Water in confined geometries

Abstract

The study of water confined in complex systems in solid or gel phasesand/or in contact with macromolecules is relevant to many importantprocesses ranging from industrial applications such as catalysis and soil chemistry, to biological processes such as protein folding or ionic transportin membranes. Thermodynamics, phase behaviour and the molecular mobility of water have been observed to change upon confinement depending onthe properties of the substrate. In particular, polar substratesperturb the hydrogen bond network of water, inducing large changes in the properties upon freezing. Understanding how the connected random hydrogen bond network of bulk water is modified when water is confined in small cavities inside a substrate material is very important for studies of stability and the enzymatic activity of proteins, oil recovery or heterogeneous catalysis, where water–substrate interactions play a fundamental role. The modifications of the short-range order in the liquid depend on the nature of the water–substrate interaction, hydrophilic or hydrophobic, as well as on its spatial range and on the geometry of the substrate. Despite extensive study, both experimentally and by computer simulation, there remain a number of open problems. In the many experimental studies of confined water, those performed on water in Vycor are of particular interestfor computer simulation and theoretical studiessince Vycor is a porous silica glass characterized by a quite sharp distribution of pore sizes and a strong capability to absorb water. It can be considered as a good candidate for studying the general behaviour of water in hydrophilic nanopores.But there there have been a number of studies of water confinedin more complex substrates, where the interpretation ofexperiments and computer simulation is more difficult,such as in zeolites or in aerogels orin contact with membranes.Of the many problems to consider we can mention thestudy of supercooled water.It is particularly important to understandwhether the glass transition temperature could be experimentally accessible for confined water. In this respect the modifications induced by the confinement on thedynamics of water on supercooling are of extreme interest anda number of experimental and computer simulation studies have beendevoted in recent years to this topic.This special section contains papers from different groupswhich have contributed withvarious experimental and computer simulation techniquesto the progress made in the study of water in confined geometry. I thank all of the authors for their stimulatingcontributions. I am very pleased in particular that Sow-Hsin Chen agreed to contributesince he has done pioneering experimental workon the dynamical properties of confined water upon supercooling, and he is still very active in the field.The work presented by the group of J Swenson concernsalso the glass transition of confined water.The Messina group (Crupi et al) is very activein the study of dynamical properties of confined waterand they present their results on water in zeolites.From the experimental side there is also a contribution from J Dore's group, one of the first to perform neutron scatteringstudies on confined water. The work of J Klein looks at the mobility of water molecules confined in subnanometre films.Important contributions on the computer simulation side comefrom the Geiger group (Brovchenko et al). They performedvery accurate simulations of water in nanopores, exploring alarge portion of the phase space. Puibasset et al were able to build a very realistic model tosimulate water inside Vycor. Zangi et al review the extensivework performed on confined water. Jedlovszky is an experton the model potential for water and studied how the hydrogenbond network of water can be modified by the presenceof an interface. The special issue is intended to stimulate interest and future workon this important subject.