MISCIBILITY OF CYCLOHEXANE AND METHYL ALCOHOL
183
5. The lyotropic order of ions has been obtained for the system cyclohexanemethyl alcohol and found to be:
I-
> SCN- > Br- > C1- > NO,REFERENCES
(1) BATES,MULLALY, AND HARTLEY: J. Chem. SOC.123, 403 (1923). (2) GILLO:Ann. chim. 12, 251 (1939). (3) HILDEBRAND: Solubility of Non-electrolytes, pp. 37, 51. Reinhold Publishing Corporation, New York (1936). (4) International Crztical Tables of Numerzcal Data, Vol. I, p. 202. The McGraw-Hill Book Co., New York (1926). (5) Reference 4, Vol. 111,p. 27 (1925). (6) Reference 4, Vol. 111,p. 395 (1928). (7) JONES AND AMSTELL: J. Chem. SOC.1930,1316. (8) LECAT:Thesis, Brussels, 1909. (9) LUNDAND BJERRUM: Ber. 64B, 210 (1931). (10) X~ONDAIN-MONVAL: Compt. rend. 183, 1104 (1926). (11) MOORE,RENQUIST, AND PARKS: J. Am. Chem. SOC.62, 1505 (1940). (12) SETSCHENOW: 2. physik. Chem. 4, 117 (1889). (13) SEYER,WRIGHT,A N n BELL: Ind. Eng. Chem. 31,759 (1939). (14) TIMMERMANS: Arch. n6erland. sci., Ser. IIIA, 6, 147 (1922). (15) UNMACK, MURRAY-RUST, AND HARTLEY: Proc. Roy. SOC.(London) l27k, 228 (1930). (16) WASHBURN AND SPENCER: J. Am. Chem. SOC.66,361 (1934). (17) WOLF:Trans. Faraday Soc. 33,179 (1937).
LYOTROPIC ORDER AKD THE EFFECTS O F SODIUM SALTS ON THE MISCIBILITY OF CYCLOHEXANE AND METHYL ALCOHOL' E. L. ECKFELDTSJ
AND
WALTER W. LUCASSE
Department of Chemistry and Chemical Engzneering, Universzty of Pennsylvania, Phaladelphia, Pennsylvania Received January 2, 1943
In 1888, while investigating the salting out of natural egg albumin from aqueous solutions, Hofmeister (12) found that the various ions were effective to different extents and arranged the two groups in their resulting order. Later Pauli (18) modified and extended the series, giving for the salting out of albumin:
F-
> SOY- > PO;--
> CeH60;--
> C4HaOT- > CgHaO; > C1- > NO; > C103 > Br- > I- > SCN-
1 This aiticle is based upon a dissertation submitted by E . L. Eckfeldt to the Faculty of the Graduate School of the University of Pennsylvania in partial fulfillment of the requirements for the degree of Doctor of Philosophy, February, 1942. SSincere thanks are due E. I. du Pont de Nemours and Company, Inc., for a Postgraduate Fellowship for the academic year 1940-41. * Present address : Research Department, Leeds & Northrup Co., Philadelphia, Pennsylvania.
