STUDIES ON ALUhfINURI SOAPS.
I1
23
(29) SPENCE, J.: J. Pliys. Chem. 43, 865 (1930). (30) STAUDIYGER, H. : Die liochmoleliulnren organischeu N n f i c r s l q f e . J. Springer, Berlin (1932). (31) TAYLOR, G. I.: Proc. Roy. SOC. (Londoii) A103, 58 (1923). (32) WIENER,0 . ; Ahhantll. innth. p1iJ.s. Masse siichs. Akad. Wiss. (Leipzig) 32, 1 (1!)12). (33) WEISSEXBERG, IC.: K a t u r e 159, 310 (1947). (34) Z\'ETROY, V. S.: Acta Pliysicochini. r-.R.S.S. 19, 86 (19-14). ZVETKOV,V. h-.,A N D FRIMAN. E . : Acta Physicochiin. LJ.R.S.S. 20, GI, 3G3 (1945). ZTETKOV, V. N., A N D PETROVA, A , : J. Tech. Phys. (Lr.S.S.R.) 12, 423 (1942).
STUDIES ON XLU3nIIN;UN SOAPS. I1 THECOMPOSITION, STRUCTURE, AND GELLING PROPERTIES OF ALUMINUM SOAPS' V. R. GRAY
AND
A. E. ALEXANDER
Dcpni,ttt/cnt of Colloid Sciencc, Canibi,idge 1Tniiiersify,Cambi,idge, England
Recsiued dicyzcsl 10, 10gS
The aliiminiin salts of the higher fat'ty acids find considerable use in t,he manufacture of paint.s, for thickening hydrocarbon oils (greases), and n s stabilizers for emulsions (13, 20). Their nature and composition are, ho\vever, siuprisingly obscure. Corninercially they are made by an aqueous double decomposil ion process (13, 20). The composition and thickening (often termed "gelling") pi'operties of the find product depend critically on a number of factors, siich as the pH and tempernture during the precipitat,ion, the speed of mising, the size of the particles formed, etc. One Trery important factor is the ma8nnernntl est8eiitsof t81iedrying of the precipit#uteonce it has been formed and \rashecl. In scienl ific in~estigat~ions of t,heir composition a non-aqueous double tleconiposit'ion react ion, such as that' Iiet'iveen an aluminum alkoside and rl. fatty acid, has much to i'ecommend it, as t,lie little-knonii influence of writer can be eliminated 01' conti-oiCerI. McBain and ,1IcClatchie (21), \\rho reacted aluininiun ethoside \\.it,hpnlmit ic acid, fount1 that not more ihan ti\co fatt'y acid groups coiild lie atltnclietl t,o each aliiminum :Itom, m t l they concluded th:Lt the tjrisoap does not, exist. In point, of fact, t'heir most reliable an:tlyais figures intlicnt8e2% lower extent of react ion e\-en than t,\\,ofntt'y acid groups. Thliies of 5.2-5.4 per cent) A41,0a\yere found; nluminim etlioside \\.ith 13 fnttp acid groups replaced by pa1mit)icacid coinresponds t,o 5.44per cent AI?O3. AlcBain's co\yorliers ( 2 i ) have 1,eceiitly considered t,he qiiest'ion in greater detail. From a study using the aqueous pi*ecipit8ationmet,hod t'hey conclridecl tlint bot'h the monolnurate and the dilanrate exist, ns tlefiii ite clirinical indivicliials and t'lie x-ray tlat'a are claimed t o support this T.ie\y. Eigenliei3ger and Eigenher~er-Bitt'i~er (11, 12), in n yery full esamiiintioll of l)otjh tlic nqiieous and noli-aqiieous precipitat'ion methods, conclutlecl t hat, in Iioth 1 Prcsciited at t h e Twciity-second Nntioiial Colloid Symposium, which WLS licit1 U I I C I C ~ t h e nuspices of tjhe Division of Colloid Chemistry of t h e Biiierican Clielnicnl RocictJ, nt Cniiil)t~itlge,1Inssnchuset ts, Julie 23-25, 194s.
