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L e t te r s t o t h e d i t o r s W i s s e n s c h a f t l i c h e K u r z b e r i c h t e

L e t t e r s t o t h e E d i t o r s ( S h o r t C o m m u n i c a t i o n s ) a r e t o e n s u r e t h ep r i o r i t y o f y o u r f i n d in g s . T h e r e s p o n s i b i l i t yf o r it s c o n t e n t s i s a b s o l u t e l y y o u r s . T h e s e p a p e r s w i l l b e p u b l i s h e d a p p r o x i m a t e l y w i t h i n 2 - 6 w e e k s a f t e r r e c e ip t .S u c h p a p e r s s h o u i d n o t e x c e e d 2 p a g e s t y p e w r i t t e n . P l e as e , d o w r i t e y o u r m a n u s c r i p t a s c l e a r a s p o s s i b l e , e s p e c i a ll yw i t h r e s p e c t t o f o r m u l a s P l e a se , d o a v o i d f i g u r es S h o u l d t h e y b e i n e v i t a b l e f o r u n d e r s t a n d i n g , p l e a se , a t t a chc l e a r d r a w i n g s b y I n d i a i n k , r e a d y f o r r e p r o d u c t i o n . Yo u w i l l r e c e i v e 7 5 re p r i n t s f r e e o f c h a rg e . P a p e r s s h o u l d b e

s u b m i t t e d d i r ec t ly t o : P r o f. D r. F. H . M i i l l e r , H a s e l h e c k e 2 6 , D - 3 5 5 0 M a r b u r g - M a r b a c h .

K u r z - M i t t e i l u n g e n s i c h e r n r a s c h d i ePriorit~it D i e Ve r a n t w o r t u n g f ti r d e n I n h a lt t r Sg t d e r A u t o r. E r s c h e i n u n g s -t e r r a i n c a . i n n e r h a l b 2 - 6 Wo c h e n n a c h E i n r e i c h u n g . U m f a n g d e rd e u t l i c h ( F o r m e l n ) g e s c h r i e b e n e n M a n u -s k r i p t e b is z u 2 M a s c h i n e n s e i t e n . A b b i l d u n g e n v c r m e i d e n . S o n s t k l is c h ie r f~ i h ig e Z e i c h n u n g e n m i t B e s c h r i f t u n gi n w e i c h e m B l e i. S o n d e r d r u c k f re i e x e m p l a r e w i e b ei O r i g i n a l a rb e i t e n . M a n u s k r i p t e a n : P r o f. D r. F. H . M i i l l e r ,H a s e i h e c k e 2 6, D - 3 5 5 0 M a r b u r g - M a r b a e h .

Herausgeber und Verlag

Colloid Polym er Sci, 258, 97 3- 97 6 (1980) 1980 Dr. ietrich SteinkopffVeriag , DarmstadtI S S N 0303-402X /AST M-C ode n: CPMSB (former ly KZZPAF)

Inst i tut M ax v on Laue - Paul Lan gevin 156X, Grenoble 38042 (France)':an d

Schoo l of Che mistry, Un iversity o f Bristol, Bristol': :and

Roy al Signals and R adar Establishment, Ma lvern, Worcester***

h e s t r u c t u r e o f a m i c r o e m u l s i o n d r o p l etD . J . C e b u l a , L . H a r d i n g , R . H . O t t e w i l l , and P. N . Pu sey

W ith 3 figures and 1 table

(Received May 14, 1980)

Optical ly t ransparent emuls ions , usual ly termed micro-emuls ions , were f i r s t inves t igated byH o a r and Schulman(1) in 194 3 using bo th visual and light scattering observa-tions. Since that t ime these systems have attracted consider-able a t tent ion (2) and there has been some co ntroversy abo ut

the nature of microemu ls ions and the or ig in of their s tabi l i tyas colloidal dispersions (3, 4). Recently we have undertakena number of inves t igat ions on microemuls ion sys tems withtwo basic objec tives in view, (a) to determ ine the structu re o fmicroem uls ion droplets and (b) to examine the phys icalbas is of the in teract ions between the droplets in order todeterm ine the reason for their s tability as colloidal disper-sions. Our experimental approach has been based upon theuse of three scattering technique s, namely, t ime average lightscat ter ing , photon correla t ion spectroscopy (PCS) and smal langle neutro n sca ttering (SANS). T he purpo se of this note isto show h ow the use of the la t ter two techniques can providefundamental informat ion on the s t ructure of microemuls iondroplets . A mo re detailed report elaborating the use of thesetechniques to provide fundamental informat ion on in terac-tions will be published elsewhere (5).

