Anomalies in supercooled NaCl aqueous solutions: A microscopic perspective

In this work we studied the effect of NaCl on the thermodynamic and dynamic properties of supercooled water, for salt concentrations between 0.19 and 1.33 mol kg−1, using molecular dynamic simulations for TIP5P/E water model and ion parameters specially designed to be used in combination with this potential. We studied the isobaric heat capacity (Cp) temperature dependence and observed a maximum in Cp, occurring at Tm, that moves to lower temperature values with increasing salt concentration. Many characteristic changes were observed at scaled temperature T/Tm ∼ 0.96, namely a minimum in the density of the system, a reduction of the slope of the number of hydrogen bonds vs. temperature, and a crossover from Vogel-Tamman-Fulcher to Arrhenius dynamics. Finally, at low temperatures we observed that water dynamics become heterogeneous with an apparently common relationship between the fraction of immobile molecules and T/Tm for all studied systems.