24
V. R. GRAY AND A. E. ALEXANDER
cases the primary product contained lt fatty acid groups per alumilium stom, and proposed the following formulation for aluminum stearate : St s st St St’ St st st tl
I
,41-0 u1\l-- 0-A1 -0 -.AI
I
I
1
-0 --AI-O
--X1-0
I
I
--.I1-0--.U
I
St
8t This formula is derived from the idea of ‘5sopolybases” found by Jander (16) from diffusion sludies of aqueous solutions of aluminum and other metal salts just before the precipitiition of the hydroxide. However, Eigenberger’s own experiments do not give much support for this formula. Firstly he only analyzed for aluminum in his products. This is not as serious in the case of aqueous precipitation ns in the case of t,he reaction between aluminum isobutoxide and stearic acid in dry benzene. Kot only is it difficult t o imagine a compound resulting from the 1al)terreaction in which all isobutoxide groups are eliminated, but if we imagine only 1i isobutoside groups replaced by stearic acid we should get an A1203 value of 5-50per cent, whereas Eigenberger found G.GO per cent. He should clearly have analyzed a t least for fatty acid and preferably also for alcohol groups. A further curious feature of this work is that greater ratios of fatty acid t o alkoxide than 5:4 do not seem to have been tried. In the case of aqueous precipitation he distinguishes between “true” and “pseudo” aluminum stearate, the latt’er being deprived of its fatty acid completely by repeated precipitation from a solvent. 1Iost commercial samples, he concludes, are a mixture of t,hese two forms. La\\,rence (19) believes that mono-, di-, and tri-soaps all exist. He bases his claim for the existence of the trisoap on the observation that distillation of aluminum isopropoxide with 3 moles of stearic acid yields 3 moles of alcohol. Edn-ards (10) studied the precipitation of aluminum hydroxide alone and in the presence of molten stearic acid. Finding that stearic acid produced no effect upon the pH changes during the precipitation, he concluded that no compound formation occurs in aqueous solution, and that the precipitate so obtained is merely an adsorption complex of stearic acid on hydrated alumina. Here the reaction between a range of aluminum alkoxides and fatty acids with and uithont solvent’s under a variety of conditions has been followed. I n additioil t o malysis for aluminum, both free and combined fatty acid have been estimated, and in some cases alkoxide groups also. The effect of water hits also been examined. ~ L U I W N U M.ILIiOXIDES, THE TISHCHENKO REACTION, ARTD THlC MEERWEIN-I’ONNDORFVERLEY
REDUCTION
Aluminum alkoxides find iniportant use in organic chemistry, us they ctttalyze the exchange reaction between carbonyl compounds and alcohols. It R” R R‘l \
‘C-0 /
Rt’
+
R”’JC-0, /
€I‘
\
R’-C-0-
/
H
‘\
f
K“‘’
C=O
This ieactioii is called t'iie ;\Ieer\veiii-Poiiiidorf-T'erley
reduction, after tlie
:Liithoi.s \\-ho first employed it extensively (22,25,30). I t must evidently Iic tlie electron-trmsfer reaction shown, as Bdkiiis (1,3) \vas :lljle t o me:isure the ositlat,ioii-reductioii potent8ials of carbonyl compoiinds by ciit:tlyziiig the at,txinmcnt of eqiiilibriiini \vith aluminum fcrf-butoxide. Tn this c,ast! the alcohol group on t.he a.Iuminum cannot, piu'ticipate.. I n t his i\q the ~.aliieof nluminum isopropoxide a:. 21, i,eduping agent is explairiecl by the compizrnti\*o FLUE \vith ~vhich
isopropjd alcohol is conr*eitecl t,o xetone. B:tker (2), invest'igating the reaction l found a imnpitl exchange I)et'\\-eeiit'he Iiet\veen :i,luminiini nlkusitie~: n ~ esters, nlliositle gi-oup on the :zl~iniiniininnd t'hat on t'he est'er. These investigations clearly she\\- t'li;Lt aluminum alliosides ionize in organic niedin, :i,nd that this property is the origin of their mlue 212s ;;elect;ire reducing agent$, leaving non-polar reducible groups such ns double bonds unaffected. :Z foi mally cquivalent reaction to the hIeern-eiii-Ponndorf-~:erleS, reduction is t8heformation of esters from aldehydes. This is referred to LLSthe l'ishchenlto react ion (20), and since t'his author secured considernble priority over other ~ v o r k ers thew is something to be said for including bot'h reactions tindei- his iianic. l\7Icls (32), in R comprehensil-e rel.ic\v of t8hereacdon and its appIic:itioii, si~ys tliiLt tlie mediiznisin is unlmovm anti dIo\\.h the possihilit'y of the form:ition of an ;i,liiniiniim tlerivntmiveof a. lieniiacetal
Rf' R'f'-[
ion\vas also carried out by mixing the dry reactants and warming. The heat of the reaction was measured as a function of fatty acid added. I n all cases the soaps fornetl \yere recovered, escess fatty acid removed, and the soaps analyzed for fatty acid and aluminum. REACTANTS EMPLOYED
A number of aluminum alltoxides were prepared by the usual method (14) of dissolving aluminum in t'he appropriate alcohol or phenol. Iodine or mercuric chloride was usually necessary as catalyst, and the product was purified by distillation in uaczm. The alkoside eventually chosen for investigation was aluminurn sec-butoxide. It '\vas found earlier that extreme precautions had t o be tdten to eliminate water from the reacting system, as the alkosides are so very hygroacopic. The solid alltoxides and phenoxides are difficult t o keep dry, particularly as they are not 1-apidly brought into solution. The scc-but'oxide, on the other hand, is a clear gummy liquid, which is miscible with the organic liquids employed. It is easily sealed in glass tubes immediately after distillrttfion,and these can be iveiglied and used as required. That the reactions of scc-butoside are hypical is shown by early results with the other members (eg., m-cresoxide, isoproposide) which were less accurate, but which were essentially all equivalent. A check on the efficacy with which water had been eliminated from our systems was sho\vn by a negative heat of dilntion of the alltoside. If traces of water were present in the diluent' or on the vessels used, a rise in t)emperature occurred on solution. The fatty acids used were acetic, caprylic, lauric, myristic, palmitic, stearic, oleic, and ricinoleic. The oleic acid \vas a distilled commercial sample; the ricinoleic acid was a commercial product partly purified by filtration; and the rest were purified fatty acids kindly supplied by RiIessrs. Price's (Bromborough) Ltd. All acids were carefully dried, the liquids over calcium sulfate, and the solids by keeping in u desiccator containing phosphorus pentoxide. CHOICE O F MEDIUM
The organic media used had to be quite anhydrous. For one in Ivhich the soap was soluble, benzene dried over sodium was used. The soap was recovered by el-nporating off the benzene and removing fatty acid from the solid obtained, as described below. It \vas more difficult to obtain an anhydrous non-solvent. This was necessary, as the only good means of removing free fatty acid from the soap is to extract Tvith such a liquid. Non-solvents such as acetone or ethyl acetate which give condensation products \\:ith powerful dehydrating agents are not suitable. Eventually diosalle T \ ~ Schosen. It can easily be dried over sodium and its miscibility with ivater simplifies analytical operations. It was employed also as extracting
STUDIES ON dLU.\IIN;UM SO1PS.
