Reprinted horn THEJOCRN.%L OF P ~ r s r r . A \ N~D CoLLorn C K E M I S T R Y
Yo].51, No. 5. September. 194i SODIUM STEARATE IX O R G A S I C SOL V C K T S
1189
THE PHASE BEHAVIOR OF SODIUM STEARXTE I X ANHYDROUS ORGAXIC SOLVEKTS GEROULD H. SMITH’
AKU
JAMES W. McB.XIS
Deparimeiit of Chenitsrry, Stanford U n i r e r s i l y , California
Xeceioed Jfaii 20, 1847
I n contrast to the many studies of the phase diagrams of soaps in aqueous systems, very little is kno\vn as to the phase behavior of soaps in non-aqueous solvents (2, G , 8, 10, 11). It is the object of this communication to presentdata for sodium stearate in tlvelve different pure hydrorarhon solvents, ivith special attrntion given to cyclohexane and toluene. Thca relation of these studies to lubricating greases, together. ivith the important rffrCTS of water and other additives, \rill be discussed in a separate pub!ication. MATERIALS
The sodium stearate \vas prepared from sodium ethoside and Eastman Iiodak Company stearic acid (White Label; KO.102). I t was identified as the y-form by x-ray diffraction (1, 5 ) . I t \vas found t o be neutral to phenolphthalein in hoiled-out 95 per rent ethyl alcohol. Eleven of the hydrocarbon solvents \vere obtained from the Eastman Kodak Company and used vithout further purification other than drying and storing over Ilrierite (anhydrous calcium sulfate) : namely, cyclohexane, toluene, osylenr, m-xylene. p-xylene, 2 , 2 ,-i-trimcthylpentane (isooctane), n-heptane, ethylbenzene, ti-butylbenzene, cumene, and p-cymene. Baker and Xdamson reagent-grade, thiophene-free benzt-ne \vas u&d. Except for n-butylbenzene and p-cymene, the solvents shoivetl no tendenry to foam \\.hen shaken. The p-cyment’ \,vas light yt-llo\v in color and may have contained terpenes as an impurity. I: X P L l t I l f E S T A L 1’b:CHSIQUES
The samples of soap and hydrocarbon irere sealed in evacuated Pyrex tubes and studied visually, in a manner similar to the previous investigations by LlcBain and coxorkers on the anhydrous aqueous soap systems. This was supplemented by microscopic examinations under polarized light and by photomicrography. All thermometers and thermocouples irere carcfully calibrated. Temperatures, designated by T ? ,ivere those a t ivhich, on cooling, anisotropic material began to separate from the single isotropic phase. Preparation of l h ~samples Pyrex tubing (6-8 mm. outside diameter), sealed a t one end, numbered with B glass-marking pencil and the number fused into the hot glass, was used as sample Present address: Unicin Oil Company of Californin. Oleum. California
1190
GEROULD H. SMITH AND JAMES W. YcB.4IN
containers. These tubes were sealed to standard taper 12/24 outer cones and cleaned with hot concentrated sulfuric and nitric acids, followed by several rinses with distilled water and dilute ammonia water. They \\-ere dried by evacuation and stored in a desiccator over phosphorus pentoxide until use. Powdered sodium stearate was charged into the sample tubes by extruding tamped plugs of soap from a 3-mm. glass tube 18-20 cm. in length. The soap thus could be deposited a t the bottom of the tube ivithout contaminating the walls, preventing decomposition when the tube was sealed. After the soap \\-as dried (described later), the dried solvent was introduced using a hypodermic syringe of 1-ml. capacity, graduated in 0.01 ml. The solvent \vas deposited directly onto the soap, using a3-in. needle. During all these operations the tubes were closed tightly with standard-taper ground-glass plugs except for n few seconds when the solvent was being added. .A tube containing the soap and solvent \\-as attached to the vacuum pump and evacuated until the solvent began to boil. Then it was chilled \\-it11 po\vdrrcd dry ice. The pressure \\-as reduced below 1 mm. of mercury, and the tube scaled off. Both pieces of the original tube were wiped clean, dried, and lveiglied. This weight, together with the tare of the tube and the neight of the dried soap, was used to calculate the composition of the sample.
