J . Phys. Chem. 1991,95, 5353-5358
5353
A Self-Diffusion Study of the Microstructure in Didodecyldimethylammonium Bromide-DodecaneBrine Microemuisions Roald Skurtveitt and Ulf Olsson* Division of Physical Chemistry 1, Chemical Center, University of Lund, P.O.B.124, S-221 00 Lund, Sweden (Received: September 26, 1990; In Final Form: January 24, 1991)
The microstructure in the L2phase in the system didodecyldimethylammonium bromide (DDAB)-dodecantbrine was studied by selfdiffusion measurements using the Fourier transform pulsed gradient spin-echotechnique. Several electrolytecollcentratiolls were investigated, and NaBr and Na2S04were used as salt. The microstructure of the L2 phase in the ternary DDABwater-dodecane system shows a somewhat richer polymorphism than what have previously been recognized. In addition to the regions having a bicontinuous monolayer structure and roughly spherical reverse micelles, we observe a region of rodlike reverse micelles in the vicinity of a reversed hexagonal phase. Furthermore, a crossover from a monolayer to a bilayer structure appears to occur at low oil content. While the stability range of the L2phase changes dramatically with small additions of electrolyte, the added electrolyte has almost no impact on the diffusion behavior of water and oil and thus presumably on the microstructure.
Introduction random Voronoi tessellation. Introducing this constraint, together with a minimum length and radius of the cylinders, the stability Thermodynamically stable liquid mixtures of surfactant, water, range of the L2 phase, predicted from the DOC model, was reand oil are usually referred to as microemulsions.'-' While being cently calculated by Hyde and c o - w ~ r k e r s . ~A~ rather good macroscopically homogeneous, they are structured on a microagreement was found with some existing phase diagrams, indiscopic length scale (10-103 A) into aqueous and oleic microdocating that geometrical arguments indeed take us very far in mains separated by a surfactant-rich film. Depending on the understanding these systems. Furthermore, the variation in the conditions, the surfactant film may enclose a finite volume of stability range of the L2 phase with the chain length of the oil various shapes or form a 3D continuous dividing surface of highly is consistent with this geometrical approach. The effective surconnected topology. Stable microemulsions can be found in a large factant parameter decreases with increasing chain length of the number of surfactant-water-oil systems, sometimesalso containing oil, due to a decreasing degree of oil penetration into the surfactant salt and a short-chain alcohol. For general surveys on micromonolayer. emulsions see refs 1-4. The stability of the L2 phase in DDAB-water4 systems is Didodecyldimethylammonium bromide (DDAB) is a doublesensitive to small additions of electrolyte.2031 A systematic study chained cationic surfactant that exhibits some common phase behavior properties of double-chain ionic surfactants. Due to its bulky hydrophobic part, the water-rich isotropic solution phase solubilizes only small amounts of surfactant. On the other hand, (1) Fribcrg, S. E., Bothorel, P., Eds. Microemulsiom: Srrucrun and Dynamics; CRC Press: Boca Raton, FL, 1987. it may readily form monolayers of reversed mean curvature, and (2) Bowel, M.; Schechter, R. S. Microemulsions and Related Sysrcms; large oil-rich microemulsion phases (L2) are frequently found in Marcel Dekker: New York, 1988. ternary DDAB-water4 systems.5 Several studies, involving (3) Martelucci, S.,Chcstcr, A. N., Eds. Progress in Microemuhiom; different techniques such as conductivity,68 NMR selfdiffu~ion?*~ Plenum Press: New York, 1989. (4) Mittal, K. L., Lindman, B., Eds. Surfactants in Solurion; Plenum and small-angle scattering,1w12 have addressed the microstructure Press: New York, 1984; Vols. 1-3. Mittal, K. L., Bothorel, P., Eds. Surof these solutions. The microstructure is characterized by an factatus in Solurion; Plenum Press: New York, 1986; Vols. 4-6. Mittal, K. average mean curvature toward water, throughout the Llphase L., Eds. Surfacrants in Solurion,Plenum Prres: New York, 1989; Vols. 6-10. region. Furthermore, the microstructure evolves from being bi(5) Fontell, K.; Ceglie, A.; Lindman, B.; Ninham, 9. W. Acra Chem. Scand., Scr. A 1986,40,247. continuous at low water content to becoming gradually water (6) Chcn, S. J.; Evans, D. F.; Ninham, B. W. 1. Phys. Chcm. 