Critical point and phase separation for an ionic system - The Journal of

May 1, 1985 - An ionic system with critical point at 44.degree.C. Rajiv R. Singh , Kenneth S. Pitzer. Journal of the American Chemical Society 1988 11...
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J. Phys. Chem. 1985,89, 1854-1855

place at a much lower temperature in our system. We report time-resolved luminescence measurements at 130 K, which is close to the glass formation temperature and the results are shown in Figure 2. At 150 K the short (20 ns) and long ( 5 11s) time-resolved spectra are only marginally different while at 110 K they do not significantly differ, having the same (blueshifted) profile. The change in spectral profile seen in Figure 2 indicates a process occurring in the microsecond regime at 130 K. Discussion The time-resolved luminescence spectra in Figure 2 provide clear evidence of a relaxation process which takes the luminescent excited state of R ~ ( b p y ) , ~from + a delocalized ( D 3 )description to one which is localized (C2). The rate of this process is too fast to observe with our equipment above 160 K and presumably involves a relaxation of the immediate environment of the complex ion in the excited state. We speculate that a movement of the charge-compensating anions into positions which favor the localization of electron charge on one ligand would occur. This process is somewhat different to the charge compensation process suggested by Hipps3 to account for the luminescence polarization measurements in a poly(methy1 methacrylate) environment. The compensation geometry would be. controlled by the ground-state

charge distribution which has little significant charge on the ligands. The charge-compensating anion could move much closer in the excited state, if allowed by the environment. The results in Figures 1 and 2 allow a reconciliation of the resonance Raman and MCPL measurements. Charge localization in the luminescent states occurs very quickly in low-viscosity solutions and the laser beam interrogates excited-state species ions which have relaxed and are thus localized. Excited-state Raman scattering12 in glassy and crystalline media substantiate our hypothesis and further indicate that the excited state may be strongly Jahn-Teller active. We also note that solution Raman measurements with lasers of much shorter pulse length would be able to determine the dynamics of the localization process, but recently reported measurements point to a process occurring in less than 25 ps.13 Acknowledgment. M.M. gratefully acknowledges the support of the Swiss National Science Foundation through the award of a Postdoctoral Fellowship. (12) E. Krausz, Chem. Phys. Letr., submitted for publication. (13) L. A. Phillips, W. T. Borwn, S. P. Webb, S. W. Yeh, and J. H. Clark, International Chemical Congress of Pacific Basin Societies, Dec, 1984, abstract 05B50.

Critical Point and Phase Separatlon for an Ionic System Kenneth S. Pitzer,* M. Conceicao P. de Lima,+ and Donald R. Schreiber Department of Chemistry and Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 (Received: February 8, 1985)

Liquid-liquid phase separation and a critical point have been observed for a system involving a fused salt. Specifically the system is tetra-n-butylammonium picrate-1-chloroheptane for which T, = 414.4 K, x, = 0.085 mole fraction picrate, and V, = 2300 cm3-mol-' picrate. The critical mole fraction of picrate is very small, and the phase diagram is very asymmetric. The critical exponent shows the mean field value of 0.50 with no detectable departure near the critical point, although the present experiments were not designed for a very precise test of the last topic.

Introduction

Although liquid-liquid phase separation and a critical point have been reported for many two-component systems, none to our knowledge involves a fused salt. We report here the phase equilibria and critical point for the system tetra-n-butylammonium picrate-1-chloroheptane. This picrate melts at 91 OC, and its conductance-viscosity product' clearly indicates fused-salt character. Thus, we are confident that the picrate-rich phase is ionic in this two-component system. There are other highly ionic systems showing two-component critical behavior. These are the supercritical aqueous systems with NaCl and similar salts, for example, NaCl/H20 at 973 K and 1237 bar.2 It seemed of interest to seek essentially ionic systems showing critical behavior under less extreme conditions. Tetra-n-butylammonium picrate was chosen because several of its properties are known1g3and because the ionic charge is either well buried in the tetra-n-butylammonium ion or well distributed in the picrate. 'Permanent address: Department of Chemistry, University of Coimbra, 3000 Coimbra, Portugal.

0022-3654/85/2089-1854$01 .SO10

Thus, the salt should behave as a relatively simple 1-1 electrolyte with a large interionic d i ~ t a n c eapproximately ,~ 7 A. Then the corresponding-states argument discussed recently4 indicates that the dielectric constant times temperature (DT)product should be roughly 1500 K for the critical point of this picrate in a polar solvent. Exploratory experiments with several liquids boiling well above 100 O C and with dielectric constants in the range 3-5 resulted in finding critical behavior for 1-chloroheptane. Results

The phase equilibrium measurements are given in Table I and shown in Figure 1. This figure shows a conventional representation as a function of mole fraction. It shows extreme asymmetry even considering the rather large ratio of molal volumes. Also, solid solubility measurements were made to complete the phase diagram. The critical properties found for this picrate ~

(1) Seward, R. P. J. Am. Chem. SOC.1951, 73, 515-17. (2) Sourirajan, S.; Kennedy, G. C. Am. J. Sci. 1962, 260, 115-41. (3) Pitzer, K. S.; Simonson, J. M. J. Am. Chem. Soc. 1984,106, 1973-77. (4) Pitzer, K. S . J. Phys. Chem. 1984, 88, 2689-91.

