3222
Langmuir 1994,10, 3222-3229
A Surfactant-Water-Oil System with Weakly Charged Films Keiichi Fukudat and Ulf Olsson* Division of Physical Chemistry 1, Chemical Center, Lund University, P.O. Box 124, S-22100 Lund, Sweden
Ulrich Wiirz Max-Planck-Institut fur Biophysikalische Chemie, Postfach 2841,D-3400Gottingen, Germany Received February 2, 1994. I n Final Form: May 30, 1994@ We have studied the effects on microstructure and phase equilibria of a nonionic surfactant-water-oil system when replacing one per hundred of the surfactant molecules by ionic surfactant, sodium dodecyl sulfate (SDS).This degree of substitution corresponds to introducing approximately one charge per 4400 &. The nonionic surfactant system contains pentaethylene glycol dodecyl ether (C12E5),water, and decane, at a constant surfactant to oil weight ratio of 52/48. The water content is varied from 60 to 100 wt %. In this composition range the initial phase sequence consisted of L1 phase-lamellar phase-L3 phase as a function of increasing temperature. Adding the small amount of SDS has a large impact on the phase and a reverse micellar microemulsion phase (Lz)are added to behavior. A reverse hexagonal phase (Hz) the phase sequence at temperatures above the L3 phase. The microstructures in the different phases were studied by the NMR self-diffision technique and by small angle X-ray scattering.
1. Introduction Surfactant-water-oil systems show a rich polymorphism with a large variety of liquid and liquid crystalline phases and corresponding microstructures. With nonionic surfactant the phase behavior is particularly rich due to the strong temperature dependence of the surfactant properties.l-1° At lower temperatures, the surfactant forms aggregates with curvature toward oil, while a t higher temperatures curvature toward water is favored.6-10 In binary systems with water, this property is manifested by the formation of dilute lamellar and L3 (“sponge”)phases a t higher temperatures in addition to micellar and the usual liquid crystalline phases at lower temperature^.^,^ In ternary systems with oil, water-rich microemulsions are formed a t lower temperatures, oil-rich microemulsions a t higher temperatures and balanced, bicontinuous microemulsions a t intermediate temperatures. The ternary systems also contain water-rich and oil-rich L3 phases and various liquid crystalline phases simultaneously rich in water and 0il.6,7J0J1 The nonionic systems contain no long range interactions, except for the undulation force12 acting in the lamellar
* To whom correspondence should be addressed. t On leave from Kao Corporation, Tokyo, J a p a n .
phase.5 Oil-swollen spherical micelles can be prepared, which, to a good approximation, interact through a hard sphere p0tentia1.l~ Long range electrostatic forces can, however, be gradually introduced by solubilizing ionic surfactant into the nonionic surfactant film. The effects of adding small amounts of an ionic surfactant have been studied in several nonionic and zwitterionic surfactant systems. In the dilute lamellar phases with nonionic surfactant, the addition of ionic surfactant results in a sharp increase of the first-order quasi-Bragg peak intensity.14 In the case of nonswelling lamellar systems, a transition to essentially continuous water swelling can be obtained upon the addition of small amounts of ionic Studies on water-rich microemulsion and micellar systems show strong effects on the interactions while the microstructure remain essentially unchanged for small amounts of ionic surfactant.19-24 The present study concerns the effects of introducing a small amount of charge to the surfactant film in a waterrich ternary system with nonionic surfactant. The ternary nonionic system contains pentaethylene glycol dodecyl ether (C12E5), decane, and DzO,where the surfactant-tooil ratio, 52/43 by weight, is kept constant. The waterrich microemulsionphase of this section through the phase prism has recently been investigated with a number of
Abstract published in Advance ACS Abstracts, September 1, 1994. (1)Shinoda, K. InSolvent Properties of Surfactant Solutions; Shinoda, K.;Ed.;Marcel Dekker: New York, 1967. (13)Olsson, U.; Schurtenberger, P. Langmuir 1993,9,3389. (2)Kunieda, H.; Shinoda, K. J . Dispersion Sei. Technol. 1982,3,233. (14)Jonstromer, M.; Strey, R. J . Phys. Chem. 1992,96,5993. (3)Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985,24, (15)Gulik-Krzywicki, T.; Tardieu, A,; Luzzati, V. Mol. Cryst. Liq. 654. Cryst. 1969,8,285. (4)Mitchell, D. J.;Tiddy, G. J.T.; Waring, L.; Bostock, T.; McDonald, (16)Jonsson, B.; Persson, P. J . Colloid Interface Sci. 1986,115,507. M. P.J. Chem. Soc., Faraday Trans. 1 1983,79,975. (17)Larsson, K.; Krog, N. Chem. Phys. Lipids 1973,10,177. (5)Strey,R.;Schomacker,R.;Roux,D.;Nallet,F.;Olsson,U.J.Chem. (18)Rydhag, L.;G a b r h , T. Chem. Phys. Liquids 1982,30,309. SOC.,Farady Trans. 