1984,88, discontinuous when diluted with water. This latter property 1631. of the polarindicates that the average mean curvature, (H), (7) Ninham, B. W.; Chcn, S.J.; Evans, D. F. J . Phys. Chcm. 1984,88, apolar interface is roughly constant throughout phase and acts 5855. (8) Chcn, S. J.; Evans, D. F.; Ninham, B. W.; Mitchell, D. J.; Blum, F. as a constraint on the microstructure. This property is not unique D.; Pickup, S. J . Phys. Chem. 1986,90, 842. to DDAB systems but may be observed also in other systems. For (9) BIum, F. D.; Pickup, S.; Ninham, B.; Chen, S. J.; Evans, D. F. J. Phys. example, the analogous transition from bicontinuity to closed oil Chem. 1985,89,7 11. droplets when diluting with oil exists in ternary microemulsions (IO) Ninham, B. W.; Barnes, I. S.;Hyde, S.T.; Dcrian, P.-J.; Zemb, T. N. Europhys. f a r . 1987,4, 561. with nonionic s u r f a ~ t a n t . l ~ - ~ ~ (11) Zemb, T. N.; Hyde, S. T.; Derian, P.-J.; Barnes, I. S.; Ninham, B. A geometrical model, the so-called DOC model, using waterW. J. Phys. Chem. 1987,91, 3814. filled spheres and cylinders as building blocks, has been put (12) Barnes, I. S.;Hyde, S.T.; Ninham, 9. W.; Derian, P.-J.; Drifford, forward to describe the microstructure of DDAB based microM.; Zemb, T. N. J . Phys. Chem. 1988,92, 2286. (13) O h n , U.; Shinoda, K.; Lindman, B. J. Phys. Chem. 1986,90,4083. emulsion.*O.ilIntroducing the constraint of constant surfactant (14) Olsson, U.; Nagai, K.; Wennerstram, H. J . Phys. Chem. 1988, 92, ~' to the spontaneous mean curvature,18 parameter, U / ~ I , ~ & related 6675. .. . the DOC model predicts a decreasing connectivity including an (1 5) Olsson, U.; Jonstramer, M.; Nagai, K.; S6derman. 0.;Wennerstram, antipercolation threshold with increasing water content, qualiH.Prog. Colloid Polym. Sci. 1988, 76, 75. (16) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J . Chrm. Soc., tatively consistent with experiments. Faraday Tram. 2 1976, 72, 1525. The connectivity of the DOC structure may vary only within (17) Mitchell, D. J.; Ninham, B. W. J . Chcm. Soc., Faraday Trans. 2 a certain range. The lower limit corresponds to discrete w/o 1981, 77, 601. spheres, and the upper limit is set by the number of faces in a (18) Hyde, S. T. J. Phys. Chem. 1989, 93, 1458. 'On leave from: Department of Chemistry,University of Bergen, N-5007
Bcrgen, Norway. *Towhom correspondence should be addressed.
0022-3654/91/2095-5353$.02.50/0
(19) Hyde, S.T.: Ninham, B. W.: Zemb, T. J . Phys. Chem. 1989, 93. 1464. (20) Chen. V.; Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1987, 91, 1823. ~
0 1991 American Chemical Society
5354 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
Skurtveit and Olsson Dodecane
Dodecane
Water
DDAB
1.36mmolal NaBr
Figure 1. Partial pulse diagram of the DDAB-dodecane-water system
Dodecane
showing the extension of the isotropic liquid phase L,. Redrawn using data from ref 5 .
of the L2 phase stability in DDAB-brine-dodecane systems, involving both monovalent and divalent counterions, was recently presented.21 The general effect is a shrinkage of the phase from the water-rich side with increasing salinity, while the phase border on the water-less side remains roughly unchanged. It is often tacitly assumed that a dramatic change in phase behavior is coupled to a change microstructure. We have therefore investigated the microstructure in the L2 phases of some DDAB-brine-dodecane systems of various salinities where dramatic changes in phase behavior occur with increasing salinity.202' In what follows we will present results from molecular self-diffusion measurements, using the Fourier transform pulsed gradient spin-echo (FTPGSE)technique?= in these L2 phases at various salinities. The brine is composed of either NaBr(aq) or Na2S04(aq),and three different paths through the L2 phase were examined. A comparison with the ternary DDAB-water-dodecane system is made.
6.83 mmolal NaBr
Review 011 Phase Bebavior The stability of the L2 phases in DDAB-water-oil systems is very sensitive to small additions of electrolyte.20.21Recently, a systematic study of the L2 phase stability in DDAB-brine-dodecane systems was performed for several salinities with NaBr and Na2S0,.*' Here we briefly recall those results. (21) Sjbblom, J.; Skurtveit. R.; Saeten, J. 0.;Gestblom, B. Submitted for publication. (22) Stilb, P. Prog. Nucl. Magn. Reson. Spctrosc. 1987, 19, 1. (23) Callaghan, P.T. Ausr. J . Phys. 1984, 27, 359.
