A nano-scale free volume perspective on the glass transition of supercooled water in confinement

We employ positron annihilation lifetime spectroscopy (PALS) to measure the temperature dependent fluctuations in a local free volume of water confined in a variety of mesoporous materials with different pore sizes and morphologies. We demonstrate that this unconventional approach can be used to probe the dynamics and glass transition temperature, Tg, of water in confinement. In the simplest case of water confined in a 13X molecular sieve (d = 7.4Å), the confinement is so severe that it precludes crystallisation altogether, and we measure a Tg = 190 ± 2K. We also show that temperature dependent PALS measurements can be used to probe the glass transition of water molecules confined in matrices with larger pore diameters (SBA-15, KIT-6 and Vycor glass), where the confinement is expected to be less severe.