J. Phys. Chem. 1991,95, 10777-10781
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Vitrified Dilute Aqueous Solutions. 4. Effects of Electrolytes and Polyhydric Alcohols on the Glass Transition Features of Hyperquenched Aqueous Solutions Klaus Hofer, Georg Astl, Erwin Mayer,* Institut fur Anorganische und Analytische Chemie, Universitat Innsbruck, A-6020 Innsbruck, Austria
and G. P. Johari Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario U S 4L7,Canada (Received: April 16, 1991; In Final Form: July 18, 1991)
Aqueous solutions of LiBr, LiI, NaC1, NaI, KBr, CsC1, MgC12,tetra-n-butylammoniumbromide, propylene glycol, and glycerol (0.2-7 mol %) have been vitrified by hyperquenching, and their glass-liquid transition temperatures ( Tg)have been measured by differential scanning calorimetry. All solutions show similar patterns in their Tg and increase in the heat capacity at Tg.T, decreases on the initial addition of the solute, reaches a minimum value, and then increases. The concentration at the Tgminimum is characteristic of the solute. The first effect of solutes, which lowers Tg,is weakening of the H-bonded network, and the second, which raises Tg,is related to the hydration of ions, stability of ion pairs, H bonding with solutes, hydrophobic interactions, etc. The two dominate at different concentrations,and their compensation at a certain concentration produces the minimum. This Occurrence is sufficiently general to conclude that explanations for the behavior of aqueous solutions should be sought in the changes in the nature of interactions between the water molecules and the solutes on hyperquenching, as the density of water decreuses on cooling toward vitrification (the expansion coefficient of water is negative in the supercooled state) and not in the specific details of ionic charge, size, and molecular shapes.
Introduction We have recently reported1 that the initial addition of a solute lowers the glass transition temperature (T,) of water, an effect which is similar to the plasticization of organic polymers2 and the lowering of Tgof Si02,3a closely similar network structure glass, when 'flux" ions, e.g., Li+, Na', and K+, are added to it. As the concentration of the solute is increased, a second process, which has the opposite effect of increasing the viscosity, acquires a predominant role such that after a minimum value is reached, T of the solution begins to increase with increasing concentration of the solute. The depth of the minimum, its width, and the concentration where it appears all vary with the nature of the solute. These studies underscored the significance of Tgas a useful probe for the perturbation of the H-bonded network of glassy water by a solute. As part of our investigations on vitrification and properties of vitrified aqueous solutions and to examine whether the Occurrence of Tg minimum is sufficiently general, we have prepared glassy aqueous solutions of a variety of different electrolytes and of polyhydric alcohols by hyperquenching and studied their glass transition temperature and its minimum. The latter solutes were investigated because of their importance as cryoprotectants. This paper reports the study and its discussion in terms of the H-bonded network structure of hyperquenched glassy water (HGW) and shows that the Occurrenceof a Tgminimum is sufficiently general and has its origins in the remarkably different water-solute interactions in the deeply supercooled state of aqueous solutions during hyperquenching and not in the thermodynamic nonideality observed at ambient temperatures. Experimental Section Our procedure for obtaining vitrified aqueous solutions has been described in several report^.^^^^^ In brief, droplets of a solution suspended as an aerosol are allowed to enter, with nitrogen as a carrier gas, through a 200-pm-diameter opening into an assembly under high vacuum, accelerated by supersonic flow and deposited on a copper plate maintained a t about 77 K. Solutions of less (1) Hofer, K.; Hallbrucker, A.; Mayer, E.;Johari, G. P. J. Phys. Chem. 1989, 93, 4674. (2) Ferry, J. D.Viscoelaslic Properties of Polymers; Wiley: New York, 1980. (3) Kreidl, N. J. In Glass: Science and Technology; Uhlmann, D. R., Kreidl, N. J., Eds.; Academic: New York, 1983; Vol. I, Chapter 3, p 137. (4) Mayer, E.J. Appl. Phys. 1985.58.663: J. Phys. Chem. 1985,89,3474. (5) Hallbrucker, A.; Mayer, E. J. Phys. Chem. 1987, 91, 503; 1988, 92, 2007.
