Redes Sociais

FacebookTwitterYoutubeGoogleYahooFeedWordpressLinkedin

Colóquios 2014

Evento 

Título:
Breaking news on water's second critical point, and a proposed connection to the new "Ultrastable gl
Quando:
16.06.2014 15.30 h
Onde:
Anfiteatro Antônio Cabral - Porto Alegre
Categoria:
Colóquios 2014

Descrição

Breaking news on water's second critical point, and a proposed connection to the new "Ultrastable glasses" phenomenon

Austen Angell  

Dept. of Chem. & Biochem., Arizona State University

 

 

 

This is an exciting time for those interested in the mysteries of supercooled water where heat capacities and relaxation times seem to be diverging according to power laws with singular temperatures Ts of ~228K. Interpreted since 1992 in terms of a second critical point (between two liquids)1, hidden by the crystallization to ice I, the debate has focused on details of where the critical point lies in pressure, and why the empirical equations of state for water show no evidence of low temperature criticality. There will be two papers in Nature and others in Nature Materials appearing this week and a PNAS paper out last week, all central to this problem and to the special problem created by a recent challenge to the very concept of the second critical point.

The latter is due to Limmer and Chandler, who used hybrid Monte Carlo calculations to obtain liquid and crystal free energies near the "putative" second critical point, as they called it. They used the crystal symmetry parameter Q6 to distinguish liquid from crystal free energy basins. Below the critical point, for two different models of water, they found only a single free energy basin in the liquid domain of low Q6 and then a second basin at high Q6 for the crystal, so concluded that the second critical point was a fiction. For one model, mW, everyone agrees. But for the other, ST2 water, there is major discord. We will describe how Debenedetti and coauthors (in Nature this week), using a battery (5 different approaches) of free energy calculations, confirm the earlier molecular dynamics-based finding of Poole, Sciortino and coauthors, and separately Stanley and coauthors. Each of their five approaches finds the two liquid basins. So the liquid-liquid transition in this water-like system now seems secure. It is unambiguous, also, in the attractive Jagla models for higher coordination number liquids that also have water-like anomalies. These models have their second critical points in the stable liquid domain (Marcia Barbosa here in UFRGS).

Near the second critical point, or across its related liquid-liquid transition, the diffusivities change dramatically - by three orders of magnitude. This is strikingly similar to the difference in diffusivity between various fragile molecular liquids at their glass temperatures, Tg, and the same liquids in their recently discovered "Ultrastable" glass states2 at that same temperature. Is this just a coincidence, or is the ultrastable glass manifesting the existence of the sub-Tg liquid to glass transition predicted for fragile liquids by recent theories of the glass transition. After all, just above its liquid-liquid transition water is the most fragile liquid known3. We will also show how the Cp anomaly of water can be pushed down to low temperatures so we can trap the structure in the glassy state for leisurely examination. Exciting times for liquids and glasses......

 

1 Poole, P. H., Sciortino, F., Essmann, U. & Stanley, H. E. Phase-Behavior of Metastable Water. Nature360, 324-328 (1992).

2 *Swallen, S. F. et al.Organic Glasses with Exceptional Thermodynamic and Kinetic Stability. Science315, 353 (2007).

3 Ito, K., Moynihan, C. T. & Angell, C. A. Thermodynamic determination of fragility in liquids and a fragile-to-strong liquid transition in water. Nature398, 492-495 (1999).

 

Local

Venue:
Anfiteatro Antônio Cabral
Cidade:
Porto Alegre
Estado:
Rio Grande do Sul
País:
País: br

Descrição

Descrição não disponível
Sexta, Dezembro 03, 2021