L I.MELL.-\R 4KD OTHER RIICELLGS
39
TJXR1EI,L.4R XSD OTHEK JIICELLES, AKD SOLl~BILLZATION H I ' SO.4P8 XSD DE'I'ERGE?;TSL JARIES if'. M c ' B A I S
.\NU
OSC'.AR A . HOFFRIAN
I)eparfr/iatil oj' Cliprri f.dr!/, Starijbd
I ' n i / , c / , s i / { C'alifoi,/i /$ ;/I
K m e i i w l .Liigctsl 19, 1948
One of the most interesting features of solutions of cwlloidal elect'ivlytes is the variety of x-ray patterns \\,hich diffeimt systems gi1.c: some sharp but, of different types, and these changing with concent 1-at ion and the presence of solul)ilized substarices 01' of electrolytes; other sj-stexns giI.ing 110 tlist,inct pattern at all. Since a numbei, of w c h systems w e here described for the first time, and since their interpretations necessarily differ, it, is advisable to mention the kinds of micelles that ha'i-e ali,eacly heen recognized 01'suggested. When one of tis (.J. \jr. M.) discovered antl defined the class of colloidal electrolytes thirty-six years ago and shelved t'he presence of colloid, the attempt \vas made to explain the heliaj.ioi, on tJhe assuinption of onlj. one kind of colloitlitl particle present, e'i-en if it's composit'ion 'iwiecl Ivith concentration. Ho\\,ei-ei,, comparison of quantitative data olitained 1)y tliffeiwit methods foiwd the conclusion th:Lt tliffei*ent~ kinds of colloidal particles must lie present, in the same solution, antl that' t'lieir respective proportions must change n.ith concentlation. T\vo tlecacles la t,ei*,Haitley revii-etl t81iehypot,hesis of :Isingle . kind of colloitlnl part'icle-namely, a sphei-e of constant, composit ion-Iut he neglected mucnli of the qunntitati\.e data. C'onsiclerat'oii suggest,s that colloidal particles begin t o form IQ. progi.essi1.e nssociatioii of ions and ion-pairs, axid t,hat all the different sizes and niotles of pncliing can occiii* in proportion t o their efficiency in renioi-ing insoluble paits of' the molecule from contact \vit,h the solvent and in accoi,(lance\vith the respecti.\-? reqriii-enientsof t,he pi*iiicipleof mass act,ion. In 1923 AlcBain sriggestecl one form of micelle \\.liich consists of ti1.o I a ~ w sof I Pi,cw$iitetlat tlic T\vrArit,, R P C O I I ~Satioiial Colloid Sj.niposiuin, ivliiTti \vas l i c l ~ lriiitlrr, t lip :iuspicrs of t lie Divisiou of C'olloitl Clieniist ry of the .Aiiiericsii Cliciiiicitl Socic>t>.st ('ii:iiliridgv, .\Inss:wlirisct is, J u n e 33-25, 19-(8.
soap niolecules or ion-pairs partially dissociated and arra,iigetl side 11y side, \\.it11 the tl\vo hydrocarbon layers inside (15). Mat,toon, Stewns, miti Harlrins (14) haye recentl>- (1946) found x-ray e\-itlence for the esistence of t8he JlcBain micelle. The prcsc>iit,paper \vi11 ine,ntion several co1loid:d c!ect8rolytes which f u r geometrical i*eascmscannot pack in this more usnnl \\y~y, although tJhey are ftilly colloidal. From 1937 t u 1912 Hess ( i )ant1 his associates and uthers disco\rered through x-ray examination that. the I\lcBain Inmcllar micelles can repeat in parallel arrangement', separated by tlctini tc layel,:: of ivatcr, t h m gilring a long spacing equal to the tloiihle Iengt81iof the molecule phis that of the layer of \\rater. This micelle we shall refer to as t,he .Hew niicelle. For niaiiy years A'lcBain arid collahoratjors h ~ ~ tlefined ve the trrin ,'solul)ilization" to include all cases in \vliich ot8her\viseinsoluble nittt,erial i6 tjrought into solution by incorporation \\itthin or ripon slal)lc col1oitl:tl micelles (16, 19, 2 0 ) . For this there are t'hree possihilities, all of \vhich uccur. First, attachment to the esternal polar groups, generally \viLh rlisraption of the s-pay micelles, an example of \vhich will he gil-en in the present' paper in the soluhilization uf dimethyl phthalate; second, interlajvering ivithin the blcBain and TIess micelles bet\\.een the hydrophobic la,yers:,examples of ivhich \vi11 also he ,giT.en ; and, third, interpenet,ration I)et\\.een the molecules o r ions of the micelle. 'rhis is the explanation here suggested for the many colloids ivhich so1ul)ilize hydrocarbons a,nd yet do not form McBain or Hess micelles. Hydrocarbons {lo not, tlissolvc in, o r fully \vet, Iiycl !