DDAB Dodecane
A
Experimental Section
Materials. Didodecyldimethylammonium bromide (DDAB) (puriss) was obtained from Fluka AG and dodecane (zur synthese) from Merck. These chemicals were used as received. NaBr (analytical grade) was obtained from Merck and from Mallinckrodt. Na$04 was obtained from Baker Chemicals. The salts were dried in a heat box at approximately 100 OC and stored in a desiccator. Water was millipore filtered. Samples were prepared in tubes having screw caps. Samples on a line of roughly constant oil volume fraction (see below) were prepared by weighing appropriate amounts of surfactant, brine, and dodecane directly into the tubes. Samples on the two brine dilution lines were prepared from diluting, with brine, a stock solution of a stable microemulsion having low content of brine. Samples were left to stand for at least 24 h at 20 OC and were then transferred into 5-mm NMR tubes. The samples were examined between crossed polarizers. Setf-DiffusionMeasurements. Molecular self-diffusion measurements were performed on a modified JEOL FX-60 NMR spectrometer with the Fourier transform pulsed gradient spin-echo (FTPGSE) t e c h n i q ~ e . ~ 'H Z ~was ~ observed at 60 MHz, and an external 2H field/frquency lock was used. Measurements were performed at 20 f 1 OC.
DDAB
/ 27.2 mmolal NaBr
\ DDAB
Figure 2. Partial phase diagram at'different salinities of the DDABdodecane-NaBr(aq) systems showing the various extensions of the L2 phase: (a) [NaBr] = 1.36 mm, (b) [NaBr] = 6.83 mm, and (c) [NaBr] = 27.2 mm. Redrawn using data from ref 21.
A partial phase diagram, redrawn after Fontell et al: showing the extension of the L2 phase in the system DDAB-dodecanewater is presented in Figure 1. The major feature of the full ternary phase diagram (not shown) are common to doublachained surfactants. In contrast to single-chain surfactants, only a minor L,phase is observed. Instead, the phase diagram is dominated by a widely stable phase and a lamellar phase (in this particular system, in fact two lamellar phases) in the vicinity of the binary watersurfactant axis. Two other phases are found in the ternary system: a reversed hexagonal phase close to the phase boundary of maximal water swelling of the L2 phase, and a narrow cubic phase (possibly being a sequence of several cubic phases) close to the binary water-surfactant axis at roughly constant surfactant-to-oil ratio. In Figures 2 and 3, the change in stability of the L2 phase upon increasing the salinity of the brine is presented, as redrawn from Sjoblom et aLz' Figure 2 shows the effect of NaBr, and in Figure 3 the brine contains NaZSO4. As is seen, the L2 phase shrinks
The Journal of Physical Chemistry, Vol. 95, No. 13. 1991 5355
DDAB-Dodecane-Brine Microemulsions Dodecane
4 1
.......................
10'O
. i
0 0
0
%
n
0
1.33 mmolal NnBr 0 6.82 mmolal NaBr
2
0
0
10''
1
0.55 0.504 mmolal NazSOP
DDAB
Dodecane
0.6
0.65
0.7
0.75
@pb + @J Figure 4. D,, measured for various salinities, plotted versus @b/(@b + as).Data refer to lines in the phase diagrams at constant a0= 0.75. The stability range at different salinities are as follows: pure water, 0.59 I @b/(@b a,) I0.78; 1.33 mm NaBr, 0.59 I*b/(@b a,) I 0.77; 6.82 m M NaBr, 0.59 5 @b/(@b + a,) 50.72.
+
+
' 1 ~; 0
1.33 mmold NnBr
0 6.82 mmolal NnBt
" 4.88 mmolal Na2S04
0.8
.
2
DDAB
0 9.MmmolalNnBr
, . ,
0.3
0.4
:~l:N%:j
0.
p
.e, . ]
10"
Dodecane
0.2
0.6
0.5
@b
Figure 5. D,, measured for various salinities, plotted versus a b . Data refer to a brine dilution line at constant s/o = 3/7. The stability ranges at different salinities are as follows: pure water, 0.20 5 a b I 0.55; 1.33 m m NaBr, 0.20 I a b 50.51; 6.82 mm NaBr, 0.20 I a b I O . 4 4 ; 4.88 mm Na2S04,0.20 I @b 50.29.
,........................
a
4
. -
48.5 mmolal Na2S0,
DDAB
Figure 3. Partial phase diagram at different salinities of the DDABdodecantNa$304(aq) systems showing the various extensions of the L2 phase: (a) [Na2S0,] = 0.504 mm, (b) [NasO,] = 4.88 mm, and (c) [Na2S0,] = 48.5 mm. Redrawn using data from ref 21. dramatically upon very small addition of electrolyte. The shrinkage occurs predominantly from the high water content side of the L2phase. The phase boundary a t low water content remains unchanged. Furthermore, a t a certain value of the salinity, the microemulsion can no longer be infinitely diluted with oil; i.e., the L2phase becomes disconnected from the oil corner.
