Ion-specific effects on surface potential and surface tension of water solutions explained via volume exclusion effects

• A modified Poisson–Boltzmann model, including volume exclusion effects, was proposed.
• It was shown that the effects are negligible at low ionic strengths.
• However, above about 0.1 M they become important.
• Different hydration numbers can explain ion-specific effects in surface tension.

The competition of ions for available neighboring hydrating water molecules (the exclusion volume effect) is important at high ionic strengths. Because of the differences in the hydration numbers of various ions, the volume exclusion effects might be responsible for the observed specific ion effects (the Hofmeister series) at large electrolyte concentrations. The exclusion volume is taken into account in the framework of the Structure Making/Structure Breaking (SM/SB) model, in which the interface with a thickness of a few Ångstroms is assumed to be depleted of SM ions (typically, the cations, such as Na+, K+ or Li+), while the SB ions (such as H+, OH−, Cl−, Br−, I−) are adsorbed on the air/water interface via the Langmuir adsorption equations. Whereas at low electrolyte concentrations, the volume exclusion has negligible effect on both Zeta potential and surface tension, for ionic strengths above 0.1 M it leads to ion specificity for both of them, and this can explain the Hofmeister series in the behavior of surface tension at large electrolyte concentrations. Whereas the surface tension of salt solutions depends on the hydration number of SB anions, their equilibrium constants might be partially responsible for the ion specificity.

Figure optionsDownload as PowerPoint slide