1990,86, 2253. (19)Guefing, P.; Nilsson, P.; Lindman, B. J . Colloid Interface Sci. (6)Olsson, U.; Shinoda, K.; Lindman, B. J . Phys. Chem. 1986,90, 1985,105,41. 4083. (20)Gradzieski, M.; Hoffmann, H. Adu. Colloid Interface Sci. 1992, (7)Olsson, U.;Nagai, K.; Wennerstrom, H. J . Phys. Chem. 1988,92, 42, 149. 6675. (21)Lindman, B.; Jonstromer, M. InPhysics ofAmphiphilic Layers; (8) Anderson, D.; Wennerstrom, H.; Olsson, U. J . Phys. Chem. 1989, Meunier, J., Langevin, D., Boccara, N., Eds.; Springer Verlag: Berlin, 93,4243. 1987;p 235. Wennerstrom, H. Adu. Colloidlnterface Sci. l994,49, (9)Olsson, U.; (22)Nilsson, P.;Lindman, B. J . Phys. Chem. 1984,88,5391. 113. (23)Siano, D. B.; Myer, P.; Bock, J . J . Colloid Interface Sci. 1987, (10)Strey, R. Habil. Schrift Thesis, Gijttingen University, 1992. 11 7,534. (11)Olsson, U.; Wiirz, U.; Strey, R. J.Phys. Chem. 1993,97,4535. (24)Siano, D. B.; Myer, P.; Bock, J. J . Colloid Interface Sci. 1987, (12)Helfrich, W. 2.Naturforsch. 1978,33a, 305. 11 7,544. @
0743-746319412410-3222$04.50/00 1994 American Chemical Society
S- W - 0 System with Weakly Charged Films
Langmuir, Vol. 10, No. 9, 1994 3223
different experimental technique^.'^,^^ In this study, we have replaced one molecule per one hundred by a n ionic surfactant, sodium dodecyl sulfate (SDS). This small perturbation turns out to have a large impact on the phase behavior of the system. In what follows, we compare the phase behavior of the present system, containing SDS, with that of the pure ternary system, which serves as a reference. We also present results from small angle X-ray scattering (SAXS)and self-diffusion measurements, performed on liquid crystalline and liquid phases, respectively. 2. Experimental Section Materials. The surfactant,pentaethylene glycol dodecylether (C12E5), was obtained from Nikkol Ltd. Tokyo. Decane (99%) was obtained from Sigma and DzO (99.8%isotopic purity) from Norsk Hydro. All chemicals were used as received. Phase Diagram Determination. Samples for the phase diagram study were prepared in flame-sealed glass ampules containing a magnetic stirring bar. The ampules were studied in a thermostated water bath. Phase boundary temperatures were determined by visual inspection in transmitted light, scattered light, and between crossed polarizers, to detect birefringence. For the microemulsion-oil equilibrium,the kinetics of phase separation and, the reverse process, solubilization is very slow (of the order of days). This phase boundary was determined from both phase separation upon decreasing the temperature from the homogeneous microemulsion phase and by solubilization upon increasing the temperature from the microemulsion-excess oil equilibrium. Close to the phase boundary temperature, the samples were kept for several days at given, controlled temperature. The kinetics involved in the other phase boundaries are fast. The phase boundaries of the anisotropic liquid crystalline phase were determined from the appearance of static birefringence. Self-DiffusionExperiments. Self-difisioncoefficientswere measured on a modified JEOL FX-60 NMR spectrometer operating at 60 MHz (‘HI equipped with an external 2H field frequency lock,using the Fourier transform pulsed gradient spinecho (FTPGSE)technique.26 SAXS. The SAXS instrument is described in detail else~here.2~ Conversion from Weight to Volume Fraction. In the discussion of the results we will use both weight and volume fractions. The volume fractions were calculated using the following densities (g/cm3): 0.967 (C12E5);’’ 1.105 (DzO); 0.73 (decane). The effect on the surfactant density when replacing 1 mol % of the nonionic surfactant with SDS is neglected.
3. Phase Behavior In Figure l a we present a partial phase diagram of the four-component system ClzE5-SDS-water-decane. The phase diagram corresponds to the water-rich part of a section defined by a constant surfactant-to-oil ratio, 521 48 by weight, and ,B = 0.01, where ,B is the mole fraction of SDS in the surfactant mixture. The diagram is presented as temperature versus WS WO,where WS and Wo are the weight fractions of surfactant and oil, respectively. For comparison, the corresponding phase diagram for the pure nonionic system (,B = 01, redrawn from ref 13, is shown in Figure lb. The effect of introducing one SDS molecule per 100 C12E5 molecules is striking. At lower temperatures, involving the stability of the L1 phase, the two systems behave similarly, showing the same phase sequence: liquid (L1)lamellar (La) and liquid (L3) with increasing temperature. At higher temperatures, the behavior is different. Two new phases are introduced in the ,B = 0.01 system where in the pure nonioic system the Ls phase is in equilibrium
+
(25) Leaver, M. S.; Olsson, U.; Wennerstrom, H.; Strey, R. J. Phys.
II 1994,4, 515.
(26) Stilbs, P. Prog. Nucl. Mugn. Reson. Spectrosc. 1987,19,1. (27) Wurz, U. Progr. Colloid Polym. Sci. 1988,76, 153.