Results In this study we have measured the self-diffusion coefficients
of water and oil in the L2 phases of DDAB-brine-dodecane systems of different concentrations of NaBr and Na2S04, respectively. For comparison we have also amed out measurements in the ternary system, without salt. Compositions were varied along three different paths in the ternary or pseudoternary (constant electrolyte concentrations) systems. One path corresponds to a roughly constant volume fraction of oil: a0= 0.75 (corresponding to a weight fraction of roughly 0.70). The two other paths are water dilutions a t constant weight ratios, s/o =
3
'
5 , . d 1V'O 8
.
:.
o w
8
W
e
1 0
0 1.33 molnl NnBr 0 6.82 m o l a l N o t 0
14.1 m o l d NnBr
.u.. A 27.2 m o l a l NaBt
X 4.88 mmobl N a p 4
X 48.4 mmolal N%S04
0 '
...........
.:
Skurtveit and Olsson
5356 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 TABLE I: SIapk Compoaitioa and M
d W-Mffusioa Coefficients ia tbe DDAB-Witer-Dodecane
System
self-diffusion
path in the
phase diagrama @o
= 0.75
s/o
= 317
s/o =
111
OSec text and Figure I.
weight fractions, wt W DDAB dodecane brine 7 .O 7.7 8.8 9.8 11.1 11.8 22.7 20.3 17.3 14.7 12.3 39.4 35.2 29.9 25.0 22.5
70.1 69.6 69.9 69.6 70.1 69.9 53.0 47.2 40.2 34.2 28.5 39.3 35.1 29.9 24.9 22.5
22.9 22.7 21.3 20.6 18.8 18.3 24.3 32.5 42.5 51.1 59.2 21.3 29.7 40.2 50.1 55.0
coefficients, m2/s
volume fractionsb @o
@b
0.186 0.184 0.173 0.167 0.152 0.148 0.205 0.280 0.374 0.458 0.539 0.188 0.265 0.364 0.461 0.510
bCalculated using the following values for the densities?
0.757 0.752 0.755 0.752 0.757 0.755 0.599 0.543 0.472 0.409 0.347 0.461 0.4 I7 0.361 0.306 0.218 p,
@I
0.058 0.063 0.072 0.081 0.091 0.097 0.196 0.177 0.154 0.134 0.1 I4 0.351 0.318 0.275 0.233 0.212
4
4
1.08E-11' 1.17E-11 2.29E-11 6.538-3 1 1.97E-10 2.5 1E- IO 4.568- 10 3.268-10 1.48E-IO 4.18E-11 1.23E-11 4.38E-IO 4.67E-IO 4.27E-IO 3.5OE-IO 3.04E- 10
6.61E-10 6.648-10 6.438- 10 6.288-10 6.22E-10 6.15E-10 4.98E-10 4.9OE-10 4.88E-IO 4.77E-10 4.46E-IO 3.86E-10 3.93E-10 3.63E-10 3.678-10 3.39E-IO
= 0.985; p , = 1.000. pa = 0.750. 'Read as 1.08
X
IO-".