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than 15% (w/w) were nebulized to an aerosol by an ultrasonic nebulizer operating at 3 M H z (EngstrBm, Model NB 108) and more concentrated solutions by a retouching air brush (Grafo Type I) or a pneumatic nebulizer (Pari, Model 360). All reagents used as solutes were of p.A. quality. The hyperquenched deposits were examined for crystalline ice, which could have formed, by X-ray diffraction measurements.'v5 A Perkin-Elmer DSC-4 instrument was used for differential scanning calorimetry of the vitrified samples. Stainless steel sample pans with screw-on covers were used for containing the samples, and all transfers from the copper plate to sample pans were made when the sample was immersed in liquid nitrogen, as described ear1ier.I~~The base line measured with empty sample pans was deducted from the scans obtained for each sample. Cyclohexane and n-heptane (Merck, Uvasol quality) were used for calibration of the temperature and n-octane (Fluka) for determining the thermal lag. Since evaporation of water from droplets in the aerosol can alter the concentration of the solution in the vitrified deposit from that in the solution, the solute concentration of the deposits was determined by comparing their melting endotherms with those of standard solution^.^ After this concentration was known, the mass of the sample was determined by a comparison of the areas of the melting endotherms. In all measurements a heating rate of 30 K m i d was used. Results Only parts of the DSC scans that are relevant to glass-liquid transition are shown in Figures 1-5. The data for LiCl and ethylene glycol (EG) solutions taken from our previous study1 are included in Figures 1 and 6-8 to allow comparison with the data for other solutions. All DSC scans were obtained by removing first the exothermic effects of (irreversible) enthalpy relaxation,i,6 which often masks the T, endotherm for hyperquenched pure water and aqueous solutions. This was done by annealing the vitrified solutions, for varying but optimal periods and at optimal temperatures, within the DSC instrument, as is described in the figure captions. Furthermore, as in previous studies of hyperquenched dilute solution glasses, the lowering of Tgof solutions required that the initial part of the scan be as flat as possible and the decrease in the height of the Tgendotherm required a greater instrumental sensitivity than usual. Both were achieved by careful manipulation of the DSC instrument. Thus,the DSC scans shown (6) Hallbrucker, A,; Mayer, E.;Johari, G. P. Philos. Mag. B 1989,60, 1 7 9 Nature 1987, 330, 552.
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mol %LiE
-4
w, 1-0
.,_..
\
t
0
--n
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0 2
+
21
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0.33 0 8
\
I-
1.8
30 2 8
0.92
\
\J,
5.3
120
140
I/
I
I
140
160
160
VK
Figure 1. DSC scans of hyperquenched LiCI-H20 (left, from ref 1) and
LiBr-H20(right) solution glasses, demonstrating the influence of solute concentration on the glass-liquid transition endotherm. Annealing temperatures (times): for all LiCl solution glasses, at 123 K (90 min); for LiBr solution glasses, 0.2 mol 9%at 125 K (90 min); others, at 123 K (90 min). Legend common to Figures 1-5: All samples had been annealed at the given temperatures (times), and thereafter cooled to 103 K at 30 K min-I and then rewarmed and scanned at 30 K m i d . The arrows mark the onset of the glass-liquid transition endotherm as determined by broken lines. The error in determining T8depends to some extent on the amount of solute because glassy solutionsnear the Te minimum often had only a very small ACp value at Tg,but it is estimated to be &2 K. Solute concentrations are given in mole percent, increasing from top to bottom. The scans are normalized with respect to sample weight and are drawn on the same scale. The uppermost scan is of pure glassy water from ref 6. An estimate for the dependence of AC, at Tgon solute concentration can be obtained from a comparison with that of HGW in the top scan which is -1.6 J K-' The temperature scale is not corrected for the thermal lag of the instrument.
1 - 4 1 ,
120
140
' 120
160
140
6K
160
Figure 3. DSC scans of hyperquenched NaI-H20 (left) and NaCI-H20
(right) solution glasses, demonstrating the influence of solute concentration on the glass-liquid transition endotherm. Annealing temperatures (times): for all NaI solution glasses, at 123 K (90 min); for NaCI solution glasses, 0.33 and 5.3 mol %at 128 K (90 min). 0.92 and 2.7 mol % at 126 K (90 min). See the legend to Figure 1.
