1058
R. D. VOLD,
c. w. LEGGETT
AND J.
w. MCBAIN
(5) GIAUQUE, W.F.,BUFWNGTON, R. M., AND SCHULZE, W. A,: J. Am. Chem. Soc. 49,2343 (1927). (6) GIAUQUE, W.F.,AND CLAYTON, J . 0.: J . Am. Chem. SOC.66,4875 (1933). W.F.,AND JOHNSTON, H. L.: J. Am. Chem. SOC.61,2300 (1929). (7) GIAUQUE, (8) GUNTHER, P.: Z. physik. Chem. 110, 626 (1924). (9) HARRINGTON, E. L.: Nature 8, 738 (1916);c f . also Phys. Rev. 66, 230 (1939). (10) HOGG,J. L.: Proc. Am. Acad. Arts Sci. 42, 115 (1906). (11) HOUSTON, W.V.: Phys. Rev. 62,751 (1937). (12) International Critical Tables, Vol. 11, p. 459. McGraw-Hill Book Company, Inc., New York (1926). (13) JOHNSTON, H. L., A N D GIAUQUE, W. F.: J. Am. Chem. SOC.61,3194 (1929). (14) JOHNSTON, H.L., AND WEIMER,H. R.: J. Am. Chem. SOC.66,625 (1934). W.H.,AND VAN ITTERBEEK, A,: Physics 2, 97 (1935). (15) KEESOM, M , Phil. Mag. 23,313 (1937). (16) K E L L S T R ~G.: (17) LATIMER, W.M., AND GREENSFELDER, B. 5.: J. Am. Chem. SOC.50,2205 (1928). (18) LONQ,E. A.: Ph. D. Dissertation, The Ohio State University, 1934. D. LEB.: Can. J. Research 2, 388 (1930). (19) MAASS,O.,AND COOPER, V. D., AND VAJIFDAR,M. B.: Proc. Indian Acad. Sci. SA, 171 (20) MAJUMDAR, (1938). (21) MAXWELL, G. C.: Phil. Mag. 19, 31 (1866); CoZlected Works, Vol. 2, p. 1, Cambridge University Press, London (1890). (22) MEYERS,C. H., A N D VANDUSEN,M. S.: Bur. Standards J. Research 10, 381 (1933). (23) RIGDEN,P. J.: Phil. Mag. 26,961 (1938). (24) SCOTT,R. B., A N D BRICKWEDDE, F.G.: Bur. Standards J. Research 6,401 (1931). (25) SUTHERLAND, B. P., AND MAASS,0.: Can. J. Research 6, 428 (1932). (26) VANDYKE:Phys. Rev. 21,250 (1923). A.: 7th Intern. Congr. Refrig. (Cong. intern. du froid), 1st (27) V A N ITTERBEEK, Comm. intern. Rapports et Commun., June, 1936,p. 81. (28) V A N ITTERBEEK, A., AND CLAES,A.: Nature 142,793 (1938). (29) VOGEL,H . : Ann. Physik 43, 1235 (1914). (30) YEN, K.-L.: Phil. Mag. 38, 582 (1919).
SYSTEMS OF SODIUM PALMITATE I N ORGANIC LIQUIDS' ROBERT D. VOLD, CHARLES W. LEGGETT, AND JAMES W. McBAIN Department of Chemistry, Stanford University, California Received July 3, 1940
The present work xas undertaken as the first step in a systematic survey of the phase rule behavior of soaps in organic solvents.* This report 1 Presented a t the Seventeenth Colloid Symposium, held a t Ann Arbor, Michigan, - June 6-8, 1940. * For Drevious work on sodium aoaps and organic liquids see references 3, 11, 13,6, 10, 1, 4,i8, and 22.