crodomains become increasingly distinguishable. On the water-rich side at a0= 0.75 D, decreases to a value of 1 X 10-l' m2 s-I. This value is slightly higher than what is expected for a micellar diffusion constant. In the case of closed discrete micelles, D, corresponds to a particle diffusion constant. Assuming spherical particles, their radius may be estimated from the Stokes-Einstein relation kBT D = -(1 - 29) 6~qR here corrected for a hard-sphere particle obstruction.29 kBis the Boltzmann constant, T the absolute temperature, q the solvent Discussion viscosity, and the 9 the particle volume fraction (9= as+ 9,). A self-diffusion experiment is a direct source of information Identifying D, with a particle diffusion constant, we obtain R = regarding solution structure in organized surfactant 70 A. This radius may be compared with a value calculated from By measuring the self-diffusion constants of water and oil, the composition. Assuming monodisperse spheres, the radius, R,, transport properties on both sides of the surfactant monolayer are of the water core is given by the water-to-surfactant ratio. We ~ monitored simultaneously. With the FTPGSE t e c h n i q ~ e , ~ ~ , ~have macroscopic root-mean-squared displacements, typically of the order of 10 rm, are measured, meaning that self-diffusion constants are sensitive to confinement on structural length scales, which typically are of the order of 100 A. Furthermore, the Here, C, is the molar concentrationof surfactant, NA is Avogadro's existence of bicontinuous microstructures is easily identified since number, Z is the area per surfactant headgroup, p, is the surfactant they are characterized by simultaneously high diffusion constants density, and M, (=463 g/mol) is the surfactant molecular weight. of both water and 0iIo2' Due to the simultaneous probing of both Taking Z = 68 A2 and ps = 0.985 g/mL, we obtain R, = 110 sides of the monolayer, an estimate of the average mean curvature A. Then, adding 10 A for the surfactant monolayer we obtain of the surfactant film may be obtained from self-diffusion data a particle radius of 120 A, which is larger than the radius (70 in bicontinuous systems.28 A) estimated from D, but nevertheless of roughly the same Ternary System: DDAB-Dodecme-Water. Turning our atmagnitude. The discrepancy is small enough to be due to intention to Figures 4-6, we start by noting that our data from the termicellar exchange between spherical reverse micelles.M salt-free system are in accordance with previous finding^.^,^ At In the water-rich end of the s/o = 3/7 dilution line D, 1 X = 0.75 (Figure 4) and along the s/o = 3/7 dilution line (Figure 10-I'm2 s-', which is too high to be consistent with spherical w/o 5 ) the selfdiffusion constant of water decreases gradually, by more droplets. Also, in a recent dielectric spectroscopic study it was than an order of magnitude, when going from the water-less to concluded that spherical reverse micelles were incapable of exthe water-rich side of phase. At low water content the diffusion plaining the data in this concentration regime.2' Due to the constants of water and oil are both high, indicating a bicontinuous closeness in compositions to a reverse hexagonal phase (forms, structure. However, the surfactant-to-water ratio here is high, after a narrow two-phase region, when diluting the microemulsion which complicates a more quantitative interpretation. A large at s/o = 3/7 with water beyond the phase boundary) and since fraction of the water molecules hydrate the water-surfactant the solutions here show strong flow birefringence, a structure of interface where they have a reduced diffusivity. rodlike reverse micelles is more likely. In such a structure moWith increasing water content, the surfactant monolayer lecular diffusion inside the extended aggregates may offer a very gradually closes around water and discontinuous aqueous mieffective transport mechanism if intermicellar exchange may occur,3owhich may explain the relatively high D, value. This was (24) Lindman, B.; Stilbs, P. In ref 1. also concluded in a recent study of the s/o dilution line addreasing (25) Lindman, B.; Shinoda, K.; Oloson, U.; Anderson, D.; Karlstrtim, G.; the dominating diffusion process of water.3' A dominating Wennerstrh. H. Colloids Surf. 1989, 38, 205. (26) Oloson, U.; Lindman, B. In Structure. Dynamics and Equilibrium Properties of CbIloiddSystenv, Wyn-Jo~m,E.,Bloor, D. M.,Eds.; Kluwer (29) Lekkerkerker, H. N. W.; Dhont, J. K. G. J. Chem. Phys. 1984,80, Academic Publishers: The Netherlands, 1990; p 233. 5790. (27) Lindman, B.; Kamtnlu, N.; Kathopoulis, T.-M.; Bmn, B.;Nilsson, (30) Jtinsson, B.: WcnncrstrBm, H.; Nilsson. P. G.; Linse, P.Colloid PoP.-G. J. Phys. Chem. 1980,84,2485. lym. Sci. 1986, 264, 7 7 . (28) Anderson, D. M.;Wennentrh, H.J. Phys. Chem. 1990,94,8683. (31) Jonstrtimer, M.;Olsson, U.; Parker, W. 0. To be published. will therefore not be discussed further. The variation of the water self-diffusion constant, D,, along the various paths is presented in Figures 4-6. The variation of D, along a line at constant @a = 0.75 is presented in Figure 4. Here, D, is presented as a function of @b/(@b + a,), a, and ab being the surfactant and brine volume fractions, respectively. In Figure 5 we show the variation of D, as a function of the brine volume fraction, ab, along the water dilution line at constant surfactant-to-oil weight ratio, s/o = 3/7. The corresponding results from the dilution line at s/o = 111 are shown in Figure 6.
