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F o r m a t i o n o f M i c r o e m u l s i o n s i n M i x e d I o n i c-N o n i o n i c

S u r fa c t a n t S y s t e m s

Hironobu Kunieda,*,† Kazuyo Ozawa,† Kenji Arama ki,† Akihiro Na kan o,†,‡ a n dConxita Solans§

Graduate S chool of E ngineering, Yokohama National Un iversity, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240, J apan, an d Departom ento de T ensioactivos, CID, CS IC,

Jordi Girona 18-26, 08034 Barcelona, Spain

Received Ap ril 21, 1997. In Final Form : October 21, 1997 X

Pha se behavior of a mixed sur factan t, sodium dodecyl sulfate + lipophilic poly(oxyethylene) dodecylether, in a brine-decane system was investigated at a constant brine/decane weight ratio equal to 1.Solubilization capability of th e mixed surfactan t rea ches its maximu m an d microemulsion is form ed whenth e surfactan t is changed from hydrophilic to lipophilic in a given system. In th e present system , lam ellarliquid crystal (LC) intrudes in the single-microemulsion region, and three-phase microemulsions are notformed. The mixing fraction of nonionic surfactan t in t he total sur factan t in t he midst of the LC presentregion increases with increasing oil content due t o the h igh solubility of nonionic surfactan t in oil. Thepartition of nonionic surfactant molecules between the oil and the bilayer in the LC phase is analyzed byusing th e geometrical relationship of the pha se equilibria in th e phase diagra ms, tak ing into account t hesolubility. The monomeric solubility of nonionic surfactant in oil is m uch less th an tha t of an ordinarycosurfactant like hexanol, and the mixing fraction of nonionic surfactant in t he bilayer decreases with

increasing salinity. The inter layer spacing ofth e midlamellar liquid crystal between the twom icroemulsionregions was m easured by small-angle X-ray scatt ering. The avera ge effective cross-sectional area persurfa cta nt is a bout 0.37 nm 2 a nd is unc ha nged upon di lut ion. It is c onsidere d tha t the re is a s trongatt ractive interaction between the ionic group an d t he nonionic hydrophilic moiety of surfactants.

I n t r o d u c t i o n

Most ionic sur factan ts are strongly hydrophilican d formaqu eous micelles when dissolved in wat er. By combiningwith lipophilica mph iphiles often called cosurfactant s, th emixed surfactan t t ends to dissolve in oil. Three-phasemicroemulsions are formed in a brine/ionic surfactant/ cosurfactant/oil system at certain mixing ratio of ionicsurfactant/cosurfactant.1-5 Since the relative hydrophile-lipophile balance (HLB) oft he mixed surfactan t in a given

system is optimum in th e midst ofth e thr ee-phase region,the composition is alsocalled the HLB composition. Whenthe microemulsion coexists with excess wat er and oilpha ses, th e solubilization capability reaches its maximu ma n d u l t r a l o w i n t e r f a c i a l t e n s i o n s a r e a t t a i n e d d u e t ocritical solution phenomena of microemulsion.6,7 Three-phase microemulsion is also called middle-phase micro-em ulsion, bicontinuous m icroem ulsion, or surfactantp h a s e i n w h ich w a t er a n d oi l m i cr od om a i n s a r e i nbicontinuous structure 8 and surfactant m olecules areadsorbed at the microinterface.9

In th e previous stu dies, middle-chain alcohols ha ve beenmainly used as cosurfactan ts. The cosurfactan t is mainlyp a r t i t ion e d b et w ee n t h e s u r fa ct a n t l a ye r a n d t h e oi ld om a i n i n si de t h e m i cr oe m u ls ion p h a s e. S in ce t h esolubility of the cosurfactant in the oil domain is fairylarge, the appar ent solubilizing capability of the m ixedsurfactant (ionic surfactant + cosurfactan t) is low. Toavoid the loss of cosur facta nt , a long-cha in alcohol shouldbe used. However, since lamellar liqu id cryst al is formedov er a w id e r a n g e o f t e m p er a t u r e s i n a m i xe d i on i csur facta nt an d higher-alcoholsyst em, it is difficult to formisotr opicfluid microemulsions.11 The combination ofionicsur facta nt and lipophilic poly(oxyethylene) type nonionicsurfactant is considered tobe a good candidate toproducemicroemulsions. Although t he phase diagram s of water/ ionic su rfactan t/nonionic su rfactan t/oil systems werereported,12-14 the detail phase behavior of ionic-nonionicmicroemulsion systems has not been studied.

