SOLUBILITY OF SODIUM PALMITATE I N ORGANIC LIQUIDS
429
T H E SOLUBILITY OF SODIUM PALMITATE IN ORGANIC LIQUIDS C. W. LEGGETT, JR., R. D . VOLD,'
AND J. W. McBAIN Department of Chemistry, Stanford University, California
Received November 81, 1941
The solubility of sodium palmitate has been determined in eleven additional solvents over a wide range of temperature and composition, thus extending to nineteen the total number in which solubility has been investigated (11). There are three main problems of theoretical interest in this work: the first is the correlation of the solubility behavior of the typical soap, sodium palmitate, with the physical and chemical characteristics of the solvents, the second is the investigation of the properties and structure of the phases separating under different portions of the solubility curve, and the third is the relation of these facts to the new field of organic colloid chemistry. Such information is of interest in commercial applications in the manufacture of soap, refined vegetable oil, shaving preparations, shampoos, hair tonics, numerous medicinal preparations, specialized detergent solutions, cosmetics, fruit and vegetable sprays, toothpaste, lubricating greases, solid coal-dust fuel, and solutions for breaking emulsions, for in all these products a soap is dissolved or dispersed in one or more organic liquids. In general, the solubility of sodium palmitate in the solvents employed appears to vary in about the way that would be expected from their polarity (I), their internal pressure (l), and their ability to form hydrogen bonds (2, 7), just as in the case of non-colloidal, non-electrolytic solutions. METHODS AND MATERIALS
The same methods were used as in preceding work (11). Ti, the temperature of appearance of liquid crystal, was determined by slow cooling of the isotropic liquid. Where the separating phase was wax-like, the temperature of the phase boundary was determined by stepwise heating a t intervals of about 1'C. With the nearly opaque cresol systems it was convenient to supplement this determination with observation of the temperature of disappearance of stable foam. The values obtained are not very precise a t concentrations of sodium palmitate below 2 per cent because of the gelatinous nature of the systems below the solubility curve; otherwise they are probably good within 2°C. After determination of the temperature of formation of isotropic solution, all the tubes of each system were observed simultaneously at each of several temperatures for color, homogeneity, rigidity, opacity, and coherence. Although these observations are not sufficientlydefinitive to permit the deduction of phase boundaries, they do serve to characteriEe the general nature of the systems a t lower temperatures. An approximate measure of the relative viscosity was obtained by noting the 1 Present address: Department of Chemistry, University of Southern California, Los Angelee, California.
430
C. W. LEGGETT, JR., R. D. VOLD AND J. W. MCBAIN
time required for the meniscus to reach a level position after the tube had been tipped about 30” from the horizontal. A few preliminary observations of thc sodium palmitate-palmitic acid system were made by the hot-wire microscopic technique (10). The sodium palmitate and all other chemicals not otherwise mentioned were the same preparations used in previous work (11). Kahlbaum’s best quality o-, m-, and p-cresols, Merck’s C.P. ethyl acetate, and Merck’s C.P. glacial acetic acid were used as received. The n-cetane was an especially pure product generously furnished by the Shell Development Company; the values given by them for the refractive index and the density are as follows: nioo= 1.4347 to 1.4349; diO,” = 0.7741. Eastman Kodak “research quality” n-cetyl alcohol wab kept in an evacuated desiccat,or over phosphorus pentoxide for 3 days before use. The acetamide was a student preparation which had been twice steam-distilled; its melting point was 79-81°C. The n-butylamine was prepared by distillation from a Sharples Solvent commercial brand which had stood in contact with sodium hydroxide pellets for 3 days; its boiling point was 78.6-78.8’C. The ethyl alcohol was prepared by extensive refluxing and repeated distillations from calcium and magnesium ribbon until the product was anhydrous, as judged by the refractive index. Eastman Kodak “research quality” heptyl alcohol was distilled, the middle portion of the distillate being anhydrous as judged by the refrartive index. EXPERIDIEKTAL DATA
To conserve spare the individual observations are not given. Instead, the solubility curves are shoivn in figures 1, 2, 3, 4, 5, and 6, in which the various curves are grouped to show the systematic variation with the nature of the solvent. Table 1, showing the solubility of sodium palmitate a t various temperatures, was constructed from these graphs. The hot-wire microscopic observations on the system sodium palmitatepalmitic acid show the existence of several transitions below the solubility curve a t concentrations greater than 74 per cent soap.2 One of these occurs around 80°C., another around 125’C., and a third around 22SoC.,but this work has not yet been carried sufficiently far to permit the construction of phase boundaries. However, the points around‘225’C. do seem to fall on a curve isolating superneat soap from the other phases. The observations of the appearance of different types of systems mav be summarized as follows (3) : At high temperatures and concentrations of soap, the phase separating from isotropic liquid has the typical appearance of neat, superneat, and subneat soap. .4s the concentration is decreased, this is replaced by a soft, wax-like material which may be rather translucent, while a t low concentrations nearly transparent gels are formed. At intermediate temperatures a more opaque, harder, wax-like material prevails. At room temperature, except for the transparent or translucent anisotropic gels in the dilute systems, the material tends to be either like a hard wax or like an anhydroufi soap curd
* .ill
concentrations are expressed in weight per cent.
