The origin of the attraction between like charged hydrophobic and hydrophilic walls confining a near-critical binary aqueous mixture with ions

The effect of ionic solute on a near-critical binary aqueous mixture confined between charged walls with different adsorption preferences is considered within a simple density functional theory. For the near-critical system containing small amounts of ions, a Landau-type functional is derived on the basis of the assumption that the correlation, ξ, and the Debye screening length, κ(-1), are both much larger than the molecular size. The corresponding approximate Euler-Lagrange equations are solved analytically for ions insoluble in the organic solvent. A nontrivial concentration profile of the solvent is found near the charged hydrophobic wall as a result of the competition between the short-range attraction of the organic solvent and the electrostatic attraction of the hydrated ions. An excess of water may be present near the hydrophobic surface for some range of the surface charge and ξκ. As a result, the effective potential between the hydrophilic and the hydrophobic surface can be repulsive far from the critical point, then attractive and again repulsive when the critical temperature is approached, in agreement with a recent experiment (Nellen et al 2011 Soft Matter 7 5360).