Melting Point Of Water Research Articles

Molecular dynamics simulation for surface melting and self-preservation effect of methane hydrate

The surface melting process of structure sI methane hydrate is simulated at T = 240, 260, 280, and 300 K using NVT molecular dynamics method. The simulation results show that a quasi-liquid layer will be formed during the melting process. The density distribution, translation, orientation, and dynamic properties of water molecules in the quasi-liquid layer are calculated as a function of the distance normal to the interface, which indicates the performance of quasi-liquid layer exhibits a continuous change from crystal-like to liquid-like. The quasi-liquid layer plays as a resistance of mass transfer restraining the diffusion of water and methane molecules during the melting process. The resistance of quasi-liquid layer will restrain methane molecules diffuse from hydrate phase to gas phase and slow the melting process, which can be considered as a possible mechanism of self-preservation effect. The performance of quasi-liquid layer is more crystal-like when the temperature is lower than the melting-point of water, which will exhibit an obvious self-preservation. The self-preservation will weaken while the temperature is higher than the melting-point of water because of the liquid-like performance of the quasi-liquid layer.

A thermoporometry and small-angle x-ray scattering study of wet silica sonogels as thepore volume fraction is varied

Silica sonogels with different porosities were prepared by acid sono-hydrolysis oftetraethoxysilane. Wet sonogels were studied using small-angle x-ray scattering(SAXS) and differential scanning calorimetry (DSC). The DSC shows a broadthermal peak below the normal water melting point associated with the melting ofconfined ice nanocrystals, or nanoporosity. The nanopore size distribution wasdetermined from the Gibbs–Thomson equation. As the porosity is increased, asecond sharp DSC thermal peak with onset temperature at the water meltingpoint is apparent, which was associated with the melting of ice macrocrystals, ormacroporosity. The DSC result could be causing misinterpretation of the macroporositybecause water may not be exactly confined in very feeble silica network regions insonogels with high porosity. The structure of the wet gels can be described fairlywell as mutually self-similar mass fractal structures with characteristic lengthξ increasingfrom ∼1.8 to∼5.4 nm and mass fractaldimension D diminishingdiscretely from ∼2.6 to ∼2.3 as the porosity increases in the range studied. More specifically, such astructure could be described using a two-parameter correlation functionγ(r)∼rD−3exp(−r/ξ), which is limited at larger scale by the cut-off distanceξ but without a well-defined small scale cut-off distance, at least up to the maximum angulardomain probed using SAXS in the present study.