43 1
SOLUBILITY O F SODIUM PALMITATE I N ORGBNIC LIQUIDS
phase (crystalline), with heterogeneity of appearance over large ranges of concentration. The appearance of all the systems with non-polar solvents is similar a t corresponding temperatures and compositions; syneresis occurs at room temperature, TABLE 1 Solubzlzt7/ of sodzum palmitate zn orpanzc laquzds (Weight per cent of sodium palmitate a t temperature quoted) SOLWILITY
___
SOLVENT
30'C. ~
Water. . . . . . , . . . .
0.5
,
SOT.
l.---'y-
1SO'C.
90°C.
--
ca.10
I
~
-,
200°C. ,
127.5 ' 3 4 . 2 I M . 6
~
250°C.
- .ip
58.5
'
~
I
I
Palmitic acid
,
41.9
i
~
280°C.
72.0(sn)* 85.5 (sn) 90.7 (n)
I
64.4
6 3 . 3 , 73.9 (sn) 97.2 90 2 (sn) 91.7 (n) I
l
1
Glycerol.
0.3
ca 1 0
15s
338
53 6
67 4 (sn) 84 4 (sn) 85 6 ( n )
91 0
o-Cresol m-Ciesol p-Cresol
1 .o
0 2 27 1 8
1 2 100 9 0
9 2 520 602
65.1 73.0 76 4
91.0 93.0 94.0
95 0 97 0 976
2 3 ea06 0.3
9 7
1 10
696 240 31 6 5s
87.5 75.0 69.5 44.9
94.8 94.0 93.4 85.4
os
10 14 0 5?
20 0 86 4 53 3
90 5
Ethyl alcohol Isopropyl alcohol n-Heptyl alcohol n-Cetyl alcohol
1.3 0.4
n-Heptane n-C e tane SUJOl
0 3
04?
0 17
0 17 0 17
lo
17
3 1 10
I
0 5 0 3? 0 11
0 51
0 3"
'
07 0 '37 6 1975
95 0 94 5
I
Diethylene glycol
0 4
n-Butylamine
0 27
Acetic acid Ethyl acetate
ea. 1 5
I
39 9
87 2
90 0
97 3
1.0
58.0
s5 5
91
05
81.2
85 1
95 3
98 5
?
95 5
97 5?
78 5
96 2
98 0
0 4?
?
?
'755
7
?
I
?
7
I
I
Acetamide
, 9.2
?
?
* (sn) stands for superneat and (n) stands for the neat soap phase. is more marked than with other systems, and increases in amount with increasing temperature (at least up to 13OOC.); gels occur a t low concentrations; a phase like neat or subneat soap occurs over nearly the whole concentration range directly below the solubility curve; and a t lower temperatures, phases like
432
C. W. LEGGETT, JR., R. D. VOLD AND J. W. MCBAIN
waxy soap and curd fiber phase are found. In the case of the alcohols the separating phases under corresponding parts of the solubility curve in different systems are similar in appearance; a curd-like phase appears below 117OC.and a wax-like phase over varying ranges of temperature and composition; syneresis occurs over the whole temperature range in the more dilute systems; and translucent to nearly transparent milky white gels are present below 10 per cent. The cresol systems resemble the alcohol systems except that all are colored red to yellow brown; there are gels to jellies a t low concentrations; syneresis is somewhat less than with the other systems; and there are wax-like phases below 140°C.which appear to change to a curd phase around 50 per cent soap. With both hydrocarbons and alcohols the range of composition over which transparent to translucent anisotropic gels occur increases with increasing length of the hydrocarbon chain of the solvent molecule. In all cases the appearance of liquid crystalline phases, the formation of gels, and the occurrence of syneresis suggest the colloidal nature of the systems, at least at temperatures below those of transition to isotropic liquid. The viscosity of isotropic solutions containing less than 50 per cent sodium palmitate is not far different from that of the pure solvent, except in cases where gel formation tends to occur in the dilute solutions. At higher concentrations of soap there is a fairly regular increase of viscosity with increasing concentration to rather high values despite the elevated temperature. The mobility of the polar solutions appears to be somewhat less than that of the non-polar solutions. DISCUSSION
Types o j solubility curves The solubility curves may be divided roughly into three classes, depending on the polarity of the solvent. This is illustrated in figure 1, where the curve for cetane is typical of non-polar liquids, that for cetyl alcohol of liquids of intermediate polarity, and that for palmitic acid of highly polar solvents. The non-polar liquids are characterized by very low solvent power for sodium palmitate except a t very high temperatures where the solubility increases tremendously within a very small temperature interval. Solubility curves for these systems also have a sharp change of slope around 85 per cent soap, sigaifyhg a change in the phase in equilibrium with the isotropic solution. Solubility curves for sodium palmitate in liquids of intermediate polarity tend toward linearity between the melting point of the soap and about 