Charge Asymmetry at Aqueous Hydrophobic Interfaces and Hydration Shells

The solvation of ions in a dielectric continuum is symmetric with respect to the sign of the ionic charge. Yet, many phenomena show indirect evidence that water may have a preference for negative charge: the vast majority of biological membrane interfaces are neutral or negatively charged,([1]) water reorientation dynamics around anions and cations is different,([2]) and the zero charge point of water is usually well below pH 7 at hydrophobic/water interfaces.([3]) There is undoubtedly a link to the asymmetry of the water molecule. Here, we spectroscopically quantify differences between the structures of hydration shells and hydrophobic/water interfaces induced by ions of opposite charge but essentially identical molecular structure. We show that these two ions, tetraphenylborate (TPB+) and tetraphenylarsonium (TPA(+)), interact dramatically differently with water and its interface: The anion is preferentially hydrated and induces greater orientational order to water near hydrophobic interfaces. In contrast, the cation forms far fewer and weaker pi-H bonds than the anion and strongly reduces the orientational order of water near a hydrophobic interface.