55
20
10
20 Wt%(S+O)
30
40
40
35
25t=--:;k;-=-4
20
10
20
30
40
w t %(S +O)
Figure 1. (a, top) Partial phase diagram ofthe system C1&SDS-water-decane. The weight ratio WdWo = 51.9/48.1, where W Sis the total weight fraction of C12E5 and SDS and W O is the weight fraction of decane, is kept constant. The mole fractionofSDSinthe surfactant mixture,p = 0.010. (b,bottom) The pure ternary nonionic system (p = 0) having the same surfactant-to-oil ratio as in part a (redrawn from ref 13). For the various phases the following notations are used: L1 is a liquid microemulsion phase similar to a normal micellarphase. Lais a lamellar liquid crystalline phase. L3 is a liquid phase having a multiply connected bilayer structure. HZis a reverse hexagonal liquid crystallinephase. LZis a microemulsionphase similar to L1, but with a reverse micellar structure.
with excess water. The two new phases are a reverse hexagonal (Hz) and a liquid phase (Lz),respectively. The L1 and Lz are essentially normal micellar and reverse micellar microemulsion phases, respectively. In the L3 phase, the local structure is a bilayer which globally is multiply connected. This bilayer is normal and oilswollen and separates two interwoven, on the average equivalent water domains.
Fukuda et al.
3224 Langmuir, Vol. 10, No. 9,1994 Curvature elasticityz8is a useful concept to understand and predict structure and phase equilibria of nonionic surfactant-water-oil s y s t e m ~ . ~ * gThe , ~local ~ - ~ curvature ~ free energy density is generally written asz8
.
,
.
.
,
,
,
,
.
,
,
.
,
,
(
,
,
.
,
(
,
,
.
.
f
g, = ~ K ( H H0)’ iik
(1) where H is the mean curvature, HO the spontaneous curvature, K the Gaussian curvature, and K and t are the bending rigidity and saddle splay modulus, respectively. The mean and Gaussian curvatures are given by H = (c1 4 1 2 and K = clcz, respectively, where c1 and c2 are the two principal curvatures. The curvature properties of the surfactant film is for the nonionic surfactants a strong function of temperature. The general behavior is that the average mean curvature of the surfactant monolayer film, (H), decreases monotonically with increasing temperature (counting curvature toward oil as p o ~ i t i v e ) . ~In~ terms ~ - ~ ~of~the ~ curvature free energy concept, this is consistent with a monotonic decrease of the spontaneous curvature, Ho, with increasing t e m p e r a t ~ r eAt . ~ lower temperatures HOis positive. With increasing temperature it decreases and becomes negative a t higher temperatures. We can calculate the mean curvature for the various microstructures relevant to the present system. The mean curvature is evaluated a t the polar-apolar interface separating the hydrocarbon from the pentaethylene oxide part of the surfactant. For spheres we have
x -0.005
+
%
H=a3Qls and for cylinders as H=a4@lS
+
where for the normal case, a = 1and @ = @O @ d 2 , and for the reverse geometry a = -1 and @ = @pw Cpd2.l~ is an effective surfactant length, defined as 1s = udas, where U S = 702 A3 l1 is the surfactant volume and as is the area per surfactant molecule evaluated a t the polarapolar interface defined above. For lamellae, H = 0 and for a multiply connected bilayer structure, here a normal bilayer, we have
+
H = - - 0% 2.21,
This latter expression for the multiply connected structure is obtained from H = - W / 2 . 2 ~ 5 where ,~ @ (=@o @d2) is the bilayervolume fraction and 6 is the bilayer thickness, which we here take as 6 = 2(@0 + @d2)lS/@s. In Figure 2, we have plotted -H versus @ = (@s + @o) for the different microstructures. In the calculations of H , we have used 1s = 16A,as determined from SAXS data from the lamellar and reverse hexagonal phases (see below). By plotting the negative mean curvature, we capture the features of the phase diagram since the mean curvature decreases with increasing temperature. Com-
+
(28)Helfrich, W. 2.Naturforsch. 1973,28c, 693. (29)Safran, 5. A.; Turkevich,L.A.;F’incus, P. A. J.Phys. Lett. 1984, 45, L69. (30)Safran, S.A. In Structure and Dynamics of StronglyInteracting Colloiolsand SupramolecularAggregates in Solution;Chen, S.,Huang, J. S., Tartaglia, P., Eds.; Kluwer Academic Publishers: Dordrecht,The Netherlands, 1992. (31)Golubovic, L.;Lubensky, T. C. Phys. Rev. A 1990,41, 4343. (32)Porte, G.;Appell,J.;Bassereau, P.;Marignan,L. J.Phys. (Paris) 1989,50, 1335. (33)Wennerstrom, H.; Olsson, U. Langmuir 1993, 365. (34)Strey, R. Colloid Polymer Sei., in press.