J
~
~~
~~
DDAB-Dodecane-Brine
The Journal of Physical Chemistry, Vol. 95, No. 13, I991 5357
Microemulsions
TABLE II: Sam& ComWritioll d Measured SClf-Diffudoll Coeffickata in the DDAB-NaBr.BrlneDodmoe Svstem ~
path in the phase diagram' 9, = 0.75
s/o = 317
s/o =
111
= 0.75
weight fractions, wt % DDAB dodecane brine 7.0 7.9 9.3 10.2 10.9 11.8 11.9 22.8 20.6 18.0 15.7 13.3 40.1 35.0 30.2 25.0 19.8 9.1 9.8 10.8 11.9 11.5
s/o = 317
s/o = 1 / I
s/o
= 317
s/o =
111
s/o =
111
12.0 22.7 21.0 19.1 17.4 15.7 39.9 35.2 30.1 25.1 19.5 22.8 22.1 21.8 21.4 39.7 38.1 36.1 34.2 32.9 39.7 39.0 38.6 37.8
69.7 69.1 69.7 69.8 70.1 70.2 69.9 53.1 48.0 41.8 36.6 30.9 39.9 34.8 30.1 24.9 19.7 69.1 69.4 69.9 69.7 69.6 68.9 52.9 49.1 44.7 40.6 36.6 39.8 35.1 30.0 25.1 19.4 52.7 51.2 50.5
49.5 39.7 38.1 36.1 34.1 32.8 39.9 39.2 38.8 38.0
23.3 23.0 21.1 20.0 19.0 18.0 18.2 24.1 31.4 40.2 47.7 55.9 20.0 30.2 39.7 50.1 60.6 21.8 20.8 19.3 18.4 18.9 19.1 24.4 29.9 36.2 42.1 47.7 20.3 29.8 39.9 49.8 61.1 24.5 26.7 27.7 29.1 20.6 23.8 27.9 31.7 34.3 20.4 21.8 22.6 24.2
~~
~~
molar ratio salinity. mm DDAB/NaBr 1.33
6.82
489 556 717 833 934 1063 1064 1544 1068 728 536 386 3260 1888 1238 811 53 1 132 150 177 205 193 199 295 223 168 131 104 626 375 239 160 101
9.00
14.1
27.2
223 199 189 176 295 246 198 165 147 155 142 136 124
volume fractionsb 0.189 0.187 0.171 0.162 0.154 0.146 0.147 0.204 0.270 0.352 0.424 0.506 0.176 0.269 0.359 0.461 0.567 0.177 0.169 0.156 0.149 0.153 0.155 0.207 0.256 0.314 0.370 0.424 0.178 0.265 0.361 0.458 0.572 0.208 0.227 0.237 0.249 0.181 0.210 0.248 0.283 0.308 0.179 0.192 0.199 0.214
9,l
9.
0.754 0.748 0.753 0.754 0.757 0.757 0.755 0.600 0.550 0.488 0.434 0.372 0.467 0.414 0.363 0.305 0.245 0.748 0.750 0.755 0.753 0.752 0.746 0.598 0.561 0.517 0.475 0.434 0.466 0.417 0.362 0.307 0.243 0.596 0.582 0.574 0.565 0.465 0.449 0.427 0.407 0.393 0.467 0.460 0.456 0.447
0.058 0.065 0.076 0.084 0.090 0.097 0.098 0.196 0.180 0.160 0.142 0.122 0.357 0.3 17 0.278 0.234 0.188 0.075 0.08 1 0.088 0.098 0.095 0.099 0.195 0.183 0.169 0.155 0.142 0.356 0.318 0.276 0.235 0.185 0.196 0.191 0.189 0.186 0.354 0.342 0.325 0.310 0.299 0.354 0.349 0.345 0.339
self-diffusion coefficients, m2/s D, D. 1.06E-1 le 1.22E-11 3.448-3 1 9.91E-11 1.74E-10 2.5 1E- 1 0 2.6 1E-I 0 4.27E-10 2.96E-10 1.80E-10 4.92E-I 1 l.53E-11 3.96E-IO 4.67E-10 4.16E-10 3.28E-10 2.66E-I 0 1.9OE-11 4.57E-I 1 1.37E-10 2.23E-10 1.86E-10 2.1 2E-10 4.22E-10 3.37E-10 2.39E-10 1.03E-10 2.82E-11 4.34E-10 4.60E-10 3.97E-10 2.93E-10 9.59E-11 3.988-10 3.55E- 10 3.38E- 10 3.2OE-IO 4.15E-IO 4.37E-IO 4.39E-IO 4.06E-IO 4.14E-10 4.16E-IO 4.29E-I 0 4.3OE-IO 4.44E-I 0
6.488-10 6.46E-10 6.36E-10 6.36E-10 6.1 7E-10 5.78E-10 5.76E-10 4.7333-10 4.72E-10 4.698-10 4.69E-10 4.1 OE-10 3.22E- 10 3.68E-IO 3.59E- 10 337E- 10 3.4OE-10 6.OOE-IO 6.1OE-1 0 6.11E-10 5.95E-10 6.13E-10 5.56E-10 4.84E-10 4.79E-10 4.80E-10 4.71 E-10 4.92E-10 3.8 1E-10 3.56E-10 3.55E-10 3.66E-10 3.58E- 10 4.85E-10 4.84E-10 4.89E-10 4.98E-10 3.79E- 10 3.378-10 3.46E-IO 3.34E- IO 3.54E-10 3.7 1E- IO 3.76E-10 3.76E-10 3.75E-10
'See text and Figure 2. bCalculated using the following values for the densities.5 p , = 0.985; pw = 1.OOO; pa = 0.750. CReadas 1.06 X lo-".
