Dynamics of urea-water mixtures studied by molecular dynamics simulation

Abstract

We report the dynamics of urea-water mixtures at ambient conditions in the range of urea mole fractions xUREA=0.05,0.10,0.15,0.20 and 0.25 by using molecular dynamics simulation. The aim of this study is to examine how much the addition of low urea concentration in water modifies the vibrational, the orientational and the hydrogen bond dynamics of water. For this reason, we analyze the vibrational power spectra of both urea and water, the rotational autocorrelation functions for water, the rotational relaxation times, the self part of the van Hove functions for water oxygen and urea carbon, the diffusion coefficients and the hydrogen bond correlation functions for water. Our results indicate that the vibrational dynamics of water does not change significantly, unlike the rotational dynamics which becomes slower as we go to the urea-rich region of the mixture. We also conclude that the water network becomes stiffer when urea molecules are present and this leads to slower diffusion of water molecules. Concerning the hydrogen bond dynamics, the hydrogen bond between the two water molecules in the mixture lasts longer than in the pure liquid.