SODIUM STEARATE-CETANE-WATER
SYSTEMS
39
SUMMARY
The Raman and ultraviolet-absorption spectra of liquid dimethyl ether-boron trifluoride have been recorded. The complex shows a set of vibrational frequencies which differs radically from those of the parent molecules. Very little of the unassociated substances are present in the liquid phase. The complex shows a faint absorption at appreciably lower frequencies than do the ethers. However, the intensity of absorption of the complex is much less in the region below X 2300, where ethers begin to absorb strongly. An explanation is presented in terms of a molecular orbital description of the association process. REFERENCES (1) BAUER,S. H . , FINLAY, G. R., (2)
(3) (4) (5) (6) (7) (8) (9) (10)
AND LAUBENGAYER, A. W.: J. Am. Chem. SOC.66, 889 (1943); 67, 339 (1945). BRIEGLEB, G., AND LAUPPE,W . : Z . physik. Chem. B36, 42 (1937); stannic chloride dissolved in alcohol and in ether. BRIEGLEB, G . , AND LAUPPE,W.: Z . physik. Chem. B36, 56 (1937). BRIEGLEB, G . , AND LAUPPE,W . : Z.physik. Chem. B37, 260 (1937); simple alcohols and ethers with halogen acids. GERDING,H . , AND SMIT,E . : Z. physik. Chem. B61, 200 (1942); aluminum chloride (A12Cla.2HzO). LAUBENGAYER, A. W., AND FINLAY,G. R . : J. Am. Chem. SOC.66, 887 (1943). LEVY,H . A . , AND BROCKWAY, L. 0.: J. Am. Chem. SOC.69,2085 (1937). MULLIKEN, R. S.:J . Chem. Phys. 1, 492 (1933); 3, 506 (1935). PAULINQ,L., AND BROCKWAY, L. 0.: J. Am. Chem. SOC.67, 2684 (1935). THOMPSON, H. W., AND LINNETT,J. W . : Proc. Roy. SOC.(London) A166, 108 (1936).
THE BEHAVIOR OF SODIUM STEARATE WITH CETANE AND WATER ROBERT D . VOLD
AND
JOSEPH M. PHILIPSON
Department of Chemistry, The University of Southern California, Los Angeles 7 , California Received September 26, 1946
This paper presents data on the transition temperatures detected in mixtures of sodium stearate, cetane, and water, and describes the appearance and properties of these systems. Some attempt is made to represent the data in terws of a phase diagram, and to relate the changes in observed behavior to the changes in the nature of the solvent. These systems exhibit a great variety of colloidal phenomena, existing as clear solutions of “solubilized” oil, as oil-in-water emulsions, as soft liquid crystalline phases, and as gels which vary from transparent jellies t o hard wax-like solids. They are also of industrial importance in such products as cosmetic preparations, lubricating greases, etc.
401
ROBERT D. VOLD AND JOSEPH M. PHILIPSON EXPERIMENTAL METHODS
Transition temperatures were determined (4,10) by visual observation between crossed polaroids of l-g. samples sealed in glass tubes supported on a rack in an electrically heated air oven. The rack was made of two strips of thin metal screwed together between the tubes, and fastened to a shaft with an external crank. This arrangement permitted simultaneous observation of several tubes and also made it possible, by turning them end over end, t o prevent any separation into layers on cooling. This latter precaution is necessary in order t o obtain a macroscopically homogeneous material for examination. Stirring was accomplished by three adjustable fan blades set at different levels on the same shaft. No serious temperature gradients were present, since several thermometers a t different positions in the oven read substantially alike. Moreover, duplicate determinations of the melting point (Ti)made with a tube in different parts of the oven agreed within 2°C. Thermometers used were calibrated in place by comparison with a previously standardized thermometer (11) and checked a t the steam point. All reported temperatures are corrected values. The observed changes occurred at temperatures designated as follows: Ti,Tu, Tu,T,,,, and Th. Ti, defined as the temperature of formation of a liquid isotropic t o polarized light, was usually determined by slow heating. That the values so determined are reliable is indicated by the fact that reproducibility was obtained within 2"C., and by the further fact that in more concentrated systems the same value of T i was obtained on both heating and cooling. Tu,observed in the system sodium stearate-cetane, may be defined as the temperature a t which, on slow heating, an opaque, optically anisotropic gel1 becomes relatively clear (very translucent but not transparent), although remaining anisotropic. It is a very sharp change, checks within 2°C. being obtained on both heating and cooling. However, the subjective, arbitrary nature of the degree of translucency selected makes evaluation of these data difficult despite their precision. Tu,determined in the binary system, is the temperature at which a gel melts on slow heating, the system becoming sufficiently fluid t o flow when the tube is inverted. The liquid phase may be either isotropic or anisotropic, depending on the concentration. Above 50 per cent2 sodium stearate the gel melts and sets .at the same temperature, but in the more dilute systems the setting point is usually lower, leading to an uncertainty of perhaps 10°C. in Tu in these cases. T,, observed in the system sodium stearate-cetane-mater, is the temperature a t which on slow cooling the translucent gel first formed below Ti becomes slightly opaque. This change is somewhat elusive and the temperatures are rather uncertain. Th, determined in the ternary system, is the temperature where the gel changes on cooling from slightly opaque to densely opaque, the reverse change being 1 Throughout this paper gel is used in the broad sense to designate all solvated systems of sodium stearate exhibiting a yield value without distinction of the different phases which are probably present. 2 A11 concentrations are expressed in terms of weight per cent.