molecular diffusion process (in contrast to a micellar diffusion process) was found in the whole dilution range. We also studied a sample with s/o = 1/4 and 45 wt 5% water. This composition is proposed, from SAXS data, to contain monodisperse spherical reverse micelles.I2 For this sample we measured D, = 5 X lo-" m2 s-l. This value is lower than what is measured at higher and lower oil content for the same waterto-surfactant ratio, showing a minimum in D, when diluting with oil. This preliminary result indicates a sphere-to-rod transition, when concentrating the system a t constant water-to-surfactant ratio, which in fact may be suspected considering the high particle volume fractions. On the dilution line s/o = 1/1 there is only a minor decrease in D, with increasing water content. The structure remains effectively bicontinuous in the whole dilution range. It is possible that in this region of the L2 phase there is a crossover from a monolayer (microemulsion) to a bilayer (L3)32-34structure. At (32) Cates, M. E.;Roux, D.; Andelman, D.; Millner, S.T.;Safran, S.A. Europhys. Lerr. 1988. 5, 133. (33) Porte, G.; Marigan, J.; Basscreau. P.;May, R. J. fhys. (Les Ulis, Fr.) 1988,19, 51 1.
lower oil content the L2 phase takes the form of a narrow "tongue" pointing toward the water corner (see Figure 1). Most likely this *tongue" is analogous to the bicontinuous L3 p h a ~ e observed ~~-~ in a number of other system^.'^ The microstructure of the L3 phase is topologically related to bicontinuous bilayer cubic phases.',36 Indeed, such a phase., or sequence of phases, is stable at lower oil content in the present s y ~ t e m .We ~ note that a crossover to a bilayer structure is consistent with an average mean curvature of the surfactant monolayer toward water.34 To summarize our observations in the salt-free system, we note that, except at low oil content, the general trend is a decreasing connectivity of the microstructure with increasing water content. The microstructure of the L2 phase of the DDAB-water-dodecane system, however, shows a somewhat richer polymorphism than what has previously been recognized. In addition to the regions having a bicontinuous monolayer structure and roughly spherical (34) Anderson, D.; WennerstrBm, H.;Olson, U. J. fhys. Chem. 1989,93, 4243. (35) Strey, R.; Schomicker,R.; Roux, D.; Nallet, F.; Olson, U.J. Chem. Soc., Faraduy Tram. 1990, 86, 2253 and references therein. (36) Balinov, B.; Olson, U.; Werman, 0. J . fhys. Chem., in press.
5358 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
Skurtveit and Olsson
TABLE III: Sample Composition and Mcrsured Self-Diffusion CoefficienQ in the DDAB-Nag30bBrheDodeane System path in the phase diagrama s/o = 317
s/o =
111
s/o = 1 1 1
weight fractions, wt 5%
molar ratio
DDAB 22.8 22.2 21.2 20.5 19.7 40.0 37.2 33.6 30.1 24.6
dodecane 53.0 51.5 49.4 47.8 45.9 40.0 37.1 33.5 30.0 24.6
brine 24.3 26.4 29.4 31.7 34.4 20.0 25.7 33.0 39.9 50.8
salinity, m m 4.88
39.5 39.3 39.1 38.5
39.5 39.2 39.0 38.4
21.0 21.5 22.0 23.1
48.4
DDAB/Na$O, 407 364 313 28 1 248 866 628 442 327 210 84 82 79 74
volume fractionsb
a,,
self-diffusion coefficients, m*/s
0.206 0.224 0.252 0.273 0.298 0.176 0.227 0.295 0.361 0.468
9, 0.598 0.584 0.564 0.548 0.529 0.468 0.438 0.400 0.363 0.302
9, 0.196 0.191 0.185 0.179 0.173 0.357 0.334 0.305 0.277 0.230
4.2OE-1Oe 3.99E-IO 3.9OE-10 3.378-10 2.86E-10 4.13E-10 4.5 1E-10 4.768-10 4.3 1 E-I 0 2.928-10
Do 4.76E-10 4.76E-10 4.77E-10 4.97E-10 4.97E-10 3.678-10 3.60E- 10 3.92E-30 4.3 1E-1 0 3.47E-10
0.184 0.189 0.193 0.204
0.463 0.460 0.458 0.452
0.353 0.351 0.349 0.344
4.268-10 4.15E-10 4.368-10 4.538-10
4.00E-10 3.73E-10 3.73E-10 3.78E-10
0,
#See text and Figure 3. *Calculated using the following values for the densities? p s = 0.985; p w = 1.000. p , = 0.750. 'Read as 4.20
