413s
LEONARD J. CORDON . ~ N DROBERT L. SCOTT [ C O S T R I B L T I O S FROM THE
DEPARTMEVT OF CHEMISTRY, uVIITRSITY OF
I T d .7-1
C A L I F O R V I A , LOS - k S G E L r S ]
Enhanced Solubility in Solvent Mixtures. I. The System PhenanthreneCyclohexane-Methylene Iodide1 BY LMNARDJ. GORDON’.AXD ROBERTL. SCOTT RECEIVED IfARCH 8, 1953 The Hildebrand theory of regular solutions predicts t h a t whenever the internal pressure of a solid (or rather the metastable supercooled liquid) lies between those for two solvent.;, there will be a range of solvent mixtures in which the solubility of the solid is greater than in either solvent alone. This enhanced solubility has been observed in the system phenanthrerlecyclohexane--methylene iodide, although the m:Lximiim wlriliility of the plienanthrcne iii thc optiniuiii solvent mixture f i i l l , bliort of the ideal value prcdicted by t h c tlit,ory,
Introduction Kegular solution theory2 when applied to the solubility of solid non-electrolytes gives solubility relationships in terms of the thermodynamic properties of the pure substances, principally the melting point and heat of fusion of the solute, and the so-called “solubility parameters” of the various components. The solubility parameter 6 is usually defined as the square root of the internal pressure or cohesive energy density, and may be calculated for a given substance in various ways,”usually from the molar energy of vaporization and the molar ~ o l Lime 6 =
(AEV/V)’/Z
11)
n’here the solubility parameter of a solid solute (calculated for the supercooled liquid) is between those of two solvents, the theory predicts that there will be a range of solvent mixtures in which the solid is more soluble than in either pure solvent. -4s far as we know this effect has not heretofore been reported for three component systems of normal non-polar or slightly polar liquids, although an analogous enhanced solubility in solvent mixtures has previously been observed in the case of high jiolymers.4v5 The system chosen for studying this enhanced solubility consisted of phenanthrene ( 6 = 9.8, estimated), cyclohexane ( 6 = S.2) and CH.12 (6 = 11.S). I n addition to satisfying the necessary s o h bility parameter conditions, this system is reasonably free from the complications of hydrogen bonding and high polarity6 and the thermodynamic properties of the components make the system a convenient one for experimental measurements. It was of interest to investigate the system further and, therefore, the two phase solvent-solvent phase diagram was determined, and some aspects ~f the three-phase freezing diagram were determined. Theory The ideal solubility of a solid (expressed as mole fraction xi in its saturated solution) is qiven by the equation R T I n 1“ = --Alii[ 1 - (T I, 1 fL‘ (1) First presented a t the X I I t h International Congress of Pure and Applied Chemistry, New York, N Y., September 11, 1951 ( l a ) Present address: h e r o j e t Engineering Corporation, Azusa. Calif (2) J. H. Hildebrand a n d R I, S c o t t , “ T h e Solubility of Nonclectrolytes,” T h i r d Edition, .4CS Monograph # I T , Reinhold 1960 (3) Reference 3, Chapter XXIII. (4) G Gee, Trans. F a r o i i i ~ yS o r . , 40, ,468f1!144). ( 5 ) R. L. S c o t t , J . Chrnz. P h y s , 17, 268 (1940). f l j j According to C P S m y t h a n d ET., E. Rogers (THIS J O U K N < 1 ~ , 52, ,>.,or. (1930)).t h e dipole moment of C.HII2 is 1.1 Debye units _Ilj
where A I I is ~ its heat of fusion, and T , its melting point. If the solution is not ideal, RT In xi is replaced by RT In (I, where (L is the activity of the solute in the saturated solution. .According to regular solution theorv, the deviations from ideality are related to the partial molal heat of mixing 1 ~ 1 of the supercooled liquid solute with the solvent Ri- 111 I = I(r i l l (ill v1 I = A H ] = v,+:(s, - a o v ( l i l ~ y l
(3)
where the 6’s are solubility parameters, the +’s volume fractions and the v’s molar volumes of components 0 (solvent) and i (solute). For three component systems the theory’ leads (with no further assumptions than those involved in the two-component case) to equations which are identical with Equation 3 if 40is taken as the total volume fraction of solvents 2 and 8 arid 6 0 is taken as the volume fraction averaqe of 6? and 6a
m = q u - d
(5)
Consequently the thermodynamic behavior of the solute should not be affected by the iact that the solvent is a mixture arid its solubility should be equal to that in a hypothetical “single liquid” of average ptoperties. I t is easily seen that if 61 for the supercooled liquid solute lies between 62 and 6 3 , a proper choice of solvent ratios will make 6 0 approach a1 and for the solvent mixture such that 6u = 6,,the ideal solubility should be observed. Experimental All materials wcre Eastman Kodak Co. “white label ,” and were not further purilietl. (Small traces of impurities should have no effect except t o depress all temperatures slightly in a more or less uniform way.) Liquid mixtures were preparerl having different ratios of cyclohexane to methylene iodide. For each “single liquid” a series of measurements of the solubility of phenanthrene were niadc by the “synthetic” method: Solid and solvent \\?re weighed m r l then sealed i l l a glass ampule. The aniptile IWS irarrneti until the solid dissolved and then cooled qiiickly so that the solid precipitated as fiiiely divided cryst:iIs. Tlic ampule !vas again heated this time a t a rate not exceeding one-half degree per minute and the temperature a t which the last crystal of solid dissolved was noted. Figure 1 gives the experimental results plotted in the usual manner. T h e precision of the temperature measurement was within one-half degree. From the best straight line for each liquid mixture the solubility at a given temperature !vas determined. A series of solubility ciirves at different
ENHANCED SOLUBILITY IN PHENANTHRENE-CYCLOHEXANE-METIIYLENE IODIDE 4139
Aug. 20, 1952
temperatures are shown in Fig. 2; the enhanced solubility in certain mixtures is obvious. I
‘.