SODIUM STEARATE-CETANE-WATER
SYSTEMS
41
noted on heating. It is much sharper and more easily observed on cooling. The temperatures obtained on heating and cooling are different, even though both are reproducible. Temperatures obtained still checked on both heating and cooling even when rates of temperature change were varied from 0.2" to 1°C. per minute, but the difference between the two values persisted. In another experiment a system was heated until about equal amounts of light opaque and dark opaque material were hitially present, and Th was then determined on cooling. The value obtained was exactly the same as when Thwas determined on cooling in the usual manner with no dark opaque material initially present. Hence it appears that the lower value of Th found on cooling than that found on heating is not due to undercooling with respect to formation of the dark opaque phase. In summary, Ti occurs a t the highest temperature of the set of transitions. The first change occurring below this temperature is Tu. At somewhat lower temperatures is the change described as taking place a t T,. T, of the system sodium stearate-cetane-water may be due to the same transition designated as Tuin the system sodium stearate-cetane. T, was not reported from visual evidence in the system sodium stearate-water (5) and in the present work became progressively more difficult to recognize as the proportion of water in the system increased, not being detected when the solvent contained 75 per cent water-25 per cent cetane. However, it should be emphasized that the appearance is almost always different just above Th (light opaque) than just below T i (translucent and nearly clear), so that some change probably takes place between these two temperatures (at Tm), even though its occurrence is usually so gradual that the exact temperature could be determined in only a few cases. Th is the lowest change detectable visually. It probably corresponds to the change a t T , in the system sodium stearate-water, i.e., the disappearance on slow heating of the last trace of white crystalline solid. It is probably also related to the T , curves previously reported (10) for sodium palmitate in various organic liquids. T , , the temperature at which the gel melts, is generally in the vicinity of either Ti or Tu. MATERIALS
a
The sodium stearate used in the work on the binary system was the &me preparation used in a previous study (8), and had been prepared directly from Eastman Kodak best-quality stearic acid. The value of Ti was only 282"C., contrasted with a value of 288°C. for a sample prepared from Kahlbaum's best stearic acid (5). This lowering is not due to hydrolysis due to action of carbon dioxide, since the soap contained 0.08 per cent excess sodium hydroxide. It is more likely due to the presence of unsaturated impurities, since a sample of Eastman Kodak best-quality stearic acid, purchased subsequent to the Stanford sample but presumably from the same batch, had an iodine number of 3.0. Laboratory preparation sodium stearate C was used in the work on the ternary system. This soap was made from a stearic acid of capillary melting point 69.1" to 70.1"C., a setting point of 67.8"C., an iodine number of 0.0, and an equivalent
42
ROBERT D. VOLD AND JOSEPH M. PHILIPSON
weight of 285.3. This acid was obtained as one of the good-but not the bestfractions following a previously described (7) method of purification of stearic acid. During preparation of the soap by neutralization with carbonate-free alcoholic sodium hydroxide care was taken to minimize possible absorption of carbon dioxide. The wet gel was dried in a vacuum oven a t 50"C.,with frequent breaking up of the cake, until relatively dry lumps were obtained which were then crushed with a stirring rod and further dried a t 105°C. This soap melted to a clear, transparent, water-white liquid a t 287"C., the same value being obtained with another sample of sodium stearate (Laboratory Preparation A) from our TABLE 1 Transition temDeraturea i n the svstem sodium stearate-cetane SODIUM STEARATE
I
1
weight per cent
"C.
