This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys. Cite this: DOI: 10.1039/c3cp51786a Different behavior of water in confined solutions of high and low solute concentrations Khalid Elamin, a Hele ´n Jansson, b Shigeharu Kittaka c and Jan Swenson* a Water–glycerol solutions confined in 21 Å pores of the silica matrix MCM-41 C10 have been studied using differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS). The results suggest a micro-phase separation caused by the confinement. Likely the water molecules coordinate to the hydroxyl surface groups of the pores, leaving most of the glycerol molecules in the centre of the pores. This makes the dynamics of glycerol almost concentration independent up to water concentra- tions of about 85 wt%. However, at higher water concentrations no substantial clustering of glycerol molecules should occur and the glass transition related dynamics exhibit an anomalous behaviour. Instead of a common plasticization effect of water, as for the corresponding bulk solutions (when no ice is formed), it is evident that water acts as an anti-plasticizer in the confinement at high water con- centrations. We propose that the increased water concentration slows down the glass transition related dynamics in the deeply supercooled regime due to that a rigid hydrogen bonded network structure of water molecules is formed at low temperatures and low glycerol concentrations. This is in contrast to the situation in a homogenously mixed bulk solution of a high solute concentration where the water molecules will be less hydrogen bonded, and therefore are typically more mobile than the surrounding solute molecules. An almost complete hydrogen bonded network of water molecules may, even in con- finements, be sufficiently rigid to slow down the relaxation of embedded solute molecules. It can also be expressed the other way around, i.e. small amounts of glycerol act as a plasticizer for water, due to its breaking up of the nearly tetrahedral network structure. From the here observed concentration dependent behaviour of the deeply supercooled bulk and confined solutions it seems, furthermore, evi- dent that the T g value of bulk water cannot be estimated from extrapolations of aqueous solutions. 1. Introduction Water is the most studied substance due to its many peculiar properties and related biological importance. However, a full understanding of the physics and chemistry of water is difficult to reach, partly because bulk water cannot be studied in the supercooled (and glassy?) regime from 235 K down to 150 K, where it almost instantaneously crystallizes to ice. Different approaches have been used to overcome the problem of crystal- lisation in order to be able to study the properties of water in this low temperature range, also called ‘the no man’s land’. The two most common approaches, by far, are to confine water in different types of porous materials or to mix it with a salt or another glass-forming liquid. For sufficiently small pores (less than about 2 nm in diameter) or high salt/solute concentrations crystallisation can be completely avoided at all temperatures. However, the fact that ice formation is prevented is a strong indication that the structural and dynamical properties are altered compared to those of pure bulk water. By comparing with the corresponding properties of bulk water at tempera- tures above 235 K a hint of how serious the alterations are can be gained. However, since, for instance, confinement effects are strongly temperature dependent 1 it is nevertheless difficult to know how relevant experimental results of confined water, or aqueous solutions, are for bulk water in the temperature range 150–235 K. Furthermore, in the case of the glass transition and its associated structural (a) relaxation, previous studies on interfacial water in the deeply supercooled regime have indi- cated that these viscosity related features cannot even be observed in confinements. 2–13 Thus, it seems as the interesting viscosity related cooperative dynamics in deeply supercooled a Department of Applied Physics, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden. E-mail: jan.swenson@chalmers.se; Fax: +46-31-772-2090; Tel: +46-31-772-5680 b Department of Civil and Environmental Engineering, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden c Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Ridaicho, Okayama 700-005, Japan Received 26th April 2013, Accepted 4th July 2013 DOI: 10.1039/c3cp51786a www.rsc.org/pccp PCCP PAPER Published on 04 July 2013. Downloaded by Chalmers Tekniska Hogskola on 16/08/2013 13:06:20. View Article Online View Journal