Experimental study of a two-dimensional gas-liquid phase transition

The surface pressure exerted by a monomolecular layer of pentadecanoic acid spread on a pH-2 distilled-water substrate has been measured as a function of temperature and surface density in the vicinity of the gas-liquid critical point for this two-dimensional system. From these measurements, the shape of the gas-liquid coexistence curve and the isothermal compressibility along both sides of the coexistence curve and above the critical temperature are deduced. In agreement with the findings of Hawkins and Benedek obtained on pure distilled water, we find that a true phase transition exists between the liquid and gaseous states of the film and that the behavior of the film in the vicinity of its critical point can be characterized by power-law behavior with mean-field exponents. We do not, however, find it possible to interpret all of our data in terms of a simple mean-field phase transition.