I1
27
agent for uncombined f:Ltty acid in the soaps. Estraction was carried out at room temperature in a modified Soxhlet apparatus where the distilled solveiit was condensed before reaching the soap.
20
n
w
6I
%
$0
Y
d
D
w
3
0
5 I-
a
W
I
0
I
I
I
2
MOLES ADDED SUBSTANCE Fro. 1. Heat evolution (kilocalories per niole of slkoside) for t h e reactioii b e t n ~ e i i aluininiuin scc-butosicle a i d fatty acids or water. Curve A : oleic acid in scc-butyl alc,ohol. Curve B : 0 , oleic acid in diossne; 0 , lauric w i t 1 in diosane; 0, caprylic acid in dioxane; A ,oleic acid in henzene; acetic acid in benzene. Curve C : water i n diosniie.
+,
ANALYTICAL TECHNIQLTE
Aluminum was estimated in the usual way by ignition t80alumina in a silica crucible. In some cases where it n-as more convenient (eg., the acetate), 8-hgdrosyquinoline was used for its estimation. The fatstyacid in the soaps \vas est'imnted as follow: A weighed sample of soap wa,s hydrolyzed by hot caust'ic potash solution. The solution was made acid, and the fatty acid released was estracted wit'h et'her. The ether layer was washed and titrat>edwith N / 2 0 carbon dioxide-free caustic soda, using as indicator a mixture of one payt cresol red to three parts t,hymol blue, as recommended by I(leinzel1cr and Trim (15). With the acetate the acid could be estimated directly by hydrolysis with neutral boiling water and t#itrat,ion.
28
Y. R . GR.\l' .\SD
-4, E. bLES.INDER
THE HEAT O F REACTION
This \\w mcasured in diosane and benzene for a number of fatty acids, as a function of M a l fiit'ty acid added. An unsilvered thermos flask was employed, with a Beckmann t8heimometerfor temperature measurement. The heat capacitv of t h e flask \\'as determined electrically. RESULTS
\?'hen vxt>er\vas absent t'he lieitt of the reaction iw a function of niole ratio of fatty acid added wa,s repioducible and independent of the nature of the fatty acid, of tlie medium (except \\here an additional reaction \vas int'roduced), and, over a 60°C. mnge, of temperature. Judging from tlie time of lieat evolution the reaction must he :L very rapid one indeed, a point' supporting ionization of the allroxide. The results ai*eslioivn in figure 1 (curve B). The curve s h o w n linear increase up t o about 1.5 moles of acid, when it begins t o curve over and beyond about 2 moles becomes ste:idy at, nboiit 18 lical. per mole of allioside. It seems, then, that under these condit'ions of reaction no more tlia8nabout 1.5-2 moles of fatty acid react foi' eirery mole of nlltoxide. This is confirmed by analysis of the soaps formed, given in t8ables 1 and 2 . Table 2 is for some reactions \vit#haluminum m-eresoside dissolved in t'etralin, \vhich undoii1)t~edlyadsorbed some water, but possessed the advantage that' cresol groups could be directly estimated by bromination. The low values found in general for tdie allioside groups in the soaps are probably due to the absorption of moisture during handling. It is seen that are general 1.5-2 fatty acid groups are combined n.ith one aluminum atom. This confirms the result of McBain and XlcLatchie (21). The dry reztction was of great, interest. It ivhs found possible t o confirm the observation of La\vrence (19) that 3 moles of alcohol could be distilled off by addition of 3 moles of fatty acid t o 1 mole of alkoside. When, however, the special precautions t o keep the allroxide dry were observed, the whole of the alcohol could he obtained only by heating the soap t o such an extent t'liat decomposition occurred and fatty liet,one formed. In all cases the product was extracted in the cold \ v i ~ l idioxane. In no case was a final product containing more than two fatty acid groups per :duminum atom obtained. Analyses are given in table :3. Few esamples are given here of combination up to the disoap stage, since disoaps of some of the lower acids-lauric, myristic, and oleic among them--were gummy fluids from \vhich it was difficult t o remove excess fatty acid. The exist#enceof disoaps c:in be inferred from these results as they were in each case subjected t o prolonged extraction, and in the case of the acetate the existence of a disalt is ulreatly recognized. Figure 1 (curve A) shows the course of the reaction in sec-butyl alcohol. The heat evolution is increased, but the exteiit of reaction is not altered by the presence of excess of product, showing that the reason for termination of reaction is not the nttairirnent. of an equilibrium involving the reaction products. Curve C shows t,he reaction with water, added in dioxane salut'ion. ;jomewhat, contrary to expectation the heat' evolution is much less-about 12 kcal. per mole
STUDIES ON BLUMINCM SOAPS.