0.79 2.94 6.38 9.80 12.8 15.6 19.6 27.2 33.2 34.5
155 163 164 164 166 169 184 204 218 217
Tu
1
1
SODIUM STEARATE
Ti
oc.
"C.
weight per cen,
"C.
"C.
'C.
169 172 172 187 187
164 197 210 212 197 209 209 207 202 197
40.0 45.5 50.4 57.9 70.3 82.1 90.1 94.4 97.8
234* 236* 238* 240 241 248 249 260 273 282
184 185 186 209 230 235 244 250
199 210 212 238 241 248 249 263 273 282
100.0
(254)
best stearic acid (7). This agreement in Ti, although satisfactory for comparative purposes, does not establish the quality of the soap, since the setting point of stearic acid A was 1.7"C.higher than that of stearic acid C. The cetane used was a specially pure du Pont product, furnished us through the courtesy of the Shell Development Company. Its freezing point, determined as the temperature of half-melting of a gallon bottle of the cetane, was 17.32"C. The density at*20"C. was 0.7720 and the refractive index 1.4346. Tdis compares with values for the pure hydrocarbon (1) of 18.15"C. for the melting point, 0.7737 for the density, and 1.4343 for the refractive index. EXPERIMENTAL RESULTS
Transition temperatures Transition temperatures detected in the system sodium stearate-cetane are collected in table 1. Those observed in the ternary system are assembled in table 2. Appearance of binary systems Tubes containing sodium stearate and cetane were observed before homogenization after standing several weeks at room temperature. Contrary to previous
'
SODIUM STEARATE-CETANE-WATER
43
SYSTEMS
reports with sodium palmitate (3), it was found that cetane wets sodium stearate easily a t room temperature, and that swelling occurs t o a solid white gel. The same result was obtained with sodium palmitate. Systems containing less than TABLE 2 Transition temperatures in the slrstem sodium stearate-cetane-water RATIO SODIUM STEARATE
CTIGHT PER CENT CETANE WIGHT PER CENT WATER
Ti
Th
(BEATING)
Th
T,
(COOLING)
,Q. 6
h5?
I
"C
weight per cent
.
.
"C
"C
.
3.1 14.1 21.2 28.8 40.7 50.9 60.0 73.8 78.3 86.3 94.9t
0.304 0.317 0.372 0.326 0.319 0.301 0.328 0.434 0.396 0.340 0.183
74* 177 245 262 268 263 255 260 262
74 75 83 90 84 108 118 128 145
46 48 67 83 71 95 102 110 125
2.5 11.5 30.1 39.9 48.0 64.6 75.7 80.4 88.3
0.972 0.891 0.983 0.977 0.930 1.045 1.008 1.12 1.16
187* 199 250 254 245 242 237 246
75 80 85 110 120 125 167
57 64 69 95 104 117 140
2.70 2.79 2.85 2.91 2.81 3.14 2.84 3.56 3.36 1.89
204* 250 239 221 209 206 215 247 261
80 82
50 66
95 119 133 135 145
85 107 111 122 125
3.5 14.2 22.1 29.4 43.7 50.3 62.5 77.5 90.5 95.5
'
"C
.
171
187 180 130 146 160 168
* Forms two immiscible isotropic liquids.
t Not plotted since solvent ratio too discordant. 30 per cent soap consisted of a loose gel with excess free cetane. Between 30 per cent and 60 per cent sodium stearate the cetane was completely absorbed by the soap, while above 60 per cent lumps of apparently dry sodium stearate were present.
