44
W. 0.MILLIGAX, G. L. BUSHEY, AND A. L. DRAPER
3. The shape of the isotherms is well explained by Sips' theory if the heat of adsorption is said to vary linearly with pore diameters. 4. Vacuum desorption rates are greatly dependent on the degree of activation. The explanation for the wide variation is evident not in macropore structure, but rather in the micropore distribution, providing surface diffusion with restricted molecular motion on the char surface is assumed. REFERENCES (1) BRUNAUER, S.: The Adsorption of Gases and Vapors. Physical Adsorption, pp. 21823. Princeton University Press, Princeton, IUew Jersey (1945). (2) DAMKOEHLER, G.: Z. physik. Chem. A173, 35 (1935). (3) EMMETT, P. H., A N D KRAEMER, E. 0.: Advances in Colloid Science, Vol. I, Chap. I. Interscience Publishers, Inc., New York (1942). (4) FREUNDLICH, H.: Colloid and Capillary Chemistry, p. 111. Methuen and Co., Ltd., London, England (1926). (5) JUHOLA, A. J., A N D WIIG,E. 0.:J. Am. Chem. Soc. 71,2069,2078 (1949). (6) LANGMUIR, I.: J. Am. Chem. SOC.40, 1361 (1918). J. J.: Ph. D. Thesis, University of Rochester, 1946. (7) MADISON, (8)MCBAIN,J. W.: Z. physik. Chem. 88, 471 (1909). M.: Z. Elektrochem. 26, 360 (1920). (9) POLAXYI, (10) SIrs, R.: J. Chem. Phys. 16, 490 (1948). (11) VOLMAX, D., AND DOYLE, G.: OSRD Report No. 5236,April, 1945. (12) WICKE,E.: Z. Elektrochem, 44, 587 (1938). (13) WICKE,E.:Kolloid-Z. 86, 167 (1939). (14) ZEFFERT, B., A N D DOLIAN, F.: TDMR 864,July, 1944. (15) ZELDOWITSH, J . : Acta Physicochim. U. R. S. S. 1, 961 (1934).
ISOBARIC AND ISOTHERMAL STUDIES I N THE SYSTEM SOAP-WATER. I' W. 0. MILLIGAN, GORDON L. BUSHEY,z AND ARTHUR L. DRAPER' Department of Chemistry, The Rice Institute, Houston, Tezas Received August 10, 1060
Studies of the physical chemistry of soaps are complicated by the fact that even pure soap constituents, such as sodium palmitate and sodium stearate, occur in a multiplicity of polymorphic crystalline forms, each of which is considered by some investigators to exist as various hydrates. Thiessen and Stauff (14,15, 16) were the first to recognize that relatively pure sodium stearate and palmitate exist in a t least two crystalline forms, designated as alpha and beta. Although 1 Presented before the Twenty-Fourth National Colloid Symposium, which was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society at St. Louis, Missouri, June 15-17, 1950. ' Proctor & Gamble Fellow, 1947-48. Present address: Department of Chemistry, University of Illinois, Urbana, Illinois. a Proctor & Gamble Fellow, 1948-50. Humble Oil and Refining Company Fellow, 1950-51.
