Molecular simulation of the surface tension of simple aqueous electrolytes and the Gibbs adsorption equation

We present molecular dynamics simulations of the surface tension of aqueous NaCl and NaBr electrolyte solutions as a function of concentration at 300 K. With explicit water and Lennard–Jones/Coulomb ions of fixed charge, we find a positive Gibbs adsorption of both salts at higher concentrations of several molar. Although the calculated surface tensions are larger than that of pure water, positive salt adsorption is in agreement with the classic Gibbs adsorption equation because our simulations reveal a local maximum in the surface-tension isotherm. Experimental surface tensions for NaCl and NaBr, however, demonstrate a linear increase with increasing electrolyte concentration, calling into question the accuracy of the intermolecular-potential parameters chosen to represent the sodium halide salts.