NOTES
2413
of Tarasenkov and Afinogenovlo are too low in general. These last two sets nf data were not considered in the present calculation. The best straight line through the first four sets of data was drawn on a large-scale plot. Of the 71 data points for the liquid, 90% lie between two parage1 straight lines of slope 27.3 kcal/mole and separated by 0.2 unit on the 2' scale. The slope of the median line gives m " 2 9 8 (the standard enthalpy of sublimation a t 298°K). Limits of uncertainty were taken from the lines of largest and smallest slope that could be put between these parallel lines in the range of the data. The resulting value of A H " 2 9 8 was 27.3 f 0.2 kcal/mole. A S " 2 9 8 (the standard entropy of sublimation at 298°K) was taken from the value of 2' at the midtemperature of the data through eq 6. Its uncertainty was evaluated from that associated with AH and the experimental uncertainty in 2'. ASOZSS was found to be 43.7 f 0.4 eu. A least-squares treatment of the data was obtained from the institute's computation group. The values and their standard deviations by this treatment were A H " 2 9 8 = 27.20 f 0.07 kcal/mole and A S o z g s = 43.56 f 0.04 eu. The differences between these and the graphical values are small. They occur because the higher temperature points were weighted somewhat more heavily than the midrange points in the graphical evaluation. This author prefers the graphical method of evaluation of data from different experimenters because of the convenience in weighting the data visually. Therefore, the data derived graphically are considered the more reliable and are used below in preference to the least-squares data. For the solid there is only one set of vapor pressure data, those of Darnell and Yosimll by effusion. When 2' is evaluated by the above method, the values above and below the melting point should fall on the same straight line. In the high-temperahre region of their data, the values obtained by Darnell and Yosim fall close to the line defined by the liquid data; however, a t the low-temperature end their data fall below the line by as much as 15% in pressure. The enthalpy and entropy of sublimation derived from the solid data alone (29.8 kcal/mole and 49.3 eu, respectively) are substantially different from the values derived from the liquid. The difference is presumed by this author to be due to some systematic error in the measurements over the solid. The absolute entropy of BiC13(g)a t 298°K is 85.4 f 1.0 eu as calculated from molecular constants by Kelle~.~ Using A S " 2 9 8 , sublimation, of 43.7 0.4 eu from above, one calculates the 8'298 for solid BiC13 to be 41.7 f 1.5 eu. With this value one can calculate
*
the free energy function for BiC13 in the condensed phases by subtracting it from the fef incr of Table I. Values of fef are given in the last column of the table. The vapor pressure of BiCL can be calculated a t any temperature from (10). Thus R In p(atm) = 43.7 (27,3OO/T) - A fef incr, with values of A fef incr interpolated from the data in Table I.
-
(10) D. H. Tarasenkov and B. P. Aiinogenov, Zh. Fiz. Khim., 9, 889 (1937). (11) A. J. Darnell and S. J. Yosim, J . Phys. C h a . , 63, 1813 (1959).
Transition Energies for a Merocyanine Dye in Aqueous Electrolyte Solutions.
Solvent
Polarity Indicator Transition Energy-Internal Pressure Relations'
by John E. Gordon Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 0.9643 (Received January $6, 1966)
Numerous compounds are known whose electronic transition energies, ET, are more or less sensitive functions of the polarity of the solvent in which they are dissolved.2-4 That these transition energies measure a "microscopic" solvent polarity (in contrast to macroscopic" measures such as the dielectric constant) and correlate better with chemical parameters representing solvent ionizing power, e.g., the WinsteinGrunwald Y values, than with functions of dielectric constant, e.g., (D - 1)/(20 l), has been amply disc~ssed.z-~It is, however, instructive to extend these comparisons to include one further macroscopic quantity, namely, the cohesive energy density or internal = energy of pressure of the solvent, AE,/V (av vaporization, V = molar volume). Some such comparisons are made in Figures 1-3. Figure 1 employs transition energies for the blue-shifb ing, polar merocyanine, I, given the symbol XB by Brooker, et d . 2 b Figure 2 illustrates the analogous 2 ((
+
(1) Contribution No. 1643 from the Woods Hole Oceanographic Institution; supported in part by the National Science Foundation, Grant No. GP 5110, and in part by the Office of Naval Research, Contract nonr 2196(00). (2) (a) L. G.S. Brooker, et al., J. A n . C h a . Soc., 7 3 , 5332 (1951); (b) L. G. S. Brooker, A. C. Craig, D. W. Heseltine, P. W. Jenkins, and L. L. Lincoln, ibid., 87, 2443 (1965). (3) E. M. Kosower, ibid., 80, 3253 (1958). (4) K. Dimroth, C. Reichardt, T. Siepmann, and F. Bohlmann, Ann., 661, 1 (1963).
