Langmuir 1996,11, 3347-3350
3347
Mixed Micelles of the Dimeric Surfactant Ethanediyl-1,2-bis(dodecyldimethylammoniumbromide) and Its Corresponding Monomer, the DodecyltrimethylammoniumBromide: A Neutron Scattering Study F. Schosseler Laboratoire d'Ultrasons et de Dynamique des Fluides Complexes, URA 851, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France
0. Anthony, G. Beinert, and R. Zana* Institut Charles Sadron (CRM-EAHP), CNRS, 6 rue Boussingault, 67083 Strasbourg Cedex, France Received February 10,1995@ Mixed micelles ofthe dimericsurfactant ethanediyl-1,2-bis(dodecyldimethylammoniumbromide),referred to as 12-2-12,and of its corresponding monomer dodecyltrimethylammonium bromide (DTAB)have been studied by small angle neutron scattering using a contrast variation method, in order to obtain information
on the localization of DTAB in the threadlike micelles present in the mixed solutions at low DTAB mole fraction. The two surfactants are found to mix intimately, with no evidence for a preferential localization of the spherical micelle-forming DTAB at the hemispherical endcaps of the threadlike micelles. Increasing DTAB mole fraction results in a decrease of the overall rigidity and mean length of the micelles. An appropriate normalization of the scattering curves shows that this decreasing rigidity weakens the correlations between the micelles and suggests that this phenomenon is linked to a transition from a solution with some orientational order to a disordered solution.
Introduction In a recent study using tranmission electron microscopy at cryogenic temperature (cryo-TEM) it was shown' that the aqueous solutions of the dimeric surfactant, ethanediyl1,2-bis(dodecyldimethyla"onium bromide), referred to as 12-2-12below, containgiant threadlike micelles at fairly low concentrations, as low as 1%. Recall that the corresponding monomeric surfactant dodecyltrimethylammonium bromide (DTAB) forms only spherical micelles even at much higher concentrations andlor ionic The presence of threadlike micelles in 12-212 solutions explains the peculiar rheological behavior of these solution^.^ It was also observed that the addition of DTAB to a solution of 12-2-12 giant micelles resulted in the transformation of the threadlike micelles into mixed DTABI 12-2-12 spherical micelles, already at a DTAB molar fraction as low as 0.30.' The question which then arose concerned the distribution of the spherical micelle-forming DTAB in the threadlike micelles, prior to their transformation into spherical micelles. Would DTAB be mostly located in the hemispherical end-caps of the threadlike micelles (segregation)or would it be distributed somewhat uniformly between the end-caps and the cylindrical body of these micelles? Recall that segregation has been shown5 to occur in the disklike aggregates present in the lyotropic nematic phase encountered in the phase diagram of the potassium laurate-decanol-water system. The decanol,
* To whom correspondence should be addressed.
ACSAbstracts, August 15,1995. (1)Zana, R.; Talmon, Y. Nature 1093,362,228. (2) Candau, S.J.;Hirsch, E.; Zana, R. J.Phys. (Paris) 1984,45,1263. (3) Ozeki, S.;Ikeda, S.J. J. Colloid Interface Sei. 1982,87, 424. (4)Kern, F.;Lequeux,F.; Zana, R.; Candau, S.J. Langmuir 1994, 10. 1714. ( 5 ) Hendriks, Y.; Charvolin, J.; Rawiso, M. J.Colloid Interface Sci. 1984,100,597. EY Abstract publishedinAdvance
which has a smaller surface area per head group than potassium laurate, is preferentially located in the disk body ofthe aggregates rather than distributed uniformly. In an attempt to answer this question, we have performed a small angle neutron scattering (SANS)study of the mixed systems DTABf12-2-12 using the variable contrast method and a sample of DTAB with a fully deuteriated alkyl chain. The results do not support a preferential location of DTAB at the thread end-caps but suggest a rather uniform distribution of that surfactant throughout the micelles.
Experimental Section (1) Materials. The sample of 12-2-12 was prepared as described previously6 and purified by three recrystallizations from ethyl acetate-ethanol mixtures, followed by chromatography over silica gel (silica gel 60 from Merck, Germany). A small volume of a concentrated 12-2-12solution in ethanol was used to carry the surfactantover the gel and ethyl acetatelethanol mixtures ofincreasing ethanolcontent were used for the elution. The elemental analysis revealed that the purest fractions were collected for ethanol contents between 20 and 50%(viv). The sample of deuteriated DTAB (DTAB-d)was prepared by quaternization of trimethylamine (Aldrich) by ClzDzsBr (Cambridge Isotope Lab., USA) in dry ethanol for 48 h under reflux (5 "C). The deuterium oxidewas purchased fromEuriso-Top(France). (2) Methods. The distribution of DTAB in the threadlike micellescan be most easily obtained by using the SANS technique and matching the contrast between the solvent and the dimeric surfactant 12-2-12. This can be done by adjustingthe D20/H20 composition of the solvent. SANS experiments were performed at Laboratoire LBon Brillouin (Laboratoire commun CEA-CNRS) on spectrometer PACE. The wavelength of the incident neutrons and the distance between detector and sample were set respectively to 6.5 A and (6)Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1091, 7, 1702.