In this context, the phase behavior of mixed sodiumdodecyl sulfate and lipophilic poly(oxyethylene) typenonionic surfactant in water-decane was investigated.

E x p e r i m e n t a l S e c t i o n

Materials. Sodium dodecyl sulfate (SDS) was obtained from

Sigma Co., and homogeneous nonionic surfactan ts, bis(oxyeth-ylene) dodecyl ether (C12E O2) and tr is(oxyethylene)dodecylet her(C12E O3) were obtain ed from Nikko Chemicals Co. Extr a puregrade n -decane was obtained from Tokyo Kasei Kogyo Co., and

* Corresponding author.† Yokohama National University.‡ Present address: JO Cosmetics Corp., Komatsudai 1-603-36,

Hanyu 348, Japan.§ CSIC.X Abstract published in Advance ACS Abstracts, December 15,

1997.(1) Winsor, P. A. Solvent Properties of Am phiphilic Compounds;

Butterworths: London, 1954; p 68.(2) Healy, R. N.; Reed, P. L. In Im proved Oil R ecovery by Su rfactant

and Polymer Flooding; Shah, D. O., Schechter, R. S., Eds.; AcademicPress: New York, 1977; p 383.

(3) Kunieda, H.; Shinoda, K. Y u k a g a k u 1980, 29 , 676.(4) Kunieda, H . J. Colloid Interface Sci. 1988, 122 , 138.(5) Kunieda, H.; Naka mur a, K.; Uemoto, A. J . Colloid In terface Sci .

1994, 163 , 245.(6) Kunieda, H.; Shinoda, K. Bull. Chem. Soc. Jpn. 1982 , 55 , 1777.(7) Kunieda, H . J. Colloid Interface Sci. 1987, 116 , 224.

(8) Olsson, U.; Shinoda, K.; Lindman, B. J. Phys. Chem. 1986, 90 ,4083.

(9) Kunieda, H.; Nakano, A. J. Colloid In terface Sci . 1995, 17 0, 78.(10) Kunieda, H.; Aoki, R. L a n g mu i r 1996, 12 , 5796.(11) Kunieda, H.; Naka mura , K. J. Phys. Chem . 1991, 95 , 1425.(12) Sagitani, H.; Friberg, S. E. Bull. Chem. Soc. Jpn . 1983, 56 , 31.(13) Shinoda, K.; Kunieda , H. J . Colloid I nterface Sci. 1973, 42 , 381.(14) Mori, F.; Lim, J. C.; Miller, C. A. Prog. Colloid Polym . Sci . 1990,

82 , 114.

26 0 Langmui r 1998, 14 , 26 0-26 3

S0743-7463(97)00411-3 CCC: $15.00 © 1998 American Ch emical SocietyPublished on Web 01/20/1998

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extra pu re grade Na Cl was obtained from Ju nsei Chemicals Co.These chemicals were used without further purification.

P r o c e d u r e s

D e n s it y o f S u r fa c ta n t i n a L iq u i d S t a t e . Th edensities of a SDS aqueous solution and a propanol +water solution (propanol:water ) 1:4) were measu red a sa function of SDS concentr at ion at 35 °C. The values ar eextra polated to 100% SDS. The avera ge molar volum e of SDS is 258 ( 2 cm 3 /mol. The molar volume of C12E O3

(347.3 cm 3 /mol) was directly obtained by using its density.B y m e a s u r in g t h e d e n si t ie s o f t h e C12E On series, thevolum e of one dodecyl group was deter mined to be 0.354n m 3.

