Surfaces of Solids XV First‐Order Phase Changes of Adsorbed Films on the Surfaces of Solids: The Film of n‐Heptane on Ferric Oxide

Two‐dimensional first‐order changes, in which a gaseous film of normal heptane is transformed into another phase of lower molecular area with evolution of heat, have been discovered on subphases of ferric oxide, silver, and graphite. All of the critical phenomena observed in three‐dimensional systems are found to be duplicated. For n‐heptane on ferric oxide the critical constants are: σc (area) 900A2 per molecule; π (film pressure) 0.45 dyne cm−1; and Tc, 29°C. The critical constants are found to depend on the nature of the solid as well as on that of the vapor. The heat of transformation at 25°C is estimated to be 12,000±5000 cal. mole−1. This value appears to be considerably higher than the 6150 cal. mole−1 required for the formation of three‐dimensional liquid n‐heptane from its vapor at the same temperature. The volume‐pressure relations are considered for the adsorption isotherm in the case in which a second‐ or third‐order phase change occurs.