Tuesday, August 25, 2009

Nanobubbles: birth, motion, and consequences

The role of bridging bubbles in the so-called ‘long-range hydrophobic force’ seems now fairly well established. In a study of this effect, Viveca Wallqvist and colleagues argue that in cases where this is the identified mechanism of attraction, it would be preferable to call it a ‘capillary force’ rather than a ‘hydrophobic interaction’. Their study looks at the effect of surface roughness on such forces between hydrophobic surfaces (V. Wallqvist et al., Langmuir 25, 9197 (2009) – paper here). They find that the range and magnitude of the force can vary significantly at different points on nanostructured surfaces due to local variations in contact angle. A high density of nanoscale crevices leads to accumulation of air bubbles that coalesce and weaken the capillary attraction.

William Ducker offers an explanation of the low contact angle and the unusual stability of these nanobubbles, in terms of a thin film of surface-active contaminant at the air-water interface (Langmuir 25, 8907 (2009) – paper here). This, he says, will both decrease the surface tension (and thus the contact angle) and hinder gas diffusion out of the bubble. It’s an alternative to Michael Brenner and Detlef Lohse’s suggestion of a dynamic stabilization of the nanobubbles (Phys. Rev. Lett. 101, 214505 (2008)).

Yi Zhang and colleagues at the Shanghai Institute of Applied Sciences suggest that a precursor to these nanobubbles may be a multilayer (bilayer or trilayer) of adsorbed gas at the hydrophobic interface (L. Zhang et al., Langmuir 25, 8860 (2009) – paper here). They have imaged such bi- and trilayer islands of gas, of up to micron-sized lateral dimensions, at the surface of HOPG, and have watched them evolve into nanobubbles, sometimes under the influence of the AFM tip used for imaging.

And in the same vein, Bharat Bhushan and coworkers look at how the mobility of nanobubbles at hydrophobic surfaces is affected by surface heterogeneity (Y. Wang et al., Langmuir 25, 9328 (2009) – paper here). They find that surfaces partly and totally covered with polystyrene films, which form small islands or can become indented at the nanoscale by the presence of bubbles, tend to have relatively immobile nanobubbles compared with bare, smooth hydrophobic surfaces. Bubble immobility reduces the frictional drag force between such surfaces in motion, and so is sometimes desired in mechanical contexts.

How do you measure hydrophobicity? Macroscopically, that is of course done using contact angles. But what are the microscopic signatures? This question is examined by Shekhar Garde and colleagues at RPI using an extensive range of simulation studies of water at various surfaces ranging from very hydrophobic to very hydrophilic (R. Godawat et al., PNAS advance online publication - paper here). They say that water density is a poor measure of hydrophobicity, but that both the probability of cavity formation and the free energy of binding of hydrophobic solutes to the surface correlates much better with the macroscopic wetting properties. This paper adds weight to the notion that it is in the dynamic rather than the structural characteristics of water that the true nature of hydrophobicity is located.

Gerhard Hummer and colleagues have developed a rather comprehensive model of gated proton pumping in cytochrome c oxidase (Y. C. Kim et al., PNAS advance online publication - paper here). This reveals show the electrostatic interaction between the proton loading site and the electron source for reduction of oxygen at the heme site is central to the pumping efficiency, and also how gating is accomplished.

Water in protein cavities is hard to detect with diffraction methods if it is disordered. Robert Goldbeck at UC Santa Cruz, Raymond Esquerra at San Franscisco State University and their colleagues have shown that non-specific hydration of the cavities of myoglobin mutants can be detected optically via its perturbing effect on the optical spectrum of the pentcoordinate heme group (R. Goldbeck et al., JACS ASAP ja903409j - paper here).

How do membrane proteins compensate, within the hydrocarbon core of a membrane, for loss of hydrophobic interactions? One possibility is that they enhance packing efficiency and thus van der Waals interactions between the hydrophobic residues. Or they might have stronger hydrogen-bonding interactions between hydrophilic regions. But James Bowie and colleagues at UCLA report structural and thermodynamic arguments for why neither plays a strong role (JACS 131, 10846 (2009) – paper here). Rather, they suspect that the reduced entropy cost of folding in membrane proteins might be responsible for their stability.

