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

Abstract

The temperature of maximum density, TMD, 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, Δ TMD: 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 Δ TMD, 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 (Δ TMD > 0) and mostly independent of temperature at low pressures (Δ TMD < 0). Our analysis shows that the ultimate factor determining changes in the TMD 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.

Density anomaly in a minimum model of water-alcohol mixtures

“…Figure 5 shows that a lower pressure reduces the ∆T * MD (x) for both structure makers and breakers, in comparison with the case P * = 2. This behavior was observed in recent experiments [48]. We also try to understand how the attraction between solvent and solute and the fluctuation effects impact the number of solute-solute and solute-solvent neighbors and, consequently, the ∆T * MD (x) as follows.…”

“…This difference is also larger at high pressures than at low pressures. In real water-alcohol mixture the increase of pressure increases ∆T * MD (x), as shown in recent experiments [48]. One reason for our model to be unable to capture the pressure effect is the lack of directional interactions, which affects our estimates of the entropic component.…”

“…Unlike the one dimensional case shown in Figure 5, the ∆T * MD (x) decreases with the increase of pressure. Addition of pressure decreases the ∆T * MD (x) independent of the solute-solvent attraction.In real water-alcohol mixture the increase of pressure increases ∆T * MD (x), as shown in recent experiments[48]. One reason for our model to be unable to…”

Water’s Unusual Thermodynamics in the Realm of Physical Chemistry

“…Beyond nonpolar solvation, the peculiarities of liquid water may be relevant to a number of classical problems in the area of aqueous solutions that are still a matter of considerable attention. Thus, the evolution of the density maximum upon the addition of solutes has acquired a renewed interest. , This is also the case for the structural effects around solutes, as envisioned by H. S. Frank and M. W. Evans in 1945 and recently correlated with water’s ice-like order with the aid of advanced spectroscopic techniques . Furthermore, careful simulations yielding reliable values for the osmotic second virial coefficient and exact thermodynamic relations involving such property suggest that water’s unusual thermodynamics may affect the forces between nonpolar solute molecules mediated by water, which are predominantly attractive at high temperatures and repulsive at supercooling conditions.…”

The temperature of maximum density for aqueous solutions

“…Using these computational tools, a detailed study of the effect of the solutes in the water structure could be carried out, allowing reaching to deeper and more rigorous conclusions about the microscopic origins underlying the TMD behavior. [16][17][18][19] In this Perspective, we show the theoretical thermodynamic framework that relates TMD to other physical properties, we summarize the experimental data of TMD for a wide class of solutes, and we try to expose the most significant results, mainly based on molecular simulation data, that link the TMD behavior with changes in the water structure. Finally, the main conclusions about the TMD work over the years are drawn out, and future researching lines for a better understanding of TMD are proposed.…”