184
E . L. ECKFELDT AND WALTER W. LUCASSE
with the cation order the same as Hofmeister’s. Since then, these orders with minor shifts have apeared repeatedly, not only in colloidal but also in other branches of chemistry, and they have acquired the name Hofmeister or lyotropic series. In 1937, Voet (21) proposed a system of quantitative lyotropy in which the behavior of salts in a particular type of experiment, as, for instance, viscosity, salting out, and reaction rate, could be predicted from certain lyotropic data. Numerous studies have been made on the lyotropic behavior of ions in liquidliquid systems, and it is to this aspect of the subject that the present discussion is limited. In most salting-out experiments the normal order of ions is observed; for example, fluoride and chloride ions show relatively large salting-out tendency compared with iodide and thiccyanate ions. Thus, for the system n-butyl alcoholwater it has recently been found (20) that the lyotropic order in salting out is as follows: NaBOt
> NaCl > KaBr > iSaNOs > NaI > KaSCiS
Some cases have been reported, however, in which the anion order is anomalous. Kosakevich (17), in studying the solubility of carbon dioxide in ethanol and in methanol, and in solutions of these alcohols containing sodium and lithium salts (chlorides, bromides, and iodides), found the cation order the same as that usually observed but the anion order irregular in methanol and the reverse in ethanol. (Liquid-liquid systems and gas-liquid systems in general show similar salting effects, which are most likely based on the same fundamental principles.) Howard and Patterson (13) investigated the effects of salts on the systems ethyl alcohol-paraffins and methyl alcohol-paraffins and found the cation order normal but the anion order reversed. The lyotropic effect of the anions on the mutual miscibility of cyclohexane and methyl alcohol has also been found to be anomalous (5). The figures (6, 7, and 8) summarizing the phase relations of this system show three aspects of salting out, all of which indicate the same reversal of the normal order: namely,
I-
> SCN- > Br- > C1- > NO;
Although considerable attention has been given the subject of lyotropy, few fundamental suggestions have been made to explain the observed facts. Gross (10) proposed a mechanism according to which one ion salts in and the other salts out, the net effect determining the observable behavior. The presence of tri-, di- and mono-hydro1 has also been suggested (3), the equilibria between these species being influenced differently by the various ions in accord with the lyotropic series. Voet (21) believes that the lyotropic effect is determined by the magniiude of the electrical field strength of the ion, while Hildebrand (11) and Albright (1) are of the opinion that ionic size is of importance. The theory of ionic field strength and that of ionic size are obviously compatible, since each stresses a different aspect of the same fundamental condition. As the ion becomes larger, the charge is spread over a more extensive sphere and the field intensity adjacent to the ion decreases.
185
MISCIBILITY OF CYCLOHEXANE AND METHYL ALCOHOL
____ ION
ION
RADIUS
A. F- . . . . . . . . . . . . . . . . . . . . . . . . c1-... . . . . . . . . . . . . . . . . . . . . . Br-. . . . . . . . . . . . . . . . . . . . . . . I-. . . . . . . . . . . . . . . . . . . . . . . .
1.33 1.81 1.95 2.20
RADIUS
____ A.
1
Li+. . . . . . . . . . . . . . . . . . . . . . . Na* . . . . . . . . . . . ............ K+. ...............
Rb+.......................
0.78 0.98 1.33 1.49
The first, the electrostatic effect, in common systems causes a decrease in mutual miscibility. The correlation of this effect with the ion size is seen in the equation of Debye, in which the mean ionic radius appears in the denominator. Thus, it follows that salting out is inversely related to the ion size. Table 1 (6) gives the sizes of several ions and the salting-out tendency caused by this factor, the arrow indicating increasing salting-out tendency. Randall and Failey (19), in considering salting out, have emphasized the use of ionic strength as more advantageous than concentration. On such a basis, which is in accord with the Debye equation, the valency is of little consequence and other factors in a lyotropic study become comparable. The action of the second salting factor, compound formation, depends upon the influence which the added salt has upon the two basic components of the system. Compound formation through the cation is likely to be small in the case of the alkali metals. The rule of Fajans states that coordination of metal ions decreases as the ion size increases and the charge decreases. Although examples have been produced, the alkali metals show relatively little tendency to form coordination complexes. This point is further demonstrated by the relative sparsity of hydrates among the alkali metal salts compared with those of the alkaline-earth metals. In many cases, also, hydrates of the alkali metal salts, when they occur, are attributable to the anion rather than the cation.
186
E. L. ECKFELDT BSD WALTER 11'.
LUCASSE
Anions tend to participate in coordination much more readily. The order of this tendency follows roughly the polarizability of the anion. Thus, the fluoride ion rarely appears in coordination complexes, while the bromide and iodide ions are frequently so found. The highly polarizable cyanide and thiocyanate ions likewise tend to form complexes readily. I n table 2 the alkali metal ions and the halogen ions are listed in the order of their refractions, which may be used as an index of ionic polarizability. The values (8) are for so-called gaseous ions, ions hypothetically removed from the effects of solvent and neighboring ions. Polarization probably plays an important part in the structure of the polyhalides of the alkali metals. Both the tendency of this group to form large anions and the variety of compound types increase as the size of the ion increases. The hydrates of the sodium halides show increasing stability in going from the chloride to the iodide. These compounds, each having two molecules of water. TABLE 2 Refractions of gaseous cons
I
CATIONS
i
Ion
Refraction
Li* ........................ Na+. . . . . . . . . . . . . . . . . . . . . . . . K+. . . . . . . . . . . . . . . . . . . . . . . . Rb+........................
