Fluid Phase Equilibria, 83 (1993) 77-34 Elsevier Science Publishers B.V., Amsterdam

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STRUCTURE OF AQUEOUS SOLUTIONS OF AMPHIPHILIES: t-BUTYL ALCOHOL AND UREA SOLUTIONS

Hideki Tanaka and Koichiro Nakanishi

Department of Industrial Chemistry, Faculty of Engineering Kyoto University, Kyoto 606 (Japan)

Keywords: t-butyl alcohol, urea, molecular dynamics, simulation

ABSTRACT

Molecular dynamic simulations have been carried out for aqueous solutions of t-butylalcohol (TBA) and urea. TBA molecules associate with each other at very dilute to 8 mol% TBA concentration by the contact of hydrophobic groups. The association of TBA molecules at 17 mol% TBA solution arises from hydrogen bondings between TBA molecules as well as the association of urea molecules at 17 mol%.

INTRODUCTION

Hydrophobic solutes such as noble gases or hydrocarbons exhibit exother- mic hydration, although the interaction between water and solute molecules is extremely weak compared with that of water dimer. The hydration process is also accompanied by the negative entropy in excess quantity, resulting in the large positive excess free energy. This hydration is observed only when a hydrophobic solute or a solute having at least one hydrophobic group is dissolved in water and is referred to as hydrophobic hydration. This unfavor- able excess free energy is partially compensated by a self-association of solutes forcing hydrophobic groups to contact. This is called hydrophobic interaction. It is believed that the self-association due to hydrophobic interaction is not restricted to nonpolar solutes, but this interaction plays a dominant role in aqueous solutions of alcohols and amines. The hydrophobic effects are mani- fested in various anomalous properties[Franks, 19731. Among such anomalies, the presence of characteristic minimum in the partial molar volume of t-butyl alcohol (TBA) 19601.

in dilute aqueous solutions is a typical example[Nakanishi,

Urea shows a quite different characteristics in its hydration process. Sev- eral models have been proposed to account for the observed thermodynamic quantities. There is no symptom that a urea molecule acts as a breaker of the original water structure by two computer simulation studies where different urea-water interaction models are adopted[Tanaka et. al, 1984a; Kuharski and Rossky, 19841. The hydration properties in dilute urea solution indicate that the solution can be regarded as almost ideal solution whose components have quite different melting points with each other.

Thermodynamic properties for aqueous solution of urea at higher concentra- tion were explained in terms of self-association of urea molecules [Kreshek and

03783812/93/$06.00 01993 Elsevier Science Publishers B.V. All rights reserved

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Scheraga, 1965; Stokes, 19671. Th ese systems are also investigated by at least two groups by means of MD simulation. The results are qualitatively different with each other. It was concluded [Tanaka et. al, 1985a] that urea molecules tend to self-associate with each other. In the other study [Cristinziano et. al, 19871, a radically different conclusion is obtained: urea molecules cannot as- sociate. The difference in the degree of self-association seems to arise mainly from the difference in the urea-urea dimer interaction rather than the ensem- ble, the rigidity of urea molecule, or the period of simulation. The urea-urea interaction in the former study is based on the extensive use of ab initio MO calculation, while the latter potential is empirical one and is very weak com- pared with the former potential, judging from the figure depicting the time duration of urea-urea dimer; at most -23 kJ mol-‘. A strong interaction for urea dimer should be expected because of the high melting point of pure urea. Furthermore, the latter simulation contradicts the experimental observation such as the sign and the magnitude of the second virial coefficients of urea in aqueous solutions[Franks, 19731. The free energy virial coefficients, which is negative, clearly show that the association of urea molecules are more fa- vorable than those isolated in water. The enthalpy virial coefficient is also negative. This means that hydrated urea molecules tend to associate through enthalpy dominant process.