SODIUM PALMITATE IN ORGANIC LIQUIDS
1059
includes the determination of solubility curves of anhydrous sodiuni palmitate in glycerol, diethylene glycol, Nujol, n-heptane, isopropyl alcohol, and palmitic acid, and a few observations with carbon tetrachloridc. The different systems are characberized with respect t'o their appearance and nature at different temperatures, and an effort is made to correlate the type of solubility curve obt,ained, and the presence or absence of certain phases, with the physicochemical characteristics of the different solvents. The observed phenomena are interpreted for the most part in terms of the occurrence of a number of phase changes rather t,han by means of the less precise concepts of swelling and gel formation. In the aqueous soap systems which have been extensively studied, both as to phase rule behavior (8, 15, 19, 20) and colloidal nature of the solutions ( i )simplc , ions and other charged particles are always present, and their properties are important in explanations of the stability and behavior of the systems. Since soap in organic liquids forms colloidal systems in many instances in the absence of ions, as shown by the occurrence in these systems of jellies, gels, liquid crystalline phases, and syneresis, the nonionic, factors must be emphasized in explaining t hc observed phenomena. Technically, systems containing soaps and organic liquids arc important in such products as greases, shaving creams, and d1.y cleaning fluids. TECHNIQUE
The technique employed in this work was very similar to that previously used i i i the study of aqueous systems (19, 9). Systems of any desired composition were prepared by weighing out the requisite amount of soap and solvent into thick-ivalled Pyrex tubes, taking care to prevent ingress of water by using coiitainoi~sprotected by calcium chloride tubes. These tubcs were then sealed and heatrd in a small electric oven until the contents were completely melted to R homogeneous isotropic liquid. They were then cooled slowly. heing watched between crossed polaroida, until anisotropic material first began to form, this temperature being noted and designated as Ti. With more dilute systems. where some kind of waxy material was the separatiiig phase rather t,han transluceiit liquid crystal, it was iiecessary to deterniinc the temperature of the phase boundary by slow heating, or by allowing the system to stand successively a t each of several increasing teniperaturw, since results dltnined on cooling are frcciucntly erroneously low, owing to undcrc~ooling. These values arc' referred to as T,
R. D. VOLD, C. W. LEGGETT AND J. W. MCBAIN
palmitate tend t o react to form various acid soaps (10). The solubility in diethylene glycol and isopropyl alcohol is considerably smaller, while the ISOT I
PIC SOLUTI
NS
! 0.4
0.2
MOLE FRACTION SODIUM PALMITATE FIQ.2. Solubility curves of sodium palmitate in various solvents. A, transition temperatures of anhydrous sodium palmitate.
hydrocarbons Nujol and heptane dissolve so little soap, even at 100°C., that it cannot be accurately determined from the present curves.
FIG.3. Phase behavior of sodium palmitate and glycerol.’ 0,Ti;0,T,. 0 , T, points on transition to a liquid crystalline phase; e, compositions calculated from the amounts of the separated phases; A, transition temperatures of anhydrous sodium palmitate.
* An additional dotted boundary is shown below the T, curve with glycerol systems containing from 10 to 40 per cent of sodium palmitate. With sufficiently slow cooling a nearly invisible “honeycomb” structure develops a t the upper curve and the material sets t o a stiff gel, very faintly anisotropic under the polarizing microscope. On more rapid cooling no change is noted until the lower boundary is reached, a t which temperature the formation of translucent, macroscopically anisotropic material occurs. On heating, the systems change only slightly until they become isotropic at the temperature of the upper curve. Three systems were studied dilatometrically and showed changes in slope agreeing within one degree with the full curve. There were also changes in slope a t lower temperatures in the vicinity of, but not on, the dotted curve. Samples which stand a few minutes a t temperatures between the two curves develop nearly transparent small spherulites, 1 to 4 mm. in diameter, which sk ,w dark interference crosses between crossed polaroids. 1065
1066
R. D. VOLD, C. W. LEOGETT AND J. W. MCBAIN
The same general picture emerges if the values of Ti a t a given composition are taken as a measure of the mutual miscibility of the soap with the various solvents. At lower concentrations, mixtures of sodium palmitate with the polar hydroxylic solvents water, glycerol, and diethylene glycol all dissolve to form isotropic solutions below 100OC. Isopropyl alcohol occupies an intermediate position, while very high temperatures are required to bring about solution in the hydrocarbons. At higher soap concentrations the temperatures of formation of isotropic solution are all high and roughly comparable, independent of the solvent, suggesting that in these regions the structure of the liquid crystalline or waxy phase, TABLE 1 Solubility of sodium palmitate in various solvents
SYSTEM
I 1 s",ugx~'
I
I
weight per cent
palmitate-water.. . . , . . . . . , . , ca. 1 .O palmitate-palmitic acid.. . . . . Insoluble palmitate-glycerol. . . . . . . . , . . ca. 1 .O palmitate-diethylene glycol. ca. 1.5 palmitate-isopropyl alcohol.. ca. 0.6 palmitate-Nujol. . . . , , . . . . . . . Nearly insoluble Sodium palmitate-heptane. . . . . . , . . . Nearly insoluble Sodium Sodium Sodium Sodium Sodium Sodium
Sodium palmitatecarbon tetrachloride, .
sgtmg:
1
TEYPEBATUREOP FOBMATION OF ISOTROPIC SOLUTION
WEZNN~PCONCENTRATION 10
I O pel 80 per
cent
weight per cent
'C.
cent -
"C.
'C.
