J . Phys. Chem. 1990, 94, 3069-3074 important. With this radius, the value of the diffusion coefficient becomes even larger, 7.7 X IO-" m2 s-l, Changes in eq 27 could be discussed, e.g., if a somewhat smaller radius than the hydrodynamic one should be used in the second term. Such a modification only leads to minor changes, however, and it is probably necessary to assume that other collective modes of motion are important for the NMR relaxation than those considered, if the resulting diffusion coefficient shall be reduced substantially. Siiderman et a1.26consider this possibility but prefer to explain the discrepancy by the assumption that the surfactant self-diffusion really is much faster in the globular micelles than in the liquid crystalline structures. This is of course possible, and it is also possible that CPC diffusion is slower in the long micelles than in the globular ones, but the consistency between our results for k p S pand h D, as expressed in Figure 5 , is evidence against such a proposition. Concluding Remarks
The merit of the triplet deactivation measurements is that the process can be followed over a much longer time than the fluorescence quenching, and small differences in the parameter values can be picked up. This is in particular the case for the migration rate, as measured by the parameter 7 . As exemplified in Figure 2, this parameter only affects the last part of the decay and leaves the initial portion-corresponding to the fluorescence time range of less than 2 ps-virtually unaffected. Migration rates as those considered here would be impossible to determine from fluorescence measurements. The estimate of D would also appear to be safer from measurements over longer time. This benefit, however, is probably outweighed by the much better precision of
3069
the data obtained from time-resolved photon counting in the fluorescence measurements. A more fundamental reason for extending the measurements over a longer time comes from the model assumptions with the idea of transport to a reaction zone, which is justifiable only if the root mean square displacement along the axis of the cylindrical micelle is much longer than the size of the reaction zone or the radius of the cylinder. With a diffusion coefficient of 5 X IO-" m2 s-I, the fluorescence time scale of 1 ps gives a root mean square displacement of 100 A and 16 ps gives 400 A, which can be taken as representative for triplet deactivation. Since the cylinder radius in this case is about 21 A, it is not obvious that he fluorescence experiments fulfill the model assumption in this respect. The fact that the mutual diffusion coefficients obtained by both methods are similar for similar pairs justifies the use of the model for fluorescence quenching. The diffusion coefficients estimated from the measurements are reliable. The good agreement with diffusion coefficients from NMR self-diffusion studies of liquid crystalline phases30 is gratifying; the less good agreement with values deduced from multifield 2HNMR relaxation studies26points to the difficulties with the interpretation of results from such studies. Acknowledgment. This work was supported by grants from the Swedish National Science Research Council. Registry No. Py, 129-00-0; MeA, 779-02-2; Az, 275-51-4; GAz, 489-84-9; BP, 119-61-9; BPC6, 64357-69-3; CPC, 123-03-5; A18, 124583-88-6. Supplementary Material Available: Details for solution of y(0,t') and evaluation of .fb [l - y(O,t')] dt'from eq 21 (5 pages). Ordering information is given on any current masthead page.
Structure and Dynamics of Cetyltrimethylammonium Bromide Water-in-Oil Mlcroemulsions Jacques Lang,*,+ Giuseppe Mascolo,fg Raoul Zana,' and Pier Luigi Luisi* Institut Charles Sadron (CRM-EAHP), CNRS-ULP Strasbourg 6 , rue Boussingault, 67083 Strasbourg Cedex, France, and Institut fur Polymere, E TH-Zentrum, CH-8092, Zurich, Switzerland (Received: May 9, 1989; In Final Form: October 18, 1989)
The state of water, the droplet size, and interdroplet exchange of reactants between colliding droplets have been investigated by NMR, time-resolved fluorescence quenching, and electrical conductivity in water-in-oil microemulsions made of cetyltrimethylammonium bromide (CTAB) in chloroform/isooctane ( 2 / I , v/v). The effect of the two additives cetyl bromide (CB) and trimethylamine (TMA) on the properties of the droplets has also been investigated. The droplet size, as expected, increases as the [water]/[CTAB] molar ratio w, increases. No interdroplet exchange of reactants has been detected in the absence of additives. The addition of CB above 0.6 M has a drastic effect on the droplet properties. A large increase of size is observed which is accompanied by the appearance on the fluorescence time scale ( N 1 p ) of interdroplet exchange of reactants. Electrical percolation phenomena are also observed for the systems with CB when the value of the rate constant k , which characterizes the interdroplet exchange of reactants is above (1-2) X IO9 M-l s-' in agreement with the observation made in previous studies of other microemulsion systems. The addition of TMA slightly decreased the surfactant aggregation number .
