A Phase Rule Study of the System Sodium Stearate–Water. - The

A PHASE RULE STUDY OF THE SYSTEM SODIUM STEARATE-WATER' JAMES W. McBAIN, ROBERT D . VOLD,

AND MARY FRICK Department of Chemiatry, Stanford L7niuemity, California

Received February SI, 1940

Sodium stearate, although one of the most important constituents of commercial soap, has not previously been studied from a phase rule point of view. This paper presents the results of an exploratory survey. Although this soap has not yet been investigated as extensively as sodium oleate ( 8 ) , it has nevertheless been possible to construct a considerable portion of the phase rule diagram. Recent work ( 6 , 7,8, 11) has shown that all the saturated sodium soapb and likewise sodium oleate pass through a sequence of mesomorphic states between room temperature and the melting point, instead of having only the single liquid crystalline form formerly recognized. Since the number of phases in this sequence is not always the same for different soaps, the generalization (1) that phase rule diagrams for different soaps do not differ from each other qualitatively must break down to some extent in the regions of concentration involving these new phases. One purpose of this work was to investigate the extent of the difference? thus caused in the soap-water diagram. A new maximum had been found in the solubility curve for sodium oleate (8), resulting in a curve for thiq soap qualitatively different from those previously reported for sodium palmitate (3) and sodium lauratc (2, 5). When this same maximum was found for sodium stearate,'showing that it was not a special characteristir of the unsaturated soap sodium oleate,--sodium palmitate and sodium laurate were reinvestigated. The new maximum mas found to be present in both these cases also. All four sodium soaps, --lawate, palmitate, stearate, and oleate,-have curves defining the limits of existence of ordinary isotropic solution which are qualitatively comparable in shape, despite the newly discovered feature of the curve. It may therefore be possible to relate quantitative differences from soap to soap, at corresponding points in the phase rule diagram, to specific differences between the soap molecules. Parts of this work were presented at the meetings of the Pacific Division of the American Association for the Advancement of Science at San Diego in June, 1938, and at Stanford University in June, 1939. 1013

1014

J . U’. MCBAIN, R. D. VOLD, .ViD M, FRICK

MATERIALS AND EXPERIhtENTAL METHODS

Sodium stearate was prepared* from stearic acid “Kahlbauni” by neutralization (phenolphthalein as indicator) of a hot alcoholic (95 per cent) solution of the acid with an alcoholic solution of sodium ethylate. A41cohol and water were driven off by heating in an air oven a t 105’C‘. with frequent stirring to prevent burning. The acid from which the soap was made melted a t 69.0’C. and had a molecular weight (by titration) of 284.3. Analysis of the soap (as used) by standard methods (2) gave the following results: real soap, 99.53 per cent; water, 0.41 per cent; free sodium hydroxide, 0.06 per cent. Allowanee was made for the water in making up samples. The sodium laurate and sodium palmitate were the same stock whose preparation has been described in previous publications (10, 11). Details of the methods of study used in this work have been described previously (8). Values of T,, the temperature a t which isotropic soap solution separates an anisotropic liquid crystalline phase on slow cooling, were determined by visual observation of systems of known composition, sealed in thick-walled Pyres tubes, and hung in an electric oven (4). Values of T,, the temperature at which the last trace of curd fiber phase disappears on slow heating,-about one degree per five minutes in the vicinity of transitions,--were also determined visually. This change was a great deal more definitive and easily observed with sodium stearate than had been the case with sodium oleate (8). These results are given in table 1. h few investigatiom w x e carried out with the polarizing microscope on samples sealed in small glass capillaries and examined in an electric furnace on the microscope stage (8, 11). These data are summarized in table 2. The temperatures To, a t which a liquid crystalline phase first begins to melt to isotropic liquid on slow heating, are easily determined and v&y useful, since a few such points suffice to determine the phase boundaries cf the fields of homogeneous liquid crystal. Other changes in microscopic appearance can also be detected, as indicated in the table, but these cannot be regarded as proof of phase changes without other substantiating evidence. Table 2 also shows the transition temperatures of anhydrous sodium stearate taken from the work of Vold, Macomber, and Vold (7). -4very useful cheek on the interpretation of the results was obtained by hanging a series of tubes in a viewing thermostat and examining their appearance by ordinary light and between crossed polaroids. Flow properties were also observed after the systems had remained a day or two a t

* The authors are indebted to Dr. M. E. L. McBain for the preparation of this soap.