I
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%CsCl
mol % MgCI,
0
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I!
mol %Lil
2.7
mol % KBr
0
0.4
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120
I
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120
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H 2 0(right) solution glasses, demonstrating the influence of solute concentration on the glass-liquid transition endotherm. Annealing temperatures (times): for all CsCl solution glasses, at 127 K (120 min); for MgC12solution glasses, 0.2 mol % at 121 K (90 min), 1.2 and 2.0 mol % ' at 123 K (120 min), 4.0 mol %at 125 K (120 min). See the legend to Figure 1.
2.3
1
160
Figure 4. DSC scans of hyperquenched CsC1-H20 (left) and MgCI2-
2.0
120
140
I! '
1
120
140
160
VK
Figure 2. DSC scans of hyperquenched LiI-H20 (left) and KBr-H20
(right) solution glasses, demonstrating the influence of solute concentration on the glass-liquid transition endotherm. Annealing temperatures (times): for LiI solution glasses, 0.4, 1.O, and 2.0 mol% at 126 K (150 min), 2.3 mol% at 130 K (90 min), 3.2 mol % at 128 K (105 min); for all KBr solution glasses, at 123 K (90 min). See the legend to Figure 1 in Figures 1-5 are ones obtained after several trials on different samples of the same stock and/or from different stocks.
The glass transition temperature (as a n operational quantity, it refers to a heating rate of 30 K m i d here) was determined by drawing tangents to the base line and endotherm parts of the DSC scans as shown by dashed lines in Figures 1-5, and the Te is marked by the arrow at the point of intersection of the two lines. In all figures a corresponding scan for pure water is included in order to draw attention toward changes in Tgon addition of the solutes. At temperatures of 10 K above Tg,all solutions showed a n exotherm indicating the onset of crystallization. The Tg values were obtained from the intersection of the (broken) lines as shown in Figures 1-5 and are plotted against the mole percent of solute in Figures 6-8. These plots also include additional values obtained in this study and not shown in Figures
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The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 10779
Effects on Glass Transition Features
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pT 1 -
120
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125 0.40
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T/K120
I \
0
LiCl
/*
I
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4
m o l % solute
I
6
F@re 7. Glass transition temperatures of hyperquenched aqueous LiCI, NaCI, CsCI, MgC12, and tetra-n-butylammonium bromide (n-BuABr) solution glasses plotted against the solute concentration (mol %), demonstrating the influence of cation. LiCl values are from ref 1. The T8s are corrected for the thermal lag of the instrument. The lines are drawn to aid the eye. ibo
160
T/K
Figure 5. DSC scans of hyperquenched aqueous solution glasses with tetra-n-butylammonium bromide (n-BuABr, left) and propylene glycol (1,2-propanediol, PG) and glycerol (GLY, right) as solutes, demonstrating the influence of solute concentrationon the glass-liquid transition endotherm. Annealing temperatures (times): for all n-BuABr solution glasses, at 123 K (90 min); for PG solution glasses, 0.2 mol % at 130 K (120 min), 1.9 mol % at 133 K (120 min), 4.3 mol % a t 138 K (60 min), 4.6 mol % at 146 K (120 min); for GLY solution glasses, 2.2 mol % at 125 K (120 min), 5.3 mol % at 130 K (120 min). See the legend to Figure 1.
150
120 Y
c
135
130
125
Figure 6. Glass transition temperatures of hyperquenched aqueous LiC1, LiBr, and LiI solution glasses plotted against the solute concentration (mol %), demonstrating the influence of anion size. LiCl values are from ref 1. The Tgsare corrected for the thermal lag of the instrument. The lines are drawn to aid the eye.