Introduction
The cationic surfactant cetyltrimethylammonium bromide (CTAB) has been known for several years as one which is capable of building reverse micelles (or water-in-oil microemulsions at higher water content).I-" Also, a few paperss-* describe chemical reactions, mostly enzymatic which take place in CTAB aggregates in organic solvents (mostly chloroform and alkanes, e.g., isooctane and higher hydrocarbons). *To whom correspondence should be addressed.
'Institut Charles Sadron (CRM-EAHP).
3 lnstitut fur Polymere. ,Present address: CNR-IRSA, via F. De Blasio 5, 70123 Bari, Italy.
0022-3654/90/2094-3069$02.50/0
Despite this widespread interest in CTAB reverse micelles and water-in-oil microemulsions, not much is known about their ( 1 ) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromclecular Systems; Academic Press: New York, 1975. (2) Ahmad, S.; Friberg, S. J . Am. Chem. SOC.1972, 94, 5196. (3) Amire, 0. A.; Burrows, H. D. J. Chem. SOC.,Faraday Trans. I 1985, 81, 2723. (4) Fletcher, P. D. 1.; Galal, M. F.; Robinson, B. H. J . Chem. SOC.,Faraday Trans. I 1985, 81, 2053. ( 5 ) Franssen, M. C. R.; Weijnen, J. G. J.; Vincken, J. P.; Laane, C.; Van der Plas, H. C. Biocataiysis 1988, I , 205. (6) Hilhorst, R.; Spruijt, R.; Laane, C.; Veeger, C. Eur. J. Biochem. 1984, 144, 459. (7) Hoffmann, 8 . ; Jackle, H.; Luisi, P. L. Biopolymers 1986, 25, 1 1 13.
0 1990 American Chemical Society
3070 The Journal of Physical Chemistry, Vol. 94, No, 7 , 1990 physicochemical properties, in particular about their ~ i z e . ~ - l ’ On the other hand, the study of CTAB aggregates in apolar solvents offers several points of interest. For example, one may wish to compare the properties of CTAB reverse micelles, which is a cationic surfactant, with those of other cationic surfactants and of the well-known anionic surfactant AOT (sodium bis(2ethylhexyl) sulfosuccinate). Also, the knowledge of the properties of CTAB water-in-oil microemulsions may prove important to understand the mechanisms of the above-mentioned reactionP taking place inside them. This paper describes a physicochemical characterization, based on NMR, fluorescence probing, and conductivity studies, of CTAB aggregates in chloroform/isooctane mixtures. In particular, we will estimate the size of the water pools as a function of the water content and in the presence of guest molecules. We will also discuss the state of water of the water pool as a function of wo, the molar ratio of water to surfactant (wo= [H20]/[CTAB]). At this stage, we will investigate as guest molecules cetyl bromide (CB) and trimethylamine (TMA). The interest in these molecules stems from the fact that they are the chemical constituents of CTAB and have been used to perform the synthesis of CTAB in CTAB/CHCI3-isooctane/water microemulsions.]* It is therefore particularly interesting to investigate the influence of the isolated “head” and “tail” on the behavior of the entire surfactant molecule.
