IKHOJIOGESEITT O F ALUMINCJl DILAUR4TE
1471
VARIABILITY ASD ISHOMOGEKEITT O F -4LUiLIINUM DILAURATE” ’ KAROL J . lITSELS3
ASD
JAMES
W. 3lcB.4IS
Department of Chemistry, Stanford University, California
Recpiivd June 23. 1948
Technical aluminum soaps prepared from mixed fatty acids behave like complex mixtures whose properties depend greatly on minor variations in the met’hod of preparat,ion. An obvious explanation is t’hat this is due to c,hanges in composition brought about by reactions between basic aluminum soaps and fat’ty acids or by the formation of aluminum soaps of different acid combinations. In t’hepresent paper an alt’ernative possibility is pointed out through the study of pure aluminum dilaurate, whose properties may be changed drastically by mild physical treatment. This pure soap shows also complex inhomogeneous behavior, despite its apparent simplicity, and in many rcsprrts closely resembles technical aluminum soaps (7). VARIABILITY OF
AlOHL,
7’11~“original mod$catiori”i
Sluminum dilaurate may be obtained a,s a tine white pou-der by addition of A sodium laurate solution to excess aqueous aluminum chloride and extraction of the washed precipitate with acetone (8, 10). It has ash values of 11.SG-ll.R2 per cent’and t,he lauric acid content of one of the samples was 89.3 per cent. The composition corresponding t o A10HL25 is 11.49 per cent hl,OX ash and 90.5 per cent acid. The ash was determined by direct ignition at 900-1OOO”C. The lauric acid content \\-as det’ermined by Mr. R. H. Coc hy displacement with aqueous hydrochloric acid, extraction with cyclohexanc, and titration Tvith 1 I’rrsc’iited at the Symposium on Gel Formation. Iletergeney, bhulsification and Film Formation in Son-Aqueous Colloidal Systems. which x a s held under the auspices of the Division of Chemistry and the Division of Petroleuin Chemistry at the, 113th Meeting of tlir American Chemical Society. Chicago, Illinois, iipril, 1918. Study conducted under a contract hetween Stanford Uiii ity and the Office of I ~ h c ~ r gency >Ianagement, reconiniendetl 11)- Division 11.3 of t h e Sational Defense Ilcsearch Cominittec and supervised tiy Professor ,J. 11’. 1IcBain. 3 Present address: Departmeiit of (’hemistry, Univcrsity of Southern California. 1,os ilngeles 7 , California. 4 The term “modification” is used Iicrc, iii a purely empirical sense t o denote materials having clearly different properties and thc same composition n-itliout assuming anything about their structure or stability. 5 The calculated compositioiis are based not on the theoretical niolceular weights of lauric acid, (‘,?H?aO:, but on t h a t of the “lauric acid” used in the 1)rcparation and which was within 0.25 per cent of thc theoretical. This procedure seeins justified by the great difficulty of obtaiiiing completcly purc fatty acids and by our essential conccrn t o differentiate, not bctween disoaps of various f a t t y acids, b u t betvxen the various soaps (mono-, di->t r i - , e t c . ) of any given acid. Experiencc with sodium soaps suggcstcd that small amounts of other fatty acids havc. in general, negligible effects on the propertics of a soap.
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KAROL J. MYSELS AKD JhYES W. MCB.41S
alkali in the presence of water. This soap is slightly hygroscopic and may be completely dried by evacuation or storage over phosphorus pentoside at room temperature (9). I t cakes readily. It shows no distinctive features under the microscope and even under the electron microscope only irregular agglomerates were found, too thick to be translucent to electrons. The s-ravdiffractioripnttern I
15 50 50 15 I
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1
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1c 7
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5
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4
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2.5
h, ing slowly)
Glassy (Prepared by quickly)
Recovered from a
FIG.1. Microphotometer tracings of x-ray diffraction patterns of six typical modifications of aluminum dilaurate, AIOHLI, showing the same Bragg spacings but greatly differing sharpness (courtesy of Dr. Svdnei- Ross).