1-5, and all are corrected for the thermal lag of the instrument. The range of concentrations, which could be investigated, was limited by experimental problems in making aerosols of high-solute concentration because the nozzle of the nebulizer became plugged when using higher concentrations than those shown in the figures. The DSC scans in Figures 1-5, which are normalized with respect to the sample weight, indicate that the height of the T endotherm also goes through a minimum, as pointed out before.t But since only part of the endotherm is seen, the rest being masked by the onset of crystallization, this will not be discussed further. Nevertheless, an estimate for the increase in heat capacity, (AC,) at T, can be obtained by comparison of the normalized scans with that of hyperquenched glassy water (scan 1) whose AC, at T, is -1.6 J K-I mol-’.6
Discussion A general pattern of behavior of hyperquenched dilute solutions of both inorganic salts and polyhydric alcohols emerges from this study, namely that the measured T, decrease on the initial addition
I
0
2
4 mol% solute
Figure 8. Glass transition temperatures of hyperquenched aqueous ethylene glycol (EG), propylene glycol (PG), and glycerol (GLY) solution glasses plotted against the solute concentration (mol %). EG values are from ref 1. The Tgsare corrected for the thermal lag of the instrument. The lines are drawn to aid the eye.
of the electrolytes as well as the alcohols, reach a minimum value, and thereafter increase. The appearance of a minimum in Tgin dilute solutions of inorganic electrolytes and alcohols is apparently a phenomenon for which our studies have found no exceptions. In view of the variety of solutes and their different heats of solution, it seems that interactions between water molecules as a result of change in H bonding take on a more prominent role than the ionsolvent interactions to which departures from thermodynamic ideality of solution behavior have generally been attributed. T, is the temperature a t which configurational contributions from molecular degrees of freedom become detectable in the C, during the heating of a glass at a certain rate, as its viscosity decreases to a value near lOI3 P. The m i n i u m in the Tgobserved in Figures 1-8 means that, for a fixed temperature measurement, the viscosity of the solution initially decreases on the addition of a solute, reaches a minimum value, and then increases. Alternatively stated, the molecular diffusion time at a constant temperature first decreases on addition of a solute, reaches a minimum value, and then increases. It would be interesting to examine whether the TBvalues of the network structure of SiOz or GeOz also show similar minima in Tgwhen network breaking oxides are added. In an earlier paper’ we pointed out that the decrease in viscosity on initial addition of a solute is in accord with Lang et al.’~’,~ observations of spin-lattice relaxation time of electrolyte solutions at 238 K and 1 bar, which indicated an increase in the mobility of H20molecules and gradual removal of the anomalies in the properties of supercooled water with increasing amount of the solute. Their studies showed that this increase in mobility continues to concentrations much higher than those for which the
10780 The Journal of Physical Chemistry, Vol. 95, No. 26, 1991
Tgminimum has been observed in our study. In particular, the increase in mobility on the addition of LiCl continues up to 11 m concentration according to their studies.’ But, according to our studies here, the mobility reaches a maximum value (Le., a minimum in 7g) at a concentration seemingly characteristic of the solute, but at 14 mol% and thereafter decreases (or Tg increases). Thus, there seems to be conflict between the interpretations in terms of molecular mobility of spin-lattice relaxation time measurements at 238 K and Tgmeasurements. It seems that the Occurrence of a Tg minimum in aqueous solutions is a consequence of two effects, one predominating a t lower concentrations and the other at higher concentrations. The first effect decreases the viscosity and therefore Tgof water, and the second raises it. As these effects occur in solutions containing both monovalent and divalent cations of different sizes and polyhydric alcohols, we suggest that explanations for their Occurrence should transcend the details of ionic charge, size, nature of interactions, and molecular shapes. Although the effects may seem similar to those observed for viscosity and density of dilute aqueous solutions at ambient temperatures, the two are unrelated, as mentioned below, for the reason that the same solutes affect the properties of water at, e.g., 298 K differently and show no general pattern in their effects