The relationship between molecular structure and surface activity of ionic surfactants

A new model is developed to quantify the surface tension of ionic surfactants from surface affinity and ionisation equilibrium. The model successfully predicts two important molecular structure-surface activity factors: the length of single-branch homologues and the nature of counter ions. The modelling results also clarify the underlying mechanisms of the two processes. Changing the counter-ion only affects the ionisation, not the affinity. On the other hand, increasing carbon length dramatically increases the affinity while having a small effect on ionisation. The modelling framework consistently resolves structure-activity observations, some of which have been reported since the 19th century. The model can be extended for surfactants with more than multiple ionic states and surfactant/electrolyte mixtures.