Anomalous surface behavior of hydrated guanidinium ions due to ion pairing

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 SO₄^2- ions form pairs. This finding can be used to explain the decreased surface affinity of Gdm⁺ when co-dissolved with SO₄^2- ions. Since SO₄^2- ions avoid the surface due to the double charge and strong water interaction, the Gdm⁺-SO₄^2- ion pair resides deeper in the solutions' bulk than the Gdm⁺ ions. Since TPA⁺ does not form ion pairs with SO₄^2-, the TPA⁺ ions are instead enriched at the surface.