Non-exponential kinetic behaviour of confined water

Abstract

Abstract. -We present the results of molecular dynamics simulations of SPC/E water confined in a realistic model of a silica pore. The single-particle dynamics have been studied at ambient temperature for different hydration levels. The confinement near the hydrophilic surface makes the dynamic behaviour of the liquid strongly dependent on the hydration level. Upon decrease of the number of water molecules in the pore we observe the onset of a slow dynamics due to the "cage effect". The conventional picture of a stochastic single-particle diffusion process thus looses its validity.The normal limit of supercooling and the possibility of vitrification of water is the subject of a longstanding scientific debate [1,2]. At T H ∼ 236 K homogeneous nucleation, due to the presence of impurities, drives bulk supercooled water into its solid, crystalline phase. Experiments sustain the hypothesis that the amorphous phase can be connected to the normal liquid phase through a reversible thermodynamic path [3]. Recent molecular dynamics (MD) simulations of bulk supercooled SPC/E [4] water also showed a kinetic glass transition as predicted by mode coupling theory (MCT) [5] at a critical temperature T C ∼ T S [6], where T S is the singular temperature of water [7], which is T S = 228K or 49 degrees below the temperature of maximum density.A comparison of the behaviour of the bulk liquid with the same liquid in a confined environment is highly interesting for the development of both biological and industrial applications. Understanding how the dynamics of liquid water is perturbed by the interaction with hydrophilic or hydrophobic substrates at various levels of hydration lays the physical basis for the prediction of stability and enzymatic activity of biologically important macromolecules [8]. Many experimental studies have been performed on confined and interfacial water. They all showed evidence of a substantial degree of slowing down of water when confined in the proximity of a polar surface [9]. Particularly significant in this context are the indications of Typeset using EURO-T E X

Component dynamics in polyvinylpyrrolidone concentrated aqueous solutions

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