29
I1
TABLE 1 d v a l y s i s of soaps FATTY ACID
~
MOLECULAR \VEIOET
::$y:&mAE
~
BUTOXIDE PER ALUMINUM ATOM B Y DIFFERENCE
~
-
-~ ~~ _ _ _ _ ~ . (a) Precipitations with aluminiuni see-butoxide in dry cliosmie; all a t 20°C. unless otherwise stated __ . . ___222.5 1.26 i 1.13 L:iuric*.. . . . . . . . . . . . . . . . . . . . . . . . . i 211.5 1.51 0.25 1.25 0.60 201.2 1.0; 0.80 Oleic.. . . . . . . . . . . . . . . . . . . . . . . . . . 281.0 284.0 1.32 1.13 , 284.0 1.56 0.49 I 284.0 1.50 0.42 (at 40°C.) 284.0 1.51 0.36 (at 40°C.) 1.50 0.80 I 281.0 292. 0 1.51 0.0 NIyristic . . . . . . . . . . . . . . . . . . . . . . . . 228. S 1.01 0.61 Caprylic. . . . . . . . . . . . . . . . . . . . . . 147.5 1.68 i 0.48 Acetic. . . . . . . . . . . . . . . . . . . . . . . . . . 60.0 1.71 1 1.20 ~
~
~
~
_______~
~
(b) Precipitatioxs in dry benzene, evaporated and extracted
__________
I
Oleir
....
................
Caprylic. . . . . . . . . . . . . . . Acetic.. . . . . . . . . . . . . . . . . . . . . .. i.
284.0 284.0 281.0 254.0 147.5 147.5 (30.0
1.30
1.16 0.65 1.33 1.34 1.33 ( a t 60"C.j 0.90 0.92
I
1.84
i
1.68 1.68 1.22 1.40 1.02
'
(c) Precipitations in butyl alcohol ~
I
Oleir, . . . . . . . . . . . . . . . . . . . . . . . . .
-
~
292
1.50
- -~
0.37
-____~
~
~
TABLE 2 Soaps from alzminum ?n-eresozidc ntid
fcltljl
acids
iii,
dioxnw ~
FATTY ACID
i
1____~__ Lauric, . . . . . . . . . . . . . . . . . . . . . . . . . 201.2 201.2 I 201.2 Oleic.. . . . . . . . . . . . . . . . . . . . . . . . , I 284.0 I 284.0 Myristic. . . . . . . . . . . . . . . . . . . . . . . . 228.8 Caprylic.. . . . . . . . . . . . . . . . . . . . . ., I 147.5 Acetic.. . . . . . . . . . . . . . . . . . . . . . . . . I 60.0
1
__
PER ~ ~ ~ ~ ~ N A & m A ~ , " CRESOXIDE ~
MOLECULAR WEIGET
I
#-.
~
~
1 ~
i
1.51 1.40 1.70 1.82 1.55 1.65 2.00 1.78
ALUXINUM ATOM
_ _ _ _ _ ~ ~ _
0.36 0.65 0.59 0.35 0.51 0.83 0.81 0.90 - _--- -.
-
of nlkoside-and reaction is complete on addition of about 2 moles of water per mole of alkoxide.
30
V . R. G R d T AND iz. E. ALEXANDER
A number of soaps were prepared by adding less than 1 mole of fatty acid per mole of alltoxide in dioxane and centrifuging off the soap formed. Analyses are given in table 4.
BUTOXIDE P E R ALULLIBULI A T O M 5 Y DIFFERENCE
F.tTTY ACID
I
Palmitic. . , , . , . . , , , . , ,
S t e a r i c . .. . . . . . . . . . . . . . . . . . . . . . .
286.7 284.0 284.0 285.0 60.0
i
i Acetic. . . . . . . . . . . . . . . . . . . . . . . . . .
Soaps FATTY ACID
I .72 2.00 1.67
, , , , , , , , , ,
1,
1
1
,f/,oiii
2.00 1.97
1
GO.0
0.91 0.66
1
T.4BLE 4 l e s s lhnn 1 ?i?olc oJ f n / l ~cicid
LCOLECL‘LAR !!‘EIGHT
1
AC1,?$E:$:ER
ACID P E R ALUbIlNUIIZ ATOM I N SOAP
I
Oleic . . . . . . . . . . . . . . . . . . . . . . . , / Caprylic.. . . . . . . . . . . . . . . . . . . A c e t i c . ,. . . . . . . . . . . . . . . . . . . . Lauric . . . . . . . . . . . . . . . . . . . . . .