consists of a series of relatively sharp lines and a halo, as shown by the top tracings in figure 1. This and the other patterns here presented were obtained by Dr. Sydney Ross. Reproducible behavior upon heating was obtained if the soap was sealed after drying in an evacuated tube by a technique essentially as described by Marsden (2). Gradual heating was effected either in a forced-draft air oven or with much better control in a Hershberg melting-point apparatus (1). Under these condi-
IXHOJIOGEBEITY O F ALCMISCM DIL.4L-RATE
1473
tions the powder sintered to a white opaque mass a t 13--170”C., and became transparent rather sharply over less than 1°C. at a temperature between 190” and 193°C.; this is taken as the melting point because the resulting mass is homogeneous and isotropic although of exceedingly high viscosity. It withstands heating for short periods to 300°C. and flom slon-ly at this temperature. Prolonged heating at 300°C. in a sealed tube or even at much lower temperatures with drastic temperature gradients results in decomposition. The soap decomposes readily before melting unless the tube containing it is evacuated and sealed.
The “crystallized” mod$ication The original appearance of the soap is not restored u-hen cooled from the molten state; instead, a hard brittle mass is formed whose properties depend on how it is cooled. The mass becomes strongly birefringent and markedly opalescent when the soap is cooled slo~vlyand kept for 15 min. to an hour between 160”and 170°C. It melts sharply upon heating again, losing its opalescence and birefringence over a range of less than 1°C. betiyeen 193” and 196”C.,i t . , about 3” higher than the “original,” the exact range of temperature depending on the sample of soap and on the way it n-as crystallized. The cycle of melting and crystallization may be repeated many times with a single sample. This modification, upon cooling, yields a bluish I\ hite, slightly translucent, brittle solid. The x-ray diffrartion pattern, when compared with that of the “original” modification, shows the same but much sharper lines and the halo is reqolved into dktinrt lines (figure 1). The “glassy” vzorlijication On cooling rapidly the melt obtained by heating either the “original” or the *‘cryFtallized” modification, a clear, slightly broivnish, transparent, brittle glass is produced. Its x-ray diffraction pattern I$ c~mparablein sharpness to that of the “original” modification (figure 1).
The ~‘recoie,.cd-~fro,,i-ycl“modification The “original” modification imbibes large nmountq of cyclohexane ut loom temperature, forming a turbid gel (3) from nhich the soap may be recovered quantitatively by vigorous evacuation, yielding an almost transparent brittle glass having n less qharp x-ray pattern. l’he ‘ L r e c o ~ e ~ c d - f r o m - ~ e ?nodtJicatioii lly”
The turbid gel formed by either the “original” or “cryitallized” or .‘recoveredfrom-gel” modifications in cyclohexane changes upon heating t o a clear jelly (3). This jelly generally remains clear for some time after cooling and the soap may be recovered therefrom by evacuation, whereupon it becomes transparent and brittle, and its x-ray diffraction pattern shows but faint lines. The position of the lines is, however. dill the same as in the other modifications.
