SULLIV.\S S.JIARSDES, .JR.,? 42;D JAIICS W. ~ I c B A I X Departitieid of C h e i n i s t r y , S t n r i j o r d C t i z c e r s i t y , C a l i f o r n i a Received A u g u s l 25, 1947
During the past dccade w number of papers havc \ m n published, first in Germany (5-9, 11-13, 19, 21, 22) and then in this country (3, 4, 10, 17, Z)), on the diffraction of x-rays by aqueous solutions of anionic and cationic drtergents. The experimental results described in these papers have established the presence of lamellar micelles in these solutions. Supplementary work on the diffraction of x-rays by detergent solutions containing solubilized hydrocarbons ha,-. intlicated that these hydrocarbons are incorporated n-ithin the micelle. In recent years a number of non-ionic detergents and other surface-actire agents have becomc commercially available. Instead of having a n ionizing hydrophilic group at one end of a long hydrocarbon chain, as do the uwal detergents, thew non-ionic materials have either a glyceryl group or a polyethylene oxide chain ending in a hydroxyl group. It was of interest to knon- the h e structure of both aqueous systems of these materials and aqucaus containing solubilized hydrocarbons. .\PP.YR.ZTUS
The source of s-radiation was a General Electric SRD-1 unit having a tube with copper target and beryllium u-indows. The radiation was filtered through nickel foil and colliniatetl through guarded pinholes placed T.Fj cm. apart. The pinholes used for thc Iletergcnt “S”solutions were about 0.025 in. in diameter; those used for the other systems were 0.010 in. in diameter. For thc side spacings, a sample-to-film distance of 50 mm. was iisrd, but for the long spacings this distance was iricreased to 163 or 200 mm. ALITERIALS
Detergent “S”, a nun-ionic detergent, is the product of condensation of isoijcttylphenol and rt’hylene oxide. It) is a yellow, slightly viscous liquid \\hose molecular weight, as measured by Dr. IC. Gonick in this laborat’ory, is aboutm636. On the basis of t’his molecula,r weight., it is estimated that’ the a,verage ethylene oxide chain in this preparation is ten units long. The samples of diglycol Inonolaurate, polyethylenr glycol (400) niotiolaurate, 1 Presented a t the Twenty-first Sational (lolloid Symposium, ivhich was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society at) I’alo Alto, California, June 1s-20, 1917. This paper is based upon a dissertation subnlitted by Sullivan S.Marsden, dr., to t h e Department of Chemistry and the Graduate School of Stanford University in partial fulfillment of the requirements for the degree of Doctor of Philosophy, August, 1947 2 Lever Brothers Company Fellow in Chemistry.
and glyceryl monolaurate were kindly supplied by Glyco Products, Inc. The former two are slightly vivoii- yellow liquids n-hose propertie-, :LS well as tho5e of the latter, are described in the catalog of the donor. These materials, bince they are merely commercial products, are not chemically pure. The Emulphor 0, a white wax-like solid, was supplied by General Dyestuff Co. ant3 iws described a4 the condensation product of oleyl alcohol and ethylene 0side. Triton S-100, supplied bj’ Kohni and Haas, is the condensation product of diisobntylphenol and ethylene oside. From its molecular weight, of about, 600, it is eqtimated that there is an average of nine or ten ethylene oxide groups per molecule. This material has a chemical formula somewhat similar to that of the Igepals, the German non-ionic detergents developed by I. G. Farbenindustrie. The a-n-decyl glyceryl ether is a white crystalline solid whose melting point is 38°C‘. The benzene was c.P.,thiophene free, and was dried over sodium and distilled. The dimethyl phthalate was supplied by the Eastman Kodak Compaxiy and wa. used 11 ithout further purification. METHODS
The t n o-component hyhtems were prepared by n-eighing the detergent and water into 2-dram g l a s vials having screw tops lined with pure tin foil. They were niised for 8-24 hr., the length of time being greater for the more viscous samples. For the multiphase systems, the vials were then Centrifuged in order to separate the phases and determine their properties and relative proportions, The three-component systems (Triton X-100-water-benzene) were prepared 11y making n stock solution of the detergent in either benzene or water and then adding the third component to portions of this stock solution. Thus the ratio of the two components could be kept constant xhile the third was varied. Samples of the systenis were then sucked up into Pyrex-glass capillaries having I: wall thickness of 0.03-0.04 mm., a diameter of about 0.8 mm., and a length of about 2 in. The “wet” end of the capillary was sealed first and then the ‘(dry” end sealed and warmed until a small glass bubble formed, which was used as an indication that the capillary was completely sealed. At all times the liquid n-as kept more than h in. from the flame, a procedure which decreased thermal decomposition and vaporization to a negligible amount. -1fter preparation, the capillaries were again centrifuged to ensure phase separation, escept in the cases where patterns of mixed phases were desired. The capiliaries vere then mounted in front of the pinhole Iiy means of a small piece or moulding clay, by a method previously described (16 . Exposures of 10 or 12 hr. were used for the long spacings of the more concentrated systems, nhereas exposures of 20-24 211. were necessary for the more dilute. Exposures of 10 or 12 hr. were used for all short spacings. lleasurements of the long spacings were made directly from the films by use of drafting dividers: it is believed that greater accuracy could be obtained by
112
S U L L I V l S S. hIARSDEN, JR., A S D JAMES
W. IICB.'IIS
this method than by measurement of the peaks of the microphotometer tracings, particularly for patterns of the more dilute samples. Since most of the surface-active materials used in this xork were liquids and gave no long spacing, it was decided t o try to cool them below their melting points and obtain pictures of the solids. It has been reported (14) that liquids could be x-rayed belox their melting points by playing a stream of liquid nitrogen on a glass capillary containing the liquid. This was successfully modified by using a stream of cold carbon dioxide gas from subliming dry ice
0
FIG.1. Diethylene glyrol monolaurate: plot of long spacing versus the weight fraction of detergent.
