Corresponding-States Analysis of the Surface Tension of Simple, Polar, and Ionic Fluids

The liquid–vapor interfacial tension of various simple, polar, and ionic fluids is studied in a corresponding-states analysis that was originally suggested by Guggenheim. Data for real fluids are compared to results of simulations and theoretical predictions for model fluids of each of the three types (namely, the Yukawa fluid, the square-well fluid, a fluid consisting of dipolar hard spheres, and the restricted primitive model of ionic fluids). As already demonstrated by Guggenheim, the data for simple and weakly polar fluids map onto a master curve. Strongly dipolar, associating fluids, which may also exhibit hydrogen-bonding (e.g., water), show deviations from this master curve at low temperatures. In addition, the surface tension of these fluids shows a characteristic sigmoid behavior as a function of temperature. A similar behavior is found from simulations of the ionic model fluid, but not from the electrolyte theories available up to now, for which we present new results here. Exceptionally low values of the reduced surface tension are obtained for hydrogen fluoride and for the Onsager model of dipolar fluids, which, however, agree remarkably well with each other in a corresponding-states plot.