44
ROBERT D. VOLD AND JOSEPH M. PHILIPSON
It was likewise possible to separate the systems into groups on the basis of their appearance after standing two weeks at room temperature after having been heated to formation of isotropic liquid and having been thoroughly mixed. Up to 10 per cent sodium stearate systems consisted of opaque white gel surrounded by clear cetane which had separated by syneresis, the amount of free cetane decreasing with increasing concentration of soap. Between 12 and 30 per cent sodium stearate there was present a soft smooth white gel with numerous holes in it. From 30 to 50 per cent soap the gel was still smooth, white, and with holes, but looked harder and had shrunk away somewhat from the walls of the tube. Between 60 and 90 per cent sodium stearate the gel looked still harder, had a very light buff color, and had not shrunk away from the walls of the tube. At 95 per cent and 100 per cent there was formed a harder looking, somewhat darker colored cake. Appearance of ternary systems3
*
Ternary systems were prepared such as to determine qualitatively the chief phenomena encountered with approximately constant solvent ratios of 25 per cent cetane75 per cent water, 50 per cent of each, and 75 per cent cetane25 per cent water. The appearance of these systems after aging for a month a t room temperature after having been thoroughly mixed as isotropic liquid is summarized in table 3. The systems described in table 3-but not all those listed in table 2-were also observed a t intervals of 25” to 50°C. as they were heated and cooled (at about 0.5” per minute). It should be noted in connection with these observations that the appearance under these conditions is not necessarily the same as a t equilibrium at the respective temperatures. In these descriptions the terminology “dark opaque,’) “light opaque,” and “translucent” (meaning fairly clear) is used to indicate an increasing degree of transparency of the system. Where observations were made at temperatures between the heating and cooling values of T h (cf. table 2), the appearance depends on whether the given temperature has been reached on heating or on cooling. In such cases the appearance on heating is given in the following descriptions. The gels, which may be wax-like or crystalline solids a t the lower temperatures, are optically anisotropic unless otherwise noted. These observations will be described in turn for each solvent ratio. In order to conserve space, only the changes in appearance a t each successive temperature are noted. These data are adequate to make possible the construction of a table showing the condition of every sample at each temperature, which facilitates comparison of the different systems. For systems in which the solvent is 25 per cent cetane75 per cent water a t 50”C., the sample with 3.1 per cent sodium stearate has changed to a stable emulsion, the other compositions still appearing substantially the same as a t 8 A more complete description of these systems can be found in the Research Report of Joseph Philipson, Spring, 1944, which is on file in the Library of the University of Southern California, LOSAngeles, California.
TABLE 3 Appearance at room temperature of.~systems of sodium stearate-cetane-water WEIGHT PER CENT SODIUM STEARATE
3.1
14.1
28.8
SYSTEM I :OMPOSITION OF SOLVENT: 25 PER CENT CETANE 75 PER CENT WATER
Appearance Soft-appearing non. homogeneous gel Two layers; bottom appears t o contain more solid which may indicate settling. No free liquid due t o syneresis. Soft, opaque, smooth , homogeneous gel. Looks like bottom layer of preceding sample. Like preceding, only harder and dryer.
50.9
Looks dryer than preceding; has holes in it, and ar incipient visual structure.
73.8
White fibrous solid with glossy sheen
86.3
Like preceding but not so glossy. Resembles anhydrous sodium stearate which looks crystalline and glossy when sufficiently pure.
WEIGHT ?ER CENT SODIUM iTEARATE
2.5
SYSTEM I 1 'OMPOSITION OF SOLVENT: 50 PER CENT CETANE 50 PER CENT WATER
Appearance Like 3.1 per cent tube in system I, but two layers arl easier t o distinguish. Some l i q uid (probably ce tane) has synerized from gel.
11.5
Like 14.1 per cent tube in system I. Some syneresis.
30.1
Like 28.8 per cent tube in system I. No syneresis.
48.0
Soft gel harder thai preceding sample
64.6
Soft, smooth, whit gel. Some striations. Intermediate appearanc between preceding and following samples.
75.7
88.3
Very different from 48 per cent sample; harder and more glossy. Ha a pattern of streaks. No hole€ Is less glossy tha: 73.8 per cent tub i n system I.
WEIGHT 'ER CENT SODIUM ITEARATE
3.5
Appearance Two layers. Bottom similar t o 3.1 per cent tube in system I. Top speckled with solid sodium stearate in cetane gel. Small amount of syneresis.
14.2
Like 14.1 and 28.8 per cent tubes i n system I. No syneresis; smooth unctuous appearance.
22.1
Intermediate between preceding and following samples. No holes.
29.4
Like the two preceding samples. Many small holes in gel.
50.3
Like preceding sample but dryer and lumpy.
62.5
Hard white solid with streaks like 73.8 per cent tube of system I. Very glossy.
77.5
Very dry white solid. No holes. Shrinks from walls of tube. Very small droplets visible on glass where gel has shrunk away. Less sheen than preceding sample.
90.5
Like 88.3 per cent tube of system 11.
Like anhydrous so. dium stearate.