All preparations and photographs were made at room temperature, which was 29°C. f 3". -411 concentrations are expressed on the basis of weight units, i.e., either weight per cent or weight fraction. RESULTS .%ND DISCTSSIOX
Aqueous systems of diglycol monolaurate at concentrations from about 45 per cent to 85 per cent consist of two distinct phases: one is isotropic and gives no long spacings; the other is liquid-crystalline and gives a long spacing which is not a linear function of the concentration as expressed in per cent by Ti-eight (figure 1, middle curve). However, when long spacing is plotted w s u s the reciprocal of the concentration in per cent by weight, a straight line results (figure 2). The ratio of liquid-crystalline phase to isotropic phase decreases rapidly n-ith incyeasing concentration. Above 85 per cent the systems are single phase and isotropic and give no long spacings; from 25 per cent to 45 per cent the systems
are extremely viscous pastes or emulsions n-hich show weak hirefringence arid give long spacings down t o about 35 per cent. Relow 25 per cent the systems arc thin emulsion$. The transition from isotropic liquicl t o the two-phase cystem at 85 per cent is quite iharp, whereas the chanpc from t v o distinct phases to emulsified phases is gradual. The i-ariation of long spacing with the reciprocal of the concentration i* decidedly in contraRt to that of ionic detergents, wherc the relation is almost linear with concentration. It is similar t o that found l y Palmer and Pchmitt 118) in their study of aqueous emulsions of nerve lipidq. an aid to understanding these systems, one may set up a model of an ideal, two-component, onephase, smectic, liquid-crystalline system. One component is a long-chain
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RATE
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hydrocarbon compound which ha- a hydrophilic group ut one end, siicli as :I tletergent or similar vrface-active d i s t a n c e . 'l'h.other component is water. ('on4tler first the pure anhydrous detergent in the bmectic liquid-cryitalliIiIc state ifigiire 3). It vi11 hai-e a liquid or average oricntation in the .a-plane and L*cry+tnllinc" or definite orientation in the y or 1.ertical direction. It will give a long x-ray diffraction spacing equal to L,.sin p, where I,sis the double length of the detergent moleculc and 3 is the angle of tilt ot the detergent molecule to 111c f~aialplane. Consider the addition of u ater to this liquid-crystalline detergent making the simplifying assumptions that ( 1 ) the density of thc detergent equals tliut of water, that is, weight per cent equals volume per cent; and ( 2 ) nll of the water that enters the system forms uniform layers betn-een the hydrophilic ends of the
114
SuLLIV.LV S. MARSDEN, JR., Ah'D JAMES W. MCBAIN
detergent molecules. Then these aqueous systems will give a new long spacing L,which is the sum of the detergent layer, L,-sin 8, and the water layer, L, (figure 4).
