ABSTRACT
Ion-surface interactions are of high practical importance in a wide range of technological, environmental and biological problems. In particular, they ultimately control the electric double layer structure, hence the interaction between particles in aqueous solutions. Despite numerous achievements, progress in their understanding is still limited by the lack of experimental determination of the surface composition with appropriate resolution. Tackling this challenge, we have developed a method based on X-ray standing waves coupled to nano-confinement which allows the determination of ion concentrations at a solid-solution interface with a sub-nm resolution. We have investigated mixtures of KCl/CsCl and KCl/KI in 0.1 mM to 10 mM concentrations on silica surfaces and obtained quantitative information on the partition of ions between bulk and Stern layer as well as their distribution in the Stern layer. Regarding partition of potassium ions, our results are in agreement with a recent AFM study. We show that in a mixture of KCl and KI, chloride ions exhibit a higher surface propensity than iodide ions, having a higher concentration within the Stern layer and being on average closer to the surface by ≈1-2 Å, in contrast to the solution water interface. Confronting such data with molecular simulations will lead to a precise understanding of ionic distributions at aqueous interfaces.
ABSTRACT
We report here the swelling and relaxation properties of confined poly(n-butyl methacrylate) (PBMA) films having thicknesses of less than 70 nm under supercritical carbon dioxide (scCO2) using the X-ray reflectivity technique. Swellability is found to be dominant in thinner films compared to thicker ones as a consequence of the confinement-induced densification of the former. Swellability is proportionately increased with the density of the film. PBMA films exhibit a more significant swelling than do PS films, and their differences become more prominent with the increase in film thickness. A comparison between the results obtained for polystyrene (PS) and PBMA ultrathin films reveals that the swellability is dependent upon the specific intermolecular interaction between CO2 and the chemical groups available in the polymers. Owing to strong Lewis acid-base interactions with scCO2 and the lower glass-transition temperature (bulk Tg ≈ 29 °C), PBMA films exhibit a greater amount of swelling than do PS films (bulk Tg ≈ 100 °C). Though they reach to the different swollen state upon exposition, identical relaxation behavior as a function of aging time is evidenced. This unprecedented behavior can be ascribed to the strong bonding between trapped CO2 and PBMA that probably impedes the release of CO2 molecules from the swollen PBMA films manifested in suppressed relaxation.