1
4
i
-0.015 -O.O1 0
0.1
0.3
0.2
0.4
0.5
@ Figure 2. Variation of the negative mean curvature, -H, with @ for different structures: (a) normal sphere; (b) normal cylinder; (c) plane (lamellae):(d) multiply connected bilayer structure; (e) reverse cylinder; (0 reverse sphere. Curvature toward oil is counted as postitive and a value 1s = 16A has been
used in the calculations.
paring Figure 2 with the phase diagram illustrates the approximately linear relation34between H and temperature. A similar calculation for the Gaussian curvature reveals that this parameter has a nonmonotonic variation when inverting the microstructure. K > 0 in the micellar L1 and LZ phases. In the lamellar (La) and cylinder (Hz) structures K = 0, and in the multiply connected bilayer structure of the L3 phase K < 0. This behavior is general for nonionic surfactants and indicates that it is the mean curvature energy ( 2 d H - H o ) ~which ) mainly dictates the microstructure and phase e q ~ i l i b r i a . ~ Replacing 1mol % of nonionic surfactant, in a ternary system, with ionic surfactant molecules extends the phase sequence as a function of temperature with two additional phases: a reverse hexagonal phase and a reverse micellar liquid phase, respectively. The extended phase sequence follows the general behavior of nonionic surfactant systems, with a decreasing average mean curvature of the surfactant monolayer film with increasing temperature. The normal and reverse spheres correspond to the maximum and minimum mean curvature structures, respectively. In the case of spherical normal micelles, the mean curvature can only be increased by phase separation with excess oil, involving a decrease in the sphere radius, and vice versa for the reverse micelles. This corresponds to the “emulsification failure”,discussed by Safran and c o - ~ o r k e r s , ~ ~and , 3 5explains ~ ~ ~ the L1 0 coexistence at lower temperatures and the analogous Lz W coexistence a t higher temperatures. In the pure nonionic system the phase sequence terminates with a W L3 phase coexistence a t higher temperatures. In the multiply connectedbilayer structure of the L3 phase, the mean curvatures of the surfactant monolayers are on the average weakly negative (i.e. toward water) and become increasingly negative the higher the bilayer volume fraction.8 With increasing temperature the spontaneous curvature decreases, and in order to minimize the mean curvature energy, proportional to ( H - H#, the L3 phase moves to a higher bilayer concentration the higher the temperature (recall that (H) % - W / C ~ , ~ where 6 is the bilayer thickness). Thus the L3 phase has a nonswelling behaviour and coexists with excess water a t higher water content^.^,^^ The effect of adding small
+
+
+
(35)Turkevich, L.A.; Safran, S. A,; Pincus, P. A. In Surfactants in Solution; Mittal, K. L.,Bothorel, P., Eds.; Plenum Press: New York, 1986;Vol. 6;p 1177. (36)Safran, S.A.Phys. Rev. A 1991,43, 2903.
S- W - 0 System with Weakly Charged Films amounts of ionic surfactant can be understood as an electrostatic effect arising from the entropy associated with the sodium counterions of SDS. In the presence of counterions a phase separation with excess water, corresponding to a compression of the counterion volume, becomes less favorable. Here it is more favorable to minimize the mean curvature energy by forming alternative structures. As shown in Figure 6, the mean curvature of the the multiply connectedbilayer, the reverse cylinders, and the reverse spheres have different dependence on CP. The same mean curvature as in the multiply connected bilayer structure can be accommodatedby forming reverse cylinders or reverse spheres with a slightly higher water content. In the pure nonionic system the W L3 coexistence is favored over the formation of homogeneous phases of reverse cylinders and spheres. This is probably due to a relatively low entropy in the liquid crystalline H2 phase and the densly packed L2 phase. By introducing the charge, and thereby making the W L3 coexistence less favorable, the Hz and L2 phases have become stable. Similar electrostatic effects can be observed in aqueous mixtures of nonionic polymers and weakly charged polye l e c t r o l y t e ~ . ~In' ~this ~ ~ case the addition of a small charge density to a polymer by chemical synthesis, increases greatly the stability of the homogeneous solution phase. Counterion entropy has also been suggested to be important for the stabilization of the L2 phase in a ternary system with ionic surfactant39and probably also plays a large role in the dramatic increase of the cloud point when adding small amounts of ionic surfactant to a binary nonionic surfactant-water system.22
Langmuir, Vol. 10, No. 9, 1994 3225
109
" L,
N
E . n
10'0
10''
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+
20
25
30
40
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45
SO
55
45
SO
55
TIT
+
4. Structural Inversion with Temperature
-
v1
N
E . fi
20
25
35
30
40
TI'C
Figure 3. (a, top) The self-diffusioncoefficients of water (01, C12E5(A), and decane (01,measured at various temperatures in the L1,Ls, and Lz phases. The lines are only guides t o the eye. The composition is WS WO= 0.30. (b, bottom) The same as in part a but with the concentration Ws + WO = 0.38.