45
SYSTEM I11 OMPOSITION OF SOLVENT: 75 PER CENT CETANE 25 PER CENT WATER
46
ROBERT D. VOLD AND JOSEPH M. PHILIPSON
room temperature. At 75°C. the 3.1 and 14.1 per cent systems are emulsions, the 28.8 per cent system is a light opaque gel, and the more concentrated systems are all dark opaque' gels. There is little change on heating to lOO"C., only the 50.9 per cent system changing from dark opaque to light opaque gel. At 125°C. the 3.1 and 14.1 per cent systems are emulsions and the remaining compositions all light opaque gels except for the 86.3 per cent system, which is still a dark opaque gel. At 150°C. the appearance is the same as a t 125"C., except that the 28.8 per cent system has changed from a light opaque to a translucent gel and the 86.3 per cent system has become light opaque. At 200°C. the 3.1 per cent system consists of two immiscible isotropic liquid layers, the 14.1 per cent system is still a stable emulsion, the 28.8 per cent system is an isotropic solution, and all other compositions are translucent gels. The appearance a t 250°C. is similar except that the emulsion of the 14.1 per cent system has broken into two immiscible isotropic liquid layers, and the 40.7 per cent system has melted t o isotropic solution. In systems in which the solvent is 50 per cent cetane-50 per cent water, at 50°C. the 2.5 per cent sodium stearate system has turned to a stable emulsion but the appearance of the other systems remains unchanged from that a t room temperature. At 75°C. the 2.5 per cent system is an emulsion, the 11.5 per cent system an opaque gel, the 30.1 per cent system a light opaque gel, and the more concentrated systems dark opaque gels. The appearance a t 100°C. is about the same as a t 75"C., except that the 48.0 per cent system has changed to a light opaque gel. By 125°C. the 64.6 and 75.7 per cent systems have also changed from dark to light opaque gels. At 150°C. the appearance is still the same, except that the 30.1 per cent system has become a translucent gel. At 200°C. both the 2.5 and the 11.5 per cent systems consist of two immiscible isotropic solutions, the 30.1 per cent system is a single isotropic solution, and all other compositions are translucent gels. The appearance a t 250°C. remains unchanged from that a t 200°C. except for the 64.6,75.7, and 88.3 per cent systems, which melted to isotropic liquid. In systems where the solvent is 75 per cent cetane25 per cent water the appearance a t 50°C. was essentially the same as a t room temperature, except that the system containing 3.5 per cent sodium stearate changed to a stable emulsion. At 75°C. the 3.5 per cent system is an emulsion, the 14.2 per cent system an opaque gel similar to the 11.5 per cent sample in the 50 per cent water-50 per cent cetane system, while the more concentrated systems are dark opaque gels. On further heating to 100°C. the 22.1, 29.4, and 50.3 per cent systems change from dark to light opaque gels. By 125°C. the 62.5 per cent system has also changed to a light opaque gel, all others remaining unchanged. L4t150°C.the (52.5 and 77.5 per cent Rystems have changed to translucent gels, and the 90.5 per cent system from dark to light opaque gel. At 200°C. the 3.5 per cent system has separated into two isotropic liquid layers, the 14.2 per cent system is still an opaque gel, and all other compositions are translucent gels. At 250°C. the 3.5 and 14.2 per cent systems consist of two isotropic liquid layers, and all other compositions have melted to homogeneous isotropic solutions.
,
SODIUM STEARATE-CETANE-WATER
47
SYSTEMS
It is apparent that the systems more dilute in sodium stearate (15 per cent and less) behave similarly irrespective of the ratio of cetane t o water in the solvent. None of them melt to a single isotropic liquid. At low temperatures these systems are opaque or semitranslucent gels, and remain in this state on heating until formation of an emulsion or two separate immiscible liquid layers. Likewise independent of solvent ratio there is a great difference in the appearance a t room temperature between systems containing more than about 50 per cent sodium stearate and those containing less. All the more dilute gels appear to be alike superficially, and have an ointment-like appearance and texture. All the more concentrated gels resemble one another in being striated and "solid," and in having a high sheen. DISCUSSION '
Comparison of sodium stearate and sodium palmitate
It was earlier shown (4, 10) that sodium palmitate is more soluble in polar than in non-polar solvents, judged either by the equilibrium concentration at a given temperature or by the temperature required to cause complete miscibility a t a given concentration in different solvents. The present work, although confirming this general picture, necessitates some revision of the details. Specifically, there appears to be somewhat more interaction of non-polar cetane with sodium soaps with resultant complete solution a t lower temperatures than previously reported. This fact is apparent from the solubility curve of sodium stearate in cetane shown in figure 1, since the curve falls off much more steeply with decreasing soap concentration than had been reported previously for sodium palmitate. Consequently, a few experiments were carried out using the same preparation of sodium palmitate as in the previous work (4), with the following results:
#er cent
"C.
"C.
"C.
