June, 1957
789
sO1211RILITY OF I O D I N E IN OCTAXIETHTLCYCLOTETRASILOXANE
TILE SO1,UnILITY AND ENTROPY OF SOLUTION OF IODINE I N OCTnRI~TTIYT,C'~~,~T~Tl~ASILOXANE AND TETRAETHOXYSILANE BY Kozo S H I N ~ D AND A J. H. HILDEBRAND Dcpnrl1)tent oJ ChernialrU and Chemical Engineering, IJniversil!/ of California, Bcrkeley, California Received January 51, 1967
Iodine dissolves in octnmcthylcyclotetrasiloxane, ( CHl)8Sid04,t o give a violet, "regrilnr" solution, but in tetraethoxysilane, (C*HbO)rSi,t o give a solvated, brown solution. Its solubility, in molo fraction, conforms closelv t o the equations, respec140 LR/T and log z2 = 1.499 816.9/7'. Tho respective mole ercentago and molal entropies of tivclv: log 2 2 = 2.820 solniion at 25' are 0.8104, 22.5 e.u., and 5.747, 12.5 e.u. T h e entropy of solution in (CH3y8SirOdshowe no divergence from thc vathies in other violet solutions, in spite of the unprecedented disparity in mold volume of solvent and solute, 312 and 59 cc., indicating that tho entropy of solution is a function of mole fraction, not voliime fraction, for quasi-spherical molecules of different size. The solubility corresponds t o a solubility pnrnmeter of 8.2 instead of the vnlue given by the energy of vaporization, 6.4. T h e zrtial mol:rl volume, like the entropy, of iodine in the violet, d o x a n e solution shows no divergence from its value in naigrboring solvcnts attributable to differences in solvent mold volumcs. I n the silane, however, both properties are considerably smdler, in accord with the evidence for complex formation.
-
-
The molecule of octaniethylcyclotetrnsiloxane, (CH3)RRi404,has an nctagonal core of alternnting silicon and oxygen atoms with two methyl groups attached to each silicon atom. A scale model (LaPine) shows that t,hc methyl groups are most uniformly spaced if the four silicon atoms are twisted about half way between a square and a tetrahedral orientation, making the whole molecule quasi-spherical. The ovygen atoms are then so shielded that an iodine molecule cannot, approach closely enough to form a charge-transfer complex, as shown by the pure violet of its solution. A former member of our group, D. N. Glew, found that its maximum light absorption occurs a t 518 mp. The masimum is a t 517 mp in carbon tet,rachloride and a t A22 mp in n-CVF18, "f-heptane." The solution is therefore quite I' regular." The molal volume of this silicone is 312 cc. a t 25", much greater even than that of f-heptane, 225 cc., and over 5 times that of liquid iodine, 50 cc. Its iodine solution therefore affords a rare opportunity for testing the effect, of disparate size upon the entropy of solution of quasi-spherical molecules. The molecules of this silicone, furthermore, present a wide departure from a radial attractive potential and this puts a severe strain upon expressions for enthalpy of solution based upon radial fields. We included in this study a second silicone, tetraethoxysilane, (C2H60)4Si,in order to emphasize, by contrast, the exceptional properties of the above compound. The oxygen atoms of this silicone are not buried by the ethyl groups, hence the iodine solution is brown in color, indicating the presence of a charge-transfer complex, with non-regular behavior. It absorption maximum is a t 475 mu, near to that for iodine in ether,' 462 mp. Materials.-The (CH,),SiqO, was urc material frirniRhed b y the General Electric Compttiy ttrough the kindness of
R . C. Osthoff. It was vacuum distilled shortly bcforc use in order to remove small amounts of polymer, evidcnt from the higher viscosity of the residue. The ( C ~ H 5 0 ) q Swas i ohtaincd h o m Dow-Corning. It was dried, then vacuum distillrd.