on the kinetic and thermodynamic properties of solutions. The differences between the effects of solutes on the structure of water at 298 K and at its Tg,we conclude, underscore the need for considering the possibility that water’s topological structure in a dilute solution continually changes during hyperquenching as its density decreases from 1.00 to -0.95 g cm-3 (ref 9) and that the two effects whose compensation at a characteristic concentration produces a minimum in Tgbecome important in the deeply supercooled water prior to its vitrification. In this context two other studies should be mentioned: First, from a neutron diffraction study, Dorelo has concluded that the structure of hyperquenched pure glassy water (HGW) is closely related to a fully H-bonded network with tetrahedral connectivity. Second, from molecular dynamics simulations, Geiger and co-workers” have concluded that as the density of water is decreased, it acquires the structure of a fully H-bonded tetrahedral network with strongly decreased molecular mobility. In light of these observations, the lowering of Tgof HGW and the increased mobility of the water molecules in dilute solutions may be seen as perturbation and breakup of HGW’s H-bonded tetrahedral network structure by the solute. If so, this Occurs in a concentration range characteristic of the solute, as can be seen in Figures 6 and 7 where the T,s of the solutions are plotted against the mole percent of lithium halides with varying anion size, and of chlorides with varying cation size, and in Figure 8 where Tgs of polyhydric alcohol solutions are plotted. Although the complexity of the effects involved and the error in our measurements do not permit a further detailed analysis, it is evident that there is a remarkable difference between the H20-H20 interaction in an aqueous solution in ambient conditions and in its deeply supercooled state, which needs to be understood, and its significance in c r y ~ f i x a t i o n ’ ~and , ~ cryopreservation of biological materials to be recognized. For binary solutions of a variety of substances including alcohols, exhaustive studies by LesikarI4 and Angell and Sare15have shown that Tgof a solution generally increases on addition of the high-T, (7) Lang, E. W.; Fink, W.; Radkowitsch, H.; Girlich, D. Ber. Bunsenges. Phys. Chem. 1990, 94, 342. (8) Lang, E. W.; Fink, W.; Liidemann, H. D. J . Phys. (Paris) 1984, 45, C7-173. (9) Mayer, E. Unpublished results. The density was measured by using a flotation method, involving a hydrometer, in a liquid N?/O? mixture. (10) Dore, J. C . J . Mol. Strucr. 1990, 237, 221. (1 1) Kowall, Th.; Mausbach, P.; Geiger, A. Ber. Bunsenges. Phys. Chem. 1990, 94, 279. (1 2) Bachmann, L.; Mayer, E. In Cryotechniques in Biological Electron Microscopy; Steinbrecht, R.A,, Zierold, K., Eds; Springer: Berlin, Heidelberg, New York 1987; Chapter I, p 22. ( 1 3) Plattner, H.; Bachmann, L. Int. Reu. Cyfol. 1982, 79, 237. (14) Lesikar, A. V . J . Phys. Chem. 1976,80, 1005; J . Chem. Phys. 1977, 66, 4263; 1978, 68, 3323. (IS) Angell, C. A,; Sare, E. J. J . Chem. Phys. 1970, 52, 1058.
Hofer et al. component and that the rate of increase depends upon the concentration of the solution. The latter produces a departure from linearity in the plots of Tgagainst concentration. This behavior, which was first regarded as an indication of association and complex formation,I4 has been later interpreted by Gordon et a1.16 and by LesikarI4 in terms of a thermodynamic theoryI6 for the glass transition according to which the configurational entropy of a liquid tends to become zero on its supercooling to a temperature known as T K ,the Kauzmann temperature,” which bears a fixed ratio to Tgin most cases (we have reservations on the use of terms “fragile and strong” for liquids which is based on limited data and their unacceptably large extrapolation). We first consider implications of this theory for the Tg minimum. In a “regular” or a nonideal solution, pairwise interaction between molecules differs from that in an ideal solution; Gordon et a1.I6 considered binary solutions as thermodynamically nonideal and derived the expression
