The affinity of amphiphilic molecules to air/water surface

The affinity of amphiphiles to the water/air surface was modelled by extending Eberhart’s equation. The equation successfully describes surface tension for all amphiphilic structures, including alkanols, carboxylic acids, non-ionic, ionic and Gemini surfactants. The model is more effective than the conventional analysis for amphiphiles with multiple ionic states. The prediction was consistently validated at different temperatures and non-aqueous solvents. The modelling results show a linear correlation between surface affinity and hydrophobicity/hydrophilicity. For alkanols, the affinity increment is 2.84 kJ/mol per CH2 group, the same as the reported energy from monomer to aggregate for non-ionic surfactants. For carboxylic acids, the affinity increment per CH2 group is 3.18 kJ/mol, incorporating the degree of acid dissociation. The affinity-hydrophilicity correlation is approximately -0.22 kJ/mol per oxyethylene group.