Results.-The solubility of iodine was determined by eqiiilibrating in the apparatus described by Glew and Hildebrand,2 and titrating weighed 11. A . Benesi and J. 11. Hildebrand, J . Am. Chem. Soc., 71, 2703 (1949). (1)
port'ions of saturated solution with thiosulfate. Oiir results for iodine in (CH3)8Si404are given in Table I. The calculat,ed values have been srnnot#hedout by the eqiiation: log 5 2 = 2.820 14G1..3/T. This gives 100 x2 = 0.8104 a t 25", and Z2(d log xz/d log 2') = 22.5 cal. deg.-'. TABLE I SOLUBILITY OF I,
IN
(CH&Si404, MOLEPERCENT.
20.0Ao
25.06O
29.96''
34.97'
39.98'
0.667 ,672 .663 .665
0.816 .816 ,805 .826
0.973 0.975
1.176 1.168 1.178
1.388 1.389 1.385
0.667 Calcd. 0,670
0.816 0.813
0.974 0.975
- - - - -
Av.
1.174 1.170
1.387 1.392
Table I1 gives the corresponding figures for iodine in (C2H60)4Si,with log z2 = 1.499 - 816.9/T and R(d log zz/d log T ) = 12.5 between the two higher temperatures. No measurements were made above 25" because a slow secondary reaction occurs as shown by a gradual decrease in the height of the absorption band. TABLEI1 SOLUBILITY OF Ip I N (CzHsO)Bi, MOLEPERCENT. 11.85'
17.07O
24.99'
4.28 4.35 4.33
4.83 4.81
5.73 5.73 5.76
-
-
Av. 4.32 Calcd. 4.323
4.82 4.848
-
5.74 5.747
The measurements are plotted in Fig. 1 in a manner similar to that used in a recent paper by Hildebrand and G l e ~ . We ~ have omitted the point for 4F,a there shown in order to show the remaining points on a larger scale. Instead of using -log za as abscissa, we here use -R In x2, which represents the entropy of ideal solutions of liquid iodine. As ordinate we plot the excess of R(d In x 2 / bIn T)sad over the entropy of fusion of iodine, in cal. deg.-l mole-'. This likewise represents the entropy of ideal solution of liquid iodine, and yields points on a 45" line starting a t the origin. (2) D. N. Glew and J. H. Hildebrand, THIEJOURNAL, 60, 616 (1956).
(3) J. H.Hildebrand and D. N. Glew, ibid., 60, 618 (1956).
I
I
TABLE I11 ENTROPY O F SOLUTION OF IODINE, 25'
16Solvent
csz CHCI, CClr 8iC14 (CHs)&irOl
(%3)T R(%%)satd 0.85 ,92 .9G .98
52
18.8 20.3 21.0 23.3 22.5
.1)9
- 8;
16.0
18.7 21.0 22.8
22.3
violet solutions, whercns the point for the brown solution in (CZH60)k%diverges strongly. It is near that for p-xylene. The solvation yields higher solubility and smaller entropy of solution. The correspondence of the former solution with other violet solutions is very significant in view of the large ratio of the molal volumes of solvent and solute, 312/59. For the neighboring solvents, cc14 and c-CgH12,the ratios are 07/59 and 109/59, rcspectively. The equation
3, - SZ'
= -RIln
+
+1(1
-
u~vl-')]
(3)
for entropy of solution, (of component 2) where v ' s arc molal volumes, yields an excess over the ideal entropy, -R In x2, of 0.15 e.u. for the CCh solution -R h x , and 1.60 e.u. for the silicone solution. The plot Fig. 1.-Iodine solutions. shows no such discrepancy. The inescapable inThe entropy of fusion of iodine a t 25" is calcu- ference is that the entropy of solution of quasilated from its value a t the melting point and ACp, spherical molecules of different size is a function of the excess molal heat capacity of liquid over solid. mole fraction, not volume fraction. I n a paper This requires extrapolation of t,he heat capacity of now in preparation on entropy of solution of gases the liquid over the interval from the melting point, we will present further experimental evidence in 113.5", to 25", and the result is thereforesomewhat support of this conclusion. It indicates that the uncertain. The data of Frederick and Hildebrand4 entropy of solution is not a function of relative that we have used heretofore, give 8.0 e.u. for the size, but rather one of configuration, as envisioned entropy of fusion a t 25". A lat,er critical survey by in the "string of beads" model used for deriving the Kelley6 yields 8.2. Bccxurje the small difference is " Flory-Huggins" equation. The derivation by unimportant, in view of the uncertainty, we retain Hildehrande based only upon molecular sizes, while our original figure for the snke of consistency. Point probahly valid for gas mixtures, is invalid for the A corresponds to the logarithm of the activity of more highly condensed liquid mixtures. The second fact to which we invite attention is liquid iodine, and point B the activity of iodine in an ideal solution, where it is the same as the activit8y that the position of the point for (CH3)8Si404is far from that indicated by the solubility parameter, of solid iodine, ai = 