| Summary: | 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, T _g , of water in confinement. In the simplest case of water confined in a 13X molecular sieve ( d = 7.4 Å), the confinement is so severe that it precludes crystallisation altogether, and we measure a T _g = 190 ± 2 K. 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.
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