~ociirI)onchains oE paidleloriented fatty acids or soaps, such :ts films on uxter, o r spherical niicelles. Schulman and h.IcRobei,ts (21) she\\- t h a t ultramici.oscopica1 tli-oplets of oil so confined to the inirldle of spherical-sh:tped filins may forin a. continuous hridge t'o oulinary emulsions. In 1945 Hughes, Sa\\yer, and Vinograd (8) olitainecl definitive proof of the presence of Hess micelles in isotropic soap solutions, whether or not other colloidal payticles are also present ; \\'e shall here adduce similar elridence for isotropic solutions of a cation-active detergent. Later Hughes, in addresses before various scientific socities (unpublished), suggest,erl for t'he more conwntrated anisotropic potassium laurate solutions i~ different arrangement of micelles: namely, a hexagonal array of parallel e1ong:ttecl part,icles, analoguus t o the tactoids of tohacco mosaic virus as deduced by Bernal and Fanla.ichen (1). ITo\vewr, \ve shall here present evidence t h a t even in these soltit'ions the Hess micelles itre prominent. Klevens (9) and Hsrlrins (4) have suggested that the elongated micelles might be cylinders or ellipsoids. We prefer to suggest, in accordance with t.he \yellest'ablished tendency of soaps t o crystallize in rod-like or lath-like forms, Lhnt elongated micelles are merely narro\v McBtiin micelles, which for geometrical reasons is the most, efficient forni of packing. Definitive s-ray proof of the hesagonal array of elongatJed micelles was obtained hy McBain and Marsden (12, 13, 17) for certain anisotropic solutions of lauryl sulfonic acid. Large flat McBain micelles would arrange themselves in parallel as in the Hess micelle but the narrower ones, through the operation of the
41
L.I&IELL.iR .AND OTHER MICELLES
same forces, \voiild pass into hexagonal array. Mere concentration through crowding can change n hexagonal into a pseudohexagonal or pseudo-Hess array. However, those of t'he many systems discussed in the present paper are clear-cut examples of hexagonal array. X o pattern corresponding t o spherical micelles! or even equidistantmmicelles, has ever been reported. -4ft,er having in 19-46 confirmed and endorsed the fiiidings of the German inJrestigators, in 1947 Harliins and collaborators expressed doubt as t o the existence of the Hess micelle, nnd instead adopted tfhe view that the IlicBain micelles are \vholly independelit'. To emphasize this they renamed the Hess long spacing d l the " I band." Since this spacing increases ivit'h t'he solubilization of oils, they :Ire forced also t,o assmne t'liat the niicelles become larger and feLver, in adtli tioii t o chnngiiig ivith concentration. This makes the interpretat'ion quite elastic, unless other evidciice is atld~icecl. DIHC'ITSSION
OF BOTH
IXTI?"ITT
.\ND P O R I T I O S O F T H E UR.J,GG
SPAC'ING t / r OR
I.ONG
"1"
IVheii h\~drocnrhmsai'e solrililized in ortliimry soap solutions, the long spucing increases, but. so does t'lie iiitcnsity. This can be explained on the familiar Hess model, but were it merely ; ~ ninterpart'icle spacing between llIcBain micelles, the increased intensit'y \vould correspond to more niimeroiis instead of fewer micelles. Furthermore, t'he increase in t8he long spacing is nlnxys linear ivitli ratio of hydrocarbon to detergent. Hughes, Sa\vyer, ;mtl \ h g r a d (8) have slio\vn that for hydrocarbons solubilized in potassiiim laurate, as the ratio of oil to soap is increased the increase of long spacing is independent of tlie concentration of the soap hit is directly proportional t o t,he rnolar Twlurne of the oil, wit'li a snialley proportionality coilstant for aliphatic tliaii for aromat,ic oils. They also reported t8he highly significiuit resrilt tho t the i / ~ t w s i L yof the long spacings tlepmitfs tipon t,lie electron density of the s\il,atance solubilized. i3ul)stances of lo\v elect'ron density, such as hydrocarbons, intensify the long spacing. \Vith progi,essi].c aclditJions of substances of high electron density the intensitjr of the first ortler of tlie long spacing can he diminished to tlie Jwiishing point ; upon iiurther idtlit.ion the first urtier reappears \\.it h increasing iiitensity. This sho\vs that t,he solubilized inaterial is arranged in part of the pattern that giJ.es rise to the long spacings; and this appears to iis t o Le definiti\.e e\+lence of the presence of tlie Hess micelle in ordinary isotropic potassium laurate solutjions. We slid1 present similar e\.idencae f u r the cat,ionic detergent, cetylpyridinium chloride. The IIess micelle (figure 1) has the advantage that one can predict how tlie micelle will affect' a beam of S-rays. For potassiuni burate, tjhe distributiori of electroii density can be siniplg calculated (\\.it11 the help of scaled models and x-rtly slicirt-spacing data) t o be roughly t'liat' s1ion.n in figure 2. From this one derives an idealized diffraction lattice such as represented in figure 3 . The situation lye are interested in is that obtained when the rays reflected from planes LY and a' are just, in phase. We shall nssiime that' tmheBragg reflect'ion from these
42
JAMES JY. RICnIIN .IND OSCAR .I. HOFFMAN
WATER
31
F I ~ 1; .. Idcalized HESSlamellar micelle made of two 1IcBain lamellar niiccllcs or “sandw i c h cs . ’’
FIG.2. Electron density of micelle in figure 1 planes is of first order. Then the ray from plane /3 will trail behind that from Q! by a fraction of a wave length A?+.