TABLE 1 Vista1 observations PBOBABLE BEPARATINO PHASE

SODIUM STEARATE

Ti

weight per ecnl

"C

'C. 62.7 66.8 68.8 69.8 71.8 72.8 75.3 75.6

19.9 24.8 29.6 34.9 38.0 39.8 41.9 43.8

87 124 152 166 169 171 164 147

76.8 78.1 78.8 80.3 81.3 81.9 82.4 82.4

45.0 47.8 49.5 55.0 60.3 64.9 71.9 74.5 79.8 84.7 88.6 89.8 91.2 92.6 92.8 93.2

170 212 226 256 277 280 284 293 286 281 274 273 272 276 276 267

94.6 97.1 99.6 100.0

274 277 284 285

TC

A T Ti

0.37 0.81 1.49 2.51 5.10 7.51 16.8 15.3

IJ

82.4

1 1

83 83 86 86 87

Either soap-boiler's

90 93 104 112 119

Neat soap

J

SODIUM PALMITATE

1

weight psr a n t

97.2 94.6 92.2 85.4

286 276 274 280

1 Keat soap Neat soap Superneat soap Superneat soap

SODIUM LAURATE

95.3 89.5 85.4 81.3

1

I

weight per ccnl

313 296 294 290

Neat soap h'eat soap Superneat soap (7) Superneat soap

1016

J. W. MCBAIK, It, D. VOLD, AND

11. FRICK

each of several temperatures (8). The results of observation of nineteen rystemu, ranging in composition from 15 per cent to 100 per cent sodium stearate a t temperatures from 96" to 161"C., are given i n table 3. EXPERIMENTIL RESULTS

The phase rulc diagram constructed from these data is ahown in figure 1. The T,curve, which gives the limiting concentrations and temperatures TABLE 2 PosszbEe LransttzoiL lenipeiatures obtained bu inzcroscopic observatzons T'

60DIUY 6T~,RATB

1

tNCIPIENT BRIGHTENING

TO

I

_--_I

"C.

'C

161 165 171 228 231 256

41.5 40.0 49.5 51.0 5.5 .0

270 265

91.5

____

32.6

90.5

158 143 158

142 209 245 267 263

76

'

.

'C.

138-143 T 180 1 168 or 183

199(?)

78 86

I 52

i

'C.

79

90-98

125

The arrows in the table indicatewhether the temperature was rising ( T ) or falling ( 1) when the change in appearance was observed a t different temperatures in the two cases. Anhydrous sodium stearate underwent the following changes in appearance: a t 123°C. the original dull material was replaced by a bright, fine-grained, pebbled structure; a t 166°C. the visible structure became coarser and brighter, and the color was still predominantly golden; a t 203°C. there was some softening, the structure became markedly coarser and brighter, and some red, green, and blue color appeared; a t 256OC. there was marked softening, bubbles became practically circular, there was marked orientation at the edges of the tube and a great increase in brightness, the variety and depth of color and the size of the structural units increased, and some small focal conics developed; a t 285°C. the field became completely dark as the soap melted cleanly t o isotropic liquid. I n addition, there was a gradual brightening of the field around 90°C. and a gradual appearance of brighter golden material from 108' to 118°C. The established transition temperatures (7), plotted in figure 1, are 69", 90", 117", 132", 169", 203", 256", and 288°C.

of existeiice of isotropic soap solution, is fairly well defined. The two maxima in the curve are presumptive evidence for the existence of two aqueous soap phases, middle and supelneat soap, which have no counterpart in the anhydrous soap. This conclusion receivcs further support from the qualitative observation that the change in viscosity a t T,is less between ca. 45 per cent aiid 95 per cent sodium stearate than a t higher or lower soap concentration8.

