Effect of monovalent and divalent salts on the interfacial and structural properties of vapor-liquid water interface in presence of sodium dodecyl sulfate

The impact of divalent cations on the interfacial and structural properties of the vapor-liquid water interface covered with an anionic surfactant such as sodium dodecyl sulfate (SDS) is of great relevance to several industrial applications. In the present work, all-atom molecular dynamics simulations are performed to investigate the interfacial and structural properties of the vapor-SDS-liquid water interface with divalent salts viz., CaCl2 and MgCl2. The surface tension rises with the rise in divalent salt concentration, whereas it remains almost constant with NaCl concentration. In the absence of divalent salts, the accumulation of Na+ ions near the SDS headgroups is observed. Conversely, in the systems with divalent salts, the divalent cations (Ca2+ and Mg2+) enriched beside the SDS monolayer cause the monovalent cations (Na+) to displace from the interface into the bulk. The structural analysis shows that Na+ and Ca2+ cations enter the first hydration shell of the sulfate group of SDS. On the other hand, Mg2+ usually coordinates with the sulfate group of SDS. The potential of mean force (PMF) shows that the escape free energy of divalent cations is higher than monovalent cations, and the interaction between the cations and sulfate head group of SDS follows the order: S-Ca2+ > S-Mg2+ > S-Na+.