Adsorption Isotherms for Mobile Monolayers

Abstract

A three‐dimensional gas is pictured in contact with a liquid, forming at the interface a mobile monolayer. The density of the monolayer is maintained in equilibrium with the pressure of the gas. An equation is deduced giving as a function of temperature, the gas pressure which is just sufficient to initiate condensation in the monolayer. The only physical parameters involved in the relationship are the mass of the molecule and the heat of evaporation per molecule from the completely clustered monolayer. The fatty acids which exhibit monolayers with negligible external gas pressures, must have heats of evaporation greater than about 100×10^—14 erg. Molecules as small as 20 atomic mass units and heats of evaporation less than about 75×10^—14 erg should require pressures of the order of 10^—2 atmosphere. An adsorption isotherm equation is found for monolayer concentrations less than the saturation value. Its form is similar but not identical with Langmuir's isotherm for immobile monolayers.