Drying the sodium stearate and solvents The sodium stearate, after iveighing into the tubes before sealing, was dried for 2 4 hr. by heating in an Abderhalden drying pistol over phosphorus pentoxide, using a reduced pressure of less than 1 mm. of mercury. Quinoline was used as the heating medium. Before removal of the samples after drying the apparatus was allowed to cool, and air, dried by slowly passing through a dry ice-cellosolve coldspot, was admitted until it was equal t o atmospheric pressure. Then the pistol was opened and the tubes were capped immediately. Experience demonstrated that even small traces of z:ygen resulted in charred soap during the drying. Because of this, the pistol was flushed five times v i t h nitrogen, evacuating between each flush, before heating the soap. The groundglass pistol cap was sealed \\-ith Apiezon grease “L”, and it maintained the pressure a t less than 1 mm. for 12 hr. or more a t the boiling point of quinoline. The hydrocarbons were dried satisfactorily over Drierite. Indicator-impregnated Drierite changes color in n few days if allowed to stand a t the bottom of an open vessel of a hydrocarbon solvent. This change is immediate if water is added to the solvent. Storage of the hydrocarbon solvents and indicator Drierite in stoppered reagent bottles resulted in no color change for the indicator over more than sixteen months.
Observation of the samples The samples were studied while being heated in a thermostatic bath of white oil, or in a Freas Precision forced-draft oven, or in a Pyrex-tube furnace. The oil bath maintained a temperature regulated to f 0.25”C. and could be used over a temperature range of 0” to 200°C.
SODIUM STEARATE IK ORGANIC SOLVENTS
1191
The Freas oven was equipped with a glass window having three parallel panes, spaced t o allow the heated oven air t o pass betxeen them, thus minimizing heat losses through the window, but still allowing observation of the samples, which were fastened perpendicularly t o a horizontal glass spindle. Wires operating a reel attached to this spindle were led through the air vent of the oven and permitted the samples t o be thoroughly rotated and mixed while in the heated oven. A Pyrex-tube furnace was used for observations a t temperatures greater than 310°C. It consisted of five concentric Pyrex tubes which were fastened alternately to two headers, causing an air current passing over a heater between the two outermost shells to pass back and forth inward, until it was vented from the innermost shell which contained the tube under observation. The temperature of the sample was measured with a calibrated, iron-constantan thermocouple, placing the junction next to the sample tube. The entire furnace was rotated t o mix the sample. The temperature was varied by controlling both the air flow and the applied voltage to the heater. Transmitted polarized light was used to illuminate all samples, and was analyzed by manual operation of a second Polaroid disk. GENERAL OBSERVATIOXS
1. Gnheated anhydrous systems
Sodium stearate does not swell in pure anhydrous hydrocarbons a t room temperature, but it is capable of soaking up nearly its own volume of solvent. Even when stored for several years in excess of solvent the particles of soap are separate and discrete and are readily redispersed on shaking, although the settled layer looks compact and gelled.
2. Anhydrous systems o n heating When sodium stearate is heated in the prevence of an excess of solvent, it b-gins to swell into the solvent a t 8045°C. It needs 5-8 per cent of soap to fill the liquid unless it has been heated so high that all of it dissolves to form isotropic solution. -it 98OC. all anhydrous systems containing below about 45 or 50 per cent of soap ediibit a sharp, sudden change in appearance. The opaque, white, swollen gel becomes translucent and liquid crystalline, transmitting polarized light with simultaneous structural change a t 98°C. results bright t'an or golden color. in the engagement of all free liquid if the sample is mixed, distributing the soap during the change. Failure to distribute the soap during the transformation always results in the formation of a compact, gelled layer which is very resistant t o dispersion in supernatant liquid. This transformation at 98°C. is reversible only very slowly on cooling, because of undercooling. Systems containing more than 50 per cent of soap change a t about 90°C. to a liquid-crystalline phase n-hich transmits light and is translucent, white, and wax-like in appearance. The samples are dry or wet powder before heating and some difficulty is encountered in observing this change in systems not previously
1192
GEROULD H. SMITH .4XD JAMES 7%'. MCB.4Ih-
I
oTi
VALUES
B C R I T . TEMP.TOLUENl * G E L T O LIQ.CRYS.
