APRIL. 1955
TEXTBOOK ERRORS' I: The Mechanism of Vapor-pressure Lowering KAROL 1. MYSELS University of Southern California, los Angeles, California
THEfact that the vapor pressure of the solvent is lower over a solution than over the pure solvent and that the lowering is proportional t o the concentration, is frequently2 explained as follows: A dynamic equilibrium exists in both cases, so that the rate a t which molecules of solvent evaporate from the liquid is always equal to that at which they condense. The rate of condensation is proportional to the partial pressure of the vapor, while that of evaporation is unimpaired in the pure solvent but is impaired by solute molecules in the surface of the solution. Hence, the rate of escape is reduced in proportion to the concentration of the solute and maintenance of equilibrium requires a corresponding lowering of the rate of condensation and therefore of the partial pressure of the vapor phase. There are a t least four arguments showing the incorrectness of this explanation. (1) The above reasoning implies that larger mole-
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cules which will obstruct the surface more effectively should cause a greater lowering of the vapor pressure when present in same numbers. This, of course, is contrary t o Raoult's law. (2) The explanation violates the principle of microscopic re~ersibility,~ since i t makes the molecules of solute obstruct the escape but not the arrival of the molecules of solvent. (3) In fact, accumulation of molecules a t the surface, such as is known to occur in solutions of surface-active agents, has no effect on the vapor-pressure lowering of these solutions, which is normal in very dilute solutions and very. low,once micelles are f ~ r m e d . ~ (4) An insoluble material, such as cetyl alcohol, which can have no significant effect on the equilibrium vapor pressure of water, can nevertheless reduce the rate of evaporation by a factor of 6000 as it forms a tightly packed monomolecnlar layer on the s u r f a c e . V t must therefore reduce the rate of condensation in the same ~ro~ortion. a TOLMAN, R. C.,"The Principles of Statistical Mechanics," Oxford University Press, 1938, p. 163. H u m , H., J. W. MCBAIN,AND A. P. BRADY, J. Phgs. Chem., 55,311 (1951).
' LANGMUIR, I., AND V. J. SAAEFER.J. F~anklinInst., 235, No. 2 (1943): also in F. R. MOULTON. Ed.. "Surface Chemistrv."
