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-MD, 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, Delta T-MD: 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 Delta T-MD, 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 (Delta T-MD > 0) and mostly independent of temperature at low pressures (Delta T-MD < 0). Our analysis shows that the ultimate factor determining changes in the T-MD 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.

Automated summary

The temperature of maximum density of aqueous solutions of tert-butanol has been experimentally determined, and molecular dynamics simulations have been performed to study the effect of pressure and temperature on the density anomaly. The results show that the addition of tert-butanol promotes the low density water structure, and this promotion is influenced by pressure and temperature.