0.258. The partial molal entropy of iodine in a solution 6.4, calculated by Osthoff and Grubb' from its energy of vaporization. This would make it nearly saturated with solid is as poor a solvent, for iodine as is f-heptane. Its poSa - 8 = R(d In x2/h In T)sntd( b In a& In 5%) (1) sition in Fig. l, however, in relation to Cc14, cThe factor on the right, which expresses the devia- cSFIl2 and SiCl,, whose &values are 8.6, 8.2 and tion from Henry's law, is not far from unity for such 7.6, respectively, corresponds to a solubility paramedilute solutionb. It may be evaluated by aid of ter practically ident'ical with t,hat of c-CBHI~, and the approximate relation t,his we have found to accord with ita behavior with two fluorocarbons, as described in the following In m = In 5 2 + k+,Z (2) paper. where 4 denotes volume fraction. Using ab = Substituting the known values in the equation 0.258 and the measured values of x gives values for IC by aid of which we obtained the values of ( b In log 0; = log 2 2 -t o*r#Jly6? - 61)2/4.575T (4) az/d In X ~ ) Tshown in Table 111. Multiplying the gives 8.2, agreeing within the experimental values of R(d In x2/d In T z ) by these and solving for factors gives the entropv of transfer of solid iodine limit of error with it8sclose neighbor, ?-CgH12. Thirdly, it is evident that tthe"measured" values to saturated solution, &!. - Sl in the last column. of t8he entropy of solution increase more rapidly These values are indicated by stars in Fig. 1. Discussion,-We invite attention, firstly, to the than - R In x2 in going from the better to the poorer fact that the point for iodine in (CHJ8Si404 falls solvents. This is evidently related closely t o inexactly upon the line through the points for other creases in the partial molal volume of iodine and /-
A
0
2
I
I
I
4
6
8
I
I
1
0
1
2
1
4
(4) K.J. Redcrick and J. H. Hildebrand, J . A m . Chem. Soc., 60, 1436 (1938). 15) K. K . Kolley, U. 8 . Biir. Minea Bull. 470.
(0) J. H. Hildebrand, J. Chem. rhus., 18, 225 (1947). (7) R. C. Oathoff and W. T. Grubh, J. A m . Chem. Soc.. 76, 309 (1954).
the attendant, e n h p y of expansion, p o i n t d out by TIildehrantl and Rcotk.* Jn order t o investigat,e this contrihut,ion IVP have detjermined the partial molal voliinie of iodinc in several selected solvent,s inchding the two silicones here used. Instead of making t8he precise det,erminations of densit,y and composition required in t'he usual method, me adopted a simple, direct, dilntometric procedure. The dilatometer consists of a cylindrical biilb of 150-cc. napacit,y contJa.ininga Iargc glass ball, It, has a capillary sttcm witjh an internal diameter of 1.8 mm. This is filled at 25" witjh solvent, exhnding int,o the lower part of the cnpilInry. A weighed amount of iodine is sealed in a long khin glass capsule narrow enough t,o slip tlomri tho capillary. After the mpsule is i n t,roduc.ed it, is hroken and the iodine is dissolved by twirling t,hr hall inside the bulb. A f h r t,emperat>iireis rest(nrec1, the change of level in the capillary is read. From the change in voliime, corrcnted for the volume of the glass of the capsiIIc, tJhepartid mold volume of the iodine is calculated. The work is being extended t,o a number of solvents and will be reported lat,er; we give here onl,y the few results that cont)rihut,edirectly to int,erpretation of the entropy of solut,ion of iodine in the two solvent,s here involved. Table IV givm results for (C2H50).Si and (CHR)8Si404t,oget.her witfh seve r d others for comparison. The contrihution of expa,nsion to the ent,ropy of soliltion may be evaluated hy aid of t.he re11hatthe size of the (CH3)RSi404 rnoleciilc introduces no depnrt.ure in either fjz or A,R1';xp. from t,he behn,vior of solvents witth neighboring values of & a.lthough A S 1 c x p , is so large as to aecoiint, for rnuch of th? difference bet,ween the actual and the men,snr.ed entropy of solution, as in the cnses reported by Hildebrnnd and Scott! and by Reeves and ITildehrand.l* The fact t,hat both tjhe pa,rt,ial molal volume a8nd the entropy of solution of iodine in t,he ethoxysilane is so much smaller than i n t,he siloxane is in accord with the formation of a strong charge transfer complex. We wish to thank Dr. R. C. Osthoff for t,he siloxane, Marilyn Angel for her exploratory rnea,surements, Dr. ,J. E. ,Jolley for advice and the National Science Foundation for its support,. (0) Adjiisted. (10) lMiinat,rd from density and boiling point,. (11) L, W, Rpnven and ,I. H. Hildobrand, Tim JOURNAL, 60, 949 (IRrIO).
