In this award, funded by the Experimental Physical Chemistry Program of the Division of Chemistry, Professor Anders Nilsson of Stanford University, together with his graduate student researchers, will investigate the nature of the hydrogen bond network in liquid water. The main experimental techniques to be used include x-ray absorption spectroscopy, x-ray raman scattering measurements, small-angle x-ray scattering and x-ray diffraction measurements. These experiments will be carried out at the SSRL synchrotron radiation facility as well as with equipment in the Nilsson laboratory. The work will be augmented with a theoretical collaboration with a Swedish computational chemistry group.
Water is the reaction medium for the most important environmental and life processes. Many of the unique chemical and physical properties of water are due to its propensity to hydrogen-bond with itself and other species. The work of Prof. Nilsson and his group is aimed at developing a better description of the hydrogen bond network in liquid water. The students working in the Nilsson laboratory will have the opportunity to work in one of the premier light sources in the country, and will further benefit from an international collaboration. Besides being intimately involved in the research work, Prof. Nilsson is an outstanding ambassador for science, participating in a number of public outreach activities at Stanford as well as at the Exploratorium in San Francisco.
Take a tall glass of water and add an ice cube to it. Let it stand for a while before measuring the temperature of the water at the bottom of the glass and you will find that it is warmer, at around 4°C, than the water at the top around the melting ice cube. Contrary to most other, "normal", liquids the density of water does not continuously increase upon cooling, but shows a maximum at 4°C; the solid (ice) is also much less dense than the liquid and floats on top. These very simple facts have enormous consequences for life in the north as lakes then freeze from the top down rather than from the bottom up. The density maximum at 4°C is only one of about 66 anomalies that taken together distinguish water from all other liquids. Another example is the unusually high heat capacity of water which stabilizes the temperature of the oceans and allows the Gulf Stream to transport heat to Europe. Yet another example is the unusually high surface tension of water, which allows droplets to form, spiders and striders to walk on water, and gives capillary forces that allow small fibers in plants and trees to transport water to great heights. As a final example, we show in figure 1 the trends in boiling points for sequences of similar liquids. Clearly, water stands out as breaking the trend strongly, but why? When water freezes we get ice. The molecules have found their lowest energy positions and are held in place in a tetrahedral coordination where each hydrogen is attracted to a lone-pair of a neighboring molecule and each of the two lone-pairs of the oxygens in turn has attracted a hydrogen from a neighboring molecule. With all molecules oriented in specific directions we get a lattice with a lot of empty spaces and thus a low density. It is clear that the density will increase if we break up this lattice and allow molecules to enter the empty spaces. This is what happens when we put in heat to melt the ice to water. The question is how much of the network remains in the liquid and how is the energy we put in distributed? Obviously, with the higher temperature and a liquid (water) instead of a solid (ice) the bonds must be distorted, but is the energy leading to distortion evenly distributed among all bonds and molecules? That would be the classical textbook picture which is supported by all theoretical simulations of the liquid and interpretations of previous experiments. This is, however, not what we observe when we apply new experimental synchrotron radiation techniques to zoom in and really measure what happens around individual molecules in the liquid. Instead we find that the molecules in the liquid divide up into two specific hydrogen-bonding situations: either very tetrahedral ("ice-like") where the favorable bonding, which minimizes the energy, is maintained, or very disordered and flexible with less specific bonds between molecules creating a situation which instead maximizes the entropy. These two situations exist in a fluctuating dynamic, temperature-dependent equilibrium where the tetrahedral component forms specific regions of dimension 1-1.5 nm corresponding to some perhaps 50-100 molecules that stick together on some yet unknown time-scale. A much more complex and interesting picture than the standard one, but how can we know this and what does it mean? A synchrotron is a particle accelerator where electrons are accelerated to velocities approaching that of light, they are stored in a circular ring and at certain positions strong magnetic fields make them curve back and forth upon which light is emitted. When the energy of the electrons is high enough the light which is emitted has wave length in the x-ray region, short enough and energetic enough to interact with the most compact electronic levels in an atom; in the case of water it is the 1s level of the oxygen atom. When the photon is absorbed and a 1s electron promoted to an unoccupied level this occurs with a probability that depends on the character and availability of the empty levels around that atom. Since this is affected by hydrogen-bonds and neighboring waters we get information on the local structure around that molecule through this x-ray absorption spectroscopy (. The absorption process leaves the molecule in a very highly excited state and, after a very brief time, an electron in an occupied level, again on that molecule, will find a way to fill the hole that was created near the nucleus. This releases energy which is most often taken up by another electron which gets ejected in a complicated (Auger) process. Sometimes, however, an x-ray photon gets emitted instead and this gives a very direct picture of the electronic structure of that particular molecule.
