Electrolytes at interfaces: accessing the first nanometers using X-ray standing waves

Separating Hofmeister Trends in Stern and Diffuse Layers at a Charged Interface

Understanding the role of pH and ions on the electrical double layer (EDL) at charged mineral oxide/aqueous interfaces remains crucial in modeling environmental and industrial processes. Yet the simultaneous contribution of pH and specific ion effects (SIEs) on the different layers of the EDL remains unknown. Here, we utilize zeta potential measurements, vibrational sum frequency generation, and the maximum entropy method to ascertain the detailed structure of the Stern and diffuse regions of the EDL at the silica/water interface with varying pH values for different alkali chlorides. Both at pH 2, when the surface is nearly neutral, and at pH 12, when the surface is highly charged, we observe that Li+ and Na+ disrupt while Cs+ enhances existing water structures within the Stern layer. Moreover, the SIE trends for the diffuse and Stern layers are opposite to one another at pH 2 (in the amount of ordered water) and at pH 12 (in the amount of net oriented water). Finally, we observe an inversion in Hofmeister (SIE) trends at low and high pH in the zeta that impacts the diffuse layer structure. These results indicate that SIEs play critical yet separable roles in governing both the electrostatic and water-structuring capabilities of the EDL.

Investigating the Conformation of Surface-Adsorbed Proteins with Standing-Wave X-ray Fluorescence

Protein adsorption to surfaces is at the heart of numerous technological and bioanalytical applications, but sometimes, it is also associated with medical risks. To deepen our insights into processes involving layers of surface-adsorbed proteins, high-resolution structural information is essential. Here, we use standing-wave X-ray fluorescence (SWXF) in combination with an optimized liquid-cell setup to investigate the underwater conformation of the random-coiled phosphoprotein β-casein adsorbed to hydrophilic and hydrophobized solid surfaces. The orientation of the protein, as determined through the distributions of sulfur and phosphorus, is found to be sensitive to the chemical nature of the substrate. While no preferred orientations are observed on hydrophobized surfaces, on hydrophilic Al oxide, β-casein is adsorbed as a diblock copolymer with the phosphorylated domain I attached to the surface. Our results demonstrate that targeting biologically relevant chemical elements with SWXF enables a detailed investigation of biomolecular layers under near-physiological conditions.

Anion Specific Effects at Negatively Charged Interfaces: Influence of Cl-, Br-, I-, and SCN- on the Interactions of Na+ with the Carboxylic Acid Moiety

Unlike counterion interactions with charged interfaces, the influence of co-ions is only scarcely reported in the literature. In this work, the effect of SCN- and the halide co-ions in the interactions of Na+ with carboxylic acid Langmuir monolayers is investigated by using vibrational sum frequency spectroscopy. At 1 M concentrations in the subphase, the identity of the anion is shown to have a remarkable influence on the charging behavior and degree of deprotonation of the monolayer, with ions ordering in the sequence I- > SCN- > Cl- ≈ Br-. The same trend is observed at both pH 6 and pH 9 when the monolayer is intrinsically more charged. Spectroscopic evidence is found for both the presence of I- and SCN- in the interfacial region at levels close to their detection limits. The results contradict electrostatic theories on charged interfaces where co-ions are not expected to play any significant role. The higher propensity for the large polarizable anions to deprotonate the monolayer is explained in terms of their ability to modify the cations affinity toward the carboxylic acid groups present at the surface.

In situ study of the competitive adsorption of ions at an organic-aqueous two-phase interface: the essential role of the Hofmeister effect

Understanding of the microcosmic essence of the competitive adsorption of different ions at liquid/liquid interfaces is of crucial importance for the elucidation of the unique chemical reactivities or selectivities of ions in numerous heterogeneous chemical processes. However, the knowledge of the microscopic mechanism behind the competitive adsorption of ions at the liquid/liquid interface is lacking. Herein, the competitive adsorption of various inorganic salt anions at organic-aqueous two-phase interfaces has been investigated as compared to that of the CrO42- ions by total internal reflection UV-visible (TIR-UV) spectroscopy since CrO42- ions are detectable by UV-visible spectroscopy and have a relatively poor interface propensity as compared to other chaotropic ions. Experimental results indicate that the interface propensities of different salt anions to the organic/aqueous phase interface follow the Hofmeister series. Molecular dynamics simulations further provided molecular-level evidence for role of the Hofmeister series of ions in the competitive adsorption of salt anions at organic-aqueous two-phase interfaces; the present study provided the first-hand experimental evidence demonstrating the occurrence of the Hofmeister series effect at the organic/aqueous two-phase interfaces, influencing the competitive adsorption of different salt ions; moreover, it is expected to offer a basis for the development of new strategies for the regulation of the chemical reactivity and selectivity of ions at organic/aqueous phase interfaces by introduction of other ions for competitive adsorption.