0.33 0.00 1.25 0.00
202.0 147.4 60
’
0.01 0.85
~
1.14 1.09 1.26
, ,
BI‘TOXIDE P E R A L U I I l h ’ U b l ATOP1 B Y DlFEERESCE
1
O.SG
0.57 1.50
DISCUSSION
It seems that the salt usually formed by interaction of fatty acid and aluminum alkoxide contains about 1.5-2 fatty acid groups to one aluminum atom. The ratio varies in a way which is not completely understood; it is lowered \\.hen ivater has partially reacted with the alltoxide, as might he expected; it is not affected, for reaction in organic media, by temperature or time of leaving the reaction mixture. When insufficient fatty acid is present to react ivith all the allcoxide the soap formed has over 1 mole of fatty acid per aluminum atom. These observations resemble those from similar experiments by Eigenbeiger (11). It i5 proliable that in his work he caused precipitation of unreacted alkoxide with the soap when the latter was removed from the benzene by addition of absolute alcohol. However, his view that about 1.25 moles of fatty acid react when in small amounts is t o some extent borne out here. As for the existence of “well-defined” salts of aluminum with fatty acids, we still have t o be cautious. The oiily one which appeays t o have any possibility of existence is the disoap. The evidence given here is limited to three samples, which were nevertheless definitely established. The acetate seems to be the
STUDIES ON ..\LUhIIMJM SOAPS.
I1
31
most \vel1 defined of the di-salts. Extraction of acetic acid could hnrclly be slow under any conditions, and in any case aluminum diacetate has heen prepared many t,imes finom aqueous solution, and is u d l defined a t least in it's acid-alumilium ratio (15). The heat data indicate that acetic acid combines irith aliiminlim in exactly tmhesame manner as for t'he other fntt8yacids. We may therefore conclude that they t,oo form cli-salts, irhich are, lion.e\.er, very difficrilt t80prepare. The t'iisoaps certainly cannot8he made hy m y lmo\rn method. The monosoaps piesent' a, prohleni. There is no discont>iniiityin the heat' of renct'ion ctirve for the at1dit)ion of 1 mole of fatty wid, \\.hich shorild be expected if a definit'e monosoap formed. When less t h m 1 mole of fatty acid is ndcled, the soap precipitated al\va.vs ~ R decidedly R more t'han 1 mole of combined acid per aluminum atom even when inireacted allcoside remains in the supernatant liquid. The indications are that the monosoap, too, does not exist as a definite chemical indi.i*itliial. The reaction curve u 3 h water shons; that only 2 moles of water react. It seems probable that the init'ial product of t'he hyclidj-sis is the s:tme a s that found ~ v h e naluminum is precipit'nted from nqueoiis solution (31). In tjliis cme the primary pi-otliict is I)ohmit'e, AlO(0H). The equation for its formatmionin o ~ i r case ~vouldbe : Al(0R)z
+ 2Hz0 = AlO(0H) + 3ROH
No doubt in the presence of excess ivnter other products form as in aqueous solut,ions. It, \redd be interesting t o confirm this by the x-ray techniques cleveloped by Weiser (31). THICKENING PROPERTIES O F A\LUJZINUM SO.IPS AND THE R 6 L E O F T17ATER
The thickening power of aluminum soaps in organic solvents depends upon t,hree factors: the nature of tjhe fatt)yacid group, the presence of peptizing agents (n.at,er Iieing: a special case), and the nature of t'lie organic liquid used as dispersion medium. By Twintion of these t'hree fact'ors it is possible to obtain a range from heterogeneoiis systems v.ith completely precipitat'ed soap, t8hrough thick gels or jellies, t o completely mobile solut,ions. For coni7enience the product of t,he reaction may be regarded as falling into one of the following physical states: Insolut~le non-swollen precipitate
Paste of Friable microcrystals gel
Strin gy thicli Mobile solution solution
In geneml, interconversion bet'ween these forms can readily be effected by changes in the factJorsmentioned, and on aging some moT-ement from right to left is commonly observed. Thus, addit)ionof a few per cent of an ordinary commercial nluminum lauibat,e t o benzene gives first, a pnst'e of microciaystnls, which ages into a friable gel and finally to a st'ringy thick solution. If a short-chain soa,p such as aluminum butyrate is treated in the same way it remains completely insoluble. Increase of chain length promotes solubility, but an optimum is
32
V. R. GRAP AND A . E. ALEXANDER