Lang et al. aggregation number N a n d their size, i.e., the water pool radius Rw . The fluorescence decay curve has been shown to obey the eq~ation’~.’’ I ( t ) = I(0) exp(-A2t
- A,[l
- exp(-A,t)]]
A2 = ko;
A3
= [Q]/[M];
(8) Samama, J . P.; Lee, K . M.; Biellmann, J. F. Eur. J . Biochem. 1987, 163, 609. (9) Seno, M.; Sawada, K.; Araki, K.; Iwamoto, K.; Kise, H. J . Colloid Interface Sei. 1980, 78, 57. (IO) Atik, S. S.; Thomas, J . K. Chem. Phys. Left.1981, 79, 351; J . A m . Chem. Soc. 1981, 103, 3543, 4367. (II ) Lianos, P.; Zana, R.; Lang, J . ; Cazabat, A. M. I n Surfactant in Solulion; Mittal, K. L., Bothorel, P., Eds.; Plenum Press: New York. 1986; p 1365. (12) Mascolo, G . ; Giustini, M.; Luisi, P. L.; Lang, J . Submitted for oublication. ( 1 3 ) Bridge, N. J.; Fletcher, P. D. I . J . Chem. Soc., Faraday Trans. I 1983. 76. 2161. ( 1 4 ) Ganz, A. M.; Boeger, B. E. J . Colloid InterfoceSci. 1986, 109, 504. ( 1 5 ) Lang, J . ; Jada, A.; Malliaris, A. J . Phys. Chem. 1988, 92. 1946.
A4
= k,
(2)
where [Q] and [MI are the quencher and micelle molar concentrations, ko is the rate constant of the fluorescence decay of the probe in the micelles without quencher, and k , is the pseudo-first-order rate constant for intramicellar quenching of the probe. In this case the mean surfactant aggregation number, N , that is, in the present case, the average number of surfactants per droplet, is directly given by
Materials and Methods
Materials. Cetyltrimethylammonium bromide (CTAB, Fluka) was twice crystallized in a mixture of ethyl acetate and ethanol. Cetyl bromide (CB, Aldrich) was distilled under vacuum before use. Chloroform and isooctane (Fluka puriss. p.a.) and deuteriochloroform (ICN Biomedicals Inc., Cambridge, MA) were used as received. Aqueous solutions of trimethylamine (TMA, Fluka) were prepared by dilution of a stock solution. The concentration of these solutions was obtained by titrating with HCI, using methyl orange as indicator. Water was freshly deionized and distilled. Ruthenium( 11) tris(bipyridine) chloride (Rubpy) was a gift from Dr. Ziessel (University Louis Pasteur, Strasbourg, France). Methylviologen chloride (MV) was from Aldrich. All solutions were thoroughly deoxygenated prior to each fluorescence measurement by carrying out at least four freeze-pumpthaw cycles. Methods. I . N M R . The 200-MHz ‘ H NMR spectra were recorded in the Fourier mode and quad detection on a Bruker AC 200 P spectrometer, equipped with an Aspect 3000 computer. For these measurements CDCI, instead of chloroform and TMS as external standard were used. 2. Conductiuity. The electrical conductivity was measured by using an automated autobalanced conductivity bridge (WayneKerr B 905) operating at 1 kHz. 3 . Fluorescence. CTAB reverse micelles have been characterized by time-resolved fluorescence quenching. In this technique the fluorescent probe (Rubpy) is solubilized in the water pool of reverse micelles, and its fluorescence is quenched by methylviologen chloride also solubilized in the water pool. The fluorescent probe and the quencher are referred to as reactants in the following. As it has been shown already in several s t u d i e ~ , l ~ *this ’~-~~ technique affords a reliable and sensitive method to determine the concentration [MI of micelles in solution and, thus, their
(1)
where I ( t ) and I(0) are the fluorescence intensities at time t and t = 0, respectively, following excitation. AZ, A3, and A, are time-independent parameters which are obtained, together with I(O), by fitting eq I to the decay data, by using a nonlinear weighed least-squares procedure. In the case where the probe and quencher distributions are frozen on the probe fluorescence time scale (no detectable intermicellar exchange of the reactants), the expressions for A*, A,, and A, are18
(3) where C is the total surfactant concentration. Equation 3 assumes that all the surfactant is involved in the making of the droplets. The expressions for A2, A,, and A, for the case where fast (comparable to r0 = ko-’, the unquenched fluorescence lifetime of the probe) interdroplet exchange of reactants takes place by collision between droplets with temporary coalescence of the colliding micelles were given as10*19320
(4)
A4
= k,
+ k,[M]
(6)
where k, represents the second-order rate constant associated with collisions giving rise to exchange of reactants between droplets. From eqs 4-6, N , k,, and k, are obtained as a function of the experimental quantities A2, A,. A,, and ko as follows:
k, =
A3A4
+ A2 - ko (9)
In all experiments, [Q]/[M] was kept between 0.6 and 1.2 and the molar ratio [Rubpy]/[M] was below 0.07. If it is further assumed that all the water is in the form of water pools and that the water droplets are spherical, monodisperse, and separated from the continuous oil phase by a monolayer of N surfactant ions, the expressions for the radius of the water pool, (16) Infelta, P.;Gratzel, M.;Thomas, J. K. J . Phys. Cfiem. 1974, 78, 190. (17) Tachiya, M. Chem. Phys. Left. 1975, 33, 289. (18) Atik, S. S.; Nam, M.; Singer, L. Chem. Pfiys. Left. 1979, 67, 75. (19) (a) Dederen, J. C.; van der Auweraer, M.; De Schryver, F. C. Chem. Phys. Letf. 1979, 68, 451. (b) Grieser, F.; Tausch-Treml, R. J . Am. Chem. Soc. 1980, 102, 7258. (20) Zana, R. In Surfacfanf Solutions;New Methods of Investigarion; Zana, R., Ed.: Marcel Dekker: New York. 1987; Chapter 5.