A remarkable observation in the /3 = 0.01 system is the large number of phase transitions the system undergoes as a function of temperature. The sequence of phases with increasing temperature correspondenceto a complete 4E+004 inversion of the microstructure from O N micelles in the 25°C L1 phase to reverse W/O micelles in the L2 phase. The 43OC SO'C structural inversion can be conveniently studied by following the variation of the molecular self-diffusion 3E+004 coefficients of surfactant (Ds), water (Dw),and oil (D0).40,41 In Figure 3 we have plotted Ds, Dw, and DO in the three liquid phases versus temperature for two samples with the compositions WS Wo = 0.3 and 0.38, respectively. At lower temperatures in the LI phase, DW >>DO= Ds, which demonstrates the presence of normal oil-swollen micelles. The surfactant and oil diffuse together as aggregates and hence has identical self-diffusion coefficients, corresponding to the micellar self-diffusion coefficient. In the L2 phase, on the other hand, DO >> Dw, indicating that the structure has been inverted to a W/O OE+OOOd -1-7r-7 n v micellar structure. However, the condition DW = DSis 0 000 0 020 0 040 0 060 not fulfilled here due to the large solubility of the 9 / A-' surfactant in the continuous oil phase. In the L3 phase, Figure 4. Desmeared SAXS spectra from the Ll(25 "C), LB(43 DW = DO showing that the structure is simultaneously "C), and Lz (50 "C) phases. The composition is Ws + WO= 0.38. water and oil continuous. This is consistent with the multiply connected bilayer structure, here a n oil-swollen The three liquid phases were also studied by small angle normal bilayer, which has been proposed for this phase. X-ray scattering (SAXS). In Figure 4 we show typical desmeared SAXS patterns of a sample with Ws W O= 0.38 in the L1, the L3, and the Lz phase. A broad correlation (37) Iliopoulos, I.; Frugier, D.; Audebert, R. Polym. Prepr. 1989,30, 371. peak is observed, which moves to lower values of the (38) Perrau, M.B.; Iliopoulos, I.; Audebert, R. Polymer 1989,1989, scattering vector, q , when going from the L1 to L3 and 2112. further to the Lz phase. This variation of the peak position (39) Skurtveit, R.;Olsson, U. J. Phys. Chem. 1992,96,8640. (40)Lindman, B.; Shinoda, K.; Olsson, U.; Anderson, D.; Karlstrom, is consistent with a n inversion of the microstructure. G.; Wennerstrom, H. Colloids Surf. 1989,38, 205. The structures of the two liquid crystalline phases were (41)Olsson, U.; Lindman, B. In The Structure, Dynamics and investigated by SAXS. In Figure 5 we show a slit smeared Equilibrium Properties of Colloidal Systems; Bloor, D. M . , Wyn-Jones, SAXS pattern from the La phase a t 37 "C. The sample E., Eds.; Kluwer Academic Publishers: The Netherlands, 1990; p 233.
+
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4
+
1 -I
______~ ~
+
Fukuda et al.
3226 Langmuir, Vol.10,No. 9, 1994
r4
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6
I
'4 -
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I
1
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l
-
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0 . 0 5 0 . 1 0 . 1 5 0 . 2 0.25 0 . 3 0 . 3 5 0 . 4
a) Figure 7. Variation of the relative self-diffusioncoefficients (A) and decane ( 0 )with CP = CPS + @o, where CPD and @O are the surfactant and oil volume fractions,respectively, in the L1phase at 25 "C. The infinite dilution self-diffision coefficientD o= 2.2 x lo-" m2s-l. The broken line corresponds to DID"= 1 - 3.6@ 3.5CP2which describes the concentration dependence of the relative micellar self-diffision coefficient in the pure nonionic @ = 0) system.13
DID"of &E5 Figure 6. Slit smeared SAXS spectrum obtained from the La phase at Ws W O= 0.38 and 37 "C. The high q part of the scattering curve is also shown on an expanded ( x 10) intensity scale (right-hand scale).
+
40'0
'-.1 1'
'i
L
3
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\
2
'r 20.0
0
,
0
3
1
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I
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I
0.020
,
I
4'
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1
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'0
0.100
Figure 6. Slit smeared SAXS spectrum obtained from the H2 phase at Ws Wo = 0.38 and 47 "C. The high q part of the scattering curve is also shown on an expanded ( x 10)intensity scale (right-hand scale).
+
+
composition is WS WO = 0.38. A strong first-order and a weak second-order peak is observed, consistent with a n La phase. In Figure 6, we show a SAXS pattern from the same sample a t 47 "C. Two peaks with the relative positions 1:43 are observed, consistent with a twodimensional hexagonal lattice. From the relative position of this phase in the phase diagram we conclude that this phase corresponds to a reverse hexagonal phase. This conclusion is further supported by a n analysis of the surfactant area per head group, as will be discussed below. 6. TheL1 Phase
In the L1 phase DWis close to its value in pure water m2 s-l) and essentially temperature independent, demonstrating that the structure is water continuous. Close to the lower phase boundary Ds = D O and very low ( m2s-l) demonstrating that the microstructure here is oil and surfactant discontinuous; i.e. it consists of discrete oil-swollenmicelles. With increasingtemperature DO > Ds, and the self-diffusion coefficients increase dramatically showing that the microstructure gradually becomes connected (bicontinuous) with increasing temperature. Hence, the average mean curvature of the surfactant film decreases with increasing temperature.