MODEL

We have already reported the results of simulation of 3 mol% and 0.5 mol% (an infinitely dilute) aqueous solutions of TBA and 8 mol% and 0.5 mol% aqueous solutions of urea at 298.15 K[Nakanishi et. al, 1984; Tanaka et. al, 1984a,b 1985a]. N ew simulations have been carried out for aqueous solutions of 8 mol% and 17 mol% of TBA and for aqueous solution of 17mol% of urea as summarized in TABLE 1. Models and methods of computation are the same as those previous simulations. The water dimer potential is MCY potential [Matsuoka et. al, 19761. TBA-water[Nakanishi et. al, 19841, urea- water Tanaka et. al, 1984a], TBA dimer[Tanaka et. al, 1984b], and urea dimer Tanaka et. al, 1985a] potentials are those previously prepared. t

TABLE 1

Summary of Molecular dynamics simulations of aqueous solutions of TBA and urea at 298.15

K. N, NW, and N. are number of molecules, number of water molecules, number of solute

molecules, respectively. d is density of the system in g cmm3.

system TBA 8 mol% TBA 17 mol% urea 17 mol%

N 216 216 216

NlU 199 181 181

N, 17 35 35

d 0.9567 0.9125 1.1018

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RESULTS AND DISCUSSION

Potential energy values for three systems calculated are given in TABLE 2. The potential energies of water molecules for both TBA solutions are almost the same as that of pure water (-35.75 kJ mol-l). However, the interaction between water molecules is substantially lower for both TBA solutions com- pared with pure water. This lower potential energy among water molecules is a general observation for the NCY water in the hydration of hydrophobic solutes.

Time-independent configurations in solution structure can be expressed quantitatively by radial distribution functions (RDF). RDF’s for interaction sites on water, goo, goH, and QHH are shown in Fig. 1 for those in aqueous solutions of TBA and urea. The height of the first peak of each RDF shows a large increase in comparison with that for pure water or water in 3 mol% TBA solution. This is certainly a result of structure enhancement of water due to the hydrophobic hydration by the existence of TBA, but we should take ac- count of the fact that these higher first peaks are due partly to the tendency that water molecules associate with each other caused by TBA-TBA (also urea-urea) association and these RDF’s are normalized by the bulk (uniform) density.

TABLE 2

Thermodynamic properties for aqueous solutions of TBA and urea. U, VW, U, are potential

energy of the whole system, potential energy for water, potential energy for solute,

respectively. All the units are in kJ mol-‘.

system TBA 8 mol% TBA 17 mol% urea 17 mol%

u -35.44 -31.96 -40.67

GU -35.55 -35.33 -37.38

US -24.50 -14.54 -57.70

Trajectories over 10 ps for both 8 mol% TBA and urea solutions are given in Fig. 2. All the trajectories are almost isolated with occasional overlap during that period. Self-association of both TBA and urea is evident, though the degree of association is not the same. Of importance is the fact that the pattern of the association is quite different. In the case of TBA, the mode of association is largely of solvent separated type and water molecules adjacent to a TBA are firmly coordinated; some water molecules enter into small region between two TBA molecules. The hydration structures seem to be very rigid. On the other hand, self-association of urea is more extensive than that of TBA. The structure of water in urea solution is seen to be distributed rather uniformly over the entire system. No clear distinction between hydrated and free (bulk) water molecules can be observed.

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Fig. 1 Radial Distribution functions for water interaction sites in aqueous (a) 8 mol% TBA

solution, (b) 17 mol% TBA solution, (c) 17 mol% urea solution. solid line: in solution,

dashed line: in pure water.

The difference in the origin of self-association can be seen more qualitatively by the pair interaction distribution function (PIDF). Fig. 3 shows the PIDF for 8 mol% TBA solution, 17 mol% TBA solution, and 17 mol% urea solution. Those for 3 mol% TBA and 8 mol% urea solutions have already been reported. General characteristics of the PIDF’s for 3 and 8 mol% TBA solutions are essentially the same. One may find that the change in the PIDF for water pair is fairly large from 3 to 8 mol% solutions and it indicates a more pronounced peak in the hydrogen bonding energy region. However, the pronounced peak in the higher TBA concentration does not necessarily imply the structure enh~cement of water as discussed earlier.