0 1985 American Chemical Society

The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 1855

Letters TABLE I: Liquid-Liquid Phase Boundary for Tetra-n -Butvlammonium Picrate- 1-ChlorobeDtaae

molar wncn 0.06 0.11 0.16 0.21 0.23 0.33 0.39 0.46

OC 72.0 98.5 116.8 129.0 129.8 134.0 139.8 141.2 138.2 133.5 133.0 116.7 99.0 60.8 1,

XP

0.0099 0.0183 0.0285 0.0381 0.0414 0.0654 0.0692 0.0930 0.1066 0.1274 0.1283 0.1645 0.1879 0.233 I

I

I

1

Figure 2. Logarithmic plot to show the critical exponent 8. The line

0.50

shows @ =

0.61 0.68 0.73 0.90 0.95

I

0.2

log [ ( T c - T ) / T c ]

I

0.6

0.4

I

I

0.0

I

I .o

XI

Figure 1. The equilibrium liquid-phase mole fraction of picrate for the system tetra-n-butylammonium picrate-1-chloroheptane.

system are T, = 414.4 K (141.2 “C), V, = 2300 cm3.rnol-’, and x, = 0.085 mole fraction picrate. One important feature is the shape of the phase boundary near the critical point. It is customary to consider the relationship (XI’’

- XI’) = B[(T,

- T)/T,]@

(1)

where xl” and xl’ are the mole fractions of picrate in equilibrium at T and j3 and B are constants. It is the exponent j3 which is of particular interest. Near the critical point and for short-range forces the value 0.325 is expected. But the value 0.50 is obtained from various theories which should be valid when the range of interparticle forces is longer than the range of fluctuations. In view of the long-range character of ionic forces, one expects the value j3 = 0.50 to be valid relatively close to the critical point, and Figure 2 shows that this is the case for the present system. In the range above 110 OC,the curve in Figure 1 corresponds to the equation

-

(XI” XI’)

= 0.028(141.2 - t)’/’

(2)

with t the temperature in OC. The curve shows considerable asymmetry as noted above. The present experiments were not designed for precise measurements very close to the critical point. Thus, a range of uncertainty is indicated for two points in Figure 2. If eq 1 is fitted to the full set of data, the extreme range for j3 is 0.46-0.60 which yields an unambiguous choice of 0.50 between the theoretical

alternates. A more pertinent question, however, is the possibility of a shift in slope such as was observed5 for the system Na/NH3 at (T,- T)/T, = 0.01. In that case j3 is 1 / 2 in the range further from T, and 0.34 closer to T,. For the present system such a change of slope could occur at (T, - T)/T, I0.005 without exceeding the estimated uncertainty. But the probable values for all of the points close to T,indicate continuance of the slope 0.50. It is also interesting to compare this system with the strongly ionic pure NaCl on a corresponding-states basis! At present this can be done only on a qualitative or semiquantitative basis, and the behavior is intermediate between that of pure NaCl and that of a nonelectrolyte. This is expected since the relative role of nonionic forces as compared to ionic forces is presumably larger in the present system than in pure NaCl. We expect to make further measurements on this and other ionic systems showing critical behavior and then to discuss various corresponding-states comparisons.

Experimental Details Tetra-n-butylammonium picrate was prepared by neutralizing tetra-n-butylammonium hydroxide with picric acid. The picrate salt was purified by recrystallization from ethanol and dried in a vacuum desiccator. 1-Chloroheptane (99.8%) from Aldrich Chemical Co. was used without further purification. The desired amount of each substance was added to a graduated glass cell fitted with a Teflon valve. After degassing by evacuation at 0 OC, the cell was placed in a silicone oil bath. The temperature was regulated hO.1 OC by varying the rate of heating. Stirring of the solution was accomplished magnetically by movement of two small Teflon-coated magnetic bars inside the cell. Measurements were made to determine the temperature at which the meniscus disappeared by movement off one end of the sample or, at the critical composition, by disappearance midway in the tube. Volumes were also measured. Equilibrium, which was sluggish, was checked by approach from both higher and lower temperature. Acknowledgment. We thank Joyce Olsen for preparation of the picrate salt. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Divisions of Chemical Sciences and Material Sciences of the U.S.Department of Energy, under Contract No. DE-AC03-76SF00098. Registry No. Tetra-n-butylammonium picrate, 914-45-4; l-chloroheptane, 629-06-1. ( 5 ) Chieux, P.;Sienko, M. J. J. Chem. Phys. 1970, 53, 566-70.