LDEN LIQUIDS T A L L I N E PHAS
GEL --C. RY s T A L
a
SOL
sa
lob
30
80
70 bo SO 40 30 PERCENT SODIUM S T E A R A T E
eo
10
FIG. 1 . Phase diagram of anhydrous sodium stearate in dried toluene. Legend for two-phase areas: A , neat soap and liquid-crystalline phase; 8,subneat soap and liquidcrystalline phase; C, superwaxy soap and liquid-rrystalline phase; D, waxy soap and liquid-crystalline phase; E, suhwaxy soap and liquid-crystalline phase; F, supcrcurd soap and liquid-crystalline phase. (No attempt has been made t o represent how the various phases t h a t exist a t different temperatures i n 100 per cent sodium stearate extend into the phase diagram, although i t is known that they do so estend. The black horizontal lines of figure 1 and the corresponding square dots of figure 2 mere1.y represent the phase transitions in the'absence of any solvent. Tie-lines adjacent t o these phases are devoid of significance.) I
melted, because of reflected light. The powdery soap usually sinters to a porous plug at 185-200°C. Both of the liquid-crystalline phases melt to isotropic sol or jelly at higher
1193
SODIUM STEARATE IN ORGANIC SOLVENTS
temperatures. The golden, liquid-crystalline phase acquires a “mosaic” pattern a few degrees below the Tivalue, which changes to vibrating, dancing particles on a further slight increase in temperature. These dancing and sometimes
0
< VALUES
0 CRIT. TEMP. OF PUR CYCLOHEXANE
GEL TO
u a . CRYST
W H I T E WAXY L l Q CRYSTALLINE P H A S
-
EL -CRY TAL B SOL
___: - - - I . ; I O 0
30
80
70
60
50
40
30
20
IO
P E R C E N T Na S T E A R A T E FIG.2 . Phase diagram of anhydrous sodium stearate and dried cyclohexane. A , neat soap and liquid-crystalline p h a w ; B, subneat soap and liquid-crystalline phase; C, superwaxy soap and liquid-crystalline phase; D, was? soap and liquid-crystalline phase; E, subwasy soap and liquid-crystalline phase; F, supcrcurd soap and liquid-crystalline phase. (See note at t h e end of legend for figure 1.)
brilliantly colored particles settle rapidly in the supernatant, isotropic medium if the sample is not stirred. They dissolve on further heating in the isotropic solution. These changes are reversible. The tempratures a t which the disappearance of the last anisotropic, dancing particle, on heating, and the re-
1194
GEROULD H. SMITH AKD JAMES W. MCBAIIC
appearance of the first one on cooling the isotropic sol, occur are within 1' for most observations. The white, wax-like, rigid, liquid-crystalline phase melts to an isotropic liquid at temperatures considerably above the true melting temperature of the pure anhydrous soap, 288°C. White, anisotropic particles separate reversibly from the isotropic melt on cooling, and, concurrently, the system again becomes rigid. Systems containing 85-00 per cent or more of soap melt at about 205°C. to a form resembling the subneat phase of anhydrous sodium stearate. This becomes isotropic below the melting temperature of the anhydrous soap. I n figures 1 and 2 the full lines in the figures are those which are carefully authenticated by experimental points; the dashed lines are those whose positions are less accurately determined but nevertheless knoll-n t o be approximately vhere indicated. For toluene and cyclohexane the dashed lines around the golden liquid-crystalline phase are corroborated by the experimental points obtained with p-xylene. 