A fluid-dynamical model for the ''anti-surfactant'' behaviour of salt solutions

We formulate and analyse a novel fluid-dynamical model for the flow of a solution with a free surface on which surface tension acts. This model, which uses the concept of surface excess, can describe both classical surfactants and aqueous salt solutions. These latter solutions have the anomalous property that in thermodynamic equilibrium the surface tension increases with increasing salt concentration, i.e., so-called ``anti-surfactant'' behaviour. We demonstrate the utility of the model by considering the stability of a deep layer of initially quiescent fluid, and identify the possibility of an anti-surfactant instability driven by Marangoni effects.