Simulation and Spectroscopy of Solvation in Water from Ambient to

Chapter 5

Simulation and Spectroscopy of Solvation in Water from Ambient to Supercritical Conditions 1

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Keith P. Johnston , Perla Β. Balbuena , Tao Xiang , and Peter J. Rossky 2

Downloaded by UNIV OF ARIZONA on June 6, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0608.ch005

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Department of Chemical Engineering and Department of Chemistry and Biochemistry, University of Texas, Austin, TX 78712 A review is presented of recent computer simulation and UV-visible spectroscopic studies of the solvation of nonelectrolytes and electrolytes in ambient and supercritical water. In addition, new simulation and fluorescence results are presented to focus on subcritical water solutions at temperatures from 298 to 573 Κ at densities on the coexistence curve of pure liquid water. The solvation of Cl is examined with molecular dynamics computer simulation and that of pyrene is probed with fluorescence spectroscopy. ForCl there is little loss in the coordination number from ambient to 523 K; this behavior continues up to the critical temperature (647.13 K) due to the strong attractive nature of the chloride-water interaction. Furthermore the negative heat and entropy of hydration are relatively constant up to about 573 K. In contrast, the solvation of pyrene decreases significantly with temperature. For nonelectrolytes such as Xe, the enthalpy of hydration changes from exothermic to endothermic as the coordination number decreases. The corresponding entropy changes from negative to positive as the hydrophobic effect dissipates, and the Gibbs free energy goes through a maximum. -

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To develop hydrothermal technology, a better understanding is needed of molecular interactions in high temperature aqueous solutions and how they influence thermodynamic properties and chemical kinetics. Although a number of simulation studies have investigated ambient water (AW) (i.e. 298 Κ and 1 bar) and supercritical water (SCW) (T = 647.13 K, p = 0.322 g/cc, P =220 bar) solutions, far less attention has been given to subcritical liquid water in the interesting temperature regime of 373 to 573 Κ where the structure changes so dramatically. In many cases, organics are orders of magnitude more soluble in liquid water at 573 Κ than 298 K. Thus subcritical water may be used as a substitute for organic solvents without the need to go all the way to supercritical temperatures and pressures. The objective of the new simulation and spectroscopic studies reported in this work is to examine changes in solvation of nonelectrolytes and electrolytes in liquid subcritical water from 298 to 647 K. In order to put the new studies into c

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0097-6156/95/0608-0077$12.00/0 © 1995 American Chemical Society In Innovations in Supercritical Fluids; Hutchenson, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

78

INNOVATIONS IN SUPERCRITICAL FLUIDS

perspective, the following section provides a review of the current state of the art for simulations of pure water and dilute solutions of nonelectrolytes and electrolytes. Thermodynamic properties of hydration are analyzed in terms of solvation structure over this wide range in temperature. The next section is a review of very recent UV-vis spectroscopic studies of organic solutes. After providing these reviews of simulation and spectroscopy, we present a new experimental study of solvation of pyrene in sub- and supercritical water by fluorescence spectroscopy. In the final section, we examine chloride solvation with molecular dynamics simulation at subcritical temperatures not considered previously. The conclusions section synthesizes the results for solvation of nonelectrolytes and electrolytes from both simulation and spectroscopy.


