Estimation of effective Debye Temperature of Water over Extended Pressure Range; 0.25 GPa -3.5 GPa

Abstract

The motion of molecules in liquids is more complex as compared to that in solids. Hughes [1] in his famous cold neutron scattering experiment confirmed that the molecules of water vibrate about fixed positions for about 10^-13 sec. It means the molecules of water follow an ordered arrangement for a fraction of picoseconds and attain solid like behaviour. In case of liquids, this behaviour is termed as quasi crystalline state. However the Debye temperature is mostly evaluated for solids but as it is related to the atomic motion of the molecules, therefore it can be a significant parameter to study the quasi-crystalline behaviour of liquids. Most of the data for Debye temperature determined on the basis of quasi-crystalline model is available only at atmospheric pressure [2]. Kor et al. [3] had made an attempt to evaluate the Debye temperature for water, methanol and ethanol using quasi-crystalline model at elevated pressure (0.098 MPa - 294 MPa) and in the temperature range 0 ^oC – 50. He concluded that the pressure has significant effect on the atomic motion of the liquids. The insight of literature also indicates that the thermodynamic properties of water at high temperature and pressure are also of considerable importance in Science and technology [1-3]. Based on Quasi-crystalline model for liquids, Debye temperature for water has been estimated in the high pressure region (0.25 GPa -3.5 GPa) and in the temperature range; T= 353.15 K – T=473.15 K.The required data   ultrasonic velocity, density have been taken from Wiryana et al. [4]. The obtained results infer that θ_D is increase ~1.5 times as the pressure changes from 0.25 G Pa - 3.5 G Pa, whereas no significant increase is observed with increase in temperature range from 353.15 K - 473.15 K. A minimal change of about 2.4% is observed when the temperature increases from 353.15 K to 473.15 K. To the best of our knowledge, study of water at elevated pressure ranges from 0.25 G Pa - 3.5 G Pa have not yet reported in literature. The present study also reveals that quasi crystalline behaviour of water is much probable at elevated pressure.