A singular behavior at the electrolytes solution surfaces: Experimental and simulation investigation over an extended range of temperature

•Surface properties of NaCl, KCl, and KI aqueous solution measured experimentally.•Dependence on conc. in a wider thermodynamic states led to interesting findings.•A singularity in excess surface conc. within 35–40 oC was found as an essential behavior.•MD simulation could decipher microscopic peculiarities underlying this singularity.•This singularity is closely relevant to the intravascular fluid/cell tissue interface. The aqueous electrolytes solutions have an essential role in a variety of systems including natural, biological, and industrial processes. The vital role in human bio-activity and the enhanced oil recovery are two familiar examples with the features that mainly root from the special organization of ions in the solution surface. Despite research works that have been reported on the electrolyte's solution surface, the dependence of surface properties on the thermodynamic state and the chemical nature of constituting phases is still an open field of ongoing research. In this study, the surface properties of halides (NaCl, KBr, and KI) aqueous solutions at the interface with vapor-saturated and at n-octane phase are investigated by surface tension measurement and molecular dynamics simulation. Surface entropy, surface enthalpy, and surface excess concentration (Γ) are explored in a wider range of temperature (up to 343.15 K) than those already have been reported. The surface tension measured under vapor-saturation conditions decreases with temperature and increases with concentration linearly, though shows some characteristics fluctuations. For both (NaCl and KI) systems, variation of Γ versus temperature peculiarly shows a maximum centered in the range of 308.15–313.15 K at all concentrations. These peculiar singularities involve a wealth of information related to the alteration of the surface structure that roots from the nature of Cl− and I− anions at vapor-saturation. To decipher this peculiarity for microscopic details and to account for the factors underlying this singularity, molecular dynamics simulation of NaCl, KBr, and KI aqueous solution was performed over the range 298.15–343.15 K. The simulated bulk and surface molecular structure, as well as the bulk transport properties of 1.0 M (mol/lit) NaCl, KBr, and KI aqueous solution, confirm a singular behavior in the range of 308.15–313.15 K which accounts for the behavior and trend of thermodynamic and surface excess concentration we measured experimentally as a function of temperature