Ion specific interactions between pairs of nanometer sized particles in aqueous solutions

For the classical DLVO theory, which deals only with electrostatic forces acting between ions and colloids, all ions in solution with the same charge should result in the same force between colloids. Ion specificity does occur in the opposing attractive Lifshitz forces but only very weakly. Ion size parameters, inner and outer Helmholtz planes are used to fit the specificity but that do not work. At, and above, biological salt concentrations other, non electrostatic (NES) ion specific forces act that are ignored in such modeling. To exemplify the general ideas we use a system that corresponds to pairs of nanoparticles. We show that ion specific double layer forces can be understood once NES forces acting between ions and colloids are included consistently in non-linear theory and in Monte Carlo simulations.