Any discussion on water slips into the domain of philosophy. Most mundane at the same time most sublime, scientific aspects of water are equally fascinating. Chemically water is an innocuous molecule, with the simple formula H2O. Alas! don't be fooled by the simple formula, because it conceals incredulous complexities that made Life itself possible on this planet. The chemical formula shows one oxygen atom linked two hydrogen atoms, but in reality there is extensive sharing of hydrogen atoms. Hydrogen bonding. is the term chemists would prefer. Hydrogen bonding is a very fast dynamic process, breaking and making happens in a jiffy, in picoseconds (10 -12 second). Almost all the unusual properties that water exhibits, have been be linked to hydrogen bonding. For decades Ander Nilsson and his team have been investigating the weirdness of water. In his opinion Water is the strangest liquid known. His .Public Lecture Water The Strangest Liquid is a nice refresher on the uniqueness of water.
273K (more popularly known as 0 degree Celsius), is the melting point of ice or freezing point of water. But under certain circumstances water can be cooled to well below 273K without ice formation. In other words water can be coaxed into a supercooled liquid state. Here is the only catch : water should be ultrapure, and shoulld very carefully cooled. Till about 232K (-41 degree C) water remains a liquid and then homogeneous crystallization sets in. This temperature referred to as TH, is the lowest temperature at which rate of crystallization of ice can be measured reliably. Below TH, ice crystallizes in microseconds. It must be mentioned that experimental variations do affect TH by a unit or two. The pertinent question is what is the structure of water below TH?.
A very difficult question indeed. Because of experimental limitations the domain below TH has so far remained a blackbox and is generally referred to as No Man's land. There are indications that below TH heat capacity and compressibility of water increase abnormally and several structural models have been proposed to account for these phenomenal changes. In the recent issue of Nature magazine(1) Nilsson's team at the SLAC National Accelerator Laboratory, Menlo Park, California report an an elegant experimental platform with ultrafast X-rays as the probing eye which can blink in femtoseconds, (Femtosecond= 10 -15 of a second) . Water droplets of diameters ranging from 9 to 37 microns were cooled rapidly by evaporation as they were propelled through vacuum.The team could detect liquid water even at temperature as low as 229K but noticed that a crop of ice crystals too appeared very quickly. The X-ray pattern suggested tetrahedrally coordinated liquid phase. which could the trigger homogeneous nucleation.
When experimental hurdles block your way , the next best thing is to resort to simulation and modeling. In the same issue of Nature a group from Princeton University, headed by Pablo Debenedetti report (2) Monte Carlo simulations of deeply supercooled water. They performed calculations on ST2 model of water with temperature fixed at 228.6K and pressure at 2.4kbar . Profile of the energy surface was monitored as a function of density and a structural parameter called Q6 . They propose that the system oscillates between two liquid forms, High Density Liquid (HDL) and (LDL) with no inclination to crystallize, though a crystalline phase too is present.
The same issue of Nature carries a write-up on the scarcity of drinking water and unconventional methods being used to address this challenge, such as construction of qanats as practised by ancient Persians and the modern fog harvesting technique.
A very difficult question indeed. Because of experimental limitations the domain below TH has so far remained a blackbox and is generally referred to as No Man's land. There are indications that below TH heat capacity and compressibility of water increase abnormally and several structural models have been proposed to account for these phenomenal changes. In the recent issue of Nature magazine(1) Nilsson's team at the SLAC National Accelerator Laboratory, Menlo Park, California report an an elegant experimental platform with ultrafast X-rays as the probing eye which can blink in femtoseconds, (Femtosecond= 10 -15 of a second) . Water droplets of diameters ranging from 9 to 37 microns were cooled rapidly by evaporation as they were propelled through vacuum.The team could detect liquid water even at temperature as low as 229K but noticed that a crop of ice crystals too appeared very quickly. The X-ray pattern suggested tetrahedrally coordinated liquid phase. which could the trigger homogeneous nucleation.
When experimental hurdles block your way , the next best thing is to resort to simulation and modeling. In the same issue of Nature a group from Princeton University, headed by Pablo Debenedetti report (2) Monte Carlo simulations of deeply supercooled water. They performed calculations on ST2 model of water with temperature fixed at 228.6K and pressure at 2.4kbar . Profile of the energy surface was monitored as a function of density and a structural parameter called Q6 . They propose that the system oscillates between two liquid forms, High Density Liquid (HDL) and (LDL) with no inclination to crystallize, though a crystalline phase too is present.
The same issue of Nature carries a write-up on the scarcity of drinking water and unconventional methods being used to address this challenge, such as construction of qanats as practised by ancient Persians and the modern fog harvesting technique.
1.Ultrafast X-rayprobing of water structure below the homogeneous ice nucleation temperature : Sellberg et al: Nature 19 June 2014( Vol. 510, pp381-384)
2.Metastable liquid-liquid transition in a molecular model of water, Palmer etal: Nature 19th June 2014 ( Vol. 510, pp385-388)
3.Water on Tap , Schiermeier: Nature 19th June 2014 ( Vol.510 pp 326-328 )
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