Credit: © 2010 ACS

The ubiquity of liquid water has resulted in a great effort to understand its complex and dynamic hydrogen-bonded structure. Focus has recently shifted to study how this intermolecular network is altered by solvated ions and interfaces because they play a direct role in many important aqueous biochemical processes. Now, Akira Yamakata and Masatoshi Osawa from Hokkaido University, Japan, have investigated1 the extent to which the structure of interfacial water at a hydrophobic surface changes when different cations are present.

They used a model hydrophobic surface of carbon monoxide molecules adsorbed on a platinum electrode and exposed it to perchlorate solutions containing different cations. The cations were drawn towards the negatively charged platinum surface and the resulting interfacial water structure was scrutinized using infrared spectroscopy. They specifically examined infrared bands associated with 'free' OH bonds: those that do not form part of the water network but point towards the hydrophobic surface. At low electrode potentials the observed behaviour of hydrophilic cations (Na+, Mg2+, Zn2+, Me4N+) was different to that of the hydrophobic cations (Et4N+, Pr4N+).

The hydrophilic cations caused a decrease in the number of free OH bonds (in comparison with a solution with no cations) suggesting that they replace water molecules at the interface, disrupting the interfacial structure. The hydrophobic cations, however, did not cause the same disruption. Yamakata and Osawa explain that water molecules in the primary hydration shell of a hydrophilic cation form directional hydrogen bonds that can interact strongly with a second hydration shell and consequently affect local water structure. The equivalent interactions for hydrophobic cations are too weak to have this effect.