Surface Activity of n-Carboxylic Acids

This study combined simulation and reported data to describe the ionization of carboxylic acids at an air/water surface. The model consistently explained the correlation between acid length/ionization and surface activity. With a surface affinity increment of 2.84 kJ/mol per CH₂ group, the model predicts the surface ionization for six acids, from formic to hexanoic. The correlation clarifies the century-old analyses of Traube and Langmuir. The acid ionization is massively enhanced at the air/water surface, which means the surface pK_a is lower than the bulk pK_a. The acidity enhancement is supported by the simulated submolecular changes as the carboxylic groups move from the bulk to the surface. The surface enhancement increases linearly with increasing carbon chain length and is consistent with experimental data. The result also explains the deficits of Stefan's rule on the surface enthalpy of polar molecules. The insights into the surface activity of carboxylic acids explain the enhanced surface activities and consequences on aerosol properties and climate effects.