|M.Sc Student||Morag Jonathan|
|Subject||Progress towards the Resolution of the Hofmeister|
|Department||Department of Physics||Supervisor||PROF. Uri Sivan|
|Full Thesis text|
This experimental study deals with ion specific adsorption (ISA) to protic surfaces, a critical mechanism for describing ion specific effects (ISE) present in the interaction between bodies submerged in electrolyte solution. Despite the prevalence of these effects in a multitude of applications in fields such as biology, medicine, nanotechnology, and industry, the interface between solid surfaces and saline solution is only partially understood. This gap in knowledge is especially apparent in ionic adsorption, which eludes a predictive theory after over a century of research. Our goal is to shed some light on the driving mechanisms that lead to adsorption, which are universal for many surfaces.
We direct our attention to the wetted surface of silica, an archetype protic oxide, onto which, like other surfaces, ionic adsorption affinity consistently ranks according to a recurring series, known as the Hofmeister series. These surfaces are especially interesting, because despite their large variance, adsorption onto them is chiefly determined by a single parameter, the surface’s pH of zero charge (PZC). Oxides that become charge neutralized at low pHs, that is, those that portray low PZC’s, promote ISA according to the original Hofmeister series. Meanwhile, high PZC oxides exhibit the exact reverse order.
We perform AFM measurements of the forces acting between two silica surfaces in the presence of varied alkali-chloride salts and pHs, which show that the series can be reversed on a single sample simply by altering the pH of the solution. At slightly acidic pH, the adsorption of Li, Na, and Cs increases with ionic bare radius, while above pH 8, this order is exactly reversed. To our knowledge, this study represents the first direct measurements of surface charge in silica that investigate the effect of high pH and observe a reversal of the series.
We use the data to elucidate the origin of the Hofmeister adsorption series on silica and its reversal. At low pH, the weak hydrogen bonds between water and neutral silanols promote adsorption of the weakly hydrated Cs and repulsion of the strongly hydrated Na and Li. As pH is increased, a tightly bound hydration layer forms on deprotonating silanols. Cs is now expelled and adsorption of small ions is encouraged. The deduced role of surface hydration agrees with hydration repulsion observed at high pH, surface over-charging previously observed by our group at low pH, as well as data in other oxides.