DEHYDRATION ISOBARS IN THE SYSTEM SOAP-WATER
45
Thiessen and Stauff believed both forms to consist of the anhydrous soap, Buerger, Smith, de Bretteville, and Ryer (1) and Gardiner, Buerger, and Smith (9) demonstrated that the alpha form is actually a hemihydrate. Numerous other polymorphic and hydrated forms of soap crystals have been reported (for a survey of the literature consult references 2 and 6). Ferguson, Rosevear, and Stillman (6, 8) recognized four clearly defined and characterized crystalline forms of soaps, designated as alpha, beta, delta, and omega. On the other hand, Buerger, Smith, Ryer, and Spike (2), McBain, Vold, and Johnston (lo), Vold (17), and others believe that a multiplicity of additional polymorphic forms of anhydrous soaps and hydrates exist, differing more or less one from the other. The minor differences are manifested experimentally by small changes in x-ray diffraction patterns, slight variations in water content and vapor pressures, and irregularities or inflection points in heating or cooling curves. The purpose of the present paper is to obtain precise dehydration isobars for a limited number of carefully chosen soap samples of known history, rather than to attempt to unravel completely the complex pattern of the known or suspected multiplicity of polymorphic and hydrated forms. EXPERIMENTAL
Preparation of samples The samples of soaps employed in this investigation were especially prepared by the Procter & Gamble Company. The following details of preparation were supplied to us by Dr. F. B. Rosevear: All samples of sodium palmitate and sodium stearate were prepared from the purest grades of palmitic and stearic acids available. Alpha phases of both sodium palmitate and sodium stearate were prepared from 1 per cent solutions in 75 per cent alcohol. The thin slurry of crystals obtained by cooling the clear hot solutions to 0°C. was filtered by suction and allowed to dry in air. Beta phases were prepared from a mixture of 60 per cent soap in water, homogenized to molten “neat soap” a t 10O-12O0C. in a sealed glass tube. The samples were cooled to room temperature rapidly to minimize the “fibering” effect often observed in x-ray diffraction patterns, and then tempered for an hour a t 70°C. The resulting mass of crystals was chopped, air-dried a t room temperature (40-60 per cent relative humidity), and finally lightly ground to a powder. The delta phases were prepared by extruding twenty times a homogenized mixture of 60 per cent soap in water through a 1/32-in. orifice a t room temperature. The bar of extruded delta phase was chopped and air-dried as for the beta phase. Omega phases were prepared by heat-treating of air-dried sodium palmitate or stearate for 1-2 hr. in an eiectric oven a t 150°C. A pure sample of calcium palmitate monohydrate was also obtained from the Procter & Gamble Company. The identity of these alpha, beta, delta, and omega phases with corresponding phases described by Rosevear (7, 8) was determined by comparing the powder x-ray diffraction patterns of these preparations with the original x-ray negatives of Rosevear. This identification was carried out independently in the Procter &
46
W. 0 . MILLIGAN, G. L. BUSHEY, AND A. L. DRAPER
Gamble Company laboratories and in this laboratory. The results agree in every respect. Isobaric dehydration apparatus Dehydration isobars in the temperature range -20°C. to +80°C. were obtained for the above-described samples in an apparatus already described (11, 13). This device consists of an assembly of fifteen silica springs, a constanttemperature water bath, and the associated vacuum and control devices. The temperature control is t o approximately fO.OOl°C. The isobars were obtained a t a partial pressure of water vapor corresponding to the water vapor in the residual vacuum of approximately lo-‘ mm. of mercury. This particular procedure was adopted in order to make possible the determination of the isobars at relatively low temperatures, thus avoiding the possible decomposition of the anhydrous soap phases or decomposition of possible traces of temperature-sensitive impurities in the soaps. Samples of the nine crystalline soap phases described above were allowed to come to equilibrium at room temperature with water vapor in the air. One series of aliquot portions in platinum buckets were loaded into the dehydration apparatus and a second series of aliquot portions were taken for water analysis. Three complete sets of dehydration isobars were obtained, with 48 hr. or more required for equilibrium a t each temperature. In the first set (3), the preliminary water analyses were based on loss of weight (“volatile material”) a t 150°C. for 2-hr. periods. Later investigztions demonstrated that more accurate results could be obtained by determining the original water content from the loss of weight occurring in