The increment of the temperature of maximum density of water by addition of small amounts of tert-butanol: Experimental data and microscopic description revisited.

The temperature of maximum density, T, of aqueous solutions of tert-butanol has been experimentally determined in the pressure range of 0-300 bars and up to 0.025 tert-butanol mole fraction. At atmospheric pressure, this quantity increases for low alcohol mole fractions, reaches a maximum at intermediate concentrations, and then quickly falls. The new experimental results are basically in agreement with previous data in the literature by Wada and Umeda [G. Wada and S. Umeda, Bull. Chem. Soc. Jpn. 35, 646 (1962)], except at very low mole fractions, where these authors reported a stronger density anomaly. Our measurements also confirm the known effect of pressure, p, on the variation in the temperature of maximum density with respect to that of pure water, ΔT: this quantity increases with p over the whole composition range. In addition, molecular dynamics simulations were performed between 0 and 2000 bars and from 238 to 328 K using a recently proposed model for the tert-butanol/water system. It has been found that our model reproduces qualitatively the experimental behavior of the ΔT, but for pressures above 1000 bars. A detailed structural analysis showed that the addition of tert-butanol promotes the low density water structure, and this promotion is somewhat hampered as the temperature increases at high pressure (ΔT > 0) and mostly independent of temperature at low pressures (ΔT < 0). Our analysis shows that the ultimate factor determining changes in the T is the temperature dependence of the low density water structure enhancement. We have also carried out a local structure analysis in which in addition to solid-like structures, low density liquid water ones have also been considered.