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A recent experiment has measured the viscosity of water down to approximately 244 K temperature and up to 300 MPa pressure. The correct viscosity and translational diffusivity data at various temperature-pressure (T-P) state points allow checking the validity of the Stokes-Einstein (SE) relation, which accounts for the coupling between translational self-diffusion and medium viscosity. The diffusion-viscosity decoupling increases with decreasing temperature, but the increasing pressure reduces the extent of decoupling. Earlier simulation studies explained the breakdown of the SE relation in terms of the location of the Widom line, emanating from the liquid-liquid critical point (LLCP). Although these studies made a significant contribution to understand the above phenomena, a detailed molecular picture is still lacking. Recently our group has explained the diffusion-viscosity decoupling from jump-diffusion perspective. The jump-diffusion coefficient - emanating from jump translation of water molecules - is calculated using a quantitative approach for different temperature at ambient pressure. It has been observed that the jump-diffusion is the key factor for diffusion-viscosity decoupling in supercooled water. The same method is adopted in the present work to estimate the jump-diffusion coefficient for different T-P state points and thereby explain the role of jump diffusion for different extent of SE breakdown at different pressures. The residual diffusion coefficient - the other component of the total diffusion which originates from small step displacement and which is calculated by subtracting the jump-diffusion coefficient from the total diffusion - is seen to be fairly coupled to the viscosity at the entire range of temperature and pressure. Furthermore, we have calculated average number of H-bonds per water molecule and the tetrahedral order for different T-P state points and investigated an approximate correlation between the average local structure and the contribution of the jump-diffusion to the total diffusion of water. This study, therefore, puts forward a new perspective for explaining the SE breakdown in supercooled water under different pressure conditions.
This article was published in the following journal.
Name: The journal of physical chemistry. B
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