7.4 29.9 86.8 87.9
228 235 250 255
167 213 244 246
218 223 245 246
These data show that sodium palmitate has the same type of solubility curve The discrepancy in the older work is not so extreme in the more concentrated systems but becomes serious in the more dilute regions. It is in the dilute systems that undercooling is most persistent, and also in this range that T,, the temperature of gel formation, is far above Ti. It therefore seems likely that gel formation was confused with true melting in the older work, resulting in falsely high values for Ti in the more dilute systems of sodium palmitate in cetane, nujol, and heptane.
. in cetane as does sodium stearate.
ROBERT D. VOLD AND JOSEPH M. PHILIPSON
I
Q
k f G H T PCRCE",
SODIUM STEARATi
FIG.1. Phase behavior of sodium stearate and cetane: 0, Ti; 0 , Tu;A,To.
Comparison of cetane and water systems of sodium stearate The melting point curve (Ti)of sodium stearate in cetane falls continuously with decreasing concentration of soap, remains a t a relatively high temperature even in dilute systems, and shows several discontinuous changes in slope indica-. tive of changes in the equilibrium phase separating from the isotropic solution. In water (5) the Ti curve has two maxima (cf. figure 2), corresponding t o the presence of superneat and middle soap, which are the equilibrium phases separating over a very wide concentration range on cooling the isotropic liquid. It seems that water, through interaction with the polar heads of the soap molecules, makes the existence of the multiplicity of phases found with anhydrous sodium stearate
SODIUM STEARATE-CETANE-WATER
SYSTEMS
49
impossible, except at very high soap concentrations. In the case of cetane i t seems quite possible that the changes in slope of the T i curve are related t o the
WEJGHT PERCENT, SODIUM STEARATE
FIG.2. Solubility of sodium stearate in mixtures of cetane and water. Curves pertain t o different solvents according to the following legend: - - - -, water; 0-, 0.75 weight per cent water-0.25 weight per cent cetane; A - - -, 0.50 weight per cent water-0.50 weight per cent cetane; 0 - -, 0.25 per cent weight water-0.75 weight per cent cetane; -.-.-. , cetane. L , transition temperatures of anhydrous sodium stearate.
-
transition temperatures of the anhydrous soap. Apparently cetane can not overcome the attractive forces between the carboxylate groups, so the possibility of stepwise melting (9, 11) may still persist even in the highly solvated systems.
50
ROBERT D. VOLD AND JOSEPH M. PHILIPSON
Consequently a large number of transitions may be expected in the cetane system, with various phases related in structure to those of anhydrous sodium stearate present over most of the composition range. This difference in behavior between polar and non-polar solvents seems t o be rather widespread (4, 10). In ethyl alcohol, the most polar of the series, sodium palmitate has a smooth solubility curve, while in the less polar alcohols the curves show a number of breaks indicative of changes in the equilibrium phase. Similarly, glycerol gives a solubility curve much like that of water, while the less polar cresols give curves with several breaks. In aqueous systems there is a transition a t fairly low temperatures (the T , curve) abov6 which no crystalline or waxy phases are found. In cetane systems such a transition was not determined by visual observation even though its presence may be assumed, since systems a t temperatures just below Tulooked very different from their appearance a t room temperature. In any case, relatively opaque material (presumably waxy or crystalline) persists to much higher temperatures in the cetane than in the water system. This fact may be of considerable importance in terms of a dependence of the properties of the cetane system on mechanical working, since “solid” phases may persist a t elevated temperatures even in dilute systems (2). The course of the Tu curve in the cetane system, if it can be accepted as a phase boundary, is of interest. First, it would require that the phases in equilibrium with isotropic liquid all contain relatively large amounts of cetane as part of the homogeneous phase. Further, since Tu appears to be independent of composition from about 30 to 50 per cent sodium stearate, there would have to be three condensed phases coexisting in this range. However, since Tu is merely the temperature of an increase in transparency as determined by visual observation, it need not necessarily refer a t all to a phase change.