3. dnhydroiis systems during recooling The systems containing less than 15 per cent of sodium stearate a t first become rigid isotropic jellies as they begin to cool from the fluid isotropic sols. This change is not sharp and occurs near 200-220°C. Further cooling t o 100-120°C. usually results in an anisotropic appearance caused by internal strain in the jelly, and is manifested as a floiving pattern with a gradual change of intensity rather than the sharply defined discontinuous anistropy characteristic of the crystalline and liquid-crystalline forms. The hot isotropic solutions containing from 15 per cent t o about 85 per cent of soap immediately begin to separate out anisotropic particles on cooling. These then solidify to the liquid-crystalline phases. Systems containing more than 85 per cent exhibit the changes characteristic of the pure anhydrous sodium strarate (7). The appearance of the systems after cooling to room temperature depends on the concentration of the soap and the particular hydrocarbon used in the systems. This appearance may change on storage, giving a measure of the relative stabilities with respect to 110th the concentration of the soap and the nature of the hydrocarbon. Systems containing less than 15 per cent of soap, on cooling from the isotropic state first form a jelly which, on further cooling, becomes anisotropic from internal strain vithout any liquid-crystalline phase being present, until a t about 80°C. they contract violently and eject isotropic solvent as a spray. The volume of the resultant opaque gel is comparable with that of the original unheated soap. A portion of such a cooled gel examined by S . Ross, using x-ray diflraction a t room temperature with (:u Zim radiation filtered through nickel foil gave a powder pattern exactly similar to that of the original dried soap, apart from general scattering by solvent (cyclohexane, glycerol, or S u j o l ) . Results with cyclohexane shoiv that even after being heated in hydrocarbon the sodium stearate retains the original crystalline form of pure y-sodium stearate.
1195
SODIU3f S T E A R A T E IK ORGAKIC S O L V E K T S
Sorption and desorption isotherms determined by Shreve (9), using the LIeBain-Bakr sorption balance, show that a t 40°C. sodium stearate takes up only 3 per cent by weight of cyclohexane a t 100 per cent relative vapor pressure, increasing to 5.5 per cent a t 50°C. At 80°C. the sorption amounts to 14 per cent and a t llO"C., 33 per cent. At about 110°C. the sorption and desorption is reversible, whereas a t lower temperatures hysteresis occurs. The isobaric curves in figure 3 were constructed from the isothermic data, and they showed that marked change in the affinity of the soap for the cyclohexane occurred betn-een 80" and 110°C. This was assumed to be the transformation to the liquid-crystalline phase a t 98°C
>
> )
40
50
60
70
90 TEMPERATURE 'C. 80
100
110
FIG.3. Isobaric curves for system of pure cyvloheznne and sodium stearatc
In general. three prominent effects are found to occur on allowing the systems to stand a t 24-20"C. after cooling from the isotropic state Ivithoiit stirring These arc syneresis, :I decrease in the volume of the rigid portions of the system, and an increase in opalescence or opacity. These changes are more rapid in samples containing less than 20 per cent uf soap. I n general, the stability of th(, cooled systems appcars to increasc from both sides toivard the middle of thv liquid-crystallinc phase areas. .I11 three Fffncts may be exhibited if enough time is allowxl for the changes to occur, and the occurrence of syneresis a n d increased opacity are simultaneous.