L.iRIELL.4.R .\SD OTHER MICELLES
43
We may no\\‘ treat the i . n j * h froin p1;11ie~a nr:d p as n composite m y , ivhich we sli:i11 c.nll my ap. The plinse relntion of thi:, composite ray to rays cy mid 13 will be PIIC’I1 that : =
3 AAp
FIG.3 . Sclieinatic tliffractioii I)), lattice of Inmell:ir, iiiicelle
So\\. consider the y-plaiie in the lntt,ice; \\.hether 01’ not, this plane nctun!ly exists \\.ill be deteiaminecl 1 i ) r the r e l n t i ~ ~elect,roii e densities of the solahilizecl oil and the hydiocarhon part’ of the micelle. If there is n disting~~islial)le rnj, from y i t \\.ill al\\.nystie just) out of phase lrit81ithe composite ray ap. It’is immediutely obl.ioris that) the intensities of the long-spacing line from this lamellar micelle \vi11 tie determined 114’ the difference het\reen the nmplitudes of t’he electric vectors of [lie i x ~ -q‘ap’’ s fiiitl “y.” We ma). \\,rite, then, iisiiig aifor the nmplitutle of thv i ‘ ” 13y:
-4 simple addition of sine c u r v w sho\\-sthat
The second term in equation 3 iiiay he ~~pprosirn:itecl closely 1)y nidiing use of t'he fact that, tlie diffmction angles \\.e ni'e coiic:ernetl \\-it'li a r lrery ~ small. We ma\: say then that' the amplitude of t'lie y-ray is proportional 1 0 tlie n i i m l ) t \ r of elcctims in the plane. .-I,. \\.liere li
=
fi
\,/,'p,fc
(3)
proport8ionalitj'coiista,nt, p , = cliffeience lwtween electron tlensities of the oil a n t l u t t8he h j r t l i u caibon p,zrtJof the micelle, f = molar \.ohme of thc oil, and = ratio of moles of oil per mole of soap present' i i i t,he soliition. From cqriat8ions3 , 1,and 5 : =
This equation gii.es t'he,cpalitative i ~ i i l that t the intensity of the r l l line from a tvi-o-component, syst'em-soap in Ivat'rr-is rletei*minetl 1)y the relative magnitudes of t'he thickness of tlie water layer a,nd t'he clouble length of the soap molecule. I n :I, three-component system--\\nter, soup, w,nd oil-t,he same t\vo quantities plus the molal, volume antl eler,tron density of t,he oil determine the intensity of' t,he r l i line. Whenever an oil is solubilized by the mechanism postulated, the Tralue of ( I , in equation ti increases. Since Hiighes, Sawyer, and Yinograd have shoivn that this increase in di leaves dtt,unchanged, the first term 011 the right side of equation 6 always increases from some finite value. The second t,ei,m on tlic right side of eqiiat,ion G also increases, lint it &arts from zero. Hence, as (1, increases ou.ing t,o solubilimt ion, t'he long-spwiiig line can 1)ecomr moixe intense, fatlc oiit, or fade out :tiid retuim \\-it>li:I,phase shift of 180" as ohserved I)y Hughes, Sa\vyer, and Trinograd. It niust be pointed out' liere that the : ~ b o ~discussion ,e of the ].ariation of line intensities holds for odd-order difl'ractions only. In fact \\-hen the odd-order lines fade becausc of solubilization of a n oil of high elect,ron density, the even orders should become stronger, ns \ y e here report foi* cetylpyridinium chloride. The analysis just gixw points out clearly another contrast betn.een the Hess micelle and an intlerpaiticle interpretation. That' is, of course, that \\.it11 the Ress lnmellnr inotlel, only niolwalrs captihle of :~gglonierntingt o form discs, i.e,., linear molecules, can j'ielcl t: long-spacing diffraction, \\.liereas according to the interparticle spacing hypothesis d l colloidal ele~t~rolytes should yield this diffraction line, \\.hich is clefinit,ely not t8hecme, as will be sho\vn below. .Another conseqiieiice of the 1nmell:ir model is that the relative diameters of the first-, second-, and third-order diffraction rings \\.ill be as 1 : 2 : 3 . On t.hc other