the isobaric dehydration apparatus in the high vacuum a t relatively lower temperatures, such as 30-85OC. (4),or by titration of the original solid soap with the Karl Fischer reagent. In the second and third sets of isobars, these latter two more reliable methods of water analysis were employed (4). A more detailed account of the analytical method used is given elsewhere ( 5 ) . After the completion of the isobars, several sets of water vapor isotherms a t both 12OC. and 2°C. were obtained for the dehydrated samples, in order to make a study of the surface properties of the dehydrated soaps. The results of the isothermal studies will be reported later. I n an independent set of isobars, a-sodium palmitate and a-sodium stearate were examined in greater detail. Seven identical samples of each of these soap phases were introduced into the apparatus and dehydration isobars were obtained. At various points along the isobars, the apparatus was opened and one of the samples was removed for x-ray diffraction examination. The isobar was then continued. The identity of this “stepwise” isobar with previous isobars, completed without interruption, permitted us to be certain that the x-ray samples were characteristic of the soap a t the chosen point along the isobar. This method has been discussed in greater detail elsewhere (12, 18). DISCUSSION
Calcium palmitate monohydrate Gardiner, Buerger, and Smith (9) demonstrated that calcium palmitate exists in the form of a definite monohydrate, and the isobar shown in figure 1 confirms
DEHYDR.\TIOS ISOIIAKS I N T H E SYSTEM SO.\P--\V.\TEK
'
47
in every respect this ear1it:r work. The calcium palmitate monohydrate tlecomposes rather sharply in the temperature range -9OC. to 0°C. a t the extremely lo\v partial pressure of water vapor employed in this investigation, ivhereas Buerger and coworkers observed a decomposition temperature around 90°C. to 95OC. at a much higher partial pressure of water vapor, obtained by mrans of a moist stream of nitrogen h b h l e d through a series of wash toivers containing suturated hrine solution at an unstated temperature, presumably around 23°C. It is to he expected that thelarge differencein the partial pressure of the \vater vapor \vould lead to a large difference in the observed decomposition temperature. In this present investigation, the extremely lo\\.partial pressure of \vatrr uxschoscn to make possible dehydration a t loivrr tempei~atures,thus avoiding possihle loss of
1.00
0.50
0.00 )
volatile materid, \\.hicah is known to occur at more elevated temperatures. The slightly hivariant character of the calcium palmitate monohydrate isobar in the temperature range - 10°C. to 0°C. is attributed to the adsorption or retention of ivater vapor by the finely divided crystals of anhydrous calcium palmitate which are produced by the decomposition of the monohydrate. This behavior is similar t o that of many hydrous hydroxides (19) which decompose to form anhydrous oxides of large surface \vhich may adsorb water, subsequently released as the temperature is increased. X-ray diffraction patterns of the original ealcium palmitate monohydrate and the anhydrous calcium palmitate are distinctly different, in accordance \vith the observations of Buerger and coworkers (9). The average decomposition temperature of calcium palmitate monohydrate may he taken to 1)r - 5Y'.in figure 1, and 9 5 T . from Rurrgrr's isol)ar (9).
.5@
50
\V. 0 . M I L L I G A S , G . L . I1USHEY, AND .%. L. DR.%PER
0.008
.
-0.10 0.006
%
-rnX 0.004
0.002
Ilosevear, and Sordsieck (7). However, this explanation is untenable, since the x-ray diffraction patterns obtained demonstrate that little or no crystals of the beta phases occur in the irregular region of the isohars. hnother possible esplanation is that the slight irregularity results from a distribution of crystal sizes in the original alpha phases, leading to a corresponding variation in decomposition, temperatures (cf. 19). This hypothesis may he tested by ohtaining precision ist.;bars for single crystals of the alpha phases.
Beta and delta phases The isohars for the heta and delta phases of both sodium palmitate and sodium stearate are given in figures -1-7. All of the isobars are regular, exhibiting no inflection points which are attributable to definite hydrates. The hivariant portions of the isohars at the lower temperatures correspond to the loss of sorhed
51
DEHYDRATION ISOBARS I N T H E SYSTEM SOAP-WATER
0.008
-0.10
0.006
f
%
-
0.004
0.002
a
m. 0
0.000
0
0.00
FIG.5 . Dehydration isobar for 8-sodium stearate 0.008
0.006
i 0.004
0.002
0.000 -20
-10
0
T ac.
10
20
30
FIG 6. Dehydration isobar for 6-sodium palmitate
isobars in figures 4-7. Hoivever, the identityof the x-ray patterns of the samples before and after the dehydration in the isobaric apparatus demonstrates that these materials are not definite hydrates.
