Ion pairing and Langmuir-like adsorption at aqueous surfaces studied by core-level spectroscopy

Abstract: Surface-bulk equilibria for solutes in aqueous solutions are studied using X-ray Photoelectron Spectroscopy (XPS) with high surface and chemical sensitivity. In the first part, the results show a reduction of the biochemically relevant guanidinium ions’ surface propensity with the addition of disodium sulphate due to ion pairing with the strongly hydrated sulphate ion, which could have implications for protein folding. Thereafter, the work considers amphiphilic organic compounds related to atmospheric science where the surface propensities, orientations at the surface and solute-solute and solute-solvent interactions are investigated. In the second part, two linear organic ions are investigated both as single solutes and in mixture. Both organic ions are surface enriched on their own and even more in the mixed solute solution. Due to hydrophobic expulsion of the alkyl chains, ion pairing between the organic ions and van der Waals interaction, the organic ions seem to assemble in clusters with their alkyl chains pointing out of the surface. The third part also covers linear organic compounds but one at a time probing the surface concentration as a function of bulk concentration. A Langmuir-like adsorption behavior was observed and Gibb’s free energy of surface adsorption (ΔGAds) values were extracted. An empiric model for deriving values for ΔGAds is proposed based upon the seemingly linear change in ΔGAds per carbon when comparing alcohols of different chain lengths. The fourth part investigates the acid/base fraction at the surface as function of bulk pH. The most important factor for this fraction seems to be how the surface propensity varies with the charge state of the acid or base instead of a possible difference in pH or pKa value at the surface. In the fifth part the oxygen K-edge of aqueous carbonate and bicarbonate is probed with the bulk-sensitive Resonant Inelastic X-ray Scattering (RIXS) technique.