0
1474
KAROL J. MYSELS AND JdJIES TV. MCBALU
T h e “lyophilized” m o d i f i a t i o n
A dilute sol of the soap in cyclohesane may also be prepared, and after freezing and evacuation, the soap is recovered in the “lyophilized” modification, which is a white fluffy powder composed of extremely fine films and fibres as shown by the electron microscope (5). It gives an s-ray diffraction pattern which is only faint but still indicates the same spacing. Comparison of these niodificatioris There is no change in composition during all the above transformations, as they occur either in sealed tubes or with quantitative recovery. All of them, by proper heating and cooling, give the same either “crystallized” or “glassy” modification, or may be recovered from a sol in the bulyophilized” modification. Thus these are all different modifications of the same material, corresponding to the composition *41(OH)L2. The sharp x-ray diffraction pattern and the melting over less than 1°C. of the “crystallized” modification suggest that a single definite compound is present. The unchanging composition upon fractionation by cyclohexane, described below, confirms this point of view. The differences between the above modifications are summarized in table 1. Besides their appearance, melting point, and sharpness of diffraction pattern, they also differ markedly in their behavior in cyclohexane at room temperature and upon heating. At room temperature, when placed in an excess of cyclohexane, the “crystallized” modification shows little perceptible interaction even after several months; the “lyophilized” modification, on the other hand, dissolved rapidly to a sol. Other modifications show an intermediate behavior; the “recovered-from-jelly” swell.; slon ly but without apparent limit, forming a jelly. The “original” swells to 25-50 cc. per gram; the “recovered-from-gel” to only about one-third that much. The iiglassy” swells markedly but is obviously inhomogeneous in its behavior, some parts sn-elling much more than others and most of them appearing as a jelly. I’pon heating, the viicosity of jellies decreaqes (6) and the degree of snelling of all gels increases (3), followed by a rather sharp transition from gel to jelly. The temperature of this transition is 40°C. for the “original” soap and 48°C. for the “crystallized.” For the “recovered-from-jelly” and bilyophilized”ones it is presumably belo\\ room temperature (3’1. Thus these modification.: differ markedly in their properties and beem to fall between the two estremes of “crystallized” and “lyophilized.” However, the order is not very well defined; for example, the birecovered-from-gel”swells distinctly less than the ‘.original,” despite a more amorphous x-ray diffraction pattern. The identity of perceptible s-ray diffraction lines s h o w that no more than one crystalline form is present. The variation of sharpness of these lines shows great differences in the degree of orderliness; whether this is due to particle size, crystallite size, or actual variation in regularity within a crystal cannot be stated a t present.
ISHOMOGEPiEITY O F dLUMINUM DILACRATE
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147.5
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KAROL J. MTSELS -4SD JAMES It-, I I c B I I N
I t seems apparent that by modifying the physical treatments many othcr modifications of this soap intermediate between or deviating slightly from the typical ones described above might be prepared. For example, “original” modifications prepared at different temperatures difler markedly in their resistance to hydrolysis by moist acetone (8). INHONOGESEITT O F I L C J l I i i C M DILACRITE,
;1lOHLz
Three hypotheses present themselves to account for the behavior of these modifications: ( I ) The simplest is probably that the modification.j actually observed are secondary mixtures or solutions of t x o primary modifications, one completely crystalline, the other completely amorphous, somewhat analogous to the Smith concept of the A and 1.1 forms of sulfur. ( 2 ) A more complicated hypothesis assumes that betv een the amorphous and the fully crystalline there is a continuous (or almost continuous) series of primary modifications, each with a definite degree of orderliness, and that the several modifications we have described arc homogeneous, typical, primary modifications. ( 3 ) -1still higher degree of cwmplexity may be assumed in hich the primary modifications of hypothesis 2 exist but their mixtures or solutions give the actually observed secondary modifications. This type of complexity is \vel1 known in the case of high polymers, where molecules of definite size exist but any actual preparation is polydisperse, and widely differing properties may be obtained by varying the molecular weight distribution. It is also analogous to powders which cannot be prepared with exactly uniform particle size and which show different properties depending on size. (In these t\\:o examples, however, the ease of interconvertibility of the different modifications is much smaller.) Some indication as t o which of these three hypotheses is correct is given h y the behavior of the “original” soap in cyclohexane (in which the most ordered “crystalline” modification is practically inert a t room temperature and the most amorphous ‘blyophilized”modification dissolves completely). If the modifications were a11 stable and reversibly holublc (like polynierb), then according to the Jirst hypothesis cyclohexane TT oiiltl fractionate any intermediate into varying proportions of two fractions, awording to the second each modification \vould be homogeneous and have a definite -oluhility, and according t o the third the solvent c ~ ~ i fixctionate l d each modification into a large number of different fractions. The modifications or’ aluininum tlilaiirate arc, hou ever, ccrtainly not truly stable nor rever5ibly soliible in cyclohexane. In thc dry btate, we have never noticed any vhange ovei a period uf 18 months. lTiwalob.cl?-ations of the soap in the presence of cyclohesanc in sealed tube.; shonetl that, upon raising the temperature, the interaction bctneen 3oap and solvent came apparently to n stable itate within 20 min. or lesi: once the soap was well snollen, and n-ithin 1 or 2 days if it was not yet An-ollen. Upon cooling, however. the interaction hetn-een soap and solvent qeemed to be much slower with frequent marked supersatura-
1477
ISHOJIOGESEITT O F ALUJIISUJZ D I L l U R l T E
tion. Furthermore, upon precipitation, more stable, more crystalline modifications might be formed ( 3 ) . Thus any eyuilibriuni betwen solute and .solution i- not too well defined and is probably an unstable one. The appearance arid volume of the swollen soap depend, hon-ever, on the modification considered and even after months of contact with the solvent at room temperature there i.j little indication of any change from one modification to another. Thus the rate of transformation txtiveen them is at leabt \very J o n a t room temperature, compared with the rate at TI hich cyclohexane interact\nth each. Severtheless it seema that a definite amount of ioap dissolve, under reproducible conditions, 3uch as gentle tumbling of the dried sonp (9) with cyclohexane in 25-cc. graduates, maintained within 0.1”C. in an air thermostat. The amount dissolved \vas determined by weighing the residue of tl linoivi~ weight of filtered supernatant liquid after freezing and evacuation 25%.