Of particular interest is the PIDF for TBA-TBA pair. As is clearly seen in Fig. 3a, the TBA-TBA interaction stronger than-10 kJ mol-r is completely lacking in 8 mol% solution, This means that the self-association of TBA occurs only by the hydrophobic interaction. Fundamental characteristics of the TBA association seem to remain unchanged when the mole fraction increases from 3 to 8 mol%. The non-zero value of PIDF in the region between -10 to -20

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kJ mol-’ is an evidence of hydrogen bond between two TBA molecules in more concentrated solution such as 17 mol% solution as shown in Fig. 3b. We have already reported self-association of urea in 8 mol% aqueous solution. This situation does not change in the 17 mol% urea solution as shown in Fig. 3c. A large peak near -70 kJ mol-’ which can be regarded as an evidence for doubly hydrogen-bonded dimer. This is consistent with the information from the trajectories given in Fig. 2.

Fig. 2 Trajectory diagrams of aqueous (a) 8 mol% TBA solution, (b) 8 mol% urea solution

over 10 ps.

The hydration number of water molecules around a TBA has estimated to be

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about 20-28[Tanaka et. al, 1985131. Therefore, most of the water molecules are expected to take part in the hydration shell of TBA even in 8 mol% solution. The reason why this is not the case in 8 mol% solution comes from the fact that the self- association of TBA due to the hydrophobic interaction tends to share hydration sphere with each other and the number of water molecules required to construct the hydration shell can be smaller than that expected. Of some important conclusions given above, accommodation of increasingly large fraction of water molecules into the hydration sphere with increasing TBA concentration imposes a constraint on the motions of water, thereby decreasing in the self-diffusion coefficient of water in the solution. Fig. 4 shows the dependence of the mean square displacements of water and TBA in 8 mol% solution. The decrease in the self-diffusion coefficient of water is evident. In the urea solution, water molecules can move rather freely than those in TBA solution. Therefore, the diffusion coefficient of water molecule is not so different from that in pure water.

i ‘\ i

. . . L.

-40 -20

v/kJmol-’

Fig. 3 Pair interaction distribution functions in aqueous (a) 8 mol% TBA solution, (b) 17

mol% TBA solution, (c) 17 mol% urea solution. Dash-doted line: solute-solute, solid line:

solute-water, dashed line: water-water, dotted line: pure water.

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There are many anomalous behaviors in physical properties of dilute aque- ous solutions of TBA and these are important bearing on the nature of the hydrophobic hydration and interactions. The anomalies appear in the region between 3 to 5 mol% solutions where the partial molar volume of TBA exhibit distinct minimum. In dilute solutions, TBA solution consists of two different regions, namely, free (bulk) water region and overlapped hydration sphere of TBA, owing to the hydrophobic interaction of TBA. The free water region which can be seen in 3 mol% solution decreases gradually with increasing TBA fraction and is expected to disappear at 17 mol% solution. The anomaly is undoubtedly related to the hydrophobic effects by which a rigid hydration shell around TBA is constructed. The disappearance of the anomalies arising in more concentrated solution may have something to do with the disruption of the hydration shell caused by the contact of hydrophilic groups in TBA molecules.

Fig. 4 Mean square displacements of aqueous 8 mol% TBA solution. solid line: TBA, dashed

line: water in solution, dotted line: pure water.

CONCLUSION

It is revealed that TBA molecules still associate with each other in aqueous solution at higher concentration of solute such as 8 mol% to 17 mol%. The association of urea molecules is also observed in the same concentration re- gion in aqueous media. While in TBA solution of 8 mol% aqueous solution the association of TBA molecules takes place by the contact of hydrophobic groups, hydrogen bondings between TBA molecules can be seen in 17 mol% solution. There is no difference between 8 mol% and 17 mol% solutions of urea; urea molecules associate with each other due to the strong interaction.

It should be noted that the results for solution at 3-8 mol% TBA concen- tration can be explained in terms of hydrophobic effects. In the aqueous urea solutions, the association of urea molecules is compatible with those observed in the experiment: the virial coefficients both for free energy and enthalpy indicate that the association of urea molecules is favorable on account of the interaction between urea molecules.

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ACKNOWLEDGMENT

The authors thank the Computer Center, Institute for Molecular Science, for the use of computer.

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