Electrolytes at Uncharged Liquid Interfaces: Adsorption, Potentials, Surface Tension, and the Role of the Surfactant Monolayer

The article summarizes the results of our research on the behavior of ions at uncharged fluid interfaces, with a focus on moderately to highly concentrated aqueous electrolytes. The ion-specific properties of such interfaces have been analyzed. The ion-specificity series are different for water|air and water|oil; different for surface tension σ, surface Δχ potential and electrolyte adsorption, and they change with concentration. A methodology has been developed that allows to disentangle the multiple factors controlling the ion order. The direct ion-surface interactions are not always the most significant factor behind the observed ion sequences: indirect effects stemming from conjugate bulk properties are often more important. For example, the order of the surface tension with the nature of the anion (σKOH > σKCl > σKNO3 for potassium salts) is often the result of bulk nonideality and follows the order of the bulk activity coefficients (γKOH > γKCl > γKNO3) rather than that of a specific ion-surface interaction potential. The surface Δχ potential of aqueous solutions is, in many cases, insensitive to the ion distribution in the electric double layer but reflects the orientation of water at the surface, through the ion-specific dielectric permittivity ε of the solution. Even the sign of Δχ is often the result of the decrement of ε in the presence of electrolyte. A whole new level of complexity appears when the ions interact with an uncharged surfactant monolayer. A method has been developed to measure the electrolyte adsorption isotherms on monolayers of varying area per surfactant molecule via a combination of experiments–compression isotherms and surface pressure of equilibrium spread monolayers. The obtained isotherms demonstrate that the ions exhibit a maximum in their adsorption on monolayers of intermediate density. The maximum is explained with the interplay between ion-surfactant complexation, volume exclusion and osmotic effects.


Symbols:
C el concentration of electrolyte C m molality of the electrolyte C thr threshold concentration above which the electrolyte behaves as sticky E 1 exponential integral function e elementary charge K association constant (ion in the bulk + surfactant  ion-surfactant associate) k B Boltzmann constant k is = ( + Z + 2 +   Z  2 )/2 ratio between ionic strength and electrolyte concentration C el L D Debye length, a ratio between dipolar and ionic strength, L D 2 = k B T/2k is e 2 C el L Q quadrupolar length, a ratio between quadrupolar and dipolar strength N w water molecules in the surface layer fully orientated by an ion in the subsurface layer n w refractive index of water P specifically adsorbed normal surface dipole moment p mean normal dipole moment R i minimum distance of approach of an ion to the surface z = 0; R i = R h,i z h R bare,i radius of the bare i th ion R h,i radius of the hydration shell of the i th ion R w radius of a water molecule T temperature u h,i dehydration potential acting on the i th ion u im,i image potential acting on the i th ion V el partial molecular volume of the electrolyte V w partial molecular volume of water

Abbreviations:
DDL dipolar double layer EDL electric double layer SchM modified Schmutzer's model (the minimal model of choice) X 0 referring to surface without surfactant but with electrolyte X h referring to dehydration X im referring to image force X w referring to water X  referring to the surface of dielectric discontinuity X ○ referring to the pure surfactant phase / the equilibrium spread monolayer around a crystal or a droplet of the surfactant phase W|A water|air surface W|H water|hydrocarbon interface W|M water|air surface with a surfactant monolayer

Polarization of the ion concentration profiles by the ion-specific interactions
Figure S2 illustrates schematically the profile of Br  at W|A and W|H interfaces (the effect from the EDL and the image forces is ignored for simplicity, making the schematic valid for very high concentration).At W|H, the dispersion repulsion is absent due to the similar Hamaker constants of the two phases.The hydrophobic and Debye forces are both attractive, producing some excess of bromide at the surface and negative deviations from SchM.The surface is negatively charged, and an EDL will be formed (Na + -dominated diffuse layer electroneutralizing the surface).
In contrast, at W|A, water attracts the ions with a long-ranged dispersion force; the shorter ranged hydrophobic and (the screened) Debye forces are of similar magnitude as for W|H.The excess of Br  is now zero, and no surface charge will accumulate.However, the surface is significantly polarized due to the specific forces alone, with negative excess charge in the layer dominated by the hydrophobic force, and positive charge in the dispersion force-dominated layer below.The cation will modulate the polarization but no EDL in the classical sense will form.
Figure S1.Symbols used to indicate surface potential, surface tension, electrolyte adsorption at various kinds of surfaces, and symbols used for the change of these quantities upon adding electrolyte or surfactant monolayer.

Figure S2 .
Figure S2.Schematic ion concentration profile for Br  at W|H (where it adsorbs) and W|A (where concentration polarization appears without significant adsorption).
el Volta potential change upon spreading a monolayer V 0 Volta potential change upon spreading a monolayer on pure water x density of the surface layer of molecules contributing to P divided by density of bulk Z i nondimensionalized absolute charge (valence) of the i th ion, Z i = |e i |/e z = 0 location of the -discontinuity z w shift of water's equimolecular surface with respect to the -discontinuity surface z h thickness of the hydrophobic gap  i molecular or ionic polarizability  el electrolyte adsorption (surface excess of C el )  p the number of water dipoles contributing to P per unit area  s adsorption of the surfactant (1/ s is area per molecule)  w adsorption of water  el molality-based mean activity coefficient  absolute dielectric permittivity of the solution  0 absolute dielectric permittivity of vacuum  w absolute dielectric permittivity of pure water  el chemical potential of the electrolyte  s chemical potential of the surfactant