0743-7463/95/2411-3347$09.00/00 1995 American Chemical Society
Schosseler et al.
3348 Langmuir, Vol. 11, No. 9, 1995 Table 1. Scattering Lengths and Partial Molar Volumes for the Different Components and Their Constituting Groups chemical group M v b or molecule (@mol) (cmVmo1) cm) 0.263 138.9 81.3 N+(CH& Br215 24.7 C12D25 194 296 25.0 DTAB-d 332.9 N+(CH& BrC12H25 (CHd2 12-2-12
123.9 169 28 613.8 18 20
54.6 215 32-34 572 18 18
0.15
1.915
3 m, thus allowing scattering wavevectors in the range < (A-1) < 10-1. Standard data treatment was performed and involved incoherent scattering subtraction and normalization by the intensity scattered from 1mm H20. Incoherent scattering level for each sample was obtained by assuming a one-to-one correspondence with the transmission value of the samples and using an interpolation through the data collected for a set of D20/H20 mixture^.^ The normalized data were put on an absolute scale by using the value 0.872 cm-l for the incoherent scattering cross-section of 1 mm HzO.* Samples thickness was 1mm and temperature was regulated t o 20 & 0.5 "C. In a first step, the composition of the solvent matching the contrast ofthe 12-2-12surfactant was determined by measuring the intensity Z(q) scattered from solutions with constant surfactant concentration (0.02 g/cm3, 32.6 mM) and varying solvent composition. The DzO volume fractionXD was given the values 0,0.08,0.12,0.20,0.30, and 0.40. Then, for each q value, P2(q.) was plotted against XD and the intercept with the X axis was ~ b t a i n e d .In ~ this manner the meanvaluexo = 0.056 =k 0.04was found by averaging the values of the intercepts for q values in the range < q (A-1) < 6 x 10-2 where they are less sensitive to background subtraction. This meanvalue is in good agreement with the estimation 0.059 calculated from the scattering lengths of the constituting groups10 and assuming additivity of their partial molar volumes.11 These quantities are listed in Table 1 and the agreement between measured and calculated values of XOshows their reliability. In a second step, new Solutions with a constant 12-2-12 concentration C2 = 65.2 mM (0.04 g/cm3)and increasing molar concentration C1 of the DTAB-d were prepared, the solvent composition being k e d a t the contrast matching value XO. The DTAB-d mole fraction Y = C1/(C1+ C2)took thevalues 0,0.0563, 0.101, 0.178, and 0.244 with the total surfactant concentration being given by Ct = C1+ C2. One sample containing only DTAB-d (C1=63.4 mM) was also studied for the purpose of comparison. q
Results and Discussion Figure 1shows the evolution of the scattering intensity vs q plots for the solutions with increasing DTABd mole fraction. For comparison, Figure 2 gives the intensities scattered from 12-2-12 in D2O (Figure 2a) and from DTAB-d in D20/H20 when XD= XO(Figure 2b). The scattering intensity in Figure 1 arises only from the DTAB-d since the volume fraction of DzO isX0. Thus, as expected, for the solution with Y = 0 the scattering intensity is flat and fluctuates about zero indicating a good contrast matching. At full contrast, in pure D20, the same solution (Figure 2a) shows a very strong and 0.035 A-1 that narrow scattering maximum at q* (7)Schosseler, F.; Skouri, R.; Munch, J. P.; Candau, S. J. J. Phys. 11 1994,4,1221. (8)Ragnetti, M.; Geiser, D.; Hbcker, H.; Oberthur, R. C. Makromol. Chem. 1986,186,1701. (9)Zana, R.; Picot. C.: Duplessix. R. J. Colloid Interface Sci. 1983, 93, 43. (lO)Marshall, W.; Lovesey, S. W. Theory of Thermal Neutron Scattering; Oxford Clarendon: Cambridge, 1971. (11)Zana, R. J. Polym. Sci., Polym. Phys. Ed. 1980,18,121.
. .* *
0.1
*
'E
x
1
. .
3 0.05 n e
0.717 -1.36 -0.164 -1.44 --0.169
I
0 0 0
Y
0.11 4
*.