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2CO
Tim. hours
FIG.2 . “Origirial” aluminuiii dilaurate-cyclohesane.
‘i-ariation of soap content oi
supernatant liquid with time. EFFECT O F T I J E
Figure 2 shows the effect of time over several days a t tivo temperatures on systems containing **original”soap and cyclohexane. -4fter about 2 days there is little further change. That there is a limit t o tlie amount of soap which dissolves is definitely indicated by qualitative ohservntions in sealed tuhes, rdiich s h o ~ vthat even in systems containing only 0.3 per cent of soap the solution is only partial a t room temperature even after many months. Therefore the terminal mlue after 2-4 days is quite definite for a given soap under specified conditions. EFFECT O F T-1RTISG S O l P TO SOLTEST RATIO
Figure 3 s h o m tlie change in concentration of the supernatant liquid as the proportion of soap to cyclohexane was changed, other conditions remaining the same. In one series the temperature was 23°C. and the time 2 days, in the other 21.5”C. and 4 days. Each series shows that the amount dissolved is proportional to the amount present within the range and precision of the esperiment. This is shown in figure 4. where the proport’ion of sonp dissolved is plotted and nppears constant’ when tlie ratio of cyclohexane to soap varies. I n all t’he above experiments each point corresponds t o a separate system opened only for analysis. In another esperiment 25 cc. of cyclohexane was
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KAROL J. MYSELS .4ND JAMES W. YCBAIN
0.4
oe2
0.6
0.8
w t . $ soap in system Fic. 3. “Original” aluniinum dilaurate-cyclohexane. Variation of soap content of supernatant liquid with amount of soap present and with temperature.
wt.’
p 30
of soap 20 dissolved 10
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a-
JI
25OC
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eoe2
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00 4
0.6
21.5OC 008
w t . $ soap in system F I G .4. “Original” aluminum dilaurate-cyclohesanc. Variation of weight fractioii of soap dissolved Xyith amount of soap present and viith temperature.
