Surfaces of Solids XVII A First- and a Second-Order Phase Change in the Adsorbed Film of <i>n</i>-Heptane on Graphite

Abstract

The films of n-heptane formed by adsorption on the surface of graphite between 25° and 40°C exhibit a first- and a second-order phase transition. The first-order transition occurs between the gaseous and liquid expanded phases. The second-order transition occurs between the liquid expanded and liquid intermediate phases, and between the gaseous and liquid intermediate phases above the critical temperature. Since the behavior of the film is the same as that observed with insoluble films on aqueous subphases, the nomenclature for the latter is used. The critical constants for the gaseous film were determined as: temperature, 31°C; area, 400A2 per molecule; and film pressure, 1.05 dyne cm−1. These values are different from those found for n-heptane on ferric oxide, which shows that the solid as well as the gas plays an important role in the determination of the adsorption isotherm. The heat of transition for the first-order change is estimated as 13,000±5000 cal. mole−1 at 25°C. It is found at corresponding temperatures, within this large experimental error, that the heat evolved in the first-order transition is the same on graphite and ferric oxide.