29.9 66 164 283 44.5 66 88 278 24.0 84 115 275 13.5 1 91 151 182 ea. 1.8 137 160 208 Very slightly 236 245 257 soluble Very slightly 244 260 269 soluble
-
~
When the concentration is 8.9 per cent sodium palmitate, T i = ca. 115'C.
determined by the soap, is the dominant factor which controls melting' rather than the constants of the various solvents. In five out of the seven solvents the solubilities bear no relation to the melting point of palmitic acid, 62.6" C., the so-called Krafft point. Instead, for 10 per cent solution, the temperatures of complete solution range upwards from 66' to 244" C. This and the fact that any correspondence in aqueous solutions is confined only to sodium soaps would seem to rob the Krafft point of any significance except as to a progressive influence of the length of the hydrocarbon chain in an homologous series. Carbon tetrachloride is in a separate category from the others, since a t high temperatures it reacts with sodium palmitate with decomposition and formation of tar.
106i
SODIUM PALMITATE I N ORGANIC LIQUIDS DISCUSSION
Relation between solubility and the charactcristzcs of the solvents There is a general correlation between the polarity of the solvents and the solubility of sodium palmitate in them. If the dielectric constant itself is taken as thc yardstick, it is cvident from examination of the curves of figurc 1 and the constants in table 2 that the morc polar the liquid the grcatcr its qolwnt power for sodium palmitate. Thus the solubility curves TABLE 2 Physzcochemzcal constants of tho solvents - - __
~
SOLVENT
__ ___~-_ Water Oleic acid Glycerol Diethylene glycol Isopropyl aicobol SUlOl Heptane Carbon tetrachloride -
INDYNEB PERCY.
72.75' 33.30 63.14
AT
t'C.
28 17
21.7b 33.P ca. 20' 26.8'
20 27 2.5 20 __ ;sources: iblishing Co.,
Approximate values were obtained or calculated from the follou a HILDEBRAND, J. H. : Solubility of Non-electrolytes. Reinhold S e w York (1936). * Petrohol. Standard Alcohol Co., Kew Yolk (1939). MOLL,W. L. H.: Kolloid-Beihefte 49, 1 (1939). (1 MCBAIN,&I. E. L , AND PERRY, L. H. : J. Am. Chem Soc. 62,989 (1940) 0 Handbook of Chemzstry nnd Physics Chemical Rubber Publishing Company Cleveland, Ohio (1936). LANCE.Handbook of Chevirst, y. Handbook Publishers, Inc , Sandusky, Ohio (1937). g LAWHIE, J. W : Glycerol and tho Glycols Chemical Catalog Company, Inr , Sew York (1928). International Crztzcal Tables, Vol. V I JIcGraw-Hi11 Hook Company, Inr , S e w York (1929) 1 Internatzonal Critzcal Tablcs, Vol. 11. McGraa-Hi11 Hook Company, Inc , New York (1927) f
for heptane and Sujol art. a t very high temperatures and that of isopropyl alcohol in an intermediate position, while those of water and glycerol are at low temperatures. This dependence of solubility on polarity may bc related to the possibility of secondary-valence, non-stoichiometric complev formation between the polar Polvcnt molecules and the dipole of the .nap ( 5 ) , although there is as yet no unan~biguousevidence on this point. It is interesting that the polarity of the solvents should alone sufficc to arrange them in the proper qualitative order with no explicit considcration of the force< acting between solvent and solute molecules or of the
1068
R. D. VOLD, C. W. LEQOETT AND J. W. MCBAIN
forces responsible for the formation of colloidal particles. Perhaps disregard of these factors is one of the reasons for the lack of quantitative correlation. Palmitic acid and carbon tetrachloride are exceptions, both having low dielectric constants, although they have relatively high solvent power for sodium palmitate. I n the case of palmitic acid this must be due to the tendency toward formation of acid soaps a t lower temperatures. Carbon tetrachloride also tends to react chemically with sodium palmitate. The order of solubilities is in accord also with the internal pressures ( E / V , where E is the energy of vaporization at constant volume and Y the molar volume) of the different solvents, even though some are nonpolar rand some have permanent dipoles. Since the solubility k greatest in water and least in heptane, molten sodium palmitate itself must have a m4atively high internal pressure. Moll (16),utilizing concepts of solubility which have been extensively developed for molecular solutions (2), found that, when values of u (surface tension) and p 2 / e (the square of the dipole moment divided by the dielectric constant) were used as coordinates, all solvents having an appreciable action for each of such solutes as cellulose derivatives, polyvinyl acetate, rubber, etc., had constants placing them in the same region of the graph. This scheme does not appear to apply in the present instance, there being no apparent regularity between the positions of the various liquids in a p 2 / e versus u plot and the solvent action for sodium palmitate. Also, the solubilities are obviously not governed here by the value of the permanent dipole.