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 3071
CTAB Water-in-Oil Microemulsions
TABLE I: Surfactant Aggregation Number, N, Second-Order Rate Constant for Collision with Transient Merging and Exchange of Reactants between Droplets, k,, Rate Constant of the Fluorescence Decay of the Probe in the Droplet, k m Radius of the Water Pool, R , and Surface Area per CTAB Molecule at the Surface of the Water Pool, u, for Microemulsions in Chloroform/Isooctane (2/1, v/v) for Various Values of woo
35 o c , [CTAB] = 0.075M
35 1Odko,
5 IO 153 I5 290 20 475 25 848 30 1225 (I
R, and
R,, 8, a, A2
s-I
N
Wn
[CTAB] = 0.075 M, [CB] = 0.77 M
[CTAB] = 0.15 M
1O"k0,
N
R,,
s-'
A
a,
ti2 N 65
1.47 1.54 1.70 1.69 1.81 u
23.3 32.5 41.9 54.4 65.2
44.9 149 45.9 289 46.4 479 43.9 803 43.6 1107
1.54 1.58 1.65 1.73 1.79
25 "C, [CTAB] = 0.15 M, [CB] = 0.77M
25 OC,
oc,
23.1 32.5 42.0 53.5 63.0
45.2 45.9 46.3 44.7 45.1
180
370 640 980
10-8k,, 1O+ko. M'' s-I s-' R,, 8, 4.3 1.26 14.6 1.40 24.6 2.4 1.9 1.48 35.3 3.0 1.54 46.2 2.5 1.59 57.1
a,
ti2 N
41.0 103 42.4 275 42.3 530 42.0 830 41.8 1320
1o-sk,, 10+ko, M-I s-' s-' 10.2 1.28 4.4 1.42 3.8 1.45 3.0 1.56 2.2 1.61
R,, A 17.0 28.4 39.8 50.4 63.1
U.
A2
35.1 36.8 37.5 38.5 37.9
have been calculated from eqs IO and 1 I , respectively.
t
- 3
t
0 10 20 30 10 Figure 1. Proton chemical shift relative to TMS, 6" (ppm), as a function of wo arising ( A ) from water with ( 0 )and without (0)TMA and (B) from the -CH2Br methylene group of CB ( 0 )and from the -Nt(CH3)3 group of CTAB (0).
