The thermodynamic equation of state for insoluble monolayers. I. Uncharged films

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Abstract
The applicability of a thermodynamic equation of state to the Π–A curves of insoluble monolayers is examined. The equation of state is based on the Gibbs dividing surface formalism and takes explicit account of the subphase water. It is found that the entire course of the Π–A isotherm can be accounted for qualitatively with an activity coefficient for the water in the surface region. In particular, the expanded film Π–A curves of many compounds can be quantitatively reproduced with a surface‐pressure independent activity coefficient which is evaluated from the measured value of the surface vapor pressure. This result implies that compression in the expanded film regime corresponds only to a reduction in the water content of the film. When the equation of state is applied to mixed‐surfactant monolayers, it is shown that only if the activity coefficient of water is the same in both single component films and the mixed film do we obtain the surface area additivity, which has been used as a criterion of ideality of mixing in the past.

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