+
The variation of the microstructure with temperature and compositionwithin the L1 phase of the @ = 0.01system is similar to that in the reference @ = 0 system. At lower temperatures discrete oil-swollen micelles are formed as evidenced by the equal self-diffusion coefficients of surfactant and oil. In Figure 7 we show the variation of the surfactant and oil self-diffusion coefficients, at 25 "C, with the micellar volume fraction, @ = @s @o,in the @ = 0.01 system. Here, the self-diffusion coefficients are normalized and presented as DIDO,where the value DO= 2.25 x m2 s-l is the extraplated value at infinite dilution. This value ofDo is consistent with a hydrodynamic radius of 87 A, using the Stokes-Einstein equation with a solvent viscosity of 1.11CPa t 25 "C. In the @ = 0 system, the micelles close to the lower phase boundary are spherical and interact to a good approximation as hard spheres.13 In Figure 7 the concentration dependence ofDID0observedin the@= 0 system close to the lower phase boundary (23.5 "C) is shown for comparison as a broken line. Presumably, the micelles in the /3 = 0.01 system are also spherical a t the present temperature and concentration range. The similar concentration dependence of DIDo in the two systems then indicates that the weak micellar charge (approximately 16 charges per micelle) has only a small influence on the self-diffision in the present concentration range. A slightly lower DIDOis observed at the highest concentrations in the /3 = 0.01 system. This can however be due to small micellar growth, since the temperature 25 "C corresponds to approximately 2 "C above the lower phase boundary a t higher concentrations. The micelles grow with increasing temperature (see below), in particular a t higher concentrations. In Figure 8 we show the temperature dependence of Ds and DO for three concentrations, Ws WO = 0.02, 0.05, and 0.15, respectively, in the L1 phase. At lower concentrations (Ws WO = 0.02 and 0.05) DS = DO in the whole temperature range, while a weak decrease in the self-diffusion coefficients is observed a t higher temperatures. The decrease in the D-values is consistent with a minor micellar growth. For the Ws WO = 0.15 sample, the diffusion coefficients first decrease more rapidly with increasing temperature. At 28.5 "C, they reach a minimum and then increase rapidly when the temperature is increased further. At higher temperatures, DO>>DSwhile both values are high, indicating a bicontinuous microstructure.
+
+
+
+
S- W - 0 System with Weakly Charged Films
Langmuir, Vol. 10, No. 9, 1994 3227
not ~ b s e r v e d ~ ~In- the ~ ~ ./3 = 0.01 system, on the other I hand, the lamellar phase has a very low turbidity.
Ioy
AA'
10.1~
15
26
28
21
30
19
31
T 1 "C
Figure 8. Variation of the self-diffusioncoefficients of ClzEs (open symbols) and decane (filled symbols) with temperature for some different compositions in the LIphase. The composiWs + W O= 0.050 and tions are (0,W) W S+ W O= 0.020, (0,O)
wo
(A, A) ws i- = 0.15.
*
500
I
200
t
loo
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
(F/A),l = kBTc0
(3) where co is the number density of counterions a t the midsurface between the walls. This concentration can be written as co = a(c) where (c) is the average (total) concentration and4' cO
a=-=-
(4
t/ 0
Moreover, as is seen in Figure 4,a second-order Bragg peak is resolved indicating that due to the introduction of the small charge density (roughly one charge per 4400 A2) the dominating long range interaction has switched from the undulation force to a n electrostatic repulsion. These findings are consistent with the results of Jonstromer and Strey14who in a recent study demonstrated that adding small amounts of ionic surfactant to a very dilute lamellar phase with nonionic surfactant introduced electrostatic interactions which sharpened the Bragg peak and reduced the so-called area correction to the repeat distance arising from the curved bilayer conformations. A crossover from dominating undulation to dominating electrostatic repulsion implies that the pressure associated with the counterion distribution overcomes that from the undulations. Within the Poisson-Boltzmann model, the electrostatic repulsive force per unit area between two planar walls is given b96,47
2iEGC 2
dw
s =
S2
s tan{s}
(4)
The dimensionless parameter s is evaluated from 2dW
s tan{s} = 5
10
15
1IQs
Figure 9. Variation of the repeat distance, d, in the lamellar phase ofthep = 0.01 system with the inverse surfactant volume fraction l/@s. The solid line is a least-squaresfit of the equation d = 21&s t o the three data points, yielding 1s = 15.6 A.
