| 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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