

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