Amorphous ices characterized

Results relative to a 25% hydrated Vycor sample indicates that at room temperature interfacial water has a structure similar to that of bulk supercooled water at a temperature of about 0°C, which corresponds to a shift of about 25 K [40]. The structure of interfacial water is characterized by an increase of the long-range correlations, which corresponds to the building of the H-bond network as it appears in low-density amorphous ice [41 ]. There is no evidence of ice formation when the sample is cooled from room temperature down to -196°C (liquid nitrogen temperature). 

When partially hydrated samples are cooled down to 77 K, no crystallization peak is detected by differential thermal analysis. The x-ray and neutrons show that an amorphous form is obtained and its structure is different from those of low-and high-density amorphous ices already known [5]. Samples with lower levels of hydration corresponding to one monolayer coverage of water molecules are under investigation. This phenomenon looks similar in both hydrophilic and hydrophobic model systems. However, in order to characterize more precisely the nature of the amorphous phase, the site-site partial correlation functions need to be experimentally obtained and compared with those deduced from molecular dynamic simulations. 

One of the more relevant questions related to amorphous ices is probably how to quantify the number of known amorphous states. From the structural point of view, one can identify three amorphous ices, namely LDA, HDA, and VHDA. Both experiments and computer simulations indicate that the structure of these amorphous ices is characterized by the absence of long-range order beyond 10-20 A and by local tetrahedral coordination, in agreement with the Walrafen pentamer geometry. Thus, in all these amorphous ices, the arrangement of a water molecule and its four nearest neighbors is not different from ice and the Bernal-Fowler rules... 

Ice films condensed from the water vapour on a cold substrate (T<30 K) has been characterized as a high-density amorphous form of ice, which could be a denser variant of the low-density phase obtained by deposition above 30 K. Condensation from the background pressure also leads to ice films that are highly porous at a nanoscale.This porosity is lost by warming or by direct deposition of water at T>90 K. Warming ice at 150 K induces the crystallization, whatever the initial structure is. 

This form is often referred to as smectic and was first mentioned in 1958 by Slichter and Mandell who observed a peculiar wide angle X-ray diffraction (WAXD) pattern in a sample melted and then rapidly quenched with dry ice. It is characterized by an order intermediate between those found in crystalline and in amorphous phases and is metastable since annealing at temperatures higher than 70°C leads to the crystallization of a-iPP. While density is low (0.88 g/cm ), infrared (IR) spectra indicate that iPP chains adopt the usual 3j helix conformation. Solid state nuclear magnetic resonance (NMR) shows a closer resemblance to p-iPP while WAXD patterns are in favor of a predominance of very local (pairs of chains) arrangements similar to those found in a-iPP. 

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