1017

THE SYSTEM SODIUBI STEARATE-WATER

The minimum temperature a t which isotropic liquid can be present between middle soap and soap-boiler's neat soap, shown in figure 1 as TABLE 3 AppeaTalkCe of systems at various temperatures SODlULI BTEARATEI

,

APPEARANCE OF 8 0 D I U Y STEARATE SYSTEW AT

__ 96'C.

'

~

IOB'C.

126'C.

I

144%.

~~

161'C.

____

weighf per cen4

15.3 19.9 24.8 29.6 34.9 39.8 45.0 19.5 55.0 60.3

I Ihl hl M 31 MN(?) Mix

I I M hl

A1 AI

MK

I

I

I

I

I

IM

I

I I I

&I

hl

M hl

A1N (1)

Mx

31

Mix

Mix

XI

s

S S'

xx

S S

S'

"

S

S

N'

S'

S

SC SC

"

K'

S

NC

S'

NC

S' S'

S S

?;

64.9 74.5 79.8 84.7 89.8

NC

92.6 94.6 97.1 99.6

C C C

xc C

S

NC

NC SC(1) C C

M

NC

xc

C C

XI

S

i\' S'

S' S' X' S

S

S

h-

s

SW W

w

i\'

SW

w

_-__

I n this table I represents a mobile isotropic liquid; M represents a nearly transparent, nearly rigid, anisotropic liquid crystal which is middle soap; N is a rather clear, translucent, anisotropic liquid crystal soft cnough t o slide in an inverted tube and is probably soap-boiler's neat soap; K' represents systems very similar in appearance t o those designated by N, except that they are considerably cloudier and less translucent; C is a rather lustrous, white, frequently fibrous, immobile crystalline material, which is probably curd fiber phase (very possibly not in the case of observations a t 126°C.);and W is a rather opaque, whitish, waxy material somewhat deformable on pounding the tube, and is probably waxy soap. Two letters in the same space indicate that both phases were present, although in general phase segregation did not occur under the influence of gravity (49.5 per cent and 45.0 per cent systems did separate into layers a t 161'C.). The apparent viscosities, judged by the ease with which systems flowed when the tubes were inverted and tapped, were in general greater toward the middle of the concentration range of a given homogeneous phase than towards its edges or in two-phase regions. Thus, a t 161°C. a 79.8 per cent system flows less easily than either 84.7 per cent or 74.5 per cent systems.

145OC., is riot entirely certain, since there was an indication of the possible presence of isotropic liquid at lower temperatures in a few of the samples

1018 v

J. W. YCBAIN, R. D. VOLD, AND M. FRICK

studied microscopically. Likewise, the value of the minimum temperature of existence of isotropic liquid between neat soap and superneat soap, shown as 262OC., may require slight revision. In the region of this latter minimum in the T i curve visual values of Ti were occasionally found to be as much as 10°C. higher than the repro-

XJTROPIC

SOLUTION

__

-.

100

80

60

COMPOSITION (WEIGHT

40

20

0

PER CENT SODIUM STEARATE’J

FIG.1. Phase rule diagram of the system sodium stearate-water.

0 ,T i ; a,T,;

e,dilatometric results; e,T O ;8 , other microscopic observations; A, vapor pressure results, taken from preliminary unpublished work of W. W. Lee. This portion dealing with curd and subwaxy boundaries will be revised when those measurements are completed.

ducible curve shown in figure 1. Although isotropic between crossed polaroids, the solutions were quite turbid. The turbidity can scarcely be due to hydrolysis products, since the soap actually contained a slight excess of alkali. In one instance it was noted that suspended dust particles were no longer free to move a t temperatures several degrees above