reached around the laurate, and with much longer chains, such as the stearate, the soap is insoluble although it may exhibit some swelling. Raising the temperature or adding peptizers tends t o produce swelling and eventually solution. The presence of double bonds (as in oleic acid) tends to promote solution, whereas addition of a hydroxyl group acts in the converse manner, aluminum ricinoleate, for example, being insoluble under conditions \\-herethe stearate is quite soluble. The nature of the organic dispersion medium is also of great importance. Thus, in petroleum ether the soaps are less soluble, and the laurate has now t o be heated for solution t o occur. Peptizers, usually substances with pronounced coordinating properties (see later discussion) such as amines, alcohols, fatty acids, etc., promote swelling and solution of insoluble soaps and render viscous solutions more fluid. The requisite amounts are generally very small (e.g., 1 per cent molar proport.ion of the soap). It should be mentioned that all commercial soaps contain variable quantities of free fatty acid, which is it,self a peptizer and together with the water present probably explains the differences in thicltening properties of different samples of the same soap. Water has a special r d e of its o m . ItJbehaves to some ext’ent like the other peptizers. Thus, increase of moisture content in a commercial soap gives increasingly fluid solutions (7). The special properties of water are revealed most strikingly when it is almost or completely absent, as v.ith the alltoxide decomposition method after extreme precautions to eliminate water have been taken. Under such conditions, over a wide range of variation of solvent, peptizer, alkoxide, and fatty acid, the reacted syst,em shows little change in yiscosity, remaining perfectly mobile. Addition t o this fluid system of very small amounts of water (ca. 1per cent calculated on the soap) generally produces a marked increase in viscosity, indicating that the formation of some type of linear aggregate has taken place. In a few cases (eg., aluminum sec-but,oxide reacting with lauric acid in dry ether, or with oleic acid in benzene) n viscous solution was obtained under what were believed to be anhydrous conditions. Houwer, in vieiv of the difficulties of ensuring really anhydrous conditions, it may \yell be that m t e r is essential if aluminum soaps, in low concentration, are to give viscous solutions. The action of these traces of water is presumably t o remove unreacted nllcoxide groups: e g . , ‘)Al--OR H10 -+)Al-OH ROH
+
+
t,he hydroxy-soap thus formed then
it
T H E COh’STITUTION
egating t,o form long chains (page 36). O F ALUMIhXJM SOAPS
The product of aqueous metathesis Britton ( 5 ) ,discussing the precipitation of aluminum resinate during the sizing of paper states: “The estreme weakness of sluniiniuni hydroxide as a base coupled with thr very weak nature of abietic acid, render i t highly probable t h a t very little, if an);, chemical combination could take place between them, save perhaps a little which might possibly occur through
STIJDIES O N .iLUAIIiYCM SOAPS.
I1
33
the fotmnt,ioii ot' vet'?. iiisolul)lc aluniiiiiuiu 1,esinat.e. In a. siziiig p i ~ ~ c takiiig ~ s s place iil)ovc pII 1, it, is uinlikcl>. t,hnt an>' alu~niniuinresiriatt ~ v o u l dbc coiitainetl i n t h e precipitato uti account of the hytlml>.singnct,ioii t h u t \voultl ttike place. If coiiibiiiatioii occurred bet8wceri such a weak acid and r2 \veal; base 1)y virtuc of t'he insolubilit). of tlic salt produced, then t h a t salt, would lie precipitated froiii solui.ions ~ i i i i r eacid t l i i i i i pH 4 :tnd would untlei,go decomposition as soon as t,lici pH of tlie motlirr liquor exccrtlcd 4."
These considei,iLtions apply in e \ ~ e r yrespect t o the precipit,iition of aluminum stearate in aqueous solution. E d n w d s ( l o ) , ivho drew our attention t o the above quotation, has shown that t'hc pH changes during the precipitation of :tlumina are iinaffect8edby the presence of molten stearic acid. He concluded that no conipound formation could occur. It thus seems likely that tti-uecompound fornmtion does iiot uccur in aqueous solution, :tnd that the substance formed under t'hese conditions is an adsorption coniples of fatty mid on sluniinn. The final comniercial product, ho\\-ei.er, appears tjo be u chemical compound (or mixture of compounds). A sample v;hich \vas extracted ivith cold dry dioxane $aye n constant fatty acid-aluminum ratio of 1.3 to 1. Dried commercial soaps possess all tlie properties obseri-ed for t.he soaps prepared by the aluminum alkositle-fatty acid reaction. We have shown here that the latter appe:tr to be chemical compounds. I t therefore seems that combination between the adsorbed fatty acid and the alumina must occur during the drying process, this becoming possible as the water is removed. This mould explain why the conditions of drying (e.g., time, temperature) play such an important part' in deciding t'he properties of the final soap. Our view is that the product consist,s of a mixture of the disosp KzBIOH with some free fatty acid and free alumina (or some form of Iiydrated alumina), for even when precipitation is carried out in the presence of a protective colloid it seems unlikely that all aggregation of the hydroxide n-odd be prevented, yet the particles may be too small, or too amorphous, to be detectable by x-rays.