0.2 0.5 2.1
3.5
AWOXS
I
I ~
_-_______ !
Ian
Refraction
F- . . . . . . . . . . . . . . . . . . . . . . . . . c1- . . . . . . . . . . . . . . . . . . . . . . . Br- ........................
2.5 9.0 12.5
TABLE 3 Solubilities of sodium s a l f s i n wzethyl alcohol
1
Salt . . . . . . . . . . . . . . . . . . . . . Temperature, "C.. . . . . . , , Solubility,molepercent
1
NaiLos cu.25 0.156
j i ~
T\TaCI 25
0.71
1
1
XaBr 25 5.43
~
XaSCX
24.7 13.8
~
~
XaI 25 16.2
undergo transition, losing their water of crystallization a t the following temperatures (16): sodium chloride, 0.15"C ; sodium bromide, 50°C.; sodium iodide, 68.9"C. Sodium nitrate is not hydrated. The very high solubility of sodium bromide, iodide, and thiocyanate indicates strong solvent attraction for these salts. The solubility of these salts in methyl alcohol, although lees than in water, is still high, and again strong solvent attraction is suggested. The data (14, 15) in table 3 show that the order in which the solubilities increase i s the same a5 the lyotropic order obtained n i t h the cyclohexane-methyl alcohol system. Bromides, iodides, and thiocyanates bhon remarkable solubility in many solvents of intermediate dielectric constant. Sodium iodide crystallizes from methyl alcohol with three molecules of solvent. All of these facts testify to the affinity of salts with a highly polarlzable anion for polar solvents. The effect of certain salts, in which an increase in solubility is observed, can be explained only on the basis of compound formation. The action of sodium
MISCIBILITY OF CYCLOHEXANE A N D METHYL ALCOHOL
187
thiocyanate on the butyl alcohol-water system is first salting out and then, with larger concentrations, salting in (20). Large quantities of the salt cause a lower consolute temperature to appear and the phase curve becomes closed. Lithium iodide in sufficient quantities causes the system aniline-water to become completely miscible a t room temperature, although the critical solution temperature for the pure system lies a t 167OC. (7). Even in the case where no third component is present, lower consolute temperatures and the simple case of increasing solubility with decreasing temperature are generally thought to be caused by compound formation (9). Salting in, then, can be understood through solvent-anion interaction. In a system such as phenol-water or butyl alcohol-water, the polarizable anions combine with molecules of both liquids. This occurrence leads to negative deviations which can be explained as having a dual cause. First, the number of free liquid molecules of both components is reduced and the corresponding activities decrease. Secondly, the two complexes, solvent molecules attached to highly polarizable anions, may show inordinate attraction for each other, again leading to greater miscibility. According to the second salting factor then, the lyotropic order of the ions for decreasing salting out would be in the direction of increasing polarizability. The halogen ions would be arranged thus
F-
> C1- > Br- > I-
Inspection shows this order to be the same as that required by the electrostatic effect. The third factor influencing salting-out power, the ionic polarization effect, is frequently masked because of the preponderance of the first two factors. Thus, lyotropic orders generally correlate the electrostatic effect and compound formation, both of which give the normal trend. In cases where compound formation is reduced by the nature of the system, as in those in which anomalous orders are observed, the polarization factor becomes important. In the anomalous cases cited earlier, one of the components was chemically inert,-as, for instance, cyclohexane,-and the dielectric constant of the mixed system was small compared with systems in which water was one of the components. Since the inert component shows little tendency to unite with the salt added in these cases, compound formatibn cannot operate to make the system more miscible. Thus, the effect of the salt is entirely upon the single liquid showing an affinity for the added electrolyte, and in this way the third factor comes into prominence. The internal pressure of the polar liquid is increased by the presence of the salt and greater immiscibility will result. Within the polar liquid there are possibly instantaneous arrangements of the nuclei and electrons leading to a range of dipole values. Such configurations might be shifted toward higher and less temporary dipole values by the presence of the various ions, particularly the anions with their highly polarizable electron systems. The high polarizability of the iodide and thiocyanate ions acts upon methyl alcohol, increasing the effective internal pressure and thus decreasing its miscibility with cyclohexane. Ions
