October 3, 2005 Colloquium
October 3
Dr. Anders Nilsson, Stanford Synchrotron Radiation Laboratory, Stanford University
The Structure of Liquid Water
In many processes in technology and nature, hydrogen bonding (H-bonding) is involved as an essential component. The H-bond in liquid water holds the key to its peculiar behavior, with implications for chemical, biological and geological processes. In the present talk I will show how we can address the nature of the H-bond and its connection to the structure of liquid water using a combination of X-ray spectroscopy and Density Functional Theory (DFT) calculations. X-ray Absorption Spectroscopy (XAS), X-ray Emission Spectroscopy (XES) and X-ray Raman Scattering (XRS) provides direct information on how occupied and unoccupied orbitals locally around the oxygen atom are transformed upon condensation of water to form ice, ambient water, supercritical water and water adsorbed on surfaces. In particular the recent result on the structure of the first coordination shell in liquid water (1) challenges many current models based on molecular dynamic simulations.
1. Wernet et. al. , Science 304, 995 (2004).
I really need to quit waiting a week to write these colloquium talks up, but oh well.
I've got to say, this talk started off pretty good. He had a picture of some waterfalls. Yeah. Waterfalls. Unfortunately it was all downhill from there. He talked about water, the typical coulomb-interaction model, and how it broke down. Then he assaulted us with slide after slide of spectra, comparing water, ice, steam, cold water, ice with a layer of ammonia on it, and only God knows what else.
I do admit it that it's kind of neat that the structure of water bore the most resemblance to the 'edge' of ice in the scattering. That means that whatever properties structurally liquid water has are similiar to what happens at the edge of a piece of ice. That is to say, broken chemical bonds.
You see, water is formed by H2O coming together with hydrogen bonds. But each O atom is actually bound to 4 Hydrogen atoms. These bonds are constantly breaking all over the place with a life on the order of femto-seconds I think he said. So you've got constantly jiggling hydrogen atoms bouncing back and forth (well they're not moving *that* much) between oxygen atoms. I think I saw that on a sitcom (Friends?) once.
At the cut edge of a piece of ice you get broken bonds from the process of cutting the ice. So it kind of makes sense that you should see the signature of those broken bonds in the various spectra you look at. And since liquid water is a teeming pool of broken bonds, it makes sense that you should see something similiar.
He then proceeded to talk about various models for deforming the structure of ice, and how they can generate something close to the right picture for water with one method of that.
It was boring. And very technical. And I don't care about condensed matter physics to get excited about it.