where Tg(x)is the Tg of a solution of concentration x (mole fraction of component 2), Tgland Tgzare the Tgsof pure components 1 and 2, and AC,, and ACp2are the differences between the C of the liquid and the glassy phases at T, for components 1 a n 8 2 , respectively. For all cases where AC,, and ACt2 are constant, one expects to find a monotonic change in T, with x. For producing a minimum in Tg,it is required that AC,, and AC,, themselves becomes a function of x , which would validate the assumption but would conflict with the general notions of the configurational entropy implicit in eq 1. Whereas the phenomenological approach in terms of configurational is undoubtedly successful for most mixtures of polar, nonpolar, ionic, and H-bonded liquids, its limitations for dilute aqueous solutions are obvious from our present study as in our previous report.] Because the Tg minimum occurs at concentrations which are much lower than those required for the formation of known stoichiometric compounds between water and the various solutes, as was discussed in a previous paper,, the observations cannot be readily interpreted in terms of complex formation. Therefore, it becomes necessary that a molecular, albeit qualitative, approach to the interpretation be taken. The density of HGW measured by a flotation technique in liquid N2/02mixturesg is found to be 0.95 f 0.02 g ~ m - which ~ , is slightly higher than that of hexagonal ice at the same temperature and -5% lower than that of liquid H20at 298 K. This also means that the average volume of an H 2 0 molecule in H G W is -5% more than that in liquid H 2 0 at 298 K, if the structure of HGW is to consist of a random, tetrahedrally bonded network without loosely packed molecules, patches, or clusters of molecules, as a number of studies suggest,1°,20 It is therefore expected that the H bonds are on the average longer or energetically weaker in HGW than in liquid water, although details as to whether all water molecules are fully tetrahedrally H bonded or whether loosely bonded regions of some water molecules also exist in H G W are not certain from the diffraction studies of its structure. But, our recent observation of a sub- Tgrelaxation process, which appears as a shoulder in the plots of dielectric loss against temperature for a fixed frequency, suggests that loosely bonded structures of water molecules do exist in HGW.*’ Such structures not withstanding, it seems certain that vitrification of water does not occur as a result of a decrease in its volume, and by inference in the free volume, in the same manner as vitrification of most other (16) Gordon, J. M.; Rouse, G. B.; Gibbs, J. H.; Risen, W. M., Jr. J. Chem. Phvs. 1977. 66. 497 1 .
117) Kauzmann, W. Chem. Reu. 1948.43, 219 (18) Gibbs, J. H.; Di Marzio, E. A. J . Chem. Phys. 1958, 28, 373. (19) Gibbs, J. H. In Modern Aspects ofrhe Vitrous Stare; McKenzie, J . D., Ed.; Butterworth: London, 1960; Chapter 7 . (20) Hallbrucker, A.; Mayer, E.: Johari, G. P. J . Phys. Chem. 1989,93, 4986. (21) Johari, G. P.; Hallbrucker, A,; Mayer, E. Unpublished results.
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and therefore eo cannot be linearly scaled with 1/T. To our liquids, including S i 0 2 and possibly G e 0 2 , which are regarded knowledge to values of supercooled aqueous solutions are not as topologically similar in structure to water. Evidently, successful known, but those for water are known from the work of Bertolini application of the widely accepted free volume t h e o r i e ~of~ ~glass ,~~ et al.30 But these data also have too large a scatter to provide transition to water is also doubtful. That states of water with unambiguous fit of an equation to their temperature dependence. identical macroscopic densities and therefore with identical average Therefore, changes in the degree of ion association on supercooling molecular volume have enormously different viscosities (greater cannot be estimated this way. But, infrared spectroscopic data P for water at 384 K, at the than lOI4 P for H G W and do show a n increase in the degree of ion association in hypersame density of -0.95 g ~ m - is~ revealing ) in that density alone quenched dilute aqueous solutions of perchlorate and nitrate anions does not determine the dynamic properties of water, whereas for with Li+ and Na’ as cations as given in an earlier paper.j’ This most liquids it does. This argument and the suggestion of changes increased ion association and the effective removal of charge in the tetrahedral bonding of water on its supercooling seem to carriers on hyperquenching aqueous solutions has important imwarrant exclusion of procedures that rely on extrapolation of given plications for cryofixation of biological polyion so1utions.l2J3 spectroscopic data, or radial distribution function of glassy water for inferring the structure of water at room temperature, although Conclusions the procedure is generally seen as suitable for other liquids. Studies of aqueous solutions of monovalent and divalent cations Of greater relevance to our present study is an inference that and varying size anion electrolytes and of polyhydric alcohols show in the bulkier structure of H G W both the degree of ionic assothat their glass transition temperature ( T g )reaches a minimum ciation and the extent to which ions and