45
L \.\IELL.\R . \ N D W H E R MICELLES
I i w n t l , it' the bimolecular AIcBain leaflets \yere iiidepenilent, tlieir ai*rangenient must he :Lhexagonal lattice. I n such a case tlie relative diameter:: of the cliffmction rings would 1 i a ~ ~been e as 1:.\/.3:2: 4 f : 3 . AAll recorded observations on isntropic colloidal elect'rolytes she\\. the rela,t,ive p o s i f i o m of the rings to he those mpectetl from t,lie Hess lamellar strticliire. T h r relati7.e i ; ) i t ~ ) ) . ~ n1.w . i f i:i,gi'ec ~~ ( 3 , 3 , 6 , 8, 14) \\,ith tlie : i l i m ~prediction+. G XPERIMlGST.%L
1 . Colloidal clccttdytcs thot do tiot gioc loiiq t ~ p t ~ ~ i ~ t ~ ~ ~ ~
'l'he onlji: s-ray studies of isotropic solutions of colloidal electrolytes hitherto published refer t'o sonp-like snhst8anceswith a n aliphatic hydrocarbon chain. We 1iai-e therefore in,estiga ted R number of compounds \\,hose molecular geometry seemed t o preclude t,hc existence of lamellar micelles. These;compounds \ \ w e : ( 1,
c:4Hn
II!l
C'4H9
I
.lreskap 100 (Monsan t 0)
.lresket 300 (Monaanto)
.\resklene 400 (IMonsnnto)
OH S odi I im cle s oq+ch o! nt c 4
(Riedel de Hnen, In(,.) -411 tdiese iiiaterials are highly colloidal in 20 per cent' aqueous solu tioii, as already determined by freezing point, conductivity, and solubilizat'ioii of dye. They \\-ere x-rayed for 12-16 hr. in 20 per cent solut'ion, \\.it,li n film-to-sample distance of 200 mm. None of them gave any suggestion of a long-spacing line, and therefore nothing corresponding t o a McBain micelle, a Hess micelle, or an interparticle spacing. Evidently in these solutions the colloid consists of particles too irregular in shape and size and distance from each other to give either a lattice spacing from \vithin the micelles or an interpa,rticle spacing bct,ween t,hem.
46
JAJIES TV. RlCB.\IN .IND OSC'.\R A . HOFFAIhS
i n contrast to this, every aliphatic colloidal electrolyte eT.er photographed has shoi\m in sufficiently concentrated isotropic solution a definite long-spacing linp, u.itli t'lie exception of certain lnu1y1 sulfonic acid solutions. This includes soaps, sull'onates, pyridinium salts, nritl quaternnry ammonium snltts. Ilarsden ant1 1,IcBnin (11) 1iax.e obtnined patteixs even f i w m ethers and esteims of the proper shape. TABLE 1 X-my data f o r isolropic systems of cetylpyridinium chloride-water m-b
C E T Y L P Y R I D I N I U U CBLORWL
I
.S >S
>S
+s.3
.~
~
I
(p. =
~~~
~-
~
0.29)
________
0.0 +5.9
. ~-
S
I
+n.o
110.0 +17.2 +23.6
~
,
-___________
~-~
t h e esperinients with CHnClZ
>S
_______
5. Oil = cumene
0.7% 1.07 _. .~
NOIl,
~
. - ~ .-~.
\I.
0.0 0.0 +0.4 +0.7 +I . 2 +2.0
_
-
-.
S
~-
3. Oil = CHlClp
0.000 0.2025 0.4156 0.704 0.8S4 1.12 1.52 . .
__ __ __
-
+7.9 +11.4 12.2 ~
~-
I
-
-
~
0.67)
0.0 +3.3
~
0.000 0,0445 0.31 0.525 0.775 1.04
( I ) ~=
_ _ _ ~-~ _
~
0.000 0.195 0.425 0.535 0.725 0.026
.
None vvw vw
I
2. Oil = CHsBrl
.
IV
-
_~~
~
S
~
R
i
vs
25 per cent solution of the detergent was used.
aqueous cetylpyridiniuni chloride solutions was investigated, using five different solubilized “oils.” The results are presented in table 2 and figure 7. The following features may be noted: First there is the usual linear increase in long spacing.
50
JAMES W. MCBAIN .4ND OSChR h. HOFFMlZN
\ jx
. .