Volume. 70, Number 7 July 2966
NOTES
2414
95 90 85
I
-
0
-
CH3-N3(=CH-CH)8$Z
1
--
8075
1
1
I
IS
0
5
-
13 ,oll 0 00
0
90t
8ol 70k 85
m
75
values measured by Kosower3 for 1-ethyl-4-carbomethoxypyridinium iodide. Two specially interesting families of solvents have been extracted from the more extensive compilation2bof transition energies for the redshifting dye I1 (XR values) to make up Figure 3.5 Consideration of the internal pressure at least provides an alternative view of this solvatochromism, for if one considers a dipolar solvent polarity indicator molecule comparable in dimensions and moment to the surrounding solvent dipoles, transition to a nonpolar electronic state (“blue-shifting” solute) is related to the creation of a hole in the solvent, and the transition
+
Figure 1. Plots of XB vs. D, (D - 1)/(2D l), and U v / V : 0, hydroxylic solvents; 0, dipolar aprotic solvents; a, nonpolar and other solvents; 1, toluene; 2, dichloromethane; 3, pyridine; 4, acetone; 5, N,N-dimethylformamide; 6, acetonitrile; 7, %propanol; 8, 1-butanol; 9, ethanol; 10, methanol; 11, water. Heats of vaporization taken from: (a) “Landolt-Bornstein Physikalisch-Chemisch Tabellen,” Springer, Berlin, 1929-1936; (b) A. Weissberger, E. S. Proskauer, J. A. Riddick, and E. E. Toops, Jr., “Organic Solvents,” 2nd ed, Interscience Publishers, Inc., New York, N. Y., 1955; (c) J. H. Hildebrand and R. L. Scott, “The Solubility pf Nonelectrolytes,” 3rd ed, Dover Publications, Inc., New York, N. Y., 1964; (d) S. T. Preston, Jr., J . Gas Chromatog., 1 (3), 8 (1963).
The Journal o j Phy8k.d Chmistry
d
7
65
60
0.2
I
03
L
-
,
I
0.4
05
D -1 ZD+/
+
Figure 2. Plots of Z vs. U v / V and (D - 1)/(2D 1). Symbols are as in Figure 1: 1, 2,2,4trimethylpentane; 2, chloroform; 3, pyridine; 4, dichloromethane; 5, acetone; 6, N,N-dimethylformamide; 7, acetonitrile; 8, t-butyl alcohol; 9, 2-propanol; 10, 1-butanol; 11, 1-propanol; 12, acetic acid; 13, ethanol; 14, methanol; 15, ethylene glycol; 16, water.
energy to the electrostatic component of the energy r e quired to create the hole working against the cohesive forces of the (This qualitative model can be generalized from total to partial dipole annihilation, hence to any change in moment, and to inclusion of (‘red-shifting” indicators.) The transition energy should thus be a function of the internal pressure of the liquid, and its solvent sensitivity a function of the change in dipole. Ample opportunity exists, with complex indicators of the sort employed, for specific solutesolvent effects, for modification of the local cohesive energy density of the solvent by solute, and for modifi(5) Only with X R was a selection of data made to illustrate a specific point; the two foregoing casea include all compounds for which heat of vaporization data could be found. The remaining XR data behave similarly to those of Figures 1and 2.