polyhydric alcohols modify value with increasing concentration of the solute, at a concentration its H-bonded structure through ion solvation remarkably differ from that of water at 298 K. The degree of ionic a ~ s o c i a t i o n ~ ~ - ~ ’ that is characteristic of the solute. This effect on Tgof the addition of inorganic and organic solutes is similar to the plasticization is generally proportional to the exponential of (l/eoT) where eo of network structures of S i 0 2 and organic polymers. There is, is the static dielectric permittivity (or constant) of the solvent and in addition, a second effect which raises Tg.Both are expected T the temperature. For aqueous solutions, an increase in T is to persist a t all concentrations, and the minimum appears as a accompanied by a decrease in eo and, therefore, the degree of ion result of compensation of the two effects. It is suggested that association varies with temperature mainly when the product eoT explanations for the Occurrence of the minimum should transcend varies. At T > 298 K, this product can be nearly constant over the details of ionic charge, size, and molecular nature of the solute a wide temperature range, as had been shown by Gilkerson26for and that this minimum is more appropriately considered in terms 673 < T < 1073 K. But, for aqueous solutions in the deeply of a change in the nature of interaction between water molecules supercooled state, the network structure of H bonds in water differs and the solute as the unbonded or weakly bonded molecules in from that at 273 K, as is evident among others from the lower liquid water gradually become fully tetrahedrally bonded on its density of supercooled water, aqueous solution^,^^^^^ and HGW9, supercooling and ultimately produce a topologically disordered structure of lower density on vitrification. (22) Cohen, M. H.; Turnbull, D. J . Chem. Phys. 1959, 31, 1164. (23) Turnbull, D.; Cohen, M. H. J . Chem. Phys. 1961,34, 120; 1970,52, Acknowledgment. We are grateful for financial support by the 3038. Forschungsforderungsfondsof Austria. (24) Fuoss, R. M.; Onsager, L. J . Phys. Chem. 1957, 61, 668. Registry No. PG, 57-55-6; GLY, 56-81-5; LiBr, 7550-35-8; LiI, (25) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions; Butterworth: London, 1959. 10377-51-2;NaCI, 7647-14-5; NaI, 7681-82-5; KBr, 7758-02-3;CsC1, ( 2 6 ) Gilkerson, W. R. J . Phys. Chem. 1970, 74, 746. 7647-17-8; MgCl,, 7786-30-3; n-BuABr, 1643-19-2. (27) Petrucci, S . In Ionic Inreractions; Petrucci, S., Ed.; Academic: New ~
York, London, 1971; Vol. I, Chapter 3, p 142. (28) Angell, C. A. In Water, A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York 1982; Vol. 7, Chapter I . (29) Sorensen, C . M. J . Chem. Phys. 1983, 79, 1455.
(30) Bertolini, D.; Cassettari, M.; Salvetti, G. J . Chem. Phys. 1982, 76, 3285. (31) Mayer, E. J . Phys. Chem. 1986, 90, 4455.
Effects of Solvent Polarity and Temperature on the Conformational Statistics of a Simple Macrocyclic Polyether Yuk Lung Ha and Arup K. Chakraborty* Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720 (Received: April 23, 1991; In Final Form: August 12, 1991) In this work, we present NPT Monte Carlo simulations of the simplest macrocyclic polyether (18-crown-6) in water and carbon tetrachloride solutions at various temperatures. The three-dimensional structure of the ring polyether plays a significant role in determining its binding characteristics. Thus, we study the conformations adopted by the cyclic polyether in various distinct environments. We find that temperature and solvent polarity have an important influence on the crown structure. Our results show that several different conformations are populated in solution. However, the conformational statistics are such that the D3dstructure is the dominant conformation in water, and the Ci structure is the dominant one in CCl., solutions. We find that the effect of temperature on the conformational statistics is more pronounced in CC14 solutions. Additionally, we determine the structuring of solvent molecules around the crown and find indications of hydrogen bonding in the case of water. We compare our results with experiment and discuss the implications for ammonium cation binding.
I. Introduction Macrocyclic polyethers have k n the subject of much attention since their discovery by Pederson’ in 1967 and the subsequent *Author to whom all correspondence should be
addressed.
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efforts of several research groups.2-E Since these cyclic compounds and their derivatives exhibit remarkably high specificity in binding ( 1 ) Pederson, C. J. J . Am. Chem. Sot. 1967, 89, 7017. (2) Cram, D. J.; Cram, J. M. Science 1974, 183, 803.
0 1991 American Chemical Society