! 2 3
i
I
Iri
'0"'
0.4 2.9
I
1197
SODIUM S T E A R A T E 13- ORGAKIC SOL VER-TS
TABLE 1 Anhydrous systems of sodium stearate in hydrocarbons P E R CENT SOAP
1
DESCRIPIlON OF COOLED SYSTEMS
I
Centl
Per w$ht
T,
1 Immediately aiter cooling
Mole
per cent,
Aiter aging
i
Cyclohexane (critics1 temperature, 281°C )
'
0.44, 0.2
2.9
0.9
11.5 1 3.5 18.3 251 33.2
6.2
i
9.1
12.8
I 39.6
i
16.2
4 j . 4 1 19.8
58.2
29.2 I
2.0
, 43.3
53.0
'
59.2
I
50.5 100.0 '100.0 93.3
Faintly opalescent particles in Opalescent particles in clear clear liquid l liquid I Slightly opalescent plug in clear Slightly opalescent plug in clear , liquid liquid (160) Faint opalescent gel plug in Cloudy plug in clear liquid clear liquid 105 i Opalescent gel plug in clear liquid i 222 I Opalescent gel; slightly syner- 1 Opalescent plug; syneretic etic 232 Clear; not syneretic Opalescent; slightly syneret,ic 240 Slightly opalescent; slightly syneretic (243) I >310 I Wasy-like; t a n and white k s y - l i k e ; t a n and white; 1 slightly syneretic i l i>310 1 White; semitranslucent; not I:nchanged I completely sintered i>300 White; semitranslucent; not Unchenge.1 1 completely sintered i>307 , White; partly translucent; L-nchanged i partly opaque , 260 T a n ; opaque; compact Unchenge.1 285 H a r d ; white; opaque; compact Cnchanged (115)
1
~
2.5
i.9
'C.
~
-
,
I
,
~
1
, 1
Toluene (critical temperature, 320.6"C.) 0.93' 0 . 3
i
(133)
Very slightly opalescent gel in clear liquid Clear gel i n clear liquid (opalescent overnight) Opalescent gel; syneretic
-
Very slightly opalescent gel particles in clear liquid 9.1 3.14 (165) 1 Very slightly opalescent gel plug i in clear liquid 1 Slightly opalescent plug; syneretic Faintly cloudy; no syneresis Partly cloudy; slightly syncr1 etic 45.4 121.2 i 235 Slightly ', opalescent; no synere- ' Opalescent plug; slightly syners1s etic 55.9 29.0 287 ' Semitranslucent gel with clear Unchanged liquid I 60.3 , 33.0 331 Opaque i n semitranslucent gel Opaque i n semitranslucent gel, slightly syneretic 335 Opaque i n semitranslucent gel Unchanged; no syneresis 257 Unchanged White; opaque 94.6 I 8 5 . 0 264 Unchanged H a r d ; white; opaque
'
~
'
1
~
1198
GEROULD H. SMITH AND JAMES W. MCBAIN
TABLE I-Continued PER CENT SOAP
pebyt/ Mole weight per cent
I ~
I Ti
I
DESCRIPTION OF COOLED SYSTEMS
i
Immediately alter cooling
After aging
p-Xylene (critical temperature, 348.5"C.) 19.5
8.4
220
28.6
13.1
232
43.7 56.3
22.5 32.6
244 216
61.9 87.8
37.8 >280 261 73.0
90.8
76.6
263
Slightly opalescent gel; slightly syneretic Clear jelly; no syneresis
Slightly opalescent plug; syneretic Very slightly opalescent; slightly syneretic Cloudy; no syneresis Cloudy; slight syneresis Very slightly opalescent; slightly Very slightly opalescent plug; syneretic syneretic White; wet; wax-like Unchanged White; wet; compact Semitranslucent; white; no syneresis White; wet; compact Unchanged o-Xylene (critical temperature, 363°C.)
10.2
4.1
Isotropic; syneretic
19.9
8.5
Clear; not syneretic
27.6
12.5
Clear; not syneretic Slightly opalescent; slight syner esis
38.1 18.7
-I
Very slightly opalescent plug; syneretic Slightly opalescent plug; syneretic Slightly opalescent plug; syneretic Cloudy; slight syneresis
m-Xylene (critical temperature, 349°C.) Clear; syneretic
10.4
4.2
20.0
8.6
223
29.4
13.5
230
20.7
244
41.1
I __ 1.31
0.5
10.3
4.1
19.0
8.1
27.8
12.6
37.4
18.3 -
Faintly opalescent; slight syneresis Slightly opalescent; not syneretic Slightly opalescent; not syneretic
Very slightly opalescent plug; syneretic Unchanged Opalescent plug; slightly syneretic Opalescent plug; slightly syneretic
Ethylbenzene Chunks of opalescent gel in clear liquii Slightly opalescent gel in clear liquid Clear; not syneretic Slightly opalescent; slightly syneretic Opalescent; not syneretic
Unchanged Unchanged Very slightly opalescent plug; syneretic Opalescent plug; slightly syneretic Opalescent; very slightly syneretic
1199
S O D I U M S T E A R A T E I N ORGANIC S O L V E N T S
TABLE 1-Continued PEECENT SOAP
1
1
DESCXIPIION OF COOLED SYSTEMS
Immediately after cooling
After aging
-
Ethylbenzeneqontinued "C.