Small-A ngle X -r ay Sc atte r ing. Interlayer spacingoflamellar liquid crystal was measured using small-angleX-ray scatt ering (SAXS) perform ed at a power of 2.25 kWfrom a Siemens generator, model Krystalloflex 760 at 35°C. The collimation was carried out with a Krat ky camerafrom MBraun an d the scattering was detected with a linearposition sensitive detector OED 50M from MBraun.R el a t iv e i n t en s it y w a s r e p r es en t e d a s a fu n ct i on of diffusion vector q. The available range of q was from 4× 10-4 t o 3 × 10-2 nm . The sam ples were covered byplastic films for t he SAXS experiment (Mylar seal meth od).Lamellar liquid crystalline pha se was also distinguishedby the SAXS peaks. The ra tio of interlayer spacing fromthe first and second peaks is 1:1 / 2 for the lamellar type.The type of liquid crystal was alsoident ified by a polarizingmicroscope.

R e s u l t s a n d D i s c u s s i o n

Thr e e -Phas e Be h avior of N aC l(aq)/SD S/C12 EO2 o rC12 EO3 /D e c ane Sys te ms . Ph ase diagrams ofN aCl(aq)/ SDS/C12E O2 a n d C12 E O3 /decane systems are shown inFigures 1 an d 2, respectively. The brine/oil weight ra tiois unity. The weight fraction of total sur factant in th esystem ( x) is plott ed horizont ally, an d the weight fractionof nonionic surfactant in total surfactant (W 1) is plottedvertically.

Surfactant forms an aqueous micellar solution phaseat low W 1, and the aqueous micellar ph ase coexists withan excess oil phase in the lower II region. However, areverse micellar solution phase is formed and coexistswith an excess water phase at high W 1 in the upper IIregion. When the pha se behavior is changed and th ehydrophile-lipophile property of the mixed surfactant is

just balanced in the given system, a single-phase micro-emulsion is formed even in a dilute r egion. In a brine/ ionic sur facta nt /middle-chain a lcohol/oil system, a t hr ee-

phase region consisting of microemulsion, excess water,and excess oil phase appears beyond the solubilizationlimit.10 However,in the present systems,instead ofthreecoexisting phases, a lamellar liquid crystal is presentbetween the two microemulsion regions as is shown inFigures 1 an d 2. Microem ulsions on both sides of the“LC” region sh ow flow birefringence, an d it is consider edtha t t he st ructur e of the microemulsion it self resemblesth at of a lamellar liquid crystal. Emu lsions are very stablein this LC present region, and we could not accuratelyidentify the num ber of pha ses except confirm ing theexistence of liquid crysta l. The lower limit of the singleLC region was not deter mined because of its na rr owness.In t he dilute ar ea of th e LC region, water and oil phasesw er e s ep a r a t e d, b u t t h e s ep a r a t i on w a s i n com p le t e.

Hence, it is considered th at th ere is a t hree-phase regionconsisting oflamellar liquid crystal, water, and oil phases.The compositions giving the two microemulsion regionsare shifted to lower W 1 values with increasing salinity.When the oxyethylene chain length ofnonionicsurfactantis increas ed, th e compositions ar e also shifted to lower W 1values. This tendency is s im ilar to that of an ordinaryionic microemulsion system.10 When t he nonionic sur-factan t and th e salinity are fixed,t he microemulsion regionis shifted to a higher W 1 with decreasing x, as is shownin Figures 1 a nd 2 due to the partit ioning of surfactantin the oil domain and that ofthe bilayer ofliquid crystal.10

A n a l y s i s o f P h a s e B e h a v i o r . As described a bove, ondecreasing the total sur factant concentra tion, the W 1values for the m icroem ulsions and the LC region are

F i g u r e 1 . Phase diagrams of the 1 wt % NaCl(aq)/SDS/C 12-E O3 /decane (4), 3 wt % N aCl(aq)/SDS/C12E O3 /decane (O), and

5 wt % NaCl(aq)/SDS/C12E O3 /decane (0) systems in a diluteregion a t 35 °C. I an d II indicate isotropic one-phase and two-phase regions, respectively. LC means the region in whichlamellar liquid crystal is present.