188
E. L. ECKFELDT AND WALTER W. LUC.4SSE
of lesser polarizability, such as chloride and bromide, have a less marked influence in increasing the effective internal pressure. When the ionic polarization effect predominates, decreasing salting-out tendency will follow the order of decreasing ionic polarization. Thus, for the halogen ions the salting-out order would be as follows:
I-
> Br- > C1- > F-
This order is the one found in the previous paper ( 5 ) for the three halides studied. In pure liquids of various types, the existence of a number of forces has been recognized. Among these may be mentioned the forces between permanent dipoles, those resulting from the moment induced in a non-polar molecule by a polar molecule (the induction effect), those produced by instantaneous arrangements of nuclei and electrons leading to attractions between non-polar molecules (the London effect), and those caused by ionic charges. In a series of liquids these attractive forces are present to different extents. Thus, London forces predominate in non-polar and in only slightly polar liquids. However, as the dipole moment increases, dipole-dipole attraction increases markedly, until in the case of water it is perhaps four times as large as the London forcefi. In a homogeneous mixture of two or more liquids, there is undoubtedly an interplay of the various forces which may be further complicated by the addition of an electrolyte. In highly polar liquids, the effect of the salt would be of lesser influence because of the more important permanent dipole attractions. With liquids of lower dipole moment than water, the ionic polarization factor would be appreciable and anomalous lyotropic order would appear, as in the cyclohexane-methyl alcohol system. The effect of an added ion in increasing the polarity of a polar liquid may be thought of as analogous to the attractions produced in non-polar liquids by the temporary dipoles within the molecules of the pure liquid. Cation, as well as anion, behavior agrees with this picture of salt effect. Since the alkali metal ions show but little tendency to form compounds and since their polarizabilities are low, a t least for the ones for which lyotropic data are available, their order follows the normal behavior. Sumerous cases exist where one or two members of a series are out of place but where the general order is correct. Such abnormalities are readily understood when the theory of salting out rests upon a number of competing and interrelated factors. The position of an ion depends upon the net effect of the salting factors postulated. SUMMARY
An explanation for the lyotropic order of salting action in liquid-liquid systems has been proposed which attempts to cover not only the normal case but order inversion such as found in the cyclohexane-methyl alcohol system.’ According to this view, salting action depends upon the net effect of three competing factors -the electrostatic effect, compound formation, and ionic polarization.
MISCIBILITY OF CYCLOHEXANE A N D METHYL ALCOHOL
189
REFERENCES
(1) ALBRIQHT: J. Am. Chem. SOC.59, 2098 (1937). (2) ALBRIQHTAND WILLIAMS:Trans. Faraday SOC.33, 247 (1937). (3) BANCROFT AND GOULD:J. Phys. Chem. 88,197 (1934). (4) DEBYE:Z.physik. Chem. l 3 0 , 5 6 (1927). (5) ECKFELDT AND LUCASSE: J. Phys. Chem. 47, 164 (1943). (6) E Y E L ~ UAND B ANDERSON: Modern Aspects of Inorganic Chemistry, p. 530. D. Van Nostrand Company, New York (1940). (7) GLASSTONE: J. Chem. SOC.1927, 635. (8) GLASSTONE: Teztbook of Physical Chemistry, p. 530. D. Van Nostrand Company, New York (1940).
(9) Reference 8, page 718. (10) GROSS:Chem. Rev. 13,Ql (1933). (11) HILDEBRAND : Solubility of Non-Electrolytes, p. 143. Reinhold Publishing Corporation, New York (1936). (12)HOFMEISTER: Arch. exptl. Path. Pharmakol. 24,247 (1888). (13) HOWARD AND PATTERSON: J. Chem. Soc.lBPB,2787. (14) HUQHEB AND MEAD:J. Chem. Soo. 1949, 2282. (15) International Critical Tables of Numerical Data, Vol. IV, p. 206. The McGraw-Hill Book Company, Inc., New York (1928). (16) Reference 15, p. 236. (17)KOIAKEVICR:2.physik. Chem. 143,216 (1929). (18) PAULI:Arch. exptl. Path. Pharmakol. 3,223 (1903);S,27 (1904). A N D FAILEY: Chem. Rev. 4,285 (1927). (19) RANDALL (20) REBER,MCNABB,AND LUCASSE: J. Phys. Chem. 46,500 (1942). (21) VOET: Chem. Rev. 20, 169 (1937).