reverse micelles, there appears to be a region where the structure consists of long rodlike reverse micelles. Quaternary Systems: DDAB-DodecaneBrine. The stability range of the L2phase is very sensitive to additions of small amounts of electrolyte as can be seen in Figures 2 and 3. Self-diffusion coefficients of water and oil were measured in the salt-containing systems. Compositions were varied along analogous paths as in the ternary system without salt. The various paths are indicated in Figures 2 and 3. Sample compositions and measured diffusion coefficients are given in Tables I1 (the NaBr systems) and 111 (the Na2S04systems). As in the salt-free system Do is only weakly dependent on the composition (cf. Tables I1 and 111) and will not be discussed here. Adding salt to ionic surfactant systems screens intramicellar headgroup interactions, leading to a decrease of the mean curvature of the surfactant monolayer. For micellar solutions a sphere-to-rod transition can be observed. In quinary microemulsion systems the microemulsion phase is stable at different water-to-oil ratios depending on the salinity. If an increased salinity was followed by a decrease in the spontaneous mean curvature of the surfactant film (counting curvature toward water as negative), or similarly an increase in u / d , we would expect D, to decrease with increasing salinity at constant ab and 9,. Following an increase in u/al, the DOC model predicts a shift in the stability region of the L2 phase in a similar way as when shortening the chain length of the oil. D, data from the various paths are shown in Figures 4-6 together with the data from the salt-free system. For 9,= 0.75 and s/o = 3/7 we observe a small decrease in D, with increasing salinity a t larger brine dilutions, indicating a small shift in the average mean curvature or u/al with increasing salinity. At the limit of dilution with 6.82 mm NaBr at s/o = 3/7, D, is about a factor of 2 lower than with pure water at the same water volume fraction (Figure 5 ) . The initial shrinkage of the L2phase with small additions of salt hence appears to follow the predictions of the DOC model. As the ionic strength increases, and the L2 phase shrinks further and, moreover, becomes disconnected with the oil corner, the consistency with the DOC model is dropped. Here only the surfactant-rich compositions are stable. The bicontinuous microstructure present a t these compositions in the ternary system appears to remain essentially insensitive to the salinity, as is seen by the very similar D, values obtained at different salinities. In
X
the system with 4.88 mm NaaO, (Figure 3b) and the system with 9.00 mm NaBr (phase diagram not shown) the structure remains bicontinuous in the whole stability range of the s/o = 3/7 dilution line (Figure 5 ) . Here, one no longer has closed, or nearly closed, reverse micelles near the brine-rich phase boundary. Hence a t higher ionic strength, when the L2 phase have shrunk considerably, the microstructure appears to be uncoupled to the changes in phase behavior. When we state that the microstructure appears to be conserved upon additions of salt, it should be noted that the dramatic changes in D, observed in Figures 4 and 5 mainly reflect a change in the average topology of the surfactant film. Small changes in the surfactant packing (u/al) for a given topology is not believed to produce any measurable change in D, and therefore cannot be ruled out. However, major changes in the surfactant packing are expected to induce changes in the average film topology and hence produce changes in D,. At lower oil content (s/o = 1/1, Figure 6) the microstructure remains bicontinuous for all brine contents. Also here, the microstructure appears to be insensitive to the ionic strength as seen by the fact that all data points fall on a common line, except for a slightly lower D, value in the most brine-rich sample with 6.82 mm NaBr. Note that at this dilution line there is an initial growth of the stability range at lower salinity before it then shrinks a t higher salinity. The salinities involved, causing the dramatic changes in the phase diagrams, are relatively small. This can be seen in Tables I1 and 111 where we also give the molar ratios of surfactant to electrolyte. In most of the cases, the surfactant concentration is much larger than the electrolyte concentration. Hence, from an electrostatic point of view, the added salt is only a minor perturbation and we do not expect any major change in the surfactant film properties. However, for a given salinity, the electrolyteto-surfactant molar ratio increases with increasing @b or @,,/(ab + 9J Hence, electrolyte effects on microstructure are expected to be stronger at higher than at lower brine content, which is observed in the present study.
Acknowledgment. This work was supported by the Swedish Natural Science Research Council (NFR). R.S. thanks the Norwegian Council for Science and the Humanities (NAVF) and Nordiska Ministerriidet for financial support.
