Density anomaly in water-alcohol mixtures: Minimum model for structure makers and breakers

We modeled the change in the temperature of maximum density (TMD) of a waterlike solvent when small amounts of solute are added to the mixture. The solvent is modeled as a two length scales potential, which is known to exhibit waterlike characteristic anomalies, while the solute is chosen to have an attractive interaction with the solvent which is tuned from small to large attractive potential. We show that if the solute exhibits high attraction with the solvent it behaves as a structure maker and the TMD increases with the addition of solute, while if the solute shows a low attraction with the solvent the TMD decreases, with the solute behaving as a structure breaker.

Figure 1

(a) solvent-solvent, (b) solvent-solute/solute-solute interaction reduced potentials as a function of reduced distance.

Figure 2

Density as a function of temperature at $P^{*}=2$ for: pure solvent (black line), $a_{AB}^{*}=0.7$ (red line) and $a_{AB}^{*}=0.9$ (blue line) at $x=0.30$. Vertical arrows show how the increase in density changes the TMD.

Figure 3

Change in the temperature of maximum density $\mathrm{\Delta }{T}_{\text{MD}}^{*}$ as a function of solute concentration at $P^{*}=2$ for $a_{AB}^{*}=0.9$ (orange), 0.8 (blue), 0.7 (red).

Figure 4

(a) Interaction correlations for different $AB$ well depths as a function of temperature. The plots were made with a concentration $x=0.30$ and $P^{*}=2$ for $a_{AB}^{*}=0.9$ (orange), 0.8 (blue), 0.7 (red).

Figure 5

Change in the temperature of maximum density $\mathrm{\Delta }{T}_{\text{MD}}^{*}$ as a function of solute concentration at $P^{*}=1.8$ (solid line), $P^{*}=1.9$ (dashed line), and $P^{*}=2.0$ (dotted line) for (a) $a_{AB}^{*}=0.7$ and (b) $a_{AB}^{*}=0.8$.

Figure 6

Interaction potential versus distance for the (a) solvent-solvent (black line), solute-solvent (red line), and solute-solute (green line) interactions; (b) solvent-solute for ${\epsilon }_{AB}^{*}=0.8$ (red line), ${\epsilon }_{AB}^{*}=1.1$ (blue line), and ${\epsilon }_{AB}^{*}=1.3$ (yellow line).

Figure 7

Density as a function of temperature at $P^{*}=0.13$ for (a) pure solvent (black circles), ${\epsilon }_{AB}^{*}=1.1$ (blue squares), and ${\epsilon }_{AB}^{*}=0.8$ (red triangles), both at $x=0.02$; (b) pure solvent (black circles), ${\epsilon }_{AB}^{*}=1.1$ with $x=0.02$ (blue squares), and ${\epsilon }_{AB}^{*}=1.1$ with $x=0.05$ (red triangles). Vertical arrows indicate the TMD. Markers indicate some of the simulation points. Curves are third degree polynomial fits.

Figure 8

Change in the temperature of maximum density as a function of solute concentration at $P^{*}=0.13$ for different values of ${\epsilon }_{AB}^{*}$: ${\epsilon }_{AB}^{*}=0.8$ (red line with circles), ${\epsilon }_{AB}^{*}=1.1$ (blue line with squares), and ${\epsilon }_{AB}^{*}=1.3$ (yellow line with diamonds).

Figure 9

Radial distribution function for ${\epsilon }_{AB}^{*}=0.8$ (red line), ${\epsilon }_{AB}^{*}=1.1$ (blue line) and ${\epsilon }_{AB}^{*}=1.3$ (yellow line), at $P^{*}=0.13$, $T^{*}=0.40$ and $x=0.03$ for (a) solvent-solute, $g_{AB}$, and (b) solute-solute, $g_{BB}$. The inset in (a) zooms in the $r^{*}\approx 2.7$ region. The inset in (b) is the solvent-solvent RDF at $P^{*}=0.13$, $T^{*}=0.40$ and $x=0.03$.

Figure 10

Change in the temperature of maximum density, $\mathrm{\Delta }{T}_{\text{MD}}^{*}\left(x\right)$, as a function of solute concentration for (a) ${\epsilon }_{AB}^{*}=0.8$ and (b) ${\epsilon }_{AB}^{*}=1.1$ for reduced pressures 0.13, 0.15, 0.18, 0.20 and 0.22.