3
‘,
\
‘.
om
(.
020
I P u 010
x 0
*6
0 01
250
0 01 370
360
350
3I0
330
320
TEMPERATURE
310
300
290
200
CHJ,
270
‘X
Fig. 1.-Solubility of phenanthrene in mixtures of cyclohexane and methylene iodide. Dotted line represents idedl iolubility.
C&I:
1
,‘OY
J
I-
0.35.
X.
Fig. 3.-Experirnental phase diagram for the binary system methylene iodide-cyclohexane.
0.30 0.25
0.20
0.15
0 10
2 50
CHJ2 X. CIH11 Fig. 3A.-Calculated phase diagram for the binary system methylene iodide-cyclohexane (using thermodynamic parameters of Table I).
0 05 -00.0
CHzL
X.
CIH12
Fig. 2.-Solubility of phenanthrene a t selected temperatures as a function of composition of the cyclohexanemethylene iodide mixture. (Circles are not t h e original experimental measurements, but are taken from the smoothed curves of Fig. 1.) The two Component system methylene iodide-cyclohexane was investigated by cooling various mixtures and observing the temperature a t which heterogeneity (milkiness) appeared. The resulting phase diagram is given in Fig. 3. The freezing points of various two-component mixtures were determined by inserting a thermocouple in the mixture and recording the maximum temperature obtained after supercooling. T h e experimental thre&component diagram is given in Fig. 4, in addition t o the experimental points shown. T h e freezing point0 in the three-component region were obtained in part from interpolation between two twocomponent curves. Approximately forty solubility determinations were made for eight different solvent ratios including t h e two pure solvents and another forty experimental poitits were determined in the solvent separation and freezing regions of the ph:iqe tliagaiii .
The solubility points were found to fall fairly well 011 straight lines on a log mole fraction ~ 3 temperature . plot (Fig. 1). Each straight line was drawn to best fit the points for t h a t particular solvent mixture and no attempt was made to draw the lines so as t o represent a consistent family; however, t h e lines are nearly parallel. The effect of the slight differences in slopes is somewhat evident in Fig. 2; if the lines had been made consistent, these curves would be smoother.
Discussion For comparison of experimental results with theory, the phase diagram Figs. 3a and 4a were calculated using the values in Table I. The properties of the three components were taken as constants independent of temperature. The solvent-solvent diagram was calculated by the method of Scatch-
arsrd.@
The ideal solubility of phenanthrene was calculated using its heat of fusionlo AH^ = 4300 cal./ (9) G. Scatchard. THISJOURKAL. 63, 2426 (1040). (10) I i i l c r ~ m f i o t r n lCritical Tables, 6, 1R4 (MrCraw-ITill. 192!1)
LEONARD J. GORDON AND ROBERT L.
41
Vol. 74
SCOTT
TABLE I1
CirHio
SOLUBILITY O F PHENANTHRENE
(All concentrations expressed as mole fractions) 5‘. OK.
Calculated Ideal 5 ~ ~in ~ solubility “ideal,, C I ~ H : ~ solvent
280 290 300 310 320
0.153
0.49
l
Observed Maximum o solubility X ~ s ~ in I n ClrHiu best solvent
0.108
0.42
.200
.49
,141
.38
.256
.49
.324
.‘a
,196 ,255
,408
-lY
.38 .35 .39
,
X36
__I
Average
Fig. 1.-Experimental phase diagram for t h e ternary system phenanthrene-methylene iodide-cyclohexane.
0.38 i 0.02
agrees with the theoretical prediction. The solubility in cyclohexane is far below t h a t predicted. The solubility curve found here, however, is still “regular” and if 6 = 9.8 is accepted as correct for phenanthrene, the apparent 6 of cyclohexane can be calculated from the theory (see Table 111). Using the value 7.0, the predicted “ideal” solvent is found to have the molar composition 0.37 cyclohexane, 0.63 CH212. This agrees with the observed value within experimental error. TABLE 111 SOLL‘BILITY OF PHEXASTHRENE I S CYCLOHEXANE Mole fraction phenanthrenr, 83 7’, ‘K. XcirrIIn 81 60 (cyclohexane)
-
286. li 393.7 396.1 3r12.8 811.7
0.0217 .0328 ,0337 ,0418
331i.13 :