ISHOMOGESEITT O F .ZLL7311SU31 DILAL-R-kTE
1.2'79
agitated n-ith 99 mg. (0.51 weight per cent) of soap at 25°C. -After 24 hr. as much as possible (19.3 cc.) of the supernatant liquid was n-ithdran-n (and analyzed) and replaced 11)- fresh solvent. This ~ r a srepeated every day and the amoiint dissolved computed. ,Again 27 per cent of soap disqolved in the fint 2 days and not more than 9 per cent in the follon-ing two, although the concentra. tion of the supernatant liquid fell from 0.115 per cent after the first day to 0.03 per cent after the fourth day. Thus again a fraction of the soap dissolves readily while the remainder does not. I n obvious question is n hether the dissolved and undissolved materials have the bame composition as the initial soap, or if they are different. To answer this a fraction soluble at 2S°C., recovered in "lyophilized" modification after the evaporation of the solvent, ivas heated in a qealed evacuated tube. I t seemed to melt at 189-190°C. -1ftei being cooled and kept at 1GO"C. until completely birefringent, it melted at 194-19S"C. X sample of the original soap melted (after crystallization at 160°C.) at 194.3-195.3"C. The x-ray diffraction pattern of the crystallized soap obtained from the dissolved portion was sharp and indistinguishable from that obtained from the original soap. Thus the dissolved portion is identical in composition with the original soap, a result which confirms the view that the material studied iq A\10HIJ2. Since the most likely impurity in the soap was lauric acid, a small amount of it was added to one of the qystems but had no noticeable effect, as shown by the cross in figures 3 and 4. EFFECT OF TEiMPERdTURE
=Is the temperature increases from 21.5" to 25°C. the proportion of soap dissolved increases from about 8 to 26 per cent, as shown by figures 3 and 4. In this connection it may be noted that visual observation of the soap as the temperature is raised s h o w no abrupt changes in the system (the gel-jelly transition temperature, while sharp and easily observed, may correspond to the disappearance of only a minute quantity of soap, of the order of 0.05 per cent) ( 3 ) . I t seems therefore that 8 and 26 per cent are simply characteristic of these particular temperatures and that the proportion dissolved would vary continuously with temperature. Experiments at lower temperatures become difficult, however, owing to the extremely low concentration of the solution, and at higher temperatures the soap swells so much as to make the separation of supernatant liquid very difficult. IXTERPRETATIOX
These facts lead to definite conclusions with respect to the structure of the soap. Were the soap homogeneous, consisting of one primary modification (hypothesis 2), it should tend to saturate the cyclohexane always to the same extent, i t . give a constant concentration in the solvent and varying proportions dissolved ab the ratio of soap to cyclohexane is varied. This is exactly contrary t o euperirnent.
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K l R O L J. 3ITSELS A S D JAMES W , 3lcB.IIS
If the soap i b a mixture of t w o (or more) primary niotlificatioiis (hypothesis 1 or 2 ) and if one ( o r more) is very soluble and the other onc (or more) relatively insoluble in cyclohesane at a given temperature, then the proportion of soap dissolved should reflect the ratio of these primary modifications and the concentration in the solvent should vary as the supply of the soluble material varies. This is in agreement with each series of experiments. If only two primary modifications are present (hypothesis l ) , the proportion dissolved should be the same a t both temperatures as long as the soluble one dissolves completely and the insoluble does not dissolve. (If the insoluble dissolves somewhat, the proportion dissolved shoiilcl vary.) This is again contrary to experiment. Thus only the third hypothesis remains: namely, that more than two primary modifications make up the soap. The experiments might agree with the esistence of three such modification\: (a)not dissolving a t 25°C. and forming 74 per cent of the soap: ( b ) dissolying a t 21.5"C. (8 per cent); ic) not dissolving a t 21.3" C. and dissolving a t 25°C. (15 per cent). It Seeins more reasonable to assume, in agreement with qualitative observations, that there is a continuous series of degree and uniformity of organization. SUhlJIART
Aluminum dilaurate, in its *'original" modification as prepared, is a fine white powder of formula AlOHL. This may be changed by physical treatments into other modifications differing definitely in their properties, such as melting point, solution and swelling in hydrocarbons, and sharpness of x-ray diffraction pattern, yet having the same composition and the same Bragg spacings and being transformable into identical modifications. When placed in cyclohesane the original soap dissolves only partially, the proportion dissolved being independent of the amount of solvent but varying with temperature. This indicates that these modifications are not themselves honiogeneous but are products covering a continuous range of degrees and uniformity of organization and therefore having different degrees of crystallinity, different stability. and different activity. These are presumably not allotropic modifications, but although thermodynamically unstable are of extremely long life in the dry State. They are not phases in the true Gibbsian sense. Their differences, due to I arying orderliness of crystal arrangement or of crystallite size, have an effect analogoil5 to that of particle size or degree of polymerization and give a continuous transition between a completely crystalline and an amorphous extreme. REFERESCES ( I ) ~IERSIIBERC, I< B Intl 1;np. Clicni , .ha1 1 3 . 8 , 312 (1036'1 (2) h I a a s n s ~S , S Rev Sei. Instruments 16, 192 (10453 (3) MCBAIS,J. Vi , MYSELS, I
