Chromatography and the Hundred Year Mystery of Inorganic Ions at Aqueous Interfaces: First Evidence of the Presence of a Kosmotrope at the Graphite/Electrolyte Solution Interface

Recent theoretical speculations and computer simulations question the validity of the Onsager-Samaras theory concerning the ion-free interface between an electrolyte solution and a hydrophobic surface. For the very first time we used chromatography not for a separative goal but as a tool to detect the adsorption of LiF, strongly expected to shun any dielectric boundary, onto porous graphitic carbon. Frontal analysis gave unequivocal experimental evidence to this unexpected phenomenon: the van't Hoff relationship and the adsorption isotherm furnished reliable thermodynamic quantities, such as the adsorption equilibrium constants, the monolayer capacity, and the standard free energy, enthalpy, and entropy of adsorption. From the influence of pressure on adsorption a very reasonable difference of the partial molar volume of LiF in the adsorbed and bulk phases was obtained. The contribution due to LiF adsorption to the interface potential difference was estimated. The main new physical insight provided by this article is however the ascertained groundbreaking adsorption of a kosmotrope onto a graphitic surface, a material that is the focus of interest of many thriving research fields in the nanoage.