Review of Computer Simulation Studies. Pure Water. As liquid water is heated from 298 to 673 K, the hydrogen bonded tetrahedral structure dissipates. Based on simulations with the SPCE (Simple Point Charge Extended) model (1), Guissani and Guillot (2) observed that the coordination number of pure saturated liquid water (Figure 1) increases with temperature from a value of about 4.5 at 300 Κ to a maximum value of 5.9 around 473 Κ and then decreases towards the critical point. The tetrahedral structure of liquid water causes the coordination number to be about 4.5 at ambient conditions, instead of a value of 12 for a typical Lennard-Jones liquid. Furthermore, molecules beyond the first shell also participate in a very large connected Η-bond network. As the temperature is increased, the Η-bond network gradually collapses, and molecules in the first shell become much more free to reorient, leaving room for other water molecules. This collapse increases the coordination number. At temperatures larger than about 500 K, the coordination number decreases due to the decrease in density. The most striking change in structure takes place between 373 and 473 Κ (2). Here the second peak in the oxygen-oxygen distribution function g shifts progressively from 4.5 to 5.5 Â. Furthermore, at 473 Κ there are very few angular correlations beyond the first shell of neighbors, indicating the extended hydrogen bond network is largely destroyed. Nevertheless, the remaining hydrogen bonds persist up to the critical temperature (3), as is also observed in predictions of a lattice fluid hydrogen bonding model (4), which utilized energy, entropy, and volume of hydrogen bonding parameters obtained from spectroscopic data at subcritical conditions (5). There is some controversy regarding the degree of hydrogen bonding in SCW. Recent neutron diffraction studies (6,7) indicate that the degree of hydrogen bonding in pure SCW is lower than the values from simulations based on rigid models (such as SPC (Simple Point Charge)(#J and SPCE). This disagreement was confirmed by new simulations with the SPC model(9) done at conditions matched to the experiments. However, significant challenges are present in interpreting the above neutron diffraction data (9JO). The neutron diffraction results agree quantitatively with ab-initio computer simulation^0) for the total structure factor, but the agreement for the spatial radial distribution functions is substantially less satisfying. Further support for an SPC-like model even in SCW comes from the fact that the ab-initio simulated molecular dipole moment is close to that of the SPC model rather than to the gas phase result. The persistence of significant hydrogen bonding among water molecules would not be surprising considering that the hydrogen bond energy considerably exceeds kT even at the critical temperature. Furthermore, previous and new experimental results by Gorbaty et al. (11-13) using IR spectroscopy and X-ray diffraction techniques indicate the presence of hydrogen bonds in supercritical water at temperatures up to 800 Κ . Thus the degree of 0 0

In Innovations in Supercritical Fluids; Hutchenson, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

5.

Simulation and Spectroscopy of Solvation in79 Water

JOHNSTON ET AL.


hydrogen bonding in pure SCW is an open question and represents an interesting theoretical and experimental challenge. Nonelectrolytes in Water. For non-polar solutes, the mechanism of hydration to explain the minimum in the solubility of rare gases and other small solutes in saturated liquid water versus temperature has been proposed by several authors (14,15). For an apolar solute such as a rare gas at low temperatures, the solvation structure is characteristic of so-called hydrophobic hydration. The hydrogen bonding groups straddle the nonpolar surface in order to maintain the hydrogen bonding available to the bulk water. This highly oriented solvent structure resembles clathrate hydrate structures locally. As shown in Figure 1, at ambient conditions the coordination number is much higher for water about Ne than about water. In contrast with the behavior of pure water, there is a pronounced decrease in the coordination number of these hydrophobic solutes as water densities are reduced and temperatures increased along the liquid branch of the coexistence curve. This observation is the first entry in Table I, which will serve as a summary of all the important results throughout this study. Guillot and Guissani (16) simulated the solvation of hydrophobic solutes. At low temperatures, a polyhedral cage of water molecules is formed around a hydrophobic solute that is favored by the highly connected Η-bond network. At temperatures higher than about 400 K, for liquid densities on the coexistence curve, the decrease in density favors the occurrence of cavities in the Η-bond network that increases the solubility. Table I. Effect of temperature on the hydration of solutes in the subcritical region from 300 to 573 Κ along the saturation curve. Property

Nonelectrolyte

Electrolyte

Coord, number

large decrease

very small decrease

ΔΗ

- to +

- (small change)

-TAS

+ to -

+ (small change)

AG

(maximum)

- (small change)

To illustrate these concepts we show in Figure 2 computer simulation (16) and experimental data (17,18) for thermodynamic properties of hydration of Xe in liquid water on the coexistence curve for the SPCE model. Extensive discussions about the choice of an appropriate reference state have been presented (19,20). Here we use the excess Gibbs free energy A G defined as the change in free energy in transferring the solute from pure ideal gas state to a state at infinité dilution in the solvent at the same temperature Τ and pressure P. It is related to the solubility of the gas in the liquid through kH, the Henry's constant, by 0

AG° = RT\nk

H

where k =\im H

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^

(1)

(2)

and f2 and X2 are the fugacity and liquid mole fraction of the solute, respectively.



