Surface activity of n-carboxylic acids

This study combined simulation and experimental data to describe the ionization of carboxylic acids at air/water surfaces. The model consistently explained the correlation between acid length/ionisation and surface activity. The affinity increment of the carboxylic series is consistent with alkanols, at 2.84 kJ per mol of CH2 groups. The correlation clarifies the century-old analyses of Traube (1891) and Langmuir (1917). The acid ionisation is massively enhanced at the air/water surface, evidenced by sub-molecular changes as the carboxylic groups move from the bulk to the surface. The surface enhancement increases linearly with increasing carbon length and consistent with previous surface experimental data. The result also explains the deficits of Stefan’s rule on the surface enthalpy of polar molecules. The insights on the surface activity of carboxylic acids explain the enhanced surface activities and consequences on aerosol properties and climate effects.