THE SYSTEM SODIUM STEARATE-WATEH

1019

T , $despite the absencc of any apparent change in the fluidity of the solution as a whole. The phenomena may be related to the apparent existence of dull anisotropic patches in isotropic sodium oleate solutions of correyonding concentration (8). Parts of the T , curvc, giving the maximum temperatures a t which solid, white curd fiber phase ran exist at any concentration, are also rather sharply defined. However, in concentrated systems where curd fiber phase may melt to a mixture of subwaxy soap and soap-boiler's neat soap, thc change in appearance is not sufficiently striking t o permit of precise visual determination. It is significant that the results of viewing experiments, with 2 days a t each temperature, are in general accord with the T Ccurve, obtained with a continuous rise in temperature of about 0.25"C. per minute. Thus, systems in the field of soap-boiler's neat soap were bemi-fluid and translucent, whilc thosc of higher soap roncentration (at temperatures below 128°C.) always had some lustrous white fibrous material, frequently with beautiful projecting individual fibers similar to those photographed in the case of sodium palmitate (9). This seems rather strange in view of the fitct that anhydrous sodium stearate undergoes a transition to the subwaxy phase a t 117°C. In a few instances the position of the T , cuive was confirmed micro5copically by the change which occurred a t this temperature from a dull, nearly opaque, featurelesp appearance t o a much brighter appearancr with a prominent structural pattern. I t is theoretically possible to determine the compositions of the interzections of the bounding curves of the middle-soap field with the T , curve from the phase rule requirement that the T , curve must be flat over the coiicentration range where two condensed phases are present on melting (3). I n the case of sodium stearate this criterion was not particularly helpful, since the increase in T , with increasing soap concentration iz so gradual over most of the concentration range that it is difficult to say \\here the ends of the flats are because of the experimental uncertainty. l h t l boundaries of the middle-soap field are nevertheless fairly well fixed by direct microscopic determination, in a few instances, of thc temperature wiierc melting rommenced t o isotropic liquid ( T o )and by the viewing experiments showing whether a t a given temperature systems were homogrneous middle soap or a mixture of phases. Thc lower boundary of the region of soap-boiler's neat soap is defined by the T , curve. The right-hand boundary of this field is dotted in, partly from the results of viewing experiments and partly from a very rough estimate of the phase composition made from thc relative volumes 3f neat soap and middle soap in the few cases where phase separation xcurred. The upper boundary of the field of superneat soap is rather well deter-

1020

J. W. MCBAIN, R. D. VOLD, AND M. FRICK

mined by the microscopic observations of To. The central portion of the diagram is left blank, since no boundaries have been determined nor

----I ISOiROPIC

SOLUT 0 4

COMPOSITON (WEIGHT PER CENT SOAP)

FIG.2. Boundaries of the field of isotropic solution in various systems of soap and water. 0,this work; 0,M. J. Vold; 0 , McBain, Lazarus, and Pitter; A, McBsin, Brock, Vold, and Vold. '-Data are shown only for sodium palmitate and sodium laurrttc and then only in the region of the new maximum in the curve.

has t,he independent existence of superneat soap and soap-boiler's neat soap yet been demonstrated in this system.

THE SYSTEM SODIUM STEARATE-WATER

1021

No adequate investigation has yet been made of the phase behavior at very high soap concentrations. Transitions known to occur in the anhydrous soap (7) have been tentatively represented as giving rise to corresponding aqueous phases as in the sodium oleate system (8), but there is as yet no evidence as to their fields of existence or concerning the question of their difference from or identity with the,aqueous phases existing at lower soap concentrations. The name “neat soap” has been retained for the phase which first separates on cooling anhydrous isotropic liquid sodium stearate and for very concentrated aqueous systems, but it seems likely, though still unproved, that this phase is not continuous with the aqueous phase known as soap-boilerls neat soap. Anhydrous sodium stearate has an additional transition (at 90°C.) so far not detected in the case of sodium palmitate. Whether this is merely a change of crystal form or a transition to still another mesomorphic phase is not yet known. No attempt is made in figure 1 to indicate the phase boundaries in the binary system arising from either this transition or the genotypic transition (69°C.)of the anhydrous soap. Nineteen systems were separated into groups on the basis solely of their visual appearance, without prior knowledge of their compositions, after they had been homogenized and cooled to room temperature. Those which had been isotropic liquid a t 100°C. at room temperature were a dull white, coarse, granular material resembling plaster. Those which had been middle soap a t 100°C. had an appearance at room temperature intermediate between the preceding and following groups. Those which had been soap-boiler’s neat soap at 100°C. had a high sheen, were macroscopically homogeneous, of a fibrous nature, and had many fine fibers visible to the naked eye. Those which had been a mixture of curd fiber phase and neat soap at 100°C.were not quite so white (slight yellowish tinge), had a very high sheen, were very fibrous, and had many welldeveloped fibers. Those which had been curd fiber phase at 100°C.had less sheen, were less fibrous in appearance, and had a slight yellowish tinge. Hence it seems that the appearance at room temperature is somewhat dependent on the phase from which the curd was formed, despite the presumption that most of these systems ultimately consist of the same mixture of phases at room temperature. The new results on the T i curves of sodium laurate and sodium palmitate are shown in figure 2, together with previous results on these systems. DISCUSSION

