| AVS 60th International Symposium and Exhibition | |
| Ions at Aqueous Interfaces Focus Topic | Tuesday Sessions |
| Session IA+BA-TuA |
| Session: | Ions and Biomolecules at Aqueous Interfaces |
| Presenter: | P.S. Cremer, Penn State University |
| Correspondent: | Click to Email |
We have employed a combination of surface specific techniques to interrogate the interactions of ions with self-assembled monolayers and proteins at aqueous interfaces. The results provide direct insight into ion pairing interactions. In particular, I will discuss the behavior of cations and anions as they relate to the Hofmeister series, which is a rank ordering of the efficacy of these species to influence the physical behavior of colloidal and interfacial systems in solution. The TiO2/water, quartz/water, alkyl chain/water, and air/water interfaces were each explored.
Experiments consisted of a combination of sum frequency generation and thermodynamic measurements. Ion specific effects at these interfaces were found to be determined by several factors. These include the sign and magnitude of the surface potential, ion pairing effects, as well as the presence of polar and nonpolar interfacial moieties. At negatively charged, hydrophilic surfaces, we found that Na+ adsorption and double layer formation was modulated by the nature of the counterion in solution. For the anions, it was found that SCN- was less depleted at the interface compared with better hydrated anions such as Cl-. The same ordering was observed for the anions whether this interface was relatively hydrophobic or hydrophilic. Changing the sign of the charge at the interface also led to a similar Hofmeister ordering. Curiously, the ordering for cations at these aqueous interfaces was found to be more sensitive to the specific surface chemistry. Moreover, at negatively charged hydrophilic surfaces, the smallest and best hydrated cations were mostly favored over more poorly hydrated cations. By contrast, well hydrated cations were repelled from more apolar surfaces. Li+ displayed somewhat anomalous behavior. All of these results will be discussed with an eye toward a broader model for interfacial partitioning of ions in aqueous solutions.