Dynamic mechanical analysis of supercooled water in nanoporous confinement

TL;DR

This study uses dynamic mechanical analysis to investigate the relaxation dynamics and glass transition of supercooled water confined in nanoporous materials, revealing distinct surface and bulk-like behaviors and their dependence on pore size.

Contribution

It provides detailed insights into the relaxation processes and glass transition of supercooled water in nanopores, identifying two relaxation peaks and their relation to pore size and surface effects.

Findings

Two relaxation peaks observed at 145 K and 205 K.

Relaxation times follow Arrhenius behavior with activation energy 0.47 eV.

Glass transition temperature scales with pore size, extrapolating to 136 K for bulk water.

Abstract

Dynamical mechanical analysis (DMA)(f=0.2 - 100 Hz) is used to study the dynamics of confined water in mesoporous Gelsil (2.6 nm and 5 nm pores) and Vycor (10 nm) in the temperature range from T=80 K to 300 K. Confining water into nanopores partly suppresses crystallization and allows us to perform measurements of supercooled water below 235 K, i.e. in water's so called "no man's land", in parts of the pores. Two distinct relaxation peaks are observed around T1 = 145 K (P1) and T2 = 205 K (P2) for Gelsil 2.6 nm and Gelsil 5 nm at 0.2 Hz. Both peaks shift to higher T with increasing pore size d and change with f in a systematic way, typical of an Arrhenius behaviour of the corresponding relaxation times. For P1 we obtain an average activation energy of Ea=0.47 eV, in good agreement with literature values. It is suggested that P1 corresponds to the glass transition of supercooled water far from pore walls, whereas P2 reflects the dynamics of water molecules near the surface of the pores. The observation of a pronounced softening of the Young's modulus around 165 K (for Gelsil 2.6 nm at 0.2 Hz) is in agreement with a glass-to-liquid transition in the vicinity of P1. In addition we find a clear-cut 1=d-dependence of the calculated glass transition temperatures which extrapolates to Tg(1/d=0)=136 K, i.e. the traditional value of water.