Reduction in mesoscopic structural fluctuations of liquid water induced by the large amphiphilicity of ionic liquid cations

Liquid water has an inhomogeneous structure in the molecular organization, due to the cooperative hydrogen bonding. Here, we report the effect of ionic liquid cations on the structural fluctuations of water using the small-angle X-ray scattering method. Aqueous solutions of imidazolium-, ammonium-, and phosphonium-based cations with butyl alkyl-chain(s) were investigated as typical cations forming ionic liquids. The investigation was performed on the basis of the concept of Mixing Scheme I, within which water maintains percolation of the hydrogen-bonding network and therefore holds the characteristic property of water even in solution systems. Dissolution of typical ionic liquid cations led to extreme reduction in the mesoscopic structural fluctuations, compared to molecular liquid. This reduction of the fluctuation is discussed both in terms of the contribution of the hydrophobic hydration of water molecules around the cations and the distorted hydrogen bonding network in bulk water due to the hydrophilic effect (C) 2018 Elsevier B.V. All rights reserved.