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,() water reorientation dynamics around anions and cations is different,() and the zero charge point of water is usually well below pH 7 at hydrophobic/water interfaces.() 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.