Abstract: | This study concerns the effect of ion type, or ion-specific effects, on the interaction between charged surfaces in aqueous electrolyte solutions. The interactions between charged bodies immersed in aqueous solutions is relevant to many fields ranging from molecular biology and bioengineering to colloid and interface science. In addition, the characteristic variation of electrolyte composition between different environments in living organisms offers vast grounds for ion-specific effects. For example, mammalian cells are characterized by high level of potassium ( 150 mM) and low level of sodium ( 10 mM) while the extracellular environments consist of high sodium ( 150 mM) and low potassium levels ( 5 mM). Fair amount of theoretical efforts have been invested in understanding the effect of ion type on the interaction between charged surfaces, but in the absence of dedicated experiments, the explanation of these dramatic effects remain 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 silica surfaces, as measured by AFM, in the presence of three 1:1, three 2:1, and three 3:1 metal-chloride aqueous solutions in a broad range of ionic concentrations. Maintaining a common chloride anion throughout the study allowed us to compare quantitatively the effects of the corresponding cations on the interaction between the surfaces. Using two complementing experimental methods, namely, force vs. separation and adhesion measurements, we were able to collect a comprehensive data set and gain significant insight into cation-specific effects on the interaction between silica surfaces from which an unforeseen, yet consistent, picture emerged.
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