(6) N. C. Den0 and H. E. Berkheimer, J . C h a . Eng. Data, 5 , 1 (1960); J . Org. C h m . , 28, 2143 (1963). (7) F. H. Stilliiger, Jr., in “Molten Salt Chemistry,” M. Blander, Ed., Interscience Publishers, Inc., New York, N. Y . , 1961, pp 45, 102 ff. (8) R. A. Pierotti, J. Phys. Chem., 69, 281 (1965).
NOTES
%a
2415
:I 44
43
+
Figure 3. Plot of X B us. ( D - 1)/(2D 1) and A&/V. Solid points (AEvlV): left ordinate, lower abscissa. Open 1): right ordinate, upper abscissa. points ( D - 1)/(2D A, substituted benzenes, ArX, X = m-,p , o-dimethyl, methyl, H, C1, Br, CN, NOz; 0, alcohols, ROH, R = l-octyl, l-butyl, 2-propyl, l-propyl, ethyl, methyl. Equations of least-squares lines and correlation coefficients: (1) for ArX, X R = 64.5 - 0.215AEv/V, r = 0.987; (2) for ArX, XR = 52.1 - 19.1(D - 1)/(2D l), r = 0.975; (3) for ROH, X R = 46.7 - 0.0175AEv/VJ r = 0.985; (4) for ROH, XR = 64.4 - 43.9(D - 1)/(2D l), r = 0.950.
+
+
+
cation of the dipole moment change by electronic changes in solventrindicator hydrogen bonds, where these exist. According to Figures 1-3, correlation with the internal pressure diminishes the split of the transition energy data into hydroxylic-nonhydroxylic families4 as compared to correlation with functions of the dielectric constant; the dipolar aprotic solvents are universally better reconciled with the other liquids. The point of introducing the cohesive energy density is, however, not that it affords better correlations than the dielectric constant or that it supplants in any way the relations with chemical or microscopic solvent polarity measures, nor is it that the above model is more attractive or even very different from that offered by previous authors. The point of importance is that the cohesive energy density is a quantity of considerable importance, and any new source of information on it should be explored. Particularly in aqueous salt solutions and in the theory of salt-induced medium effects is the internal pressure a key Thus the first satisfactory theory12 Of nonelectrolyte Salting relates salting-in Or salting-out to the effectof the electrolyte on the cohesive energy density Of the water by way Of the energy required to create a hole for the nonelectrolyte. This effect of the electrolyte is measured independently by the quantity
(V, - VSo)//3o,where (V, - V2) is the electrostriction of the salt and /30 is the compressibility of water.13 Unfortunately, electrostrictions are not known for many salts of interest, and the ambiguity of estimating the intrinsic volume occupied by the ions in solution, V,, has led to a number of electrostriction scales which, while generally agreeing in the order of the various salts, differ broadly in actual m a g n i t ~ d e . ~ J ~Any J~-~~ readily measured quantity which can be shown to be quantitatively related to the electrostriction would thus be of value in, e.g., the correlation of nonelectrolyte salting measurements, particularly in solutions of mixed or unfamiliar electrolytes. In the hope of finding such a property in the salt dependence of the transition energy of an appropriate solvent polarity indicator, we have made some preliminary measurements with aqueous electrolyte solutions of the merocyanine dye M8fL2* Experimental Section The inorganic salts employed were of analytical reagent quality. Distillation Products Industries tetran-propylammonium bromide and sodium benzenesulfonate were used as received and recrystallized three times from 90-95% ethanol, respectively. The dye was kindly furnished by Dr. L. G. S. Brooker, Eastman Kodak Co., and was used a s received. A saturated methanol solution of the dye (8 p1) was added to 3 ml of deionized water, or of a salt solution prepared therefrom, .and the visible spectrum was immediately recorded on a Cary Model 14 spectrophotometer. This was found to be the most reproducible method of preparing the solutions, as the dye is very slow to dissolve in water and ultrasonic treatment resulted in chemical alteration. The solutions were stable during the time of measurement but decolorized on standing overnight. Salt solutions of low pH (e.g., Al2(SO4)3)decolorize the dye. Results and Discussion Observed wavelengths and computed transition energies for solutions of five salts are summarized in Table I and Figure 4. Unfortunately, the shifts are not (9) N. C. Den0 and C. H. Spink, J . Phys. Chem., 67, 1347 (1963). (10) M.A. Paul, J. A m . Chem. SOC., 74, 5274 (1952). (11) F. A. Long and W. F. McDevit, Chem. Rev., 51, 119 (1952). (12) W.F. McDevit and F. A. Long, J . A m . Chem. Soc., 74, 1773 (1952). (13) Or alternatively by dP,/dc. where P, is the effective pressure exerted by the salt in solution: R. E. Gibson, J . A m . Chem. Soc., 56, 4,865 (1934); 57, 284 (1935). (14) P. Mukerjee, J . Phys. Chem., 6 5 , 740, 744 (1961). (15) J. Padova, J . Chem. Phys., 40, 691 (1964). (16) E.Glueckauf, nan8. Faraday sot., 61, 914 (1965). .
