Anomalous components of supercooled water expansivity, compressibility, and heat capacity (<i>C</i> <i>p</i> and <i>C</i> <i>v</i>) from binary formamide+water solution studies

Abstract

Recently reported heat capacity studies of N2H4+H2O and H2O2+H2O solutions, from which an anomalous component of the pure water behavior could be extracted by extrapolation, have been extended to a system NH2CHO+H2O which has the chemical stability needed to permit expansivity and compressibility measurements as well. Data accurate to ±2% for each of these properties as well as for the heat capacity are reported. The expansivity data support almost quantitatively an earlier speculative separation of the bulk and supercooled water expansivity into a ‘‘normal’’ (or ‘‘background’’) part and an ‘‘anomalous’’ part, the latter part fitting a critical law αanom=A(T/Ts−1)−γ with exponent γ=1.0. According to the present analysis, the anomalous part of the expansivity which is always negative, yields Ts in the range 225–228, γ in the range 1.28–1.0, depending on the choice of background extrapolation function. The normal contribution to the heat capacity obtained from the present work is intermediate in character to that from the previous two systems and leads to similar equation parameters. The normal contribution to the compressibility on the other hand is very different from that speculated earlier by Kanno and Angell and approximately verified by Conde et al. for ethanol–water solutions. The background component from the present analysis is ∼50% larger, with the result that the anomalous component, at least when values above 0 °C are included in the analysis, cannot be sensibly fitted to the critical point equation. The possible origin and interest content of these differences is discussed. Combination of the new thermodynamic data permit estimation of Cv values for the solution, and by extrapolation, a normal Cv component for water. The anomalous component of Cv for pure water obtained by difference has the form of a Shottky anomaly in contrast with the corresponding Cp component which diverges.