Anomalous surface behavior of hydrated guanidinium ions due to ion pairing
Victor Ekholm, Mario Vazdar, Philip E. Mason, Erik Bialik, Marie-Madeleine Walz, Gunnar Öhrwall, Josephina Werner, Jan-Erik Rubensson, Pavel Jungwirth, Olle Björneholm
Surface affinity of aqueous guanidinium chloride (GdmCl) is compared to that of aqueous tetrapropylammonium chloride (TPACl) upon addition of sodium chloride (NaCl) or disodium sulfate (Na2SO4). The experimental results have been acquired using the surface sensitive technique X-ray photoelectron spectroscopy on a liquid jet. Molecular dynamics simulations have been used to produce radial distribution functions and surface density plots. The surface affinities of both TPA+ and Gdm+ increase upon adding NaCl to the solution. With the addition of Na2SO4, the surface affinity of TPA+ increases, while that of Gdm+ decreases. From the results of MD simulations it is seen that Gdm+ and SO42− ions form pairs. This finding can be used to explain the decreased surface affinity of Gdm+ when co-dissolved with SO42− ions. Since SO42− ions avoid the surface due to the double charge and strong water interaction, the Gdm+-SO42− ion pair resides deeper in the solutions’ bulk than the Gdm+ ions. Since TPA+ does not form ion pairs with SO42−, the TPA+ ions are instead enriched at the surface.