Surface viscoelasticity of concentrated salt solutions: specific ion effects

Herein, we study the viscoelastic response of concentrated salt solutions using surface waves excited by electrocapillarity. We show that the hydrodynamic behaviour of the solutions is similar to that of water at concentrations up to 2 m-well above the concentration C*, at which inhibition of bubble coalescence occurs in these solutions. This result excludes the occurrence of changes in the slip conditions at C*, postulated to explain this inhibition. Our study is carried out on salts that both increase and decrease the surface tension. We observe that the salt that decreases the tension does not change the surface behaviour at all, whereas the other two salts essentially produce negative contributions to the surface viscoelasticity at very high salt concentrations. The effects observed are quite large and remain to be explained.