F i g u r e 2 . Phase diagrams of the 1 wt % NaCl(aq)/SDS/C 12-E O2 /decane (4), 3 wt % NaCl(aq)/SDS/C12E O2 /decane (O), and

5 wt % NaCl(aq)/SDS/C12E O2 /decane (0

) systems a t 35 °C. Iand II indicate isotropic one-phase and two-phase regions,respectively. LC means the region in which lamellar liquidcrystal is present.

Form ation of M icroem ulsion s in S u rfactan t S ystem s L an gm uir, Vol. 14, N o. 2, 1998 26 1

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deviated to th e nonionicsurfactant-rich region. This typeof distortion of the microemulsion region or three-phasebody is quite com m on in a m ixed sur factant systembecause surfactant molecules are distributed betweenwater domain, oil domain, and the interface inside themicroemulsion.14-20 Lipophilic cosur facta nt , or nonionicsurfactant , is fairly soluble in oil. As a result , withincreasing oil cont ent, cosur facta nt molecules ten d to shiftfrom the surfactant layer to the oil dom ain inside themicroemulsion. Hence, more cosurfactant is needed to

keep the HLB composition in the surfactant layer upondilution.

If i t is assum ed that the m onom eric solubilit ies of hydrophilic surfactan t in wat er a nd oil an d of lipophilicsurfactant in water are negligible, th en the followingequation holds concerning the center of one- or three-phase regions of the t hree-phase m icroemulsion

where W 1 is th e weight fraction of lipophilic sur facta nt orcosurfactant (nonionicsurfactant)in the mixed surfactant

a n d S 1 is the solubility oflipophilic sur facta nt 1 in oil. S 2

S

is the m ixing fra ction of sur facta nt 2 (ionic sur facta nt ) inth e total sur facta nt at th e microwater or microoil inter faceinside th e microemulsion (S 1

S +S 2S ) 1). R ow is the weight

fraction of oil in wa ter + oil, an d x is the weight fractionof total su rfactan t in t he system, r espectively.

In the present system, however, a three-phase body doesnot appear as is shown in Figures 1 and 2. Instead, alamellar liquid crystal is present between two one-phasemicroemulsion regions. Moreover, th e nu mber of pha sespresent in the LC region and the compositions of eachphase are alsou nkn own. However,a s stat ed before,sinceit was observed that water an d oil phases ar e present inth e dilute region of LC (Figur es 1 and 2), th ere is a thr ee-phase region consisting of LC, water, and oil phases in

the LC region. Hence, it is considered that the sam eequation for a three-phase m icroem ulsion in a m ixedsur facta nt system can be applied to th e present LC region.In fact, such three- or four-phase regions are reported inother ionic surfactan t systems.21,22 The m onomeric solu-bility of SDS in water an d th e solubility of SDS in oil areextremely low, and these values decrease in the presenceof salt or other a mphiphilic compounds. Hence, all theSDS m olecules can be considered to form bilayers of lamellar liquid crystal. However, although the criticalmicelle concentration ofC12E O2 in water is much less tha nSDS, in general, the monomeric solubility ofC 12E O2 in oil(S 1) is not negligibly low. As a resu lt, it can be as sum edtha t only the n onionic surfactan t is distribut ed betweenthe bilayer and th e oil. It is also assum ed that t he water

part of the lam ellar l iquid crystal is regarded as purewater, an d th e composition of the oil part is the sam e astha t in excess oil phase. Then, concerning the center of LC present region, eq 1 m ay h old. In t he present case,

S 1S is the mixing fraction of nonionic surfactant in the

bilayer of LC.The center of the “LC present ” region in F igure 1 (the

broken curve) is plotted against 1/ x - 1, and the result isshown in Figure 3. The straight lines are obtained, andit is considered th at eq 1 holds not only in the t hr ee-pha semicroemulsion but also in the LC region in the presentsystem. From Figur e 3, we can estima te th e solubility of C12E O3 in oil part.