T H E LIQUID-LIQUID SO1,UBILITY OF OCTAMETIIYL-CYCLOTETRASILOXANE WIT€€ FERFLUOROMETHYLCYCLOHEXANF, AND PERFLUORO-n-HEPTANE BY J. E. JOLTJCY AND J. H. HILDWBRAND DcpaitmPnl of Chem'stril, U n i v e r s i t ~of Califnrnin, Berkelry, CoE Rpwiiwd ,January S I , 1867
The Hqriid-li iiid soliibilitv ciirves of mixtiires of octamet~liylcyolotetrssiloxanc?with (a)of-met,hylcvcloheXane and ( b ) f-heptane havexeon determined. The critical t)cmpcrnturcs drr (a) 43.8G' and (b) 09.97 . The mole fractions of the silicone a t the ciirve maxima are (a) 0.38,(b) 0.44. The value of the fiolribilit,y parameter of the silicone a t 25' calculated from the meaauremrnts agrees well wit,h the vnliir 8.2 oht,ained from its solvent power for iodine (see preceding paper), which is far in CXCARS of t,he value obtaincd from ittsenergy of Vaporization ( A E V * P / V ) ~ /These ~. systems add to the mounting evidence that, the soluhilitp pnramet,ers of aliphatic hydrocn,rhons must, ht! nonsidernl,ly increased in order to conform t o regular soliit,ion t.hcoi,,yin their interaction8 wit,Ii molor~rilcsof other t,,vpcR. This onlls for a crit,iral s t d y of intcrmolecrilar potnntials.
The solnhili ty of iodinc in oc.t,nmetJhylcyclotetrasiloxane, report'ed in the preceding paper, corresponds t o a solubility paramet,er of 8.2, much larger than (3.4, the value derived hy Osthoff and Gruhb' from its energy of vaporizat'ion. The lat'ter value would lead one to exDect this mhst,ance t,n form nearly ideal solution$ with fluorocarbons. We found, however, that, it behaves like paraffin hydrocarbons in forming two liquid phases with fheptane and f-methylcyclohexane. We have de( I ) R. C. Ostlrofl and w.T. Griibb, J . Am. Chem. Soc., 76, 304 (1954).
t,ermined its mutual solubility with these two fluorocarbons in order to learn whet,her the value 8.2 i R consistent, with it8 solvent power for suhst8ancesat, both ext,remes of the parameter range. Materials.-The two fluororarhons were urified by the inrthotb desrrihatl hv Clew nnd Reeves.* &e siloxane waa from the Rtork d r s c h e d in the prereding paper. Procedure.-To contain t,he mixtures, glass tubes a p roximately 6 rnrn. in diameter and 12 rm. long were sealer!earh at nn an& of 120" to one end of a length of 1 mm. capillary tuhing, the other end of whirh rould be nttached to a round glass joint. The tubes were thoroughly washe% with (2) D. N. QIrw and L. W. Reevra,
TirrR
J O U R N A L60, , 015 (1D5fl).