100°C., below which temperature the solubility is low and not greatly influenced by temperature. Many of these curves (cj. figures 3 and 4) have a number of sharp changes in slope indicative of changes in the separating phase. Diethylene glycol and ethyl alcohol, unlike the other semi-polar liquids, appear to have entirely smooth solubility curves, although a more sensitive experimental method might demonstrate the presence of breaks. The polar liquids have the greatest solvent power for sodium palmitate, the solubility curves dropping rapidly toward lower temperatures while the sohbility is still high, Water, glycerol, and palmitic acid give rise to a phsse (super-
s o L u B I L I w OF SODIUM PALMITATE I X ORGANIC LIQUIDS
433
neat soap) different from the phases of pure sodium palmitate, the presence of which causes a maximum in the solubility curve. Water alone has an additional solvated liquid crystalline phase (middle soap), giving rise to a second maximum in its solubility curve. Table 1 gives the solubility of sodium palmitate in each of the solvents at a series of temperatures up to nearly the melting point of the soap. Above the melting point there is complete miscibility except where superneat soap is formed, and even in these cases complete miscibility is attained a t slightly higher temperatures. Where the temperature and concentration are both high, the solubility (on a weight per cent basis) is very similar in all solvents. In more dilute systems, however, the differences between the solvents become apparent, it being evident that in general higher temperatures are required to attain a given solubility the less polar the solvent. The qualitative difference in the temperature-dependence of the solubility between polar and non-polar solvents suggests the existence of somewhat different modea of interaction between solvent and solute in the two cases. In the case of the non-polar solvents, the solvent molecules can be regarded simply as interfering with the ease of orientation of the soap molecules or micelles, thereby necessitating lower temperatures before sufficient coherence occurs to bring about the formation of a liquid crystalline or waxy phase. The much more rapid lowering of the temperature with increasing concentration of solvent in the case of the polar liquids is probably due to the tendency toward combination between the dipole of the solvent molecule and the polar head of the soap molecule, which prevents coherence of the latter to form nuclei of a second phase. The same process might also take place even after nuclei had formed, thus blocking their growth. Correlation o j solubility with polarity Different measures of the polarity of the solvents are assembled in table 2. No effort has been made to obtain the most precise dielectric data, since the values are used only for qualitative comparison. Furthermore, the polarity of the actual solutions, ae yet undetermined, would be preferable anyway as a basis for exact comparison rather than the polarities of the pure solvents. Although the total polarization of a molecule can be regarded as being made up of the sum of three terms,--electronic polarization, atomic polarization, and orientation polarization (due to a permanent dipole) (Q),--only the latter seems to be important in connection with the solvent properties of polar liquids. This is strikingly evident from table 2, where it appears that water and cetane are nearly equally polar if judged by the value of total polarization. The dipole moment itself is not a satisfactory criterion since, for example, it is nearly constant for all the alcohols, even though the long-chain members are more nearly paraffinic than polar. The dielectric constant itself works reasonably well (ll), but includes in it the net result of a number of Merentfactors. Instead, we propose to use the quotient of the dipole moment by the molar volume as the criterion of the effective polarity of the different liquids as solvents for
TABLE 2 Various measures of the polarity of the solvents SOLVENT
_____~__Watei Acetic acid Acetamide Glycerol 1-Nitropropane Ethyl acetate Diethylene glycol Butylamine Palmitic acid Eihyl alcohol Isopropyl alcohol Heptyl alcohol Cetyl alcohol
'
VOLUME A I ZO'C.
IIVE OLAPlIYl
rr/v
VQ
P.+
Pav
,
pm
~
~
80 0 7 1 59 2 562
1.85 1.23 3.6 2.5k
18.0 57.2 50.9 73.1
10.3 2.15 7.1 3 4
25 Ob 6 1 ea 4OC 5 3 I