I
Volume 70, Number 7 July 1066
NOTES
2416
Table I: Wavelengths and Transition Energies of the Visible Absorption Band of the Dye M 8 8 in Aqueous Salt Solutions Salt ooncn, mole/l.
0 0.25 0.50 1.00 2.00 4.00
-NaxSObA
ET"
4877b 4845 4833 4832
58.62 59.01 59.16 59.17
--MgSOr----. A
ET(kcal/mole) = 28.59 X 104/X(A).
I
I
I
4854 4841 4828 4817
Salt YNaC1A ET"
ET'
58.90 59 06 59.22 59.35 I
4863 4852 4846 4836 4833
4908 4929 4972
-PrrNBr---
ET^
A
ET"
58.25 58.00 57.50
4961 5012 5100
57.63 57.04 56.06
* Brooker, et U Z . , ~ report 4875 A. I
I
I
cs Figure 4. Transition energy, ET (kcal/mole), of the merocyanine dye M88 in aqueous salt solutions us. molarity of the salt: A, Na&Oa; 0,MgSOc; 0,NaCl; V, NaCeH&Os; 0, (n-CaH7)aNBr.
large relative to the precision of measurement of Xm,, for this broad band ( i3-5 A). More solvent-sensitive indicators are known,2b however, and in the hope of making measurements with one of these at a later date, we did not attempt to determine quantitatively the limiting slopes dET/dc, in the present system. Qualitatively, the order of the limiting ET os. salt concentration slopes (NazSO4 > MgSOr > NaCl > CsH&30aNa > Pr4NBr) observed is the known order of
The Journal of Physical Chemistry
58.79 58.92 59.00 59.12 59.16
--CsHsSOaNaA
electrostrictions : NazS04,53;9 RSgS04,46-65;14 NaCl, 12.8;9 CsH6S03Na, -9.0;9 PsNBr, -329 ml/mole. Correct also is the sign in each case; the salts with positive electrostrictions give blue shifts, those with negative electrostrictions red shifts, corresponding to nonelectrolyte salting-out and salting-in, re~pectively.~ Kosower's model3 of the pyridinium iodide solvatochromism applied to the aqueous electrolyte systems would consist of a description of the effect of the various salts on the ability of the water to be organized into "cybotactic regions'' about the indicator molecule, which could only follow from quantitative understanding of the effect of the ions on the water structure. This actively pursued goal of solution chemistry is, on the other hand, the key to prediction of the molal volumes of ions and the electro~triction,~~-~~ and the two views of the solvatochromism are thus closely related. It is doubtful that indicator ET measurements will contribute appreciably to the underlying theory. The unknown role of possible local alteration of the native water structure by the large dye molecule itself has been pointed out earlier in this note and in the previous l i t e r a t ~ r e . ~Nevertheless, ~?~ the possibility of a useful empirical dET/dc,-electrostriction-nonelectrolyte salting correlation appears real enough to justify the study of a greater variety of salts with a more sensitive indicator.
Acknowledgment. The author thanks Dr. L. G. S. Brooker for a gift of the merocyanine M88 and Mr. Robert L. Thorne for technical assistance. (17) H. S. Frank and W.-Y. Wen, Discussions Faraday SOC.,24, 133 (1957).