The technique of PCS is now w el l developed (6) and canbe used to determine the mean diffus ion coeff ic ient of thepar t ic le , ZJ, and hence the effect ive mean hy drody nam ic

radius R h . Un der the condi t ions of the present exper imentsthe measured f ie ld correla t ion funct iongl(Q,T) is expectedto be close to a s ingle exponential, namely,

g l ( Q , ~ ) 0c e x p -D Q 2 ) [1]

whe re r is the correlatio n delay time and Q is the scatteringvector defined as,

4 : r . 0Q = T s m ~ [2 ]

where ~. = the wavelength of the l ight in the microemuls ionand 0 the scattering angle. In general, due to interparticlein teractions , D wil l be a funct ion of par tic le volume f raction0. However, there is cons iderable evidence that , when theinteractions are short-ran ge (i. e. effectively "ha rd-sp here "),D does not deviate marked ly f rom i ts zero concentra t ionvalue for q5 < 0.05 (7). Thu s the S tokes-Ein stein equ ationcan be used in the form,

R h = k T / 6 x ~ [3]

W 145

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Ceb u l a e t a l . T h e s t ru c t u re o f a mi c ro emu l s i o n d ro p l e t 975

Table 1 . Resul ts of scat tering experiments on microemuls ion droplets

Technique System Paramete r Value//~.

SANS D2 0/H -t o lu en e Core radius - R c 40 -+ 2S A N S H 2 0 / D - t o l u e n e S he ll r a d i u s -R s 48 -+ 2

PCS H20 /H- to lu ene Hydr odyn amic rad ius - R h 52 -+ 3

60

4

Eu

2

-20

I

~D2

--~-D-Toluene

. . . . . . . .

6 4

. . . . . . . . l .. .--H-Toluene

. . . . . . . . ~ ~ Oleic cid

1 . . . . . . . I --H~g 8

Radiuslnrn

Fig . 2 . Schematic d iagram to i l lus trate the coherent Scat tering lengths of the comp onent s of the microe muls ion and todemons t ra te the changes in con t ras t p roduced . Lef t -hand s i de - D20 co re in H- to luene ; R igh t -hand s id e- I - I20 in D-

to luene

able fits. The results o btai ned are recorde d in table 1,where in they a re compared wi th the hyd rodynamic rad iusob ta ined f rom PCS.

Figure 2 g ives a schematic d iagram indicat ing that withD2 0 in the system, i t is the D2 0 core which has the h ighestcoherent scat tering length whereas with a deuterated o i l , theoi l contras ts the hydrogenated material , i . e . the water dropand the surface active materials.

It is generally accepted that, for a diffusing species with a

wel l -def ined ou te r boundary, the hyd rod ynamic rad ius R hmeasured by PCS is a few .~ larger than the t rue radiusbecause of a layer of so lvent which moves with the part icleHowever, in the present case, where some flexib i l i ty isindicated in the oute r o leate chains (see below), R h could becomparable to , or even somewhat less than, the maximumradius of the d iffus ing species The values obtained bySANS, u s ing D20 , co r respond to the rad ius o f the wate r-core of the droplet , R o al though i t is likely that the hydrate dhead groups of the surface active molecules will also becontained in th is . The resul ts indicate a d is t inct water phaseand are not compatib le with the idea of a swollen micelle .The value obtained using D-to luene as the d ispers ionmedium, R s, i ies in betwee n the hyd rody nami c radius R h andthe core radius R c. The difference R s- R c is ca. 8 A, ab outthe length of a hexano] hydrocarbon chain and head group.This suggests that this reg ion, R~ - R c is com pos ed of aclose-packed monolayer of hexanol and o leate moleculesThe exper imen ts in D- to luene , however, sugges t s t rong ly