To summarize this section we can conclude that the variation of the microstructure with temperature and concentration within the L1 phase of the present system is essentially identical to that of the /3 = 0 system. 6. The Lamellar Phase Figure 5 shows a slit smeared SAXS pattern obtained for W S W O= 0.38 a t 37 "C. A weak second-order peak can be observed and the 1:2 pattern confirms the lamellar structure. The periodicity, d , of the structure depends on the bilayer area per unit volume. For a lamellar structure the position of the first-order peak corresponds to
'GC
where dw is the water layer thickness and AGC is the GouyChapman length
Here, Z = as/P is the area per charge, where as is the area per C1PE5 molecule, and AB is the Bjerrum length. For the solvent water a t the present temperatures AB = 7.0 A, and with Z = 4400 A2,Am becomes approximately 100 A. In the case of Helfrich's undulation force, the force per unit area is given by12
+
In Figure 9 we plotted the repeat distance, d , versus l/@s for three different concentrations in the lamellar phase. The straight line passing through the origin confirms the one-dimensional swelling and from a least-squares fit of the slope we obtain 1s = 15.6 A. An important observation concerns the low q part of the lamellar phase structure factor. The lamellar phase in the /3 = 0 system as well as in the binary nonionic surfactant-water system swell under the influence of the undulation force. In general such lamellar phases appear rather turbid, due to their high osmotic ~ompressibility.~~ The intense scattering a t low q may even hide the firstorder Bragg peak and a second-order peak is generally
(7) Here K is the bilayer bending rigidity and we have taken into account the excluded volume (dw = d - 6) due to the bilayer thickness, 6. The ratio of the electrostatic to the undulation induced pressure can be written
In the present system, we have @&Do = 0.815, giving = 0.45 @, where @ = @S + @O is the total volume fraction of surfactant and oil. The average counterion @S
(42)Nallet, F.;R o n , D.; Milner, S.T. J . Phys. (Paris)1990,51,2333. (43)Row, D.;Safinya, C. R. J . Phys. (Paris) 1988,49, 307. (44)Porte, G.;Marignan, J.;Bassereau, P.; May, R. Europhys. Lett. 1988,7,713. (45)Appell, J.;Bassereau, P.; Marignan, J.;Porte, G. ColloidPolym. Sci. 1989,267,600. (46)Marcus, R. A. J . C h e n . Phys. 1966,23,1057. (47)Jonsson, B.; Wennerstrom, H. J.Colloid Interface Sci. 1981,80, 482.
3228 Langmuir, Vol. 10, No. 9, 1994
Fukuda et al.
density in the water film is given by
t ~ r e . In ~ the , ~ present ~ system, which is strongly related to the binary water-ClzE5 s y ~ t e m the , ~ structure corresponds to a n oil-swollen (normal)bilayer separating two intertwined water subvolumes. As seen in Figure 3, the diffusion coefficients of all components in this phase are high, illustrating that the microstructure is simultaneously surfactant, water, and oil continuous. In SAXS, a broad scattering intensity maximum is observed a t a wave vector %, reporting on the structural length scale t = 2n/q,.54 The ratio ql/qmM 1.3, where q1 is the position of the first-order peak in the La phase for the same composition. This value is slightly smaller than the value 1.4-1.5, which is generally 0 b s e r v e d ~ ~for 9 ~this ~ ratio, presumably due to the relatively large bilayer thickness.
where U S x 700 A3 is the surfactant volume. The water layer thickness can be written as d w = d ( 1 - @)/a, where the bilayer thickness has a value of approximately 60 From eq 4 and 5 and the measured repeat spacings, we find that a takes values in the range 0.65 to 0.85 when increasing the bilayer volume fraction from 0.2 to 0.4. Approximating a with a constant value of 0.75 we find for the present system that eq 8 can be approximated by:
A.
8. The Reverse Hexagonal Phase Figure 6 shows a slit smeared SAXS pattern obtained This ratio has a value x3 K/kBTatthe highest concentration for the sample W S W O= 0.38 a t 47 "C.Two peaks can studied and increases rapidly with dilution. Since K is be resolved, corresponding to the relative positions 1:d3. expected to be larger than KBT, it follows that the By assuming a reverse hexagonal structure, we can electrostatic pressure is dominating over the undulation calculate I, and compare this value with the value obtained force over the whole concentration range. in the lamellar phase a t lower temperatures. For a twoThe addition of charged amphiphiles is also expected dimensional hexagonal symmetry the diffraction peaks t o increase the rigidity of the m e m b r a n e ~ . ~ ~ -The ~O magnitude of this effect depends on the ratio dw/A~c.~~-~Oa t q u , where (h,K) are the Miller indices, correspond to For AGC dw/2 the bending rigidity is estimated to increase 4n(h2 k2 hk)112 by the amount49 (10) qhk =
+
+ + a&
(9) while for ilw < dJ2 AK is estimated to grow linearly with d ~ . In~the~present , ~ system ~ (a < 0.4)dw/2 = AGC (dw varies from approximately 150 to 400 A and A G = ~ 100 Hence, in the more dilute regime we may expect a significant increase (several k&") in the bilayer bending rigidity from the added charged amphiphile. In the discussion of the effect of SDS on the bilayer rigidity and interactions, we have assumed that all the charged surfactant ions are incorporated in the bilayer. Surfactant ions dissolved in the water layer act as added salt and screen the electrostatic interactions, thereby decreasing (F/A)ed(F/A),,nd and AK. This will probably be necessary to take into account in a more complete analysis. Here it is also of interest to consider the experimental findings of Larche et al.51 and of Appell et al.45on the highly oil-swollen lamellar phase in the sodium octylbenzenesulfonate-pentanol-water-decane system. Upon dilution (with oil) the first-order Bragg peak first gradually becomes overwhelmed by the monotonic small angle scattering. However,upon further dilution the Brag peak reappears, and a t very high dilution, ford > 1000 also second- and third-order reflections were observed in light diffraction experiments. This scenario is consistent with a crossover from a dominating steric repulsion to a dominating electrostatic repulsion a t very high oil dilution. The origin of the electrostatic repulsion is presumably a small, although significant solubility of the surfactant ion in the oil, as suggested by Wennerstrom.52
where a is the lattice parameter (nearest neighbor distance). In the case of infinite cylinders of cross section radius Rcyland volume fraction acylwe have the relation 112
A).