We calculated the monomeric solubility of nonionicsurfactant u sing eq 1 and the data in Figures 1 and 2. Theobtain ed values are shown in Table 1. The S 1 is consider-ably smaller than with the result in the NaCl(aq)/SDS/ hexanol/decane system.10 At high salinity, th e solubility,S 1, tends to decrease in a m ann er sim ilar to that of anordinary cosurfactant system. S 1

S is also decreased athigher salinity because SDS becom es relatively lesshydrophilic upon addition ofsalt. In other words, in orderto keep a balanced stat e of th e mixed surfactan t layer atthe water-oil inter face, th e am ount of lipophilic nonionicsurfactant should be decreased. This is also in goodagreement with th e result s oft he C12E O3 /comm ercial alkylethoxy sulfate system.14 However, in the latter system,there is a three-phase body, which does not appear in the

(15) Kunieda, H.; Sh inoda, K. J. Colloid Interface Sci . 1985, 107 ,107.

(16) Kunieda, H.; Sat o, Y. In Organized Solutions; Lindman, B.,Friberg, S. E., Eds.; Marcel Dekker: New York, 1992, pp 67-88.

(17) Kunieda, H.;Ushio,N.;N akano,A.;Miura, M. J. Colloid InterfaceSci. 1993, 15 9, 37.

(18) Kunieda, H.; Yamagata, M. Colloid Polym. Sci . 1993, 271 , 997.(19) Kunieda, H.; Yamagata , M. L a n g mu i r 1993, 9, 3345.(20) Kunieda, H.; Naka no, A. J. Colloid I nterface Sci. 1995, 170 , 78.(21) Kegel, W. K.; Lekkerk erker , H. N. W. Colloids S urf., A 1993, 76 ,

241.(22) Hackett, J. L.; Miller, C. A. SPE Reservoir Eng . 1988, 3, 791.

W 1 ) S 1S+

S 1S 2S

1 - S 1

R ow(1

x- 1) (1)

F i g u r e 3 . The weight fraction of nonionic surfactan t in totalsurfactant (W 1) for the midst of the LC r egion plotted a gainst1 /

x- 1 in the 1 wt % NaCl(aq)/SDS/C

12E O

3 /decane (0), 3 wt %

NaCl(aq)/SDS/C12E O3 /decane (O), and 5 wt % NaCl(aq)/SDS/ C12E O3 /decane (4) systems at 35 °C.The value of x is the weightfraction of total surfactant in the systems.

T a b l e 1 . S 1 a n d S 1S i n t h e M i x e d S u r f a c t a n t S y s t e m s

a t 3 5 °C

surfactant1a oil

water oraqueous soln(wt % NaCl) S 1 S 1

S

C12E O3 n -deca n e 1 0.0307 0.703C12E O3 n -deca n e 3 0.0227 0.598C12E O3 n -deca n e 5 0.0184 0.525C12E O2 n -deca n e 1 0.0447 0.526C12E O2 n -deca n e 3 0.0292 0.462C12E O2 n -deca n e 5 0.0247 0.407hexanolb dodeca ne 1 0.197 0.223h exa n ol dodeca ne 3 0.0783 0.306h exa n ol dodeca ne 5 0.0490 0.319

a Surfactant 2 is SDS. b The values for SDS-hexanol systemsare at 25 °C.10

26 2 L an gm u ir, V ol. 14, N o. 2, 1998 Ku n ied a et al.

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present system . The S 1S value for C12E O3 is higher than

t h e S 1S value for C12E O2 or h exanol, because C12E O3 is

more hydrophilic than the latter two.In t he present discussion, we assume th at a queous salt

solution can be treat ed as pseudo one component , whichmeans t hat the salinity is always constant in each pha se.However, i t is possible that the salt/water ratios aredifferent in excess water phase and microemulsion phaseor lamellar liquid crystal. In order t o improve the abovetheory, we have t o consider the distribution of inorganicsalt within each phase.

I n t e r l a y e r S p a c i n g o f L a m e l l a r L i q u i d C r y s t a lb e t w e e n T w o Mi c r o e m u l s i o n R e g i o n s . According toth e phase beha vior an alysis in the form er section, 2.27 wt% of C12E O3 is dissolved in oil in the center of the LCpresen t region, an d the mixing fra ction ofC 12E O3 is 0.598in t he bilayer of the liquid crystal in the NaCl(aq)/SDS/ C12E O3 /decane system.