P
L
FIG.3. Anhydrous smectic liquid crystal
FIG.4. Hydrous smectic liquid c r p T d
Thus, for the ideal system, the total long spacing varieq as the reciprocal of the concentration times a constant, which is the product oi the double length of the molecule and sin 8. The relation between total long spacing and concentration for this ideal system
X-R.\Y
DIFFRACTION STUDY OF DETERGENTS
115
corresponds to that actually found for the aqueous systems of diglycol monolaurate. Consider again the ided system. Take the logarithms of both sides of equation 3. For a given substance, L , is constant. ,Us0 assume that p is constant. Then equation 4 is a straight line and for the ideal system, a log-log plot of I,, and c is a straight line of slope - 1 and intercept equal to L, .sin p. The experimental resiilth for diglycol monolaurate were plotted on these coordinates (figure 5 , middle curve). The slope was numerically slightly greater than - 1. The chemical purity of this material was doubtful, so it was decided to check the properties of the two phases. An 80.0 per cent system was prepared, the two phases separated, and the water removed in a vacuum desiccator 2001
0 A
LIQUID- CRYSTALLINE PHASE RESIDUES ISOTROPIC PHASE RESIDUES
040 WEIGHT
FRACTION OF
\
060 080 I DETERGENT
FIG 5 I,og-log plot of cuperiniental results f o r digl\ 01 n~onolaui’tto
m e r phu-phorus pcntoGde. ‘I‘he hotropic phase loct 1cs* than 10 per cent I)> n-eight, \\lierear the liqiiiti-crptalline phase lo-t mo1e than 30 per rent. From these re4due*, additional aqueous $ystems were prepnred and photographed. tern- difiered from the original qyhtexn.j Imth in long spacing (figure 1) m d 111 pliyical appearmcci, i.c., the nmount and type of phase present at a given concentration. This + o w i that the original diglycol monolaurate was not chemically homogeneoui, :ml that there is an unequal distrihution of the components betxveen the two phases in the original aqueous system. The residues from the iwtropic phase were somem-hat hydrophobic, and it5 aqueous yqterns contained a greater proportion of isotropic phase than did systems of the original material. The residues from the liquid-crystallinc pkiaw reatlily mixed with water to form nen- systems which w t w more strongly hiwfringent :lnd more highl!, colored than wqiieous sy+temiof the original material.
116
S U L L I V A S S. V A H b D E T , JR., A S D J.4MES \V.
?.ICBAIK
I t call bc seen that the lorig ypacings of thc’ systems iiom the isotropic p i l w tbidues increase conderably morc rapidly than (lo tlloic irom the liquid-crystalline phase residueh T h e n t h e v arc plottcc! on log-log coordinates (figure 5 ) tlic slope of the line for the iiciuid-r~rl’stallinephase residue. i s -1, wherea- that for the isotropic phase residue< i? conderably greater. .I\\vas shonm in equation 5 , the rate of decrease of the log of the total long spacing with respect to the log of the concentration equals imity when only water iq entering the structure. But if this rate is numerically greater than unity, wmethmg elbe must be happening. Work by other investigator\ with aqueous solution5 oi ionic detergents hat shown that the addition of hydrocarbon results in an increase of long spacing of the solution. This has been explained as due t o the hydrocarbon forming layers between the hydrophobic end- of the molecules similar t o the water layer4 betveen the hydrophilic ends. Therefore, it is here postulated that the greater rate of increase of long .pacing than can be attributed to the addition of water alone is actually due t o hydrocarbon entering the lamellar structure in layers between the hydrophobic ends of the molecules. This hydrocarbon is assumed t o be presrnt in the original material as an impurity, and e ~ i d e n t l ycollects in the isotropic phase, which serves as a reservoir from which it is progressively abstracted. Since the iiotropic phase residues give aqueous tems which haye a long spacing, they must ex 1dently also contain some of the iurface-active material. Returning to the aqueous systems of the original material, in the isotropic region between 85 per cent and 100 per cent water i y taken up by the siirtaceactive material without the formation of a liquid-crystalline phase. Evidently this mater penetrates between the sides of the detergent molecules, possibly v-ith hydrogen bonding to the oxygen atoms of the carboxylate group or the ether oxygen of the diethylene glycol. There is no change in the numerical value of the side spacing halo of 4.6 -1.when going from 100 to 85 per cent, but theie is an incre$se in the relatiye intensity of a halo corresponding to a Bragg spacing of 7-8 9. Considering the other extreme of the concentration range. there i- the po-ylbility that structure may persist below that found in this investigation, hut I t becomes increasingly more difficult t o detect it because of both the clecreahing intensity with dilution and the approach of the dift’raction pattern to the main s-ray beam. ,it any rate, no diffraction was ohserved for aqueous system3 of the original material below 35 per cent and for the liquid-crystalline phase r e d u e s below 21.5 per cent, although a number of sypacing of this material \\hen cooled to bcloiv it- melting point i q 49 i
.i., a \ aluc u11ic.11 agret; \ \ P H 11it11 tlic c.a~culatec~ e\ti.ntirt~tfoiible lengtli of tlie moleciile, 1.c , aboiit 50 .I l’lii\ \voultl intlicate thar in the tiozen material the molecules are practical!? pcipendiciilar t o thc basal plailei. Since the intercept of the log-log plot is 36 8 .i., this ifould indic+atean angle of about 18Ofor the diglycol monolaurate moleciile~in the h:i.di oii- 1iquid-c.rystalline striictiire of this material. I3waiise of the vnriou- tls>iimptioii>thi- x alue i y only appro-amate. It nil1 be noted that the long Xpacing- for the cooled samples of original cliglycol monolaurate and the 1\01 ropic p11a.c ir+idues are very close t o their respective interceptq, a f‘nrt 11h i d l ould indicate that the mo!cciiIc\ : i r t h +till tilted, ~vlien cooled, if there i \ hytlrocarhon prehent to allow freer arraiigenient of the hydrophobic end. oi the detergent molecule.