=Iluininum poaps as po1ymo.s A41uniinumsoaps in their general pi,operties have much iii cuminon \vitli thc natural and synthetic high polymers. They are rubberlike when in hulk, m d they me susceptible to the influence of plasticizers. S o crystals appear to h a w been obtained. Further, the properties of the soaps in solut'ion as shou.11 in tltc previous paper are best esplained on the basis of long-chain units in sohition. The distinction, in this case, from the more usual polymers is that thc particle size appears t o depend upon the concentration, presence of peptizers (e.g., alcohols, phenols), etc. However, the force of aggyegation of the individual soap units is so h g e that for modemte concentrations long chains extend through the solution. We may thus assume that aluminum soaps arc pol~~niers, but that the linl;n,ge which joiiis the monomeric units is considerably weaker than is found for carboncarbon chain polymers, and others such as silicones, based on covalent ljnknge:;,
34
V. R. GR.IY AND A . E . ALEXANDER
7'hc ttufitrc of the linkuqc joitritzg soap units .i tliiwt covdent al~iminrini-nluminiim chain is unlikely, though such a linkage, ot n sort, has I)een postulated to explnin the electron diffraction pattern of dimeric duminum t'riinethyl, AI?(CH3)6 (6). If such linkages exist bet,\veen liighly chaigetl at'oins of the same sign they \vould not be expected to form polymers. -\ direct .~I-O-~~l-polymerized chain, enviszged by Eigenbeiger ( I l ) , is ruled out as only monosoaps could form, and we have seen that furt,lier combinnt'ion occurs. However, an ~\I--O-Al-chain is the most likely possibility for a polymer chain, so we must' examine the possibility of such a chain involving other than direct coydent linkages. One possibility is that which has already been post8iilat'edas responsible for t'he association of alkoxide molecules, namely:
R
R
I
I
-+hl-0-+.21-O-+.~lThere :we other possihilities, such as A1-O--c'=O+,41
I
R This 1inl;agc can join beiyllium at,oms, as it is present in basic beryllium acetate (24). Basic. beryllium acet,at,edoes not form gels, ho\vever, and it does not seem that, this linkage could be iesponsible for an extensive polymer chain. Further, it is difficult t,o explain t,he results obtained if this linkage is admit,ted,so \re shall assume here that, the linliage i*esponsil)lefoi, t hc gelling propert'ies of aluminum soaps is t8hepi-evioris one. -2 chain of this t>ype\\odd be quite consistent with the st'rong coordin a t'ion tendency of aluminiim, as displayed in many of its compounds. Fourfold or sixfold coordinated groups are formed \vith great' ease. In the si1ic:Ltes fourfold aluminum-oxygen tet,rahedra, together ivith silicon-oxygen tetrahedra, constitiitme\\hat may \)e r e g d e t l as a \\.hole range of polymeric materials h i l t l i p from chains of t'hese t,et,r.ahedrslcomplexes, interspersed with met'allic ions for electrical stitbilit,y (4). 'l'lie tetrahedral coordination t'endency of aluminuni is so strong t,hat the aluminum halides form tlirners (23). Oct8aheclralcomplexes also form \vitJhgreat stability. The triacetylacet'onate nnd t,he trioxalate and catechol tleriiratives are examples of thesc.
TIE consfitiifionof alti~ruhi/msoaps A chemical furmula for aluminum soaps should t n k e into :mount the follo\ving points : ( f ) A polymeric formula h i l t up of AI--O+.U--O+~~l links seems most' pid,nhle. ( 2 ) 'l'hc ovci*-a11molecLilnr formula must' halve about 1.3-2 fntty acid groups per aluminum at,om, and must nllo\v of variation in t,his ratio bet,iveen 1 and 2. ( 3 ) \Ve miist explain the difference in properties between allioxides and soaps. This tliffei-ence cannot, be ascribed t o t,he acidic chnracter of the grouping, as
phenol is not, so clissiinilai~in acid ntrengt'li to stearic acid, yet' t riplicnositles f'oini whereas tjristeaw,tw do not'. The fatt?* acid 'gi3oup nlt,ers the \\.hole process of association. Xlkosicles are associated in solution in f'o~irt'olclgroiips, and they do not, form gels. Soaps associat,e in long chains giving gels. Tho reason Cor this change in n,ssocint,ion clue t'o introclrict8ionof fatt'y acid groups can only iiivolve the follon-ing factors: (a)The fatty acid group is capahle of tn.ofoltl cooi*dination,and so it's int'1.oduction rnn change the coordination niinil)ei~of aluminum from 4 to G , rtltci-ing the \rliole stiuctiire, i.e., 0
-C
/ \
\\r0
is euhstitiited for R-0--. ( b ) The long hydrocarbon chain alters the t'jrpe of association froin t,liree- t o two-dimensional by orientJedhydrocarbon chains forming micellar Iayeix. This can only be a secondary effect, as it is possible to obtain an nliuninum acetat>cgel in benzene, yet alliosides from long-chain alcohols do not gi1.e gels. (c) Possibly the associative linking is changed, as XI-O-+Al-O-+
t0
Al-o-c=0-+Al-
This is not considel-ed likely.
FIG.2. Suggested s t r u c t u l , e for :durniiium tlisonps. carbon; 0 , os~-gcn.