There is a small clutch of papers on the structure of the air-water interface and other species located there. Yi Qin Gao and colleagues at Texas A&M use vibrational sum frequency spectroscopy and MD simulations to investigate orientational ordering of water molecules, and suggest that this occurs to any significant degree only in the first two layers at the surface (Y. Fan et al., J. Phys. Chem. B ASAP jp900117t - paper here). Joyce Noah-Vanhoucke and Phillip Geissler argue that the preferential segregation of some ions at the air-water interface is due primarily to the way the ions induce deformations of the interfacial geometry, causing electrostatic fluctuations that are not accounted for in the conventional picture (PNAS advance online publication - paper here).

In water confined to nanoscale dimensions (as in the crowded environment of a cell), hydration effects can be quite different from those of the bulk. Margaret Cheung and colleagues at Houston provide an illustration of this by using MD to look at the conformations of hexane in nanoscale water droplets (D. Homouz et al., J. Phys. Chem. B ASAP jp907318d - paper here). They find that the hexane molecules are situated at the droplet surface, where disruption of the H-bonding favours the all-trans conformation.

Meanwhile, Michael Fayer and colleagues at Stanford use ultrafast IR spectroscopy to look at water dynamics close to the neutral and ionic surfaces of reverse micelles (E. E. Fenn et al., PNAS advance online publication - paper here). They find that the orientational relaxations times are rather similar in both cases, both being significantly slower than in the bulk, and conclude that it is the mere presence of an interface, rather than its chemical nature, which exerts the dominant effect.

Nanoscopic water films on metals and other simple surfaces are known often to adopt ordered structures in the first one or two monolayers that may or may not be like bulk ice. On Pt(111), for example, it seems to form a monolayer that is flat rather than having the puckering expected of a ‘slice of ice’. Now Greg Kimmel, Bruce Kay and colleagues find that water on graphene (supported on Pt(111) also forms a flat film, here two monolayers thick (G. A. Kimmel et al., JACS ASAP ja904708f - paper here). This structure has been predicted previously for confined bilayers between hydrophobic walls, but it seems that confinement is not needed to induce it. The bilayer has no dangling bonds or lone pairs on either face, and so one might anticipate that it will itself be somewhat hydrophobic, as indeed Greg Kimmel and others found previously for the flat monolayer on Pt(111) (G. A. Kimmel et al., Phys. Rev. Lett. 95, 166102 (2005).).

Jürgen Köfinger and Christoph Dellago have used MD calculations to probe the dynamics and dielectric response of single-file water chains in narrow pores (Phys. Rev. Lett. 103, 080601; paper here). This supplies a baseline for using dielectric spectroscopy to investigate the properties of such highly confined water, for example enabling the diffusion of defects in the H-bonded chain to be studied.

Kafui Tay and Anne Boutin at the Université Paris-Sud XI have studied the dynamics of hydrated electrons using MD, and say that their diffusion is dictated by fluctuations in the H-bonded network: in the temperature region where the diffusion is Arrhenius-like, the activation energy is determined by H-bond breaking (J. Phys. Chem. B ASAP jp810538f - paper here).

Lars Pettersson, Anders Nilsson and their colleagues at Stanford, Stockholm and in Japan have published a controversial paper claiming to see inhomogeneities in water structure on length scales of around 1 nm (C. Huang et al., PNAS advance online publication - paper here). They say that they see these using SAXS, and argue that the density contrast is due to the coexistence of two water structures: one tetrahedral, the other with distorted H-bonds, related respectively to low- and high-density liquid water. This is, needless to say, a revival of the very old two-state picture of water structure, which in various forms goes right back to Roentgen. It will be disputed, no doubt, but demonstrates again how remarkably tenacious this two-state notion is.