Ever since the seminal work of Franz Hofmeister in 1888 on the effect of background salt type on the precipitation of proteins in aqueous solution, fair amount of theoretical efforts have been invested in the understanding of such strong ion-specific effects. However, in the absence of dedicated experiments, the explanation of these dramatic effects remained undecided and invariably neglected in ample studies.

Motivated by the lack of systematic experimental studies of such a fundamental phenomenon we have launched a series of experiments focused on characterizing differences in the interaction between two negatively charged silica surfaces, as measured by AFM. The study included different monovalent electrolyte solutions: NaCl, KCl, and CsCl in a broad range of ionic concentrations, from 0.1mM to 5M. Force vs. separation curves reveal that the surface charge of silica is regulated by cation adsorption in addition to silanol deprotonation. Cation adsorption grows monotonically with bare cation radius. As ion concentration is increased surface charge is gradually neutralized by specific cation adsorption, hence, suppressing the repulsion due to electrostatic double layer overlap and revealing van der Waals attraction at a concentration specific for each salt. At pH5.5 the smallest ion, Na⁺, neutralizes the surface at 0.5-1 M, K⁺ at 0.2-0.5 M and Cs⁺ at ~0.1 M. At higher salt concentrations, repulsion reemerges due to surface charge reversal by excess adsorbed cations. When attraction dominates, the force curves are practically identical for the three salts, independent of their concentration.

The importance of understanding ion-specific effects of these 1:1 salts lies in the fact that living organisms maintain characteristic electrolyte compositions in their different biofluids. For example, the intracellular fluid of mammalian cells is characterized by high level of potassium (~160mM) and low level of sodium (~10mM), while extracellular environments consist of high sodium (~140mM) and low potassium (~4mM) levels. We have found that in ~150 mM NaCl two negatively charged silica surfaces repel each other but at the same concentration of KCl they will attract. The same charged objects may, hence, repel each other in extracellular ionic environment and attract each other in cell like electrolyte composition.