|Ph.D Student||Dishon Matan|
|Subject||Ion-Specific Effects on Interactions in Aqueous Solutions|
|Department||Department of Physics||Supervisor||Professor Uri Sivan|
|Full Thesis text|
This study concerns the effect of ion type, ion-specific effects, on the interaction between charged surfaces in aqueous electrolyte solutions.
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.
In the first part of the study we found that the interaction between the silica surfaces in 1:1 salts solutions was fully accounted for by ion-independent van der Waals (vdW) attraction and electric double-layer repulsion. We have traced the effect of salt on the interaction to the differences in absorption of the cations to the silica surfaces, and hence ion-specific alteration of silica surface-charge.
In the second part of the study, application of the two complementary AFM measurements, revealed the combined effects of cation size and valency on the interaction between silica surfaces in the same three 1:1 salts, three 2:1 salts: MgCl2 , CaCl2 and SrCl2 , and three 3:1 salts: LaCl3 , [Co(NH3 ) 6 ]Cl3 and AlCl3 . As with 1:1 salts the interaction between the silica surfaces in 2:1 salt solutions was fully accounted for by ion-independent vdW attraction and electric double-layer repulsion modified by cation specific adsorption to the silica surfaces. However, in the presence of 3:1 salts the attraction between the silica surfaces is too large to be attributed solely to vdW force. Furthermore, strong adsorbability of trivalent cations to silica contrasts straightforward expectations based on ranking of the cationic solvation energies, thus, suggesting a different adsorption mechanism which is inoperative or weak for monovalent and divalent cations.