Since the series of phases of the anhydrous soap is not the same for all, it follows that qualitative differences must likewise exist in their phase rule behavior with water. However, comparison of figure 1 with figure 2 and with the diagram for sodium oleate (8) shows that sodium stearate, sodium palmitate, sodium laurate, and sodium oleate all have the same

1022

J. W. MCBAIN, R. D. VOLD, AND Y. FRICK

kind of T i curve, and hence significant comparisons can be made, sincc presumably the phases involved are the same with the different soaps (except above ca. 95 per cent). I n order to compare the phase behavior it is desirable to select points a t which the different soap systems will be in a corresponding condition, rather than to compare temperatures a t some arbitrary composition or compositions a t some arbitrary temperature. For this purpose the following six “corresponding points” on the T i curve have been chosen: K , which is the temperature of maximum inflection in the solubility curve (closely related to the so-called Krafft point) ;Io, where isotropic solution is in equilibrium with both middle soap and curd fiber phase; M , where middle soap melts to isotropic liquid of the same composition; I N , w h c i ~ isotropic liquid is in equilibrium with both middle soap and soap-boiId* TABLE 4 Compositions* and temperatures at COTTespOnding points on the phase Tule diagrams of sodium laurate. sodium valmitate. sodium stearate. and sodium oleate

Sodium laurate (CIZ).. . . . . . . Sodium palmitate (CIS). ... . Sodium stearate (Cis). . , , . . . Sodiumoleate (CIS-).. . . _ . .

294’ 1.7 285 284 1.1 269 287 1.5 262 260 0.5 199 __ * Temperatures are in “C. and compositions in terms of moles of water per mole of soap. 136 159 145 70

3.6 4.5 6.0 6.1

neat soap; S, where superneat soap melts to isotropic liquid of the same composition; and ZNs, where isotropic solution, superneat soap, and neat soap (subneat soap in the case of sodium oleate) are simultaneously in equilibrium. Comparative results for sodium stearate and sodium oleate, which differ from each other by only a double bond, are shown in table 4. In all cases corresponding changes occur a t lower temperatures with sodium oleate than with sodium stearate. The unsaturated soap is also much more soluble, as shown by the higher concentration of soap in the saturated solution (point IcM) and by the very much lower temperature of the Krafft point (point K ) , the temperature below which the soapjs nearly insoluble ( < 2 per cent). Although the maximum temperature a t which middle soap and superneat soap can exist is lower for oleate than for stearate, the compositions a t these points ( M and S) are nearly the same

