Thermodynamics of liquid surfaces: Adsorption at the binary hydrocarbon liquid—vapor interface

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The surface tension and density of n-dodecane- n-hexane mixtures were determined at 25°, 30°, 35°, and 40°C. The activity coefficients of benzene— n-dodecane mixtures were determined at 30°C. The composite and individual adsorption isotherms at the liquid—vapor interface were calculated by the Gibbs adsorption equation for mixtures of benzene with n-hexane, 2,2,4-trimethylpentane, and n-dodecane; cyclohexane with n-hexane and 2,2,4-trimethylpentane; and n-dodecane with n-hexane and 2,2,4-trimethylpentane. Results are discussed on the basis of activity and of position and orientation effects at the interface. The positive excess surface tension found in the n-dodecane— n-hexane and the n-dodecane—2,2,4-trimethylpentane systems is explained on the basis of position and orientation effects at the liquid—vapor interface; isotherms for the benzene— n-dodecane system indicate that a multilayer may be adsorbed at the interface.