A study describes liquid water diffusion at molecular level

Feb 24, 2012

An article published in Physical Review E and conducted by Spanish researchers at the universities of Granada and Barcelona might lead to a revolutionary change in water desalination and filtration methods.

Researchers at the universities of Granada and Barcelona have described for the first time the diffusion of liquid water through nanochannels in molecular terms; nanochannels are extremely tiny channels with a of 1-100 nanometers that scientists use to study the behavior of (nm. a unit of length in the metric system equal to one billionth of a meter that is used in the field of nanotechnology).

This study might have an important impact on water desalinization and filtration methods. Two articles published in Science state that the introduction of graphene membranes and carbon nanolayers will revolutionize water desalinization and filtration processes, as water diffuses rapidly through these materials when their are 1nm in diameter.

Liquid water exhibits a range of unusual properties that other do not have: up to 65 abnormalities. Some of these abnormalities have been known for 300 years, as the fact that water expands below 4ºC.

Many of the abnormalities found in water have a dynamic nature -e.g. water molecules move faster as density increases-, as a result of the properties of the hydrogen bond networks that form between water molecules; hydrogen bonds lead to the formation of tetrahedral structures wherein a central atom is located at the center with four molecules located at the corners. However, this geometrical structure changes with pressure and temperature and, until now, changes in the molecular structure and properties of had not been described.

A Mystery to Solve

Particularly confusing are the results on the diffusion of water confined between two hydrophobic plates. Neither experiments nor computer-based models have clarified whether confinement increases or reduces the mobility of water molecules. However, it seems that the mobility of relies on ducts having a diameter above or below 1nm.

In a study published in the prestigious journal Physical Review, professors Francisco de los Santos Fernández (University of Granada) and Giancarlo Franzese (University of Barcelona) described the behavior of water confined between two hydrophobic plates. In their study, Franzese and Fernandez used models to demonstrate that the diffusion of nanoconfined water is unusually fast, as a result of the competition between the formation and breaking of hydrogen bonds, and the free volume available for cooperative molecule rearrangement.

In nanochannels above 1 nm in diameter, macroscopic diffusion of water only occurs if there is a cooperative rearrangement of molecules, which leads to HB breaking within a cooperative region of 1nm in size. On the other hand, increases in nanochannels below 1 nm, as fewer HBs need to be broken. Thus, this study proves that the interplay between hydrogen bond breaking and cooperative rearranging within regions of 1-nm determine the macroscopic properties of .

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More information: F. de los Santos and G. Franzese, Phys. Rev. E 85, 010602(R) (2012). link.aps.org/doi/10.1103/PhysRevE.85.010602

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Callippo
not rated yet Feb 24, 2012
The expansion of ice during freezing indicates, the molecules within bulk watter are compressed mutually by a force in range of 270 kbars. As the result, the water molecules form a dynamic clusters of icosahedral geometry, each containing about 240 molecules of water. These clusters behave like rigid, solid objects maintaining their shape like jelly at the 15 nm dimensional scale, which leads into many anomalous phenomena. The passing the water trough channels breaks these clusters and inside of pores the water is behaving like low-molecular fluid, analogous to hydrogen sulphide or similar gases. You can face the anomalous low viscosity of thin layers of water during crunching of fresh snow under your feet.
Callippo
not rated yet Feb 24, 2012
BTW you can visit the Giancarlo Franzeses group site for preprint of this study and more info.

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