Computer simulations on supercooled aqueous solutions : the effect of solutes on the liquid-liquid critical point

Abstract

Water is the most abundant substance on Earth and in the human body. It is the medium in which most biological processes take place. Yet water is an anomalous substance. In fact from a physical point of view, its thermo- dynamic, structural and dynamic properties show peculiar behaviours when compared with those of simple liquids. The anomalies of water are par- ticularly pronounced in the supercooled region, the region in which water can be found in a metastable liquid state, below the freezing point. The experimental investigation of the supercooled region is hampered by the ho- mogeneous nucleation and computer simulations are the prime mean of study of this intriguing region. It has been previously hypothesized by computer simulations that water possesses in the supercooled region a liquid-liquid critical point (LLCP) located at the end of the coexistence line between a high density liquid (HDL) and a low density liquid (LDL) phase of water. These liquid phases would be the counterparts at higher temperature of the experimentally observed two forms of amorphous ice. It is often stated that the ultimate understanding of biology requires rst the comprehension of water. This is even more true for aqueous solutions. In fact aqueous solutions are ubiquitous in nature and their properties at low temperatures can be relevant for many biological phenomena. Furthermore the investigation of aqueous solutions can also help to unravel the mysteries of bulk water. In fact the presence of solutes a ect many thermodynamic properties of water. In particular aqueous solutions of electrolytes are more easily supercooled with respect to bulk water, because of a downward shift in temperature of the homogeneous nucleation in these systems. In this Ph. D. thesis we present the results of three sets of computer simulation studies performed on bulk water and on aqueous solutions, the PR-TIP4P set, the JJ-TIP4P set and the HS-JRP set. The rst two sets of simulations were performed using the standard molecular dynamics (MD) algorithm and they di er by the solute potential employed. The third set was simulated using the discrete molecular dynamics (DMD) algorithm. The PR-TIP4P set includes the simulations performed on bulk TIP4P water and on sodium chloride solutions, NaCl(aq), in TIP4P water with con- centrations c = 0:67; 1:36 and 2:10 mol/kg. In this set the Pettitt-Rossky ionic potential was employed. The analysis of the isotherms and the iso- chores of the systems allowed the determination of important thermody- namic loci such as the temperature of maximum density (TMD) line and the liquid-gas limit of mechanical stability (LG-LMS). These results were also compared with water con ned in a hydrophobic environment of soft spheres as studied in a previous work of our group. The LG-LMS line is not signi cantly altered by the addition of polar solutes, while the TMD line moves to lower temperatures and pressures with respect to bulk water, upon increasing the concentration of salt, with also a progressive narrowing of the amplitude of the curve. In the case of water con ned in the hydrophobic environment, both the LG-LMS line and the TMD line move to higher pres- sures due to excluded volume e ects and the TMD line also moves to lower temperatures, with respect to bulk water. In the NaCl(aq) solutions, indica- tions of the approach of the system to liquid-liquid coexistence are observed, with in ections of the low temperature isotherms and the appearance of min- ima in the behaviour of the potential energy as a function of the density. The structural properties of the c = 1:36 mol/kg NaCl(aq) solution were also studied and compared with those of water con ned in the hydrophobic environment. The hydration structure of ions (hydrophilic solutes) and soft spheres (hydrophobic solutes) are compared at di erent temperatures and densities. The JJ-TIP4P set of simulations consists in the extensive MD simulations performed on bulk TIP4P water and on c = 0:67 mol/kg NaCl(aq) solution aiming to the localization of the LLCP in both systems. For this set of simulations the Jensen-Jorgensen ionic potential was used. The LLCP was found in both systems. In bulk TIP4P water it is located at T = 190 K and P = 150 MPa, in the NaCl(aq) it is located at T = 200 K and P = -50 MPa. Therefore the LLCP is found to shift to higher temperatures and lower pressures in the aqueous solution. The results obtained for TIP4P bulk water are compared with available experimental data and a shift of the simulated phase diagram is proposed to match the experimental results. On the basis of the comparison of the simulated and the experimental TMD point at ambient pressure for NaCl(aq) we hypothesize that the same shift obtained for bulk water can be applied to the solution. With these considerations we propose the liquid-liquid phase diagrams of both bulk water and c = 0:67 mol/kg NaCl(aq) as they should be measurable in experiments. In the shifted phase diagram the LLCP falls at T = 221 K and P = 77 MPa in bulk water, close to the value previously hypothesized by Mishima and Stanley from experimental results on ice. In NaCl(aq) the LLCP falls at T = 231 K and P = -123 MPa, above the homogeneous nucleation line of the solution, making an experimental observation of the LLCP in the NaCl(aq) feasible. In the solution the width of the LDL region is reduced, in accord with experimental results that show an apparent favorable solvation of ions in HDL. We also investigated the structural properties of HDL and LDL both in bulk water and in NaCl(aq). We nd that the LDL phase is more a ected by the presence of ions with respect to the HDL phase. We propose a possible mechanism of disturbance of the LDL phase in terms of the substitution of the oxygen by the chloride ion in the oxygen coordination shells. The systems studied in the HS-JRP set are idealized models for solutions of hydrophobic solutes in water. The solutes were modeled with hard spheres (HS), while the solvent was modeled with the Jagla ramp potential (JRP) particles. The JRP is a spherically symmetric potential with two charac- teristic lengths and it is known to reproduce water anomalies. For this set of simulations the DMD algorithm was employed. We studied the mixtures with HS mole fractions xHS = 0:10; 0:15; 0:20 and 0:50. By the analysis of the isochores and the isotherms planes of the mixtures, the position of the LLCP was determined for all mixtures. The LLCP is found to shift to higher pressures and lower temperatures upon increasing the solute content. On the basis of a rotation of the phase diagram performed to match the sign of the slope of the liquid-liquid coexistence line of the bulk JRP particles to that of real water, we obtain a prediction for the direction of the shift of the LLCP in experiments on solutions of hydrophobic solutes. We expect the LLCP to move to higher pressures and higher temperatures with respect to bulk water. We also studied the di usive behaviour of the mixtures. The di usivity anomaly, indicated by the presence of extrema in the behaviour of the di usion coe cient at constant temperature as a function of the density, is found up to the highest HS mole fraction. For xHS = 0:10; 0:15 and 0:20 a crossover in the behaviour of the di usion coe cient at constant pressure, is also observed above the LLCP upon crossing the Widom line. The results obtained from the three sets of simulations show that the anomalies of bulk water and the LLCP are preserved in solutions of both polar and polar solutes, at least for concentrations from low to moderate. Thus the systematic study of the properties of aqueous solutions can be a novel route for the understanding of the many unsolved mysteries of water. In particular the position of the LLCP moves to higher temperatures with respect to bulk water in the solutions we studied, in the experimentally accessible region. With these results, the experimental measurement of the LLCP in aqueous solutions appears now possible.