THE SYSTEM SODIUM STEARATE-WATER

1023

in the two systems. There is a considerable difference in composition a t point Iss, which is to be expected, since in this instance the phases involved are no longer the same in the two systems, being neat soap, superneat soap, and isotropic liquid with sodium stearate and subneat soap, superncat soap, and isotropic liquid in the case of sodium oleate. A further difference is the much larger range of composition over which sodium oleate systems exist as a mixture of phases as opposed to sodium stearate systems, particularly with respect to the equilibria between soap-boiler’s neat soap and middle soap or isotropic liquid. Table 4 also shows the effect on the phase rule diagram of increasing the length of the hydrocarbon chain of the soap molecule. The values given for points S and I N Smust be regarded as provisional until a more exhaustive investigation has been made of this portion of the diagram. The solubility in hot water (point IcM)decreases rapidly as the chain length increases. The composition of that middle soap which melts unchanged to isotropic liquid varies nearly linearly with the length of the hydrocarbon chain, This fact would seem to require that any satisfactory structural model of middle soap must dispose the water in such a way that its amount is related to the length of the chain and not only to the extent of hydration of the carboxylate groups. -4similar situation seems to prevail with respect to the composition of that superneat soap which melts to isotropic liquid without change of composition. Here, too, the concentration depends on the chain length, the water content a t the maxima in the curves being in the ratio 11:13.7: 18.3 for laurate, palmitate, and stearate, respectively, while the lengths of the hydrocarbon chains are in the ratio 11:15:17. Sodium oleate, despite its double bond, has almost exactly the same water concentration as sodium stearate a t this corresponding point. SUMMARY

A part of the phgse rule diagram of the system sodium stearate-water has been constructed on the basis of visual observations of the temperatures a t which phase changes occurred, supplemented by microscopic determinations and by viewing experiments a t selected temperatures. -4much larger number of mesomorphic phases exist than had previously been recognized with saturated soaps. Except a t very high soap concentrations the phasc behavior in this system is qualitatively similar to that found with other soap systems. Comparison has been made of the temperatures and compositions of corresponding points in the phase rule diagrams of water and the following soaps: sodium laurate, sodiurn palmitate, sodium stearate, and sodium oleate.

1024

L. A. MUNRO AND J. A. PEARCE

REFERENCES (1) MCBAIN,J. W.:Chapter V in Alexander’s Colloid Chemistry, Vol. I. The Chemical Catalog Company, Ino., New York (1926). (2) MCBAIN,J. W., BROCK, G. C., VOLD,R. D., AND VOLD,M. J.: J. Am. Chem. Soc. 60, 1870 (1938). (3) MCBAIN,J. W.,LAZARUS, L. H., AND PITTER,A. V.: 2. physik. Chem. A147, 87 (1930). (4) MACBAIN,J. W., VOLD,R. D., AND JAMEBON, W. T . : J. Am. Chem. Soc. 61, 30 (1939). (5) MCBAIN,J. W., VOLD,R. D., AND VOLD,M. J. : J. Am. Chem. SOC.60,1866(1938) (6) VOLD,M.J.: J. Am. Chem. SOC.62, December (1940). (7) VOLD,M.J., MACOMBER, iM., AND VOLD,R. D.: J. Am. Chem. SOC.62, December (1940). (8) VOLD,R. D . : J. Phys. Chem. 4S, 1213 (1939). (9) VOLD,R. D . , AND FERGUSON, R. H.: J. Am. Chem. SOC.60,2066 (1938). (10) VOLD,R. D., AND VOLD,M. J.: J. Am. Chem. SOC.61,37 (1939). (11) VOLD,R.n.,ANDVOLD, hf. J.: J. Am. Chem. SOC.61,808(1939).

T H E T I M E OF SET OF SILICA GELS. V

THE EFFECT

OF

AND

PH

I,. .\, MUNRO

AND

ALCOHOL8

ON THE

“HEAT O F ACTIVATION”

J. .4. PEARCE’

Depnrtment of Chemistry, Queen’s Uniaersity, Kingston, Canada Receaved August 1 , 1959

Hurd and Letteron (7) have discussed the application of the equations of chemical kinetics and the Arrhenius equation to the setting of silicic acid gels. A value of 16,940 cal. was obtained for the “heat of activation.” This was independent of the soda-silica ratio (8) and the weak acid used ( 5 ) but varied when strong acids were used (6). Prasad and Desai (13) have recently estimated the “‘heat of activation” of various inorganic gels of unrecorded pH. Previous investigation (12) has shown that the effect of addition agents on the time of set of silica gels is specific a t all pH values except 7.0 and changes in a regular manner as the acidity decreases. The heterogeneity of the gelating system and the difference in properties of acid and alkaline gels made it seem of interest to determine whether this change in behavior and the specific effect of addition agents would be reflected in the “heat of activation.” Present address: Department of Chemistry, &Gill Canada.

University, Montreal,