R,, and for the spherical surface area, u, occupied by each surfactant ion at the water pool surface can be obtained from simple geometrical considerations as
Rw =
[
3N(woV,
+ VBr-)
4a D
1
'I3
(10)
= 4aRW2/N
(11)
where R, is in A, D is in A2, and V, and VBr- represent the molecular volume of a water molecule ( V , = 29.9 A3 at 25 "C) and the partial molecular volume of the surfactant counterion Br( VBr- = 49.3 A3 at 25 0C),2' respectively. If an additive with partial molecular volume V, in water is solubilized in the water pool, the expression for R , becomes
where w'is the molar ratio [additive]/[CTAB] andfis the fraction of the additive in the water pool. This quantity depends on the partition coefficients of the additive between the water core, the surfactant layer, and the bulk of the microemulsion. Of course, f < 1 if only part of the additive is dissolved in the water pools. In TMA-containing microemulsions eq 12 was used with V, = VrMA= 131 A3.22
Results N M R Studies. Figure IA shows the 'H NMR chemical shift of water in the water pools of reverse CTAB micelles in chloroform/isooctane (2/ I v/v) as a function of wo. Compared with bulk water (having a chemical shift at around 4.8 ppm), the resonance of the water pool protons is shifted toward higher fields, with a tendency to reach a plateau at wo values around 30-40 in agreement with results for CTAB/chloroform/water microemulsion^.^ Similar findings (chemical shift values and curve shapes) have also been reported for reverse micelles made with an anionic surfactant, AOT?3 or with another cationic surfactant, ~
~~
~
(21) Millero, F J. Chem. Reo. 1971, 71, 147. (22) Cabani, S.;Conti, G.; Lepori, L. J . Phys. Chem. 1974, 78, 1030.
Lo b IO8 M-' s-I). Recall that in such systems interdroplet interactions are attractive and phase separation usually takes place when the temperature is increased. The increase of N with T may have the same origin as the increase of the attractive interactions between droplets. The dynamic information should be considered together with the data obtained from electrical conductivity experiments. These studies confirm the conclusion reached earlier,25a26that electrical percolation occurs only when k, becomes larger than (1-2) X IO9 M-' s-'. Thus, these results add further proof to the generality (27) Israelachvili, J . N.; Mitchell, D. 1.; Ninham, B. W. J . Chem. Soc., Faraday Trans. 2 1976, 72, 1 5 2 5 .
Lang et al. of this conclusion in w/o microemulsion. They also support the hypothesis that above the percolation threshold the conductivity is mainly due to the motion of counterions, through water channels or droplet fusion, rather than to hopping of surfactant ions from one droplet to another in droplet clusters. At this stage it must be emphasized that the presence of CB in sufficient amount (CcB > 0.6 M) promotes percolation simply by reducing the overall oil penetration in the surfactant layer, resulting in an increased fluidity and a greater easiness of this layer for "opening" upon collision with another droplet. I f we now consider the effect of TMA, it must be noticed first that the determination of the surfactant aggregation number N is independent of the value of the partition coefficient of the additive. Therefore, the decrease of N found upon TMA addition to the microemulsion (see Table 111) indicates that TMA affects the droplet structure. However, the fraction of additive solubilized in the water pool must be known for calculating R, and u. In Table I11 we have considered the two limiting cases, where TMA is assumed to be completely solubilized or insoluble in the water pools cf= 1 or 0 in eq 12, respectively). The comparison of the results obtained with and without TMA shows a decrease of R, and an increase of u regardless of whether f = 0 or 1, upon increasing temperature. This confirms the effect of TMA on the structure of the droplets. The increase of u may be due to a partial solubilization of TMA into the droplet surfactant layer; TMA would then act as a cosurfactant. This permits to explain the decrease of N upon TMA additions since then more surface is available to form the interfacial layer between water and oil, and consequently the droplet size decreases. It must be emphasized, however, that the fluorescence method cannot be used to measure the partition coefficient of TMA between water pool, surfactant layer, and bulk. Nevertheless, it will be shown in a following paperi2 that the data reported here can be used to explain the variation of droplet concentration noted when CB and TMA are simultaneously added to such w/o microemulsions. Indeed, the reaction taking place between CB and TMA leads to new CTAB molecules and therefore to more water droplets. Conclusion
The size of reverse micelles in water/CTAB/chloroform/isooctane ( 2 / l , v/v) w/o microemulsions increases with the [water]/[CTAB] molar ratio as in other w/o microemulsions, but no exchange of material between droplets was detected on the fluorescence time scale ( E 1 ps). The addition of cetyl bromide at concentrations above 0.6 M resulted in a large increase of droplet size and the occurrence of fast exchange of material. Electrical percolation was also observed in the presence of this additive. The present study confirms our previous conclusion that electrical percolation takes place only in those w/o microemulsions where the rate constant for droplet collisions with exchange of material is larger than about (1-2) X IO9 M-I s-l.
