Abstract
IT has often been assumed that van't Hoff's discovery, that the simple gas-law, PV = RT, may be applied to the osmotic pressures of dilute solutions, justifies the view that osmotic pressure is caused by the bombardment of a semi-permeable membrane by the molecules of the solute, just as gas-pressure is caused by the bombardment of the containing vessel by rapidly moving gas-molecules. A recent exposition of this view by Prof. Ehrenfest, in the Proceedings of the Amsterdam Academy (vol. xvii., pp. 1241-1245), has elicited a reply from Prof. J. J. van Laar {ibid., vol. xviii., pp. 184-190), which will be read with very great interest by all those who have seen in the mechanism of osmosis an even more difficult problem than that of expressing the magnitude of the osmotic pressure by means of a mathematical formula. Prof. van Laar's reply is of exceptional value in that it demonstrates the inadequacy of the gas-analogy from the thermodynamic point of view, and so challenges the simple kinetic theory of osmosis on what has generally been supposed to be its strongest ground. The osmotic pressure may be expressed, according to Van Laar, by the equation, where x is the molecular concentration of the dissolved substance, and a is an “ influencing ” coefficient, which expresses the consequences of the interaction of the molecules of the solvent with those of the dissolved substance. The logarithmic term is an essential feature of the thermodynamic equation, and it is urged that all kinetic theories which lead to expressions without a logarithmic member must be rejected. The thermodynamic equation, it is true, leads to an expression for dilute solutions which is identical with that of van't Hoff. But in practice it is found that in more concentrated solutions deviations appear which are much smaller than those for non-ideal gases. We may therefore surmise that the so-called osmotic pressure has an entirely different ground from that suggested by van't Hoff's application of the gas-equation, and that there is here no close relation but merely an analogy. If the osmotic pressure were actually caused by the pressure of the dissolved substance, as Ehrenfest, reviving the old theory, suggests, the pressure of the sugar molecules against the semi-permeable membrane would, in van Laar's opinion, cause the reverse effect to that which is actually observed. No water would pass from the pure solvent through the membrane into the solution, giving rise to a hydrostatic pressure in the osmometer; but, on the contrary, the inward flow of water would be checked, since the pressure in the solution would from the outset be greater than in pure water. In reality, osmotic pressure is caused by the water which penetrates through the semi-permeable membrane, giving rise to a hydrostatic pressure which prevents the further intrusion of the water. This excess of pressure is the so-called “osmotic pressure” of the solution. Generally speaking, every theory which seeks to interpret osmotic pressure kinetically must be based on the diffusion of the water molecules on the two sides of the membrane. If this is done, the logarithmic member arises of its own accord, and finds a place in the equation, whether there is interaction between solvent or solute or not, i.e. the a-term appears quite independently of the logarithmic term. In van Laar's opinion, the kinetic interpretation of osmotic pressure, which is always reappearing again in new forms, is moving, and has moved, in a wrong direction, 'and should again be founded on the simple diffusion phenomenon.
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L., T. Osmotic Pressure or Osmotic Suction? . Nature 97, 68 (1916). https://doi.org/10.1038/097068a0
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DOI: https://doi.org/10.1038/097068a0