1,
7. The LBPhase
The
L3
phase has a multiply connected bilayer struc~~
~
(48)Pincus, P.; Joanny, J.; Andelman, D. Europhys. Lett. 1990,11, 763. (49)Harden, J.L.;Marques, C.; Joanny, J.;Andelman, D. Langmuir 1992,8,1170. (50)Fogden, A,; Ninham, B. W. Langmuir 1991,7,590. (51)Larche, F. C.; Appell, J.; Porte, G.; Bassereau, P.; Marignan, J. Phys. Rev. Lett. 1986,56,1700. (52)Wennerstrom, H. Prog. Colloid Polym. Sci. 1987,74, 31.
Rc, = a ( g acyl)
(11)
The oligoethylene oxide (Ed chain of the C12E5 surfactant makes up approximately one-half of the total surfactant volume. Assuming a n inverted structure and associating the water and the EO chains with the polar domains, we have 2(@w Rcyl
=
+ QS/2)2,
(12)
@S
defining
+
QW, = q$l a s / 2
(13)
This definition (eq 13)corresponds to defining the polarapolar interface a t the interface separating the oligoethylene oxide and hydrocarbon b locks of the surfactant. With eqs 11-13 in (10) we then obtain (14) From the diffraction pattern of Figure 5 we evaluate, with eq 14,1, = 15.7 A, which is almost identical to the value obtained in the Laphase a t slightly lower temperatures and hence confirms the reverse hexagonal structure. 9. TheLzPhase Nonionic surfactants are highly soluble in oil and the surfactant diffusion coefficient, Ds,is for this reason not (53)Porte, G.; Marignan, J.; Bassereau, P.; May, R. J . Phys. (Paris) 1988,49,511. (54)Gazeau, D.;Bellocq, A. M.; Roux, D.; Zemb, T. Europhys. Lett. 1989,9,447. (55)Strey, R.;Jahn, W.; Skouri,M.; Porte, G.; Marignan, J.; Olsson, U. In Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution; Chen, S., Huang, J. S., Tartaglia, P., Eds.; Kluwer Acedemic Publishers: Dordrecht, The Netherlands, 1992;p 351.
S - W - 0 System with Weakly Charged Films a good probe for the microstructure in oil-continuous solutions.56The dominating contribution to the relatively high Ds-value comes from monomers dispersed in the continuous oil domain. The important observation from this phase is, however, that DO >> DW which indicates a reverse micellar structure. Quantitative information on the particle size and shape cannot be obtained from the self-diffusion data, due to the high (internal) volume fraction. The volume fraction of the polar domaines (@w + @d2)vanes from 0.64 at W, WO = 0.4 to 0.81 at W S WO = 0.2. From comparing these values with fcc close packing of spheres, DCp= n&6 x 0.74, and the hexagonal close packing of cylinders, mCp= n/(2&) = 0.91, it is clear that the reverse micelles cannot be spheres a t high internal volume fractionsin the area per headgroup remains unchanged. A reverse micellar solution is nevertheless the most accurate description of the phase as revealed by the relatively low water self-diffusion coefficient (Figure 3).
+
+
10. Conclusion We have studied the effect on the phase behavior of a nonionicsurfactant-water-oil system when replacing one per hundred of the nonionic surfactant molecules by a n ionic surfactant. A reverse hexagonal (H2) and a reverse (56) Olsson, U.; Jonstromer, M.; Nagai, K.; Soderman,0.; Wennerstrom, H.; Kose, G. Prog. Colloid Polym. Sei. 1988, 76, 75.
Langmuir, Vol. 10, No. 9, 1994 3229
+
micellar (L2) phase were found to replace the W Lz coexistence region when introducing the ionic surfactant. The transition from W LBcoexistence to homogeneous H2 and LQ phases essentially conserves the mean curvature of the surfactant film and the effect is attributed to the osmotic pressure associated with the counterion entropy, which destabilizes phase separation with excess water. The surfactant length, IS, associated with a n area per headgroup defined a t the polar/apolar interface separating the ethylene oxide and the hydrocarbon parts of the surfactant monolayer was evaluated in the lamellar and reverse hexagonal phases, respectively. The value is essentially identical in the two phases. Similar values were also found in the lamellar and normal hexagonal phases of the ClzE5-water-tetradecane system" and in the lamellar phase of the binary water-ClzE5 ~ y s t e m . ~ Thus, the area per surfactant molecule, a s defined here, does not appear to depend crucially on either the temperature, the aggregate geometry, or the chain length of the oil.
+
Acknowledgment. U.O. acknowledges the Swedish Natural Science Research Council (NFR) for financial support. The stay of K.F. in Lund was supported by a grant from the Swedish Institute. U.W. is indebted to Professor M. Kahlweit for support. Discussions with Bengt Jonsson, HAkan Wennerstrom, and Marc Leaver are kindly acknowledged.