that penetrat ion of to luene occurs in to the outer regions ofthe micro emuls io n droplet , i . e ., between the spaces of theprotruding o leate chains , but that i t penetrates only s l ight ly,i f a t a l l , in to the close-packed shel l region contain ing thehexanol molecules . These experiments indicate thereforethat the s t ructure of a microemuts ion droplet must be closeto that rep resented schematical ly in f igure 3 . This p icturealso suggests tha t the oute r regions of the o leate chains in theto luene phase main tain som e flexib i l i ty and that th is aids the

col lo id stabi l i ty by the mechan ism of s teric s tabi l isat ionwhich in the present context would be close to a hard-sp herein teract ion (12). In fact , a t h igher v olume fract ions of water,considerable in teract ion occurs between the droplets and adetailed study has been made of this topic (5, 13, 14, 15).

The model shown in f igure 3 represents a t ime-averagestructure of the microemuls ion droplet . Clearly the reals t ructure wil l be a dynami c one and thus there is l ikely to be,as in micel lar sys tems (6), a cont inu ous in terchange betw eenthe surface act ive molecules in the in terracial region and bot hthe o i l and water phases . For the analysis of the data theboundarie s bet ween th e core and the close-packed shel l ofhexanol and o leate molecules and between the shel l and theloosely packed outer layer of o leate chains have been takenas sharp This is an overs implif icat ion and a more complica-ted model can be developed to g ive a more sophis t icatedin terpretat ion . On present evidence, however, i t i s doub tfu lwhether th is wil l change substant ial ly the modei of amicroemuls ion d rop le t p rop osed in th i s communica t ion .

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976 Colloid and Polymer Science, Vol. 258. No. 8 (1980)

= To l u e n e

oo c3

H e x a n o l

O l e a t e

~ ,,~

Fig. 3. Schematic diagram of the structure of a microemu lsion droplet which wo uld be consistent with the SANS and PCSexperim ental results

Acknowledgements

We wish to express our thanks to the Science ResearchCoun cil foi~ suppo rt of this wo rk and to the Inst itut Laue-Langevin for neu tron beam facilities.

eferences

1) H~,;r, T. P.,J. H. Schulman, Na ture t52, 102 (1943).2) Prince, L. M., Microemulsions, Th eory and Practice

(Academic Press, New York 1977).3) Overbeek, J. Th. G.,Farada y Disc. Che m. So c. 65, 7

(1978).4) Ruckenstien, E., J. C. Chi,J. Chem. Soc. Faraday II,

11, 1960 (1975).5) Cebula, D. J., R. H. OttewiU, P. N. Pusey, J. Ral-

ston, to be published.6) Cummins, H. S., E. R. Pike,Photon Correlat ion and

Light Beating Spectroscopy (Plenu m, New Y ork 197 4);Photo n Co rrelation Spectroscopy and Velocimetry (Plenum,New York 1977).

7) New ma n, J., J. L. Swinn ey, S. A. Berko witz, L. A.Day, Bioch emistry 13, 4832 (1974);G. K. Batchelor, J. FluidMech. 74, 1 (1976).

8) Guinier, A., G. Fournet, Small Angle Scattering ofX-rays (Wiley, New York 1955).

9) Jacrot, B., Rep. Prod. Phys. 39, 911 (1976).

10) Harding, L., B. Sc. thesis, U ni ve rsi ty of Bristol(1979).

11) Ashcroft, N. W.,J. Lekner,Phys. Rev. 145, 83 (1966).12) Vrij, A., E. A. Nieuwenhuis, H. M. Fijnaut, W. G.

M. Agterof, Faraday Disc. Ch em. Soc. 65, 101 (1978).

13) Dvo laitzky , M ., M. G uyot, M. Laq~ies, J.P . LePesant, R. Ober, C. Sauteray, C. Taupin,J. Chem. Phys. 69,3279 (1978).

14) Cebula, D.J. , L. Harding, R, H. Ottewill,to bepublished.

15) Cebula, D, J., D. Myers, R. H. Ottewill, tobepublished.

16) Aniansson, E.A. G., S.N. Wall, M. Almgren, H.Hoff man n, . Kielmann, W. Ulbricht, R. Zana, J. Land, C.Tondre, J. Phys. Chem. 80, 905 (1976).

Au tho rs address :

R. H. Ottewill,School of Ch emistry,University of Bristol,Cantock s Close,Bristol BS8 ITSEngland