Mathematical models to account for the dependence on sub-phase pH of numerous surface and interfacial properties of stearic acid are proposed. They describe quantitatively the variations in cross-sectional molecular areas and twodimensional transition and collapse pressures (as determined using LangmuirBlodgett techniques), and interfacial tensions and activities (using the conventional duNoOy apparatus). Species arising as a result of acid-base dissociations have been shown to coexist independently at air/water and oil/water interfaces over the entire range of pH studied.Correspondence between theory and experiment based on correlation coefficients of linear regression analyses is excellent: R-square values range from 0.93 to 0.99. Additional evidence for the correctness of the model is gained from an application of the Phase Rule of Gibbs to two-dimensional systems.From a knowledge of (1) the apparent surface pK a values (from surface pressure-pH data) and (2) the distribution coefficient describing the partitioning of hydrogen ions between the aqueous phase and the surface (from surface potential data) absolute surface pK a values are calculated.Interfacial pK a values are obtained in a similar manner. 559

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The importance of the distribution coefficient is frequently overlooked in the undergraduate physical chemistry curriculum. Sometimes it is mentioned in connection with "systems which lend themselves nicely to straightforward applications of the phase rule." At 14

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