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ε

s β s ce


a

s* ο ο 200

300

400

500

Τ

600

700

(Κ)

Figure 1. Variation of the coordination number of pure water and Xe and Ne at infinite dilution in liquid SPCE water at densities on the coexistence curve according to MD simulation data (2,16). 15.0

10.0

5.0

'3D

0.0

S -5.0

-10.0

200

300

400 Τ

500

600

(Κ)

Figure 2. Thermodynamic properties for Xe in water (same densities as Figure 1). Continuum curves are computer simulation data from Ref. 16. Points are experimental data from Refs. 17 and 18. In Innovations in Supercritical Fluids; Hutchenson, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

5. JOHNSTON ET AL.

Simulation and Spectroscopy of Solvation in Water

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The positive AG near ambient conditions is caused by ordering of the water, as is evident in the negative AS°(21) while the energy of hydration is slightly exothermic . As the temperature is increased, the tetrahedral extended hydrogen bonding network of water is disrupted, so that there is less tendency for water to orient itself about the solute. Consequently A S decreases in magnitude and becomes positive. Also, A H becomes smaller in magnitude, until it changes sign and becomes endothermic, reflecting the energy needed to create a cavity in the solution. Here the contribution of the attractive solute-solvent interactions becomes progressively smaller relative to the repulsive forces. Since at low temperatures A H is found to change more rapidly than -TAS°, the sum, A G , increases with temperature passing through a maximum (that corresponds to a minimum in gas solubility) at temperatures near 400 Κ ( Table I). Afterwards A G decreases causing the solubility to increase reaching values higher than at ambient conditions. 0

0

0

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Electrolytes in Water. A systematic investigation of the qualitative behavior of solutes of different charge at infinite dilution in ambient water has been carried out by Geiger (22). The nature of the structure breaking effect has been investigated using molecular dynamics (MD). Static and dynamic properties have been compared for the interactions of water with otherwise equivalent spherical solutes carrying charges ranging from 0 (non-polar, Xe type) to +2 (several cations) and -1 (model anion). An increase in charge causes the first peak in the solute-oxygen pair distribution function (designated as gioW) to become much higher and more narrow reflecting the growth in structure in the first solvation shell due to the increasingly attractive electrostatic interaction. Also, an increment of about 30 Κ does not change the first peak in gio(r) for the case of a monovalent cation, although the structure beyond the first shell is flattened out. For a given charge, the first peak of gio(r) is comparable for an anion and a cation, while more structure is found in the second shell of the anion. The effect of solutes on water structure (for example on goo( )) has also been analyzed. The addition of a nonelectrolyte increases the first peak of goo(r) slightly, while ions partially destroy the water structure. A similar loss in structure occurs in pure water with an increase in temperature. Recently, MD simulations have been used to determine the equilibrium structure of water molecules about N a and CI" ions at ambient conditions and at two states near the critical point (23,24). The SPC model was used for water. It has been found that the local density of water in the first solvation shell of the ions decreases only slightly from AW to SCW conditions. The first measurements of ion hydration in SCW have been reported recently (25) using Xray absorption fine structure. Radial distribution functions have been determined for 0.2M solutions of Sr++ in SCW, and also for solutions of Kr in SCW. The decrease in coordination number from AW to SCW for S r is much larger than that for CI" in simulations. Because of this profound difference in desolvation behavior, further work is required to understand ion solvation as was the case above for pure water. The system NaCl at infinite dilution in water has been the subject of several publications (26,27). In particular, Cui and Harris (26) studied this ion pair system over a wide range of temperature and density, and also considered ion association. At 800 Κ and a density of 0.083 g/cm^, the ion association is driven by a gain in the entropie contribution to the potential of mean force (pmf) due to the loss of électrostriction upon ion pairing. The energetics oppose ion pairing as water strongly solvates the isolated ions. The results follow the same trends by continuum theory (28) at ambient conditions, but the actual values are much larger r

+

+ +


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Simulation and Spectroscopy of Solvation in Water from Ambient to

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