I
46.8 66.1
24.8 250 42.2 >291
78.9
58.4
Opalescent; translucent 1 White; sintered; opaque; slightly , syneretic f hard white, f semitranslucent; not syneretic White; semitranslucent; opaque 1 top I 1 White; opaque
258
Opalescent; not syneretic Unchanged Unchanged
~
88.2
73.6
258
95.0
87.5
260
Cnrhangetl Unchanged
Cumene 11.3
5.1
23.1
11.2
33.4
17.5
43.7
Clear syneretic gel (opalescent after 4 hr.) Opalescent syneretic gel
Opalescent
Opalescent syneretic gel plug; syneresis Slightly opalescent; slightly syneretic Unchanged
Slightly opalescent; very slight syneresis Cloudy; slightly syneretic
24.8
-
syneretic gel
n-Butylbencene
-
~
9.2
4.5
19.9 31.1
10.5 17.6
Very slightly opalescent; not syneretic 193-41 White, fractured gel 226 i Light yellow; opalescent; not
38.8
23.2
239
1I
9.5
4.7
118
1 Yellow; slightly semitranslucent
19.5
10.3
144
27.6
15.2
171
39.3
23.4
Slightly opalescent plug; syneretic White, fractured plug; syneretic Cloudy; slightly syneretic
~
Lir;;;ow; syneretic
opalescent; not
Opalescent; not syneretic
p-Cymene (critical temperature, 378°C.)
--_
opalescent; ' Trace of syneresis
Light yellow; opalescent; not syneretic Whiter; opalescent; not synere.tic Whiter; opalescent; not syneretic
Unchanged Unchanged Unchanged
n-Heptane (critical temperature, 266.8"C.)
i Opalescent Opalescent gel in clear liquid gel in clear liquid
1.48' 0.5 9.5 I 3 . 2
,
, (240)
' Opaque gel in clear liquid Opaque gel in clear liquid 1 Increased opalescence and syneresis I Vnchanged
1 White opaque plug; syneretic I ~
White; opaque; solid
~
1200
GEROULD H. SMITH AND JAMES W. MCBAIN
TABLE l-Concluded
1I
PER CENT SOAP
DESCPIPTION OF COOLED SYSTEMS
2,2,4-Trimethylpentane (isooctane) (critical temperature, 296OC.)
i
Y.
~
I
I
47.4 73.3
20.1
29.9
~
6.5
1
222
I
~
1
10.5 1>238
!
I
t
i
Clear; not syneretic Clear; not syneretic
Slightly opalescent plug; syneretic Slightly opalescent plug; slightly syneretic
than the rest. The nhite patches became the white, wax-like, liquid-crystalline phase on heating, vhile the remainder was the golden, liquid-crystalline phase. Considerable shrinkage occurred on cooling. The system !vas metastable at room temperatures and exhibited no syneresis. The samples in tubes 7 and 8 both were heated to 310°C. without melting, although sintering occurred. It may be pointed out that these systems exhibited a sharp birefringence and were rigid at that temperature, which was 30°C. above the critical temperature of pure cyclohexane and 25°C. above the temperature a t which the pure soap melts t o isotropic liquid. ' The last sample, that in tube 9, exhibited the phase changes observed in the anhydrous soap but became isotropic a t a lower temperature than did the pure soap. This showed that a eutectic area was formed between the anhydrous soap and the white, wax-like, liquid-crystalline phase. The cooled system was light tan, opaque, and compact.
