Why does water do that?

Following on from the suggestion that the �glass transition� of hydrated proteins at around 220 K is in fact a change in the dynamical state of the water of hydration, related to the second critical point of water in the supercooled, high-pressure state (see the previous post below), Gene Stanley and his coworkers have now released a preprint that aims to relate this behaviour to the molecular structure of the liquid. To recap, the idea is that the �transition� corresponds to the crossing of the so-called Widom line, the locus of the maximum in the correlation length that extends like a �ghost� of the liquid-liquid phase transition beyond the critical point at which this transition vanishes. (At the critical point itself, this correlation length diverges.) What does this mean for the nature of the hydrogen-bonded network? Well, it�s subtle. In thermodynamic terms, the crossover corresponds to a change from non-Arrhenius dynamics at high temperature (the activation energy for water diffusion depends on temperature) to Arrhenius dynamics at low temperature (temperature-independent activation energy). The simulations by Stanley and colleagues now suggest that this crossover shows up in terms of the tendency to form non-bifurcated hydrogen bonds. Bifurcated H-bonds are fairly common in the liquid state at ambient conditions, and seem to be the defects that make diffusion facile. The Widom line corresponds to the point at which the derivative of the probability of forming non-bifurcated bonds with respect to temperature is maximal. Hmm, not an easy quantity to visualize. But it does mean that above the Widom line, supercooled water has fewer non-bifurcated hydrogen bonds, and so is less �tetrahedral� and denser (like the high-density liquid phase), than below this line. That�s intuitive enough � crossing the Widom line as temperature decreases corresponds to the supercooled liquid becoming less dense, more structured and changing from a high-density-liquid-like to a low-density-liquid-like state: a ghost of the liquid-liquid phase transition itself, but with no abrupt change of thermodynamic variables.

Lawrence Pratt at Los Alamos and his coworkers have posted a preprint entitled �What is special about water as a matrix of life?�. This, as I recall, is basically the paper that Lawrence presented at the Varenna meeting, convened to discuss that very question in early 2005. The paper aims to address the title question in general, and hydrophobic effects in particular, by moving away from an emphasis on structure (for example, the classic Kauzmann model of entropically driven hydrophobic attraction due to the release of �structured water� in the space between hydrophobes) and focusing instead on what the authors call the �engineering characteristics of the liquid�, such as its equation of state. The key message seems to be that, as a solvent for life, water represents a safe bet: the liquid state exists over a wide temperature range (compared with other simple small-molecule solvents), and within that range there is rather little variation in thermodynamic variables: response functions such as the compressibility and thermal expansion coefficient, as well as the nature of the solvophobic effect, vary little. It seems to me that this raises several questions (which are not objections), such as: does life require a wide temperature range, or does it just fill up whatever niches are available? (Thermophiles seem very ancient; would life have happily persisted if all Earth�s water was warm?) Does the �structured� character of water play any necessary role in this scheme? (It does seem to be biologically important that water forms directional H-bonds.) And what does underlie the hydrophobic attraction, and how general is it?

Speaking of which, more evidence for the role of nanobubbles in the long-range hydrophobic attraction is provided in a study of nanoparticle adsorption onto a quartz microbalance in gassed and degassed water by Sangmin Jeon and colleagues in Korea (Langmuir 23, 1623-1625 (2007)).