Conclusions from the shape of the solubility curves It is possible to make some deductions concerning the phases present under the solubility curves from the appearance of the systems together with the shape of the curves themselves. For sodium palmitate and water, for example, the nearly perpendicular change in direction of the solubility curve at 26 per cent soap is caused by the change in the saturation phase from curd fiber to middle soap. The lower of the two maxima in the solubility curve is caused by the existence of the liquid crystalline solution phase, i.e., middle soap, and the higher by another liquid crystalline solution phase, Le., superneat soap. Examination of the curves from this point of view shows that water, palmitic acid, and glycerol all have a maximum which indicates the presence, in these three systems, of a phase which may be termed superneat soap. The maximum temperature of existence of the phase in the three cases is 284', 286O, and 275OC., respectively, indicating an approximately equal thermal stability independent of the solvent, and suggesting that here the structure of the liquid crystalline soap phase is of greater importance than the nature of the solvent.
SODIUM P.4LMITATE IN OHGANK LIQUIDS
1069
The ratios of the molal compoaitions under the superneat maxima in the solubility curves are in rough correspondence with the ratio of the lengths of the solvent molecules, the relative compositions being 1 :4.0:22.6 for palmitic acid, glycerol, and water, respectively (the actual values being 0.199, 0.803, and 4.50 moles of solvent per mole of sodium palmitate), while the ratio of the lengths of water, glycerol, and palmitic acid molecules is 1:3.3:10.4 (calculated from atomic radii (17)). This shows that in superneat soap the association between sodium palmitate and solvent cannot be a one-to-one molecular interaction between the dipole of the solvent and the carboxylate group of the soap. Possibly the structure is micellar, the solvent essentially merely filling the interstices between soap aggregates, more of the small molecules being required for this purpose than of the longer molecules. Only the curve for water exhibits a second maximum attributable to middle soap. I t may be significant in this connection that of all the solvents used water is the most polar, has the smallest molecule, and constitutes the most highly ionizing medium. Ferguson (1) has published a phase rule diagram showing middle soap in anhydrous glycerol, continuous in mixtures with aqueous middle soap. This seemed surprising in view of its apparent absence in the system sodium palmitate-glycerol, even though a sample containing 34.4 per cent sodium palmitate, 32.9 per rent glycerol, and 32.7 per cent water did have a phase having the typical stiffness and relative transparency of aqueous middle soap. Consequently a sample was made up containing 36.1 per cent sodium tallow soap (obtained from the National Oil Products Company) and 63.9 per cent anhydrous glycerol, melted to isotropic liquid, and then placed in a thennostat nt 90°C. I t was a great deal more turbid than aqueous middle soap and seemed to contain dispersed, yellow-white, soft wax-like material, although according to Ferguson’s diagram only middle soap and isotropic liquid should have been present. Likewise a system which should have been homogeneous middle soap (46.6 per cent tallow soap, 53.4 per cent glycerol) had a similar turbid appearance. The discrepancy may he one of degree rather than kind, hut it requires further investigation. The solubility curve of sodium palmitate in isopropyl alcohol has n number of sharp inflections. The changes in the saturation phase responsible for these inflections may be related to phase transitions of the nonsolvated soap, sinre their temperatures are close to those of the transitions of anhydrous sodium palmitate. Jellies, gels, ad syneresis For about 10°C. above the solubility curve, Nujol and heptane solutions containing less than about 25 per cent sodium palmitate, although themselves isotropic and flowing freely, flash bright between crossed polaroids
1070
R. D. VOLD, C. W. LEOOE’IT AND J. W. MCBAIN
when the tube is tapped, thereby demomtrating the existence of a strain anisotropy. Consequently, even though aggregation has not gone far enough to form a firm jelly or to constitute a second phase in the sense of the phase rule, the sodium palmitate molecules or micelles cannot be independent in the solution, but must be associated into some type of loose, ramifying structure. The solubility curve appears to mark a classical phase change since, on heating, systems change reproducibly a t this temperature from anisotropic gel to isotropic liquid. With more dilute systems, values obtained on cooling were generally 5 to 10°C. lower than the curvc, but these values were not reproducible, suggesting that the difference is duc to simple undercooling. In the case of the more dilute systems the formation of the anisotropic gel immobilizing all the solvent would seem therefore t o be due to formation of a second phase in a finely divided form, possibly in more orderly structure, with a large adsorptive surface. At lower temperatures syneresis occurs, some of the previously immobilized solvent being released. This indicates that there must have been an increase in the average particle size or in the compactness of the structure. Whether this is due to a gradual “ripening” of thc aggregate or to further phase changes undergone on cooling, as Lawrence suggests for very dilute Nujol systems (4), cannot be determined without further investigation. SUMMARY
Solubility curves have been determined over the whole range of temperature and composition for anhydrous sodium palmitate in each of the following solvents: palmitic acid, glycerol, diethylene glycol, isopropyl alcohol, Nujol, and heptane. At sufficiently high temperatures the soap is completely miscible in all proportions with these solvents, but a t lower temperatures anisotropic gels, liquid crystalline phases, and wax-likc or crystalline phases are formed. The solubility depends on the internal pressure and polarity of the solvents, higher temperatures being required in general to bring about complete solution in the non-polar than in the polar liquids. No phase completely analogous to aqueous middle soap was found in any of the organic liquids studied. A phase resembling aqueous superneat soap was found with palmitic acid and glycerol, but not with the less polar solvents. REFERENCES (1) FERQUSON, R. H . : Oil & Soap 14, 115 (1937). (2) HILDEBRAND, J. H.: Solubility of Non-Electrolytbs. Reinhold Publishing Corporation, New York (1936). (3) LAINQ,M. E.: J. Chem. SOC. 118, 435 (1918).
