(Invited) Solvent Effects on the Surface Charge Formation in Electric Double Layers
Authors: Petsev Dimiter N. , van Swol Frank , Fleharty Mark 
Published: February 2020

The solvent role in electrolyte solutions is often considered relatively unimportant since it is uncharged and therefore assumed to be unaffected by the electric fields in the vicinity of charged interfaces. While there are studies that explicitly account for the molecular structure of the solvent their aim is primarily to provide a molecular interpretation of the dielectric properties of the solution.1-4 In a recent publication5 we focused on the question of how the precise solution structure near an interface couples to the surface chemistry that govern the charge. The system we analyzed consisted of (i) Lennard-Jones type of solvent, (ii) potential determining ions (PDIs), (iii) non-potential determining ions (non-PDIs) that are not involved in chemical interaction with the surface and (iv) common counterions to both the PDIs and non-PDIs. The surface charge is a result of the chemical equilibria6

AH2
+ + BH ↔ AH + BH2
+

AH + BH ↔ A- + BH2
+

2BH ↔ B-+ BH2
+

Where AH represents the surface chemical groups that can bind or release a proton, while BH­2
+are the PDIs.
Our work shows that the solution structure very strongly couples to the surface chemical equilibrium above, and has a profound effect on the resultant charge at the interface (see the Figure). It shows dependence of the surface charge on the solvent molecular diameter. Varying the molecular diameter is equivalent to gradually “turning on” the solvent contribution to the solution detailed structure. The different curves correspond to an increasing magnitude of the non-electrostatic molecular attraction from top to bottom. If the solvent is considered a structureless continuum, the solvent diameter is zero and all curves collapse together (see the far left region of the Figure). However if the solvent diameter becomes comparable to the size of the rest of the ionic species, then the system demonstrates a complex dependence on size, Coulombic and non-electrostatic interactions (see the far right region of the Figure). Between these two limiting cases the solution exhibits a phase separation (the dotted line represents a spinodal) because the overall solution density increases with the solvent diameter. While not physical this phase behavior emphasizes how inadequate the structureless solvent model is in the description of electric double layer with surface charge controlled by a chemical equilibrium between surface ionizable groups and the potent. In some ways the ignoring the explicit solvent structure is akin to applying the ideal gas equation to a dense liquid state.
The reason for the strong solvent-structure effect is in the overwhelming number of solvent molecules in comparison o the ionic species. Despite the fact that the solvent molecules are uncharged they largely determine the liquid structure and ions can only occupy vacancies between the solvent molecules. Hence the local distribution of ions (including PDIs) in the vicinity of the charged surface will depend on the presence of the solvent and thus affect the charge regulation at the interface.
1. S.A. Adelman and J.M. Deutch, J. Chem. Phys., 1974. 60: p. 3935-3949.
2. D.Y.C. Chan, D.J. Mitchell, and B.W. Ninham, J. Chem. Phys., 1979. 70: p. 2946-2957.
3. D.Y.C. Chan, D.J. Mitchell, B.W. Ninham, and B.A. Pailthorpe, J. Chem. Phys., 1978. 69: p. 691-696.
4. S. Carnie and D.Y.C. Chan, J. Chem. Phys., 1980. 73: p. 2949-2957.
5. M.E. Fleharty, F. Van Swol, and D.N. Petsev, Phys. Rev. Lett. , 2016. 116 p. 048301.
6. D.Y.C. Chan, T.W. Healy, and L.R. White, J. Chem. Soc., Faraday Trans. I, 1976. 72: p. 2844–2865.

Figure 1