2. Selected data on systems of sodium stearate in twelve anhydrous hydrocarbons Typical individual descriptions of systems of sodium stearate in the twelve anhydrous hydrocarbons are collected in table 1,which includes the values of Ti, the temperature at which, upon cooling the completely homogeneous isotropic solution, the first appearance of heterogeneity or anisotropy occurs. The dipole moments of n-heptane, cyclohexane, benzene, p-xylene, and p-cymene are 0 ; those of isooctane, toluene, ethylbenzene, m-xylene, cumene, and n-butylbenzene are 0.4; that of o-xylene is 0.7. The critical temperatures of the pure solvents are included in the table. DISCUSSION
,Just as in aqueous systems, the phase rule of Willard Gibbs is found to apply to true equilibria among the phases here recognized, as is illustrated in the phase diagrams for sodium stearate and toluene (figure 1) and sodium stearate and cyclohexane (figure 2 ) .
1201
SODIUM STEARATE IN ORGANIC SOLVENTS
Two definite phase transformations were observed: namely, from the gel form to one of the two liquid-crystalline phases, and in turn from these liquid-crystalline phases either to an isotropic sol or to a jelly. They mere both reversible transformations, although circumstances such as undercooling could cause them to appear irreversible. Further, owing to the defective fluidity of the liquidcrystalline phases, other phases with which they are in equilibrium, such as sol, jelly, crystals, or other liquid crystals, quite frequently cannot separate out or segregate. Many writers include all such systems under the name gels, not reserving “gel,” as is done in this Laboratory, for the systems involving a solid crystailine phase, together TTith a second phase which may be liquid-crystalline, a jelly, or a sol. Sol and jelly constitute a single phase in which there is a gradual change from complete fluidity t o that of a typical isotropic, transparent, elastic, thixotropic jelly. Sol and jelly are isotropic when unstrained, but show streaming birefringence. They are reversibly and gradually transformable from one to the other by heating or cooling or by change in concentration. Laing and McBain (4)and Heyman (3) demonstrated for aqueous soap systems that they belong to the same phase. Figures 1 and 2 depict two liquid-crystalline phases in respect to their composition and the temperature range in which they are the stable phases. Their appearance was characteristic and different. Thus, their coexistence in samples containing about 50 per cent of soap \$-asobserved easily, since the samples contained patches of both. The vapor pressure of the white, xax-like, more concentrated, liquid-crystalline phase was considerably lover than that of the golden liquid-crystalline phase, shoxing that the association of solvent and soap is considerably stronger for the concentrated liquid-crystalline phase. This is in contrast to a gel phase, where the presence of free solvent asserts its full vapor pressure. The phase occurring in the lower concentration range was golden and bright, transmitting polarized light with a strong birefringence. I n systems ivhich were mobile, the liquid-crystalline material floxed when the tubes were inverted, exhibiting the most exquisite play of pastel colors. Such streaming birefringent effects also were noted in the soft jellies when they were stressed or slo~vlyflowed; but those effects could not compare Tvith the ones occurring in the liquid-crystalline phase for intensity, duration, and numerous brilliant shades of color. The vibrating or dancing particles occurring in the tlvo-phase region between the isotropic and liquid-crystalline phases also exhibited a play of colors, but it was more fleeting because of the lack of oriented flon. patterns. ,
Soluhiliiy ay‘ sodium stearate and oarious hydrocarbons The solubility of dry sodium stearate lvas found to be remarkably similar in all of the dried, pure hydrocarbon solvents. The temperatures of transformation to the isotropic solution for some of these systems have been plotted in figure 5 !see also figure 6). X curve can be drawn through the points for all but p cymene and n-butplbenzene. The deviation among the values for the various hydrocarbons was much less than that which would be caused by even 0.’
1202
GEROULD H. SMITH AND JAMES W . MCBAIN
250
1c
I
I
I SOTROPIC
230
-
2 10-
9 w
E 1902
I-