j
,_:
0
00 -3-0\0
0
z
0
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J
0 7
A 0
EVcJLPPOR 0 \ GLYCERYL N O N O L A U R A T E P 3 L Y E T H Y L E N E GLYCOL 400 M O h C L A U R A T E ‘ 0 a-N-DECYL GLYCERYL ETHER
! I
D
t 20 WEIGHT
PER C E N T
40 OF
60
80
100
DETERGENT
FIG, 6 Log-log plot of long spacing i eisris \\eight pel cent of deteigcnt for Emulphoi 0, glyceryl monolauratc, polvethylene glyc.01 400 nionolauratc, and a-~z-drrylglyceryl e t h r i
P o l y c t h y h c ylycol $00 monolairrate This inaterial is the monoester of *‘Iaiiiic’’acid with polyethylene glycol, having an average molecular ueight of 400 ii e , aliout nine rthylene oxide unit\; commercial “lauric” acid coni5ts usually of the acid o b t i n e d from cocoanlit oil). Its aqueous system\ con& of either one or t\vo phase>, depending on the concentration. One pliaie i \ clcar, iwtropic, and gives 110 long spacing; the other 13 liquid-crystalline and gi\ e+ a long . It is helieved that these aqueous sollitions of detergent ”X” contain lamellar micelle5 similar to those that esist in solutions of‘ ionic detergents. When detergent ‘*X”is frozen and photographed, it gii-e- a long spacing of about 50 ; this agrees \vel1 with the calculated single length of the average extended molecule (51 If the molecwles are arranged in both the solid material and lamellar micelles with the hydrocarbon ends together and the hydrophilic ends together, as they are in the case of the ionic detergents, then onc would expect a first order of tivice 50 ‘\. (times sin 6 ) for the solid material, and also spacings of more than this for the first order of the solutions. I-Ion-ever, these were
a.
I
1
0
I
20
PER
not
o n the I
40 CENT
60
80
100
DETERGENT ” X *
em ant1 the \.cry thorough stiidy oi the ternzene (bee helow), it i y helieiwl that the o l w r ~ c dvalues are actiially t h r second (and foiirth’l order and that the first (and third) order is either missing or very weak. ’l’hc T-alues of long spacing on the graph+ are the obher\wl ones multiplied by a factor of two, giving what is helimed t o be the true rrpeating pattern of the bystems. This same peculiarity, i.e., +trong second order of diffraction hut no first. has previouqly heen o b s c n d 1)y KO$:“and 3LlcBain (20) for aqueous sybtema of hesanolamine oleate. alternative explanation t o the above \voultl bc an angle of tilt of 30” or , nhich is con+leral)ly lesa than has l m n reported for any otlici substances. It I* helieved that the fir3t explanation is the more probable. In the course of this investigation it iva* desired to know the \,ariation of specific gravity of aqueous detergent ”X” wlutions with concentration. These were determined with a small pycnometer and are plotted in figure 8. It can be seen that in the region between 75 per cent and 100 per cent, the solutions of detergent “X” h a w :I ipecific gravity greater than that of pure detergent “ X ” ; lo11ntl
X-RAY DIFFR.4CTIOS 5TL-DT OF DI,'I'I,RGI.\
121
I'S
this indicates the strong association of the water and detergent molecules through hydrogen bonding with the ether oxygen atoms and hydroxyl group. The greatest deviation of the observed values from the expected straight line, i.e., the greatest difference between the full and hroken line, is in the region of TO per cent, which coincides with the concentrations giving the strongest and sharpest x-ray patterns. Short spacings of detergent "X" and its aqueoiii solutions consist of a halo whose maximum intensity corresponds to a Rragg sparing of 4.5 This value remains constant but the intensity decream with dilution. In dilute solutions the water halo becomes relatively stronger and this causes an apparent shift in the side
