Ions and water at interfaces: will they be understood by August?

There has been a lot of debate, some described in earlier posts, about the behaviour of hydroxide and hydronium ions at air-water and hydrophobic interfaces. Some claim the air-water interface is acidic, others that it is basic. So do protons or hydroxide ions get preferentially adsorbed at these interfaces? I’ve been sent a preprint (now published in J. Phys. Chem. C, doi:10.1021/jp800888b; paper here) by Robert Vácha, Ronen Zangi, Jan Engberts and Pavel Jungwirth that casts new light on the issue with simulations of hydroxide hydration near hydrophobic walls in aqueous KOH. They find that rigid walls create strong layering effects and a peak of hydroxide concentration about 5Å from the wall. But when the wall atoms are allowed to vibrate thermally, and when the dispersion interactions are weaker (more like the air-water interface), this structuring tends to get washed out, in some cases completely. Thus it seems one can’t generalize about hydroxide adsorption (or not) at a hydrophobic interface.

Another blow to ‘water structure effects’ at interfaces comes from a paper by Mischa Bonn and colleagues, in Huib Bakker’s group at FOM Amsterdam (M. Sovago et al., Phys. Rev. Lett. 100, 173901; 2008 – paper here). They have investigated the double-peaked vibrational sum-frequency generation (VSFG) spectrum of the O-H bonds of interfacial water in the hydrogen-bonded region, at a lipid-water interface. The two peaks have previously been interpreted as two distinct classes of hydrogen bond, ‘weak’ (ice-like) and ‘strong’ (water-like). But the FOM team, along with others from the Universities of Amsterdam and Utrecht, have evidence from isotope substitution experiments that the double peak is due to vibrational coupling between stretching and bending modes. They therefore conclude that the interfacial water is more homogeneous than has been thought.

How ions affect hydration and hydrophobic interactions is one of the thorniest problems in this field, and I won’t repeat myself by trying to summarize what has been said previously on the issue. Suffice to say that this question is intimately bound up with the notoriously puzzling ion-specific Hofmeister effects on solubility and aggregation of proteins. Neither am I going to make a poor attempt at summarizing the conclusions of a new investigation into these phenomena by Roland Netz and colleagues in Germany and Sweden (D. Horinek et al., Langmuir 24, 1271-1283; 2008 – paper here). Let’s just say that they have used single-molecule AFM experiments and MD simulations to delve into the ion-specific factors, free from complications of bubble nucleation and cavitation that may intervene for extended surfaces. The punchline is that “the most important factor determining ion-specific adsorption at hydrophobic surfaces can best be described as surface-modified ion hydration” – but you’d best read the paper to unpack that. A useful addition to a complicated story.

Lawrence Pratt and coworkers have a preprint exploring the role of dispersion forces on the potential of mean force between methane molecules in water. The find that these attractive methane-water interactions contribute a repulsive term to the pair potential (potential of mean force, pmf) between methanes. They also say that packing effects in the hydration shells make a dominant contribution to this pmf, but not in a way that can be interpreted with a perturbative approach – that is (if I’ve understood this properly), by treating the pmf as a perturbation expansion around this basic term.

Haiping Fang of the Shanghai Institute of Applied Physics and his coworkers have a very nice ‘topical review’ entitled ‘Dynamics of single-file water chains inside nanoscale channels: physics, biological significance and applications’ in J. Phys. D: Appl. Phys. 41, 103002 (2008). Some of the same issues, and some others, are addressed in a recent review by Gerhard Hummer and colleagues: J. C. Rasaiah et al., ‘Water in nonpolar confinement: from proteins to nanotubes and beyond’, Ann. Rev. Phys. Chem. 59, 713-740 (2008).

Finally, a free advert for a RSC Faraday Division discussion meeting on 27-29 August at Heriot-Watt University in Edinburgh entitled ‘Water: From Interfaces to the Bulk’ (see details here.). The announcement says that this meeting “plans to achieve a unification of views towards the goal of understanding the microscopic structure and behaviour of condensed phases of water at interfaces and progressing into the bulk.” I wish I could be there.