QUANTITATIVE CAPILLARY LUMINESCENCE ANALYSIS
1071
LAWRENCE, A. S. C.: Trans. Faraday SOC.34,680 (1938). LAWRENCE, A. S. C.: Trans. Faraday SOC.33, 325 (1937). LEDERER,E. L.: Kolloidchemie der Seijen. Th. Steinkopff, Leipeig (1932). For references see MCBAIN,J. W.: Nature 146,702 (1940). MCBAIN,J. W.: In Alexander’s Colloid Chemistry, Vol. I, Chap. 5. Chemical Catalog Company, Inc., New York (1926). (9) hfcBAIN, J. W., BROCK,G. C., VOLD,R. D., AND VOLD,M. J.: J. Am. Chem. SOC.80, 1870 (1938). (10) MCBAIN,J. W.,AND FIELD, M. C.: J. Chem. SOC.1933, 920. (11) MCBAIN,J. W., AND LAING, h.1. E.: Kolloid-2. 36, 18 (1924). (12) MCBAIN,J. W.,LAZARUS, L. H., AND PITTER, A. V.: Z. physik. Chem. Al47, 87 (1930). (13) hfcBAIN, J. W., AND MC~LATCHIE, W. L.: J. Phys. Chem. 86,2567 (1932). (14) MCBAIN,J. W.,VOLD,R. D., AND FRICK, M.: J. Phys. Chem. 44, 1013 (1940). VOLD,R . D., A N D VOLD,M.J.: J. Am. Chem. SOC.60,1866 (1938). (15) MCBAIN,J . W., (16) MOLL,W.L.H.: Kolloid-Beihefte49,l (1939). This paper also gives numerous references to earlier work of Wo. Ostwald and collaborators. (17) PAULING, L.: The Nature of the Chemical Bond. Cornel1 University Press, Ithaca, New York (1939). (18) SMITH,E. L.: J. Phys. Chem. 36, 2455 (1932). (19) VOLD,R.D.: J . Phys. Chem. 43, 1213 (1939). (20) VOLD,R.D.: Soap and Sanitary Chemicals 16, 31 (1940). (21) VOLD,R. D.,A N D VOLD,M.J.: J. Am. Chem. Soo. 61, 808 (1939). (22) WEITBY,G. S.: Colloid Symposium Monograph 4, 213 (1926). (4) (5) (6) (7) (8)
QUAKTITATIVE CAPILLARY LUMINESCEXCE ANALYSIS’ FRANK E. E. GERMANN
AND
JAMES
W. HENSLEY
Department of Chemistry, University of Colorado, Boulder, Colorado Received July 3, I940
Although attempts to determine concentrations by capillary adsorption methods have been made by Holmgren (10) and by Schmidt (21), little has been done along this line in the case of capillary luminescence analysis. Guyot (8) used the term “Quantitative ResearcH” as applied to the determination of color and intensity of fluorescence, and Neugebauer (13)stated that he would publish methods for the determination of concentrations using the methods of capillary fluorescence analysis. This question has also been considered by Eisenbrand (1, 3). Since the methods of capillary luminescence analysis are known to be valuable in identifying traces of material (4),it seemed worth while to study the quantitative aspects more closely by means of a direct vision 1 Presented a t the Seventeenth Colloid Symposium, held a t Ann Arbor, Michigan, June 6-8. 1940.