Systems of sodium stearate in mixed solvents Palit (6) has recently found that mixed solvents such as ethylene glycol and butyl alcohol dissolve up to 10 per cent soap a t room temperature, even though the solvent power of the component liquids is poor. Extending this idea he also found that liquids such as monochlorohydrin, in which polar and non-polar solvent power are combined in the same molecule, were also good solvents. In this latter case, for example, the temperature for formation of an isotropic 10 per cent solution of sodium palmitate was only 58”C., as contrasted with 84°C. in a relatively polar liquid like glycerol and 244°C. in heptane. No such simple generalizations are possible in the present case, since the two liquids involved are non-miscible. In all the mixed solvents studied (ratios of 3 to 1, 1 to 1, and 1 to 3 cetane to water), the behavior appeared to resemble that of the aqueous system rather than the cetane system, with the exception that no evidence was found for the existence of a phase like middle soap. The T i curve is free of sharp changes in slope in all the systems containing mater, and passes through a maximum similar to that due to formation of superneat soap in the aqueous system. Apparently even in the solvent containing 75 weight
SODIUM STEARATE-CETANE-WATER
SYSTEMS
51
per cent cetane there is sufficient water present to destroy the phases related t o those of anhydrous sodium stearate, replacing them by a phase resembling aqueous superneat soap. . Possibly these observations afford a clue as to the importance of a small amount of water in conferring stability and unctuousness on non-aqueous soap systems. It m w act by changing the phase in which the soap occurs, or by reducing the capacity of the system to undergo numerous phase changes on heating. Superneat soap has a very high stability, as shown by the fact that its melting point is nearly as high as that of anhydrous sodium stearate. Consequently, although first additions of water to cetane lower T i for the system below that in cetane itself, in other composition ranges Ti is actually increased by the addition of water, owing to the formation of a more stable phase different from that existing in the cetane system. In the 50 per cent cetane-50 per cent water system the Ti curve actually lies above that for pure cetane at most soap concentrations. This is in striking contrast to the results obtained with miscible solvents of polar and non-polar character (6), in which case temperatures required for formation of isotropic solution are greatly lowered. As the proportion of water in the cetane-water solvent decreases, the concentration of sodium stearate a t the maximum in the T , curve (cf. figure 2) likewise decreases. This leads to the speculation that formation of the most stable superneat soap requires a certain minimum ratio of water to soap, which can be reached only by increasing the per cent solvent as its water content decreases. The matter is not quite this simple, however, since the ratio of moles of water per mole of sodium stearate a t the maximum in the Ti curve does not remain constant, changing from 5.7 t o 8.6 to 9.7 moles of water per mole of soap as the per cent cetane in the solvent changes from 0.0 to p5 to 50 per cent.
Emulsions, gels, and syneresis In both cetane and water systems of sodium stearate sufIicient heating results in the formation of a single-phase isotropic solution a t all concentrations of soap. In the mixed solvents, however, a t low soap concentrations, heating results first in the formation of an emulsion; and at higher temperatures in two immiscible liquid layers. Sodium stearate acts as a solubilizer for these liquids, since increasing the concentration of soap permits the formation of a single isotropic solution phase. Increasing the ratio of cetane to water in the solvent seems to have some tendency to decrease the soap concentration required to permit melting to isotropic liquid instead of formation of emulsions. Thus, melting occurs to two immiscible liquids at 21.2 per cent sodium stearate when the ratio of cetane to water is 1 to 3, but to a single homogeneous phase a t 22.1 per cent soap when the ratio of cetane to water is 3 to 1. Increasing the ratio of cetane to water, while keeping the concentration of sodium stearate constant, also seems to increase the stability of the gels present a t room temperature. Thus, a system containing 14 per cent soap in 75 per cent cetane-25 per cent water rem-ains a gel
52
ROBERT D. VOLD AND JOSEPH M. PHILIPSON