W. 0 . MILLIGAN,a. L. BUSHEY, AND A. L. DRAPER
52
Omega phases Isobars were obtained for both w-sodium palmitate and w-sodium stearate. The small amount of water originally present in the samples was lost at - 19°C. in the vacuum of the dehydration apparatus. The isobars consisted of horizontal lines coincident with the abscissa; hence the curves are not reproduced here. No indication wm observed of irregularities or inflection points, and therefore, in view of the identity of the x-ray patterns before and after the vacuum dehydration, these omega phases are believed to be anhydrous. The small amount of water held a t room temperature by omega samples in contact with the moisture of the air is considered t o be adsorbed on the surface of the crystals. 0.006
0.10
0,004
I 0.002
-20
-10
0
10
20
T OC. FIG.7. Dehydration isobar for &sodium stearate
30
SUMMARY
The following is a brief summary of the results of this investigation: 1. Precision dehydration isobars have been obtained for the alpha, beta, delta, and omega crystalline phases of both sodium palmitate and sodium stearate, and for calcium palmitate monohydrate. 2. The existence of calcium palmitate monohydrate as a definite chemical individual has been confirmed, in accordance with the results of Buerger and coworkers. 3. The heat of hydration of calcium palmitate monohydrate has been nstimated t o be -33 f 5 kcal./mole. 4. The isobars demonstrate that a-sodium palmitate and a-sodium stearate exist as hemihydrates, in confirmation of the work of Buerger and coworkers. 5. The beta, delta, and omega crystallime phases of both sodium palmitate and sodium stearate are not definite hydrates, the water content being desorbed continuously. The authors are grateful to the Procter & Gamble Company, which made thege studies possible by establiihing a fellowship program at The Rice Institute.
TRANSITIORS IS SOAP-OIL SYSTEMS
53
REFERESCES (1) BUERGER, SMITH,DE BRETTEYILLE, A N D RYER:Proc. Natl. Acad. sci. C . S. 28, 526 (1942). (2) BUERGER, SMITH,RYER,. ~ N DSPIKE:Proc. Natl. Acad. Sci. U. S. 31,226 (1945). (3) BUSHEY: Ph.D. Thesis, The Rice Institute, 1948. (4) DRAPER: M.A. Thesis, The Rice Institute, 1949. (5) DRAPER A N D JfILLIGas: Texas J. sci. 2, 209 (19s). (6) FERGUSOX: Oil & Soap 21, 6 (1944). (7) FERGUSON, ROSEVEAR, A N D SORDSIECK: J. Am. Chem. SOC.89, 141 (1947). (8) FERGUSON, ROSEVEAR, A N D STILLMAN: Ind. Eng. Chem. 36, 1005 (1943). (9) GARDINER, BUERGER, A N D SMITH: J. Phys. Chem. 49,417 (1945). (IO) MCBAIN, VOLD,A N D JOHNSTON: J. Am. Chem. SOC.63, 1000 (1941). SxmsoN, BUSHEY, RACHFORD, ASD DRAPER: In process of publication. (11) MILLIGAN, (12) MILLIGAN A N D WATT: Unpublished results reported at the 110th Meeting of the American Chemical Society, which was held in Chicago, Illinois, September 9-13, 1946. (13) SIMPSON: M.A. Thesis, The Rice Institute, 1943. (14) THIESSEN: Angew. Chem. 61, 318 (1938). (15) THIESSEN AND STAUFF: Z. physik. Chem. 176A, 397 (1936). (16) THIESSEN A N D STAUFF: %. physik. Chem. 177A, 398 (1936). (17) VOLD:J. Phys. Chem. 49, 315 (1945). (18)WATT:M.A. Thesis, The Rice Institute, 1946. (19) WEISERAXD MILLIGAN: J. Phys. Chem. S8, 1175 (1934).
TRANSITIOSS I N SOAP-OIL SYSTEMS BY DIELECTRIC ABSORPTION' TODD M. DOSCHER
AND
SANFORD DAVIS
Department of Chemistry, University of Southern California, Los Angeles 7, California Received August 10, 1960 I. INTRODUCTION
Very small quantities of water and other polar molecules have a very pronounced effect on the physical behavior of soap-oil systems and their industrial utilization. Attempts to understand the role played by water have led to studies of the phase relations in these systems (5, 6, 17, 18) and studies of their stability (7). However, in previous investigations it was not always certain that the last traces of water had been removed from the soap. I t was one of the purposes of this investigation to determine the minimum quantities of water which are required to produce significant changes in the temperatures a t which phase transitions have been recorded. Further, measurements of the capacitance and dielectric absorption of these systems are capable of revealing the existence of any changes in the orderliness of packing and the freedom from intermolecular restraint of the soap aggregates a t or between transition temperatures. 1 Presented at the Twenty-Fourth National Colloid Symposium, which was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society at St. Louis, Missouri, June 15-17, 1950.