,
: d u ~ ~ 1 i ~ l u0 m ;, ii
\ ~ t l ~ ~ o0g ,( ~ ~ ~ ;
There is, then, a stilong intlicat'ioii that' aluminum soap chains are matle (111 u t linked nluniinuiii-osygeii oct'ahedm, \vi th fatt'y ncitl chains extended hicIe\\.:Lys from the main chain. T1iei.e are t \ r o possible i\q,~ in irhicli these octahedra can tic linked toget,lier: by shni,ing an apes, or by sharing an edge. If the octuhecli-a are linked throilgh an apes t~v,-ofnt'ty acid chains can conihine and \\.e have a disoap (figure 2). If
36
Ti. R . GRAT AND A . E . ALEXANDER
linked through an edge, then only one fatty acid can combine and we have a monosoap (figure 3). The linkage through an edge might not be expected to be as stable as through an spes, and this would explain why monosoaps combine readily with further fatty acid. However, complete conversion to the disoap mould be difficult, for as soon as long chains form, with lateral attraction between the chains, further reaction would be difficult. Intermediate conversions t o the disonp might thus lie explained.
FIG.3. Suggested structiiic Tor
:duiniiiuiii i i i o i i o s o t i p .
0 ,carbon ; 0 , ouvgen.
0 , nlniiiinuin; 0 , h\-droyt.ii;
Assuming that the monomeric forms in each case :we R:AlOH aud R’Al(OH), (wheye R’ denotw the fatty acid radical), the aggregated structures may be written as:
for thr disoaps and monohoaps, i,esyectivrl\.-
RUhfMARZ
The nature of uluininuni alkoxides is discubsed with special reference t o the Tishchenlco reaction and the ,Ileerwein-Ponndorf-vel.ley reduction.
38
1'. R . G R A Y .\XD .i. L;. .4LEX:.iKDER
Soaps \\.ei-cp i q ~ n i ~ eby t l reaction ))etw e n alriminum :dkositles i m l fa1 t y ncitls under ;t vaiiety of coiiriitions. The heat' of miction, measured as a function of fatty acid aclcled, showecl that not more t'han 2 fatty acid groups combine pel. nluminum atom. These i*esultms were coiifirmecl by analysis for aliiminum and fatky acid in soap free from tincomhined f;it,ty acid. It is sho1i.n t81iatthe protliict of aqueous metat,hrsis of an dkali soap a n t l s n aluminum salt' is an adsorption complex of fatty acid on a1umin:t. Coniliinntioii occiirs clrwing tho drying: process. A polymeric formula, for aluminuni so:~psis proposed, hasetl on sixfold coortlination aluminum-oxygen oct'ahedra. When these join through an edge a monosoap forms; \\.hen t'lirough an apex, R disoap. Inlermediat,e ]dues i.esult, from a combination of t'hese. The plienomenoii of peptization is discussed on the hasis of tthis formiila and some infrared measurements are shown to be consistent \vith t'hese conclusions.
The authors thank the Ministry of Supply for permission to puhlish this v-ork, which \vas carried out while they \\'ere in their employ. T o Professor Ritleal they are indebted, as a,lways, for much helpful advice and criticism. REFERENCES
ADKINS,H . , A N D c:ox, F. bv.: J . h i . Chein. Soc. 60, 1151 (103s). BAKER, R. H . : e J . h i . ChC11i. SOC.60, 2673 (1938). BAKER,R,. H., A N D ADKIKS,H.: J. Alii. Clieni. SOC.62, 3305 (1040). BRAGG,\v. L. : .ltorrric 8 t r i c c f n r e o f k f i n e d s . Osfold University Press, K ~ o ~ i t l o(1!)3T). ii BRITTOS,H . T . S . : Hydroyeji I o n s , Vol. 11, p. 275. Cliapman and Hall, L t t l . , L o i i d ~ i i (1942). (6) BROCKWAY, L. o., A X D DAVILBON, X . R.: J . Alii. Chein. Soc. 63, 3287 (1041). C. PI., C A R L I L E J., H . , ICIivr:, J. G., A N D I C I x c m s , F. E . : J. Inst,. 1'etl.olcunl (7) CAWLEY, 33, 721 (1947). (8) C o x , F. W . , ASD ADKINS,€1.: J. Ani. Chein. SOC.61, 3364 (1030). (0) D A V I D S O N ., R., H U G I L LJ., .4., SKISSER,H. A , , .*.I'D SUTTON, L. E . : Trans. Faraday SOC.36, 1212 (1040). (10) E D W A R D L. S , J.: Thesis for Fellowship, Royal Institute of Cheinistry (1046); and private coniinunication. (11) EIGESBERGER, E . : Fette. u . Seifen 49, 505 (1940). (12) E I G E N B E R G E R . , ,. ~ N DE I G E N B E R G E R - B I T TAN. E : Kolloid-%. R, 91,287 (1940). (13j ELLIOT,S. B . : -4lbaline E w l h and H e m y Jletal S o a p s . Reinhold Puhlishiiig Corporation, New Yorlc (1046). (14) G L A D S T O N E , J . H . , . ~ N DT R I B E A , . : J. Chein. soc. 29, 158 (18i6); 39, 1 (1881). (15) GMELIN,L . : H a ~ ~ t l D c ~der c A anorgunische C h c m f e . Verlag Chemic, Berlin (1034). (113) J . ~ N U E G R ., , .ixn J A H R ,Irst 89, 241 (1944). (18) KOHLRAITSCH, I
