Water is the basis for life as we know it and is fundamental as a solvent in countless chemical and geochemical processes. It is often referred to as the "strangest" liquid due to its many unique physical properties. Among these are the density maximum at 4 C and the solid (ice) being less dense than the liquid. These properties make aquatic life possible also in the far north since lakes freeze from the top, while water at the bottom remains at 4 C. Its compressibility and heat capacity show a minimum at normal temperatures and then rapidly increase when the liquid is supercooled below the freezing point. Both depend on fluctuations in the liquid that for other liquids decrease upon cooling, but for water they instead increase. The origin of this anomalous behavior has been hotly debated, but from both experiment and simulations it is becoming increasingly clear that water, far from ambient conditions, can exist as a liquid in two different forms, a high-density (HDL) and low-density liquid (LDL). This still affects water under normal conditions through dynamical fluctuations as groups of molecules switch between local HDL and LDL structures. The ultimate goal of GAS-WAT is to determine how these structures are built, how they affect chemistry and how they affect the solvation of gases, most importantly oxygen, and how oxygen is transferred to fish gills.
Water can form so-called clathrates where water molecules form cages, typically around a solvated gas molecule, like methane or oxygen, but can such structures form locally in the liquid, even without the gas molecule? Would such more open structures be preferred for, e.g. oxygen in water? Irrespective of whether they pre-existed or formed around the oxygen, once the oxygen is in the cage, the question becomes how do fish get the oxygen out and into their gills? Has evolution found a way to change the structure of water in the vicinity of the gills to make it more attractive for the oxygen? To investigate this, we perform x-ray measurements at synchrotron radiation facilities to determine the arrangement of water molecules around Argon dissolved in water. Argon has a similar size and behavior as oxygen and we selectively look at the Argon in the water. We find a basic clathrate-like environment, but to answer the question if it existed without the Argon or formed because of the Argon we must turn to theoretical simulations and modeling.
For this we will put clathrate-like structures into simulation models of water and follow them as the simulation proceeds. Will they remain for any length of time or disappear? To be conclusive, the description of how the water molecules interact has to be very reliable and to this end we are finalizing a state-of-the-art computer model of water (FCM-GAP) that we will apply.
X-ray spectroscopic data indicate that the structures we find from advanced computer models of water need to be modified. These computer models give many other properties (density, diffusivity, compressibility, etc) in agreement with experiment, but will this still be the case for the modified structures we deduce from the x-ray spectra? To answer this we have developed a Monte Carlo code to calculate all the relevant properties, both for the original and modified structures to check the consistency.
Finally, the protons in the water molecule are light enough that we need to take quantum effects into account. This leads to water molecules existing in two different forms, ortho- and para-water, depending on how the proton nuclear spins are coupled and this affects how the molecules rotate. To include this we develop and implement methods to treat the protons on an equal basis with the electrons, i.e. as properly spin-coupled wave functions. The goal is to perform computer simulations where the protons are fully quantum mechanical and their spins properly described.
