Configurational and vibrational entropies and molecular relaxation in supercooled water

Abstract

In order to investigate whether the anomalous decrease in the net entropy of water on supercooling indicates a structural change, its entropy and relaxation time data have been examined by equating the Adam-Gibbs expression with the Vogel-Fulcher-Tamman equation. This gave values of the minimum size of the cooperatively rearranging region as 4.7 molecules at 150 K, and the temperature-invariant energy as 7.42 kJ mol−1. On the premise that a liquid’s configurational entropy, Sconf, differs from its excess entropy over the ordered crystal state, Sconf of water has been estimated over the 150–273 K range by using the available value of its excess entropy at ∼150 K. Water’s Sconf at 273 K is found to be less than half of its entropy of fusion and to further decrease continuously on supercooling. This puts into question the conjecture that water structurally transforms near 228 K, as deduced by (wrongly) assuming that water’s configurational entropy is equal to its excess entropy. The analysis also indicates that the vibrational entropy of supercooled water, Svib, becomes less than the calorimetric entropy of hexagonal ice at T<193 K, which is seen as a reflection of the relatively tighter and strained intermolecular H-bonding in water than in hexagonal ice. This is supported by the known higher frequency of translational modes in water than in hexagonal ice. The ratio of Sconf to Svib for water at 273 K is 0.19, which is comparable with the corresponding ratio determined here for other supercooled liquids.