A phase diagram of the 3 wt % NaCl(aq)/SDS/C 12E O3 / decane system was constructed at an equal weight ratioof brine an d decane at 35 °C and is shown in Figure 4.

The concentrat ed region of SDS was not investigatedb eca u s e S DS i s i n a s ol id s t a t e . T h e s i n gl e-p h a s emicroemulsion region is extended from the C12E O3 apexto the brine/decane corner. Along the m icroem ulsionregion, lamellar liquid cryst al is presen t over a wide ra ngeofcomposition. If we assu me tha t the bilayer compositionis unchan ged and the solubility of nonionic surfactan t inthe oil part of the LC phase S 1 is constant in the centerof th e LC region in F igure 1, th en t he bilayer compositionwould be un changed on line Ain Figur e 4. In other words,if we measure the interlayer spacing of lamellar liquidcr y st a l a l on g t h i s l in e b y m e a n s of S A XS , a n d t h ecomposition of bilayer is considered to be un changed, th enthe following relation should hold

where d is th e interlayer spa cing measu red by SAXS, d Sis th e effective length of surfactant in t he bilayer, φS isthe volume fraction ofmixed surfactant in the system, φS′

is the volume fraction of mixed surfactant of the bilayerof the LC pha se in th e system, φO is the volume fractionof oil in the system , an d F1 a n d Fo are the densities of nonionicsu rfactan t an d decane, respectively. The secondterm in eq 3 represents the volume fraction of nonionicsurfactant in t he oil part of the LC phase. Equa tion 3 isderived from a simple volume balan ce in th e LC pha se,taking into account the above assumptions.

The SAXS result on line A is shown in Figure 5. Wep lot t e d t h e m e a su r e d i n t er l a ye r s p a ci n g a g a i n s t t h ereciprocal of the volume fraction of the mixed su rfactan tin th e bilayer of the LC phase. Since the st raight line is

obtained, th e above relation may approximatelyh old. Theaverage effective cross-sectional area per one surfactantmolecule in th e bilayer, aS, can be calculated by,

where V s is the molar volume oft he mixed surfactan t an d L is the Avogadro constant . The aS is also indicated inF i gu r e 5. T h e aS value is constant and is equal to 0.37n m 2. We also m easured the interlayer spacing of thelamellar liquid crystal in 50 wt % C 12E O3 solution, an dthe sam e value of aS was obtained at 35 °C. However,SDS alone form s hexagonal liquid cryst als in water at t hesame concentr ation. The effective cross-sectional a reaper SDS in the hexagonal phase is much larger than the

above value.23

Since the charge density in the m ixedsurfactan t layer decreases, the latera l att ractive intera c-tions increase in t he present ionic-nonionic surfactant s,compared with ionicsu rfactan t alone. Besides, the presentnonionicsu rfactan ts ha ve long lipophiliccha ins compa redwith ordinar y cosurfactant s, and the cohesive energy of lipophilic chan ges is strong. For th is reason, the bilayerstructure is maintained even in a very dilute region.

A c k n o w l e d g m e n t . The auth ors than k Mr. J. Caellesfor the SAXS measurements.

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(23) Amar al, L. Q.; Gulik, A.; Itri, R.; Maria ni, P. Phys. Rev. A: At., Mol., Opt. Phys. 1992, 46 , 3548.

F i g u r e 4 . Phase diagram of the 3 wt % NaCl(aq)/SDS/C12-E O3 /decane system at 35 °C. The br ine/decane weight r atio isunity. The line A corresponds to the broken curve in F igure 1.

d ) 2d S / φ′S (2)

φ′S ) φS -S 1

1 - S 1

F0

F1

φO (3)

Figure 5. The interla yer spacing of lamellar liquid crystal (0)and effective cross-sectional ar ea per one surfactan t molecule

in the bilayer (O) are plotted against 1/ φS on the line Ain Figure4. φS′ is the volume fraction of the mixed sur factant bilayer inthe system.

aS ) V s / d S L (4)

Form ation of M icroem ulsion s in S u rfactan t S ystem s L an gm uir, Vol. 14, N o. 2, 1998 26 3

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