Chromatography and the hundred year mystery of inorganic ions at aqueous interfaces: Adsorption of inorganic ions at the Porous Graphitic Carbon Aqueous Interface follows the Hofmeister series

• Conceptual novelty: kosmotropes do adsorb onto graphitic carbon.
• Their adsorption is in contrast to textbook knowledge (Onsager Samaras theory).
• Originality: we highlighted a non-separative goal of chromatography.
• The Hofmeister series predicts adsorption of electrolytes onto graphitic carbon.
• Research needs: MD, chromatographic and nanotechnological exploitation of adsorption.

Many physical phenomena are affected by the structure of water interfaces, yet it remains an active and controversial subject. A great deal of recent theoretical endeavour and computer simulations question the validity of the Onsager Samaras theory of the ion-free interface between an electrolyte solution and an hydrophobic surface. Experimental results play a crucial role in assessing the legitimacy of the theories. Experimental data are scarce, while simulation results suggest an increasing surface affinity of ions with increasing chaotropic character, in dramatic contradiction to the classical view. Chromatography is a powerful separative technique, but we originally used it as a tool to detect the adsorption of chloride electrolytes and sodium electrolytes, strongly expected to shun any dielectric boundary, onto an hydrophobic surface, and to rank ions according to their adsorbophilicities. Frontal analysis gave unequivocal experimental evidence to this unexpected phenomenon and it was used to quantify it. The infinite dilution equilibrium constants for adsorption of kosmotropes and chaotropes onto the interface were obtained and contrasted to the Jones–Dole B viscosity coefficients, that is a common quantifier of the Hofmeister effect. It is clear that (i) the more chaotropic the ion is, the more it contributes to the global adsorbophilicity of the electrolyte; (ii) the influence of the variable anion is more than twofold that of the variable cation, thereby confirming a robust observation in many other physical systems. Standard free energy of adsorption for each electrolyte was calculated and its reliability was commented upon. The central issue in this paper is the effective and ascertained adsorption of electrolytes onto an hydrophobic surface and the fact that the adsorbophilicity of an electrolyte may be inferred from its position in the Hofmeister series.