On the Behavior of Pure Substances Near the Critical Point

It has been suggested several times that the phenomena of condensation could be understood by considering the vapor as a system in which molecules are associating into clusters, these obeying the ordinary laws of equilibrium. One can also consider the liquid as a system in which bubbles of vapor are forming. The present paper attempts to apply these ideas to phenomena occurring in the neighborhood of the critical point. Only thermodynamic methods are used, in conjunction with some general assumptions concerning the properties of the molecules involved. Some aspects of the surface tension of the liquid near the critical point have been considered in some detail. The highest temperature Tm at which a meniscus can exist is assumed to be the temperature at which the surface tension vanishes at the same time that the condition for equilibrium between liquid and vapor phases is fulfilled. It is concluded that the pressure-volume isotherm at Tm has a finite horizontal region, corresponding to the squeezing out of surface when the surface tension is zero. The slope of the isotherm at Tm in the vapor region outside the horizontal portion is closely related to the slope in the liquid region just to the other side of the horizontal part; these slopes approach zero as the flat part is approached. Above Tm there is still a process which may be called condensation, but no horizontal part to the isotherms. This is in contradiction to conclusions reached by Mayer and Harrison on the basis of their statistical theory of condensation, but is apparently not in real contradiction to the theory.