up to 204"G., whereas in 25 per cent cetane-75 per cent water the gel breaks to an emulsion a t 60°C. It appears that the liquefaction of the gels of sodium stearate in pure cetane (T,of figure 1) is not a phase change. Up to 50 per cent soap the temperature . of liquefastion seems to be approximately constant at 200°C. In the more dilute part of this range liquefaction is from optically isotropic gel to liquid, while in the more concentrated part the system is optically anisotropic both above and below the temperature of gelation. Despite the great effect on the mechanical properties, the organization of structure required to form the gel apparently is too transitory or too irregular to constitute formation of a new phase in the sense of the phase rule. Above 50 per cent sodium stearate the temperature of gelation coincides yith that of the phase transition from isotropic liquid to anisotropic liquid crystal. The possible occurrence of syneresis, which is of such great practical importance with respect to the stability of various creams and ointments, depends in part on whether at any given temperature isotropic liquid is present as one of the equilibrium phases. Although the present data do not bear directly on this problem, some of the observations in table 3 are of interest as indicating regions where obvious syneresis occurs. The unusual behavior of the system 27.2 per cent sodium stearate-72.8 per cent cetane is also worthy of note. This composition, anisotropic a t room temperature, on heating becomes completely optically isotropic. a t 196"C., anisotropic again at 199"C., and finally becomes completely isotropic a second time a t 204°C. Whether an isotropic phase is actually formed below Ti or whether the effect is due to some kind of orientation of the liquid crystallites by the glass walls of the tube is not clear. SUMMARY
Temperatures have been determined a t which systems of sodium stearatecetane and sodium stearate-cetane-water undergo visible changes in appearance, thus making possible prediction of the physical state of the system a t any concentration and temperature. The solubility curve of sodium stearate in cetane has a number of breaks which presumably are due to phase changes related t o those undergone by the anhydrous soap, In cetane-water mixtures these breaks are absent, suggesting that the water may have diminished the number of transitions undergone by the system. Over the range investigated in mixtures of cetane and water at low concentrations of sodium stearate, increasing the proportion of cetane increases the temperature-stability of the gels. Increasing the proportion of water increases the concentration of soap required for complete miscibility in the liquid phase, and promotes the formation of emulsions rather than gels a t lower temperatures. REFERENCES (1) DEANESLY, R. M., A N D CARLETON, L. T.: J. Phys. Chem. 46, 1104 (1941). (2) FERGUSON, R. H., ROSEVEAR, F. B., AND STILLMAN, R. C.: Ind. Eng. Chem. 36, 1005 (1943). These authors discuss some of the phase complications consequent on mechanical working of aqueous soap systems.
PERMSELECTIVE COLLODION MEMBRANES
53
(3) LEGGETT,C. W., JR.: Thesis for degree of Engineer in Engineering Chemistry, Stanford University, 1941. (4) LEGGETT, C. W., JR.,VOLD,R . D., AND MCBAIN,J. W.: J. Phys. Chem. 46,429 (1942). M.: J. Phys. Chem. 44, 1013 (1940). (5) MCBAIN,J. W., VOLD,R. D., AND FRICK, (6) PALIT,S. R.: J. Indian Chem. SOC.19, 271 (1942). (7) PHILIPSON, J. M., HELDMAN, M. J., LYON,L. L., AND VOLD,R. D.: Oil & Soap 21, 315 (1944). (8) VOLD,M . J., MACOMBER, M., AND VOLD,R . D.: J. Am. Chem. SOC.63, 168 (1941). (9) VOLD,R. D.: J. Am. Chem. SOC.63, 2915 (1941). (10) VOLD,R. D., LEGGETT, C. W., JR.,AND MCBAIN,J. W.: J. Phys. Chem. 44,1058 (1940). (11) VOLD,R. D., AND VOLD,M. J.: J. Am. Chem. SOC.61, 808 (1939).
IMPROVED METHODS OF PREPARATION O F “PERMSELECTIVE” COLLODION MEMBRANES COMBINING EXTREME IONIC SELECTIVITY WITH HIGH PERMEABILITY HARRY P. GREGOR‘
AND
KARL SOLLNER
Department of Physiology, University of Minnesota, Minneapolis, Minnesota Received J u n e 21, 1046
I The preparation and properties of “permselective”2 collodion membranes which combine extreme ionic selectivity with high permeability have been described recently (3, 9), and a preliminary account was given of their usefulness in various types of physicochemical studies (6, 8, 9, 10, 15). The permselective collodion membranes were prepared (3) as follows: A solution of collodion in ether-alcohol mas poured over test tubes which were rotated slowly in a horizontal position. The film thus formed was allowed to dry for several minutes; another layer was applied in the same manner, and yet another a few minutes after the second. After several more minutes the tubes mere immersed in distilled mater. The membranes, still on the tubes, were then oxidized in 1 M sodium hydroxide3 for measured lengths of time and then soaked in water to remove the base. Still on the tubes, the membranes were dried in air. New address: The Permutit Company, Birmingham, New Jersey. Interchangeably with the term “permselective” the word “megapermselective” was used previously (3, 9); since this latter term is philologically not entirely desirable, the briefer and linguistically more convenient term “permselective” will be used exclusively here. The alkali causes a complicated decomposition of the collodion with the formation of nitrites and probably other nitrous compounds. The nitrous compounds act upon the collodion, causing thorough oxidation (12). This oxidation greatly increases the surface concentration of fixed acidic (anionic) groups on the pore walls of the membrane. This surface concentration determines the electrochemical characteristics of the membrane (9, 11, 12, 14). Thus, thoroughly oxidized membranes are electrochemically very active. 1