0.06 0.22 0.32
0 -0.05
0
0.03
0.06 0.09 q (A-9
0.12
Figure.1. Evolution of scattering intensities for mixed micelles with increasing DTAB mole fraction. The contrast of the 122-12 surfactant is matched by choosing the appropriate composition of the mixed H20/D20 solvent. 25
I
00
h
A
2 0 1L
0
0
O
0.03
0.06
0.09
0.12
q (A-9
n
'€
W
n U
0.2
0
1 0
Ooo0
0.03
0.06 0.09 9 (A-9
3
0.12
Figure 2. Scattering intensities from the pure 12-2-12 threadlike micelles in DzO (a)and from the pure DTAB spherical micelles in HzO/D20 (b).
indicates strong electrostatic correlations in these charged threadlike micelles. In fact, q* has been shown12to vary like Czm in close analogy with the well-known behavior of salt-free polyelectrolytes o l ~ t i o n s . ~Closer ~ J ~ inspection of that curve reveals, for q RZ 2q*,a weak second maximum that is discussed in details in ref 12. On the other hand, pure DTAB micellar solutions are characterized by a broad scattering intensity maximum located at q* 0.06 A-1 and characteristic of the repulsive interactions between charged spherical objects.15 Comparison of the data in Figures 1and 2 shows that the intensities scattered from mixed micellesare markedly different from that scattered from the pure DTAB micellar solution, even when Y = 0.24, and are much more similar (12)Schmitt,V.; Schosseler,F.;Lequeux,F. Submittedfor publication in Europhys. Lett. (13)de Gennes, P. G.; Pincus, P.; Velasco, R.; Brochard, F. J. Phys.
(Paris) 1976,37, 1461. (14)Williams, C. E.;Nierlich, M.; Cotton, J. P.; Jannink, G.; B o d , F.; Daoud, M.; Famoux, B.; Picot, C.; de Gennes, P. G.; Rinaudo, M.; Moan, M.; Wolff, C. J. Polym. Sci., Polym. Phys. Ed. 1979,17,379. (15)Chevalier. Y.:Zemb. T. ReD. Prop. - Phvs. . 1990.53. 279. and
references therein.
Dimeric Surfactant Mixed Micelles
Langmuir, Vol. 11, No. 9, 1995 3349
Table 2. Parameters of the Equivalent Monomers Mixtures Corresponding to the Different Compositions, Y, Used To Normalize the Scattering Curves As Described in the Text
65.2 65.2 65.2 65.2 65.2 0
0
0.0563 0.101 0.178 0.244 1
65.2 69.1 72.5 79.3 86.3 63.4
0
0.0290 0.0530 0.0979 0.1390 1.0000
130.4 134.3 137.7 144.5 151.5 63.4
286.0 286.3 286.5 287.0 287.4 296.0
-0.720 1.01 10-3 0.600 1.72 2.75 25.0
970 0.639 1.96 6.36 12.6 667
0.2
n
n
OD
0 0 0 0.10
A
8 0.15
B c
y b
0.1
0.18 0.24 . I m A
0 0
ea0.05 n
.z
VY
O t '
0
'
I
"
0.03
I
'
0.06
"
0.09
'
"
0.12
9 (A-9
0
0.03
0.06
0.09
0.12
9 (k')
Figure 3. Scattering intensities from the mixed micelles, normalized by the DTAB molar concentration.
to the intensity scattered from the pure 12-2-12 micelles. These results suggest that the DTAB molecules are distributed randomly in the mixed micelles and decorate them. This can be checked by plotting the scattering intensities normalized by the monomer concentration C1 as shown in Figure 3. The resulting curves are seen to be identical in a first approximation, the slight increase in the total surfactant concentration Ct being responsible for both the shift in the peak position to higher q values and the slight increase of Z(q*).12 On the basis of this picture, an interesting normalization of the scattering curves can be performed by considering each dimer as equivalent to two hydrogenated monomers characterized by a scattering length b1,H = bd2 and a partial molar volume U1,H = ud2, where b2 and u2 are the corresponding quantities for the dimer (Table 1). The deuterated monomers are characterized in the same way by b l p and U1,D. This approximation amounts to neglecting the existing correlations due to the linkage of two monomers into one dimer and small differences in polar head groups. Thus it should be justified for small enough q values in the range investigated here. Ifthese monomers are considered to be randomly distributed, then the average contrast factor can be calculated as
where Y' = Yl(2 - Y) is the molar fraction of deuterated monomers and uy = U1,DY' u&Y')the averagepartial molar volume. Here by is the average scattering length of the monomer mixture defined as
+
In eq 1, bo and uo correspond to the scattering length and molar volume of the solvent. The structure factor of this mixture of monomers can then be defined as S(q) I(q)lKy, the total concentration of equivalent monomers being Ce = C1 2Cz. Figure 4 shows the normalized S(q)/Ceplots based on the values of the parameters listed in Table 2. For the pure 12-2-12 sample, the scattering length of the solvent is that of pure
+
Figure 4. Normalized structure factor for mixed micelles with increasing molar fraction of DTAB. Normalization is described in the text.
D2O (Table l),while the other samples the value bo = -0.0502 x cm is used (XD= XO). Figure 4 illustrates strikingly the weakening, upon addition of DTAB, of the correlations in the surfactant mixture, in contrast with Figures 1 and 2 where this phenomenon is masked by contrast variation effects. The intensity of the peak is seen to decrease progressively as the mixture becomes richer in DTAB. However its position remains almost the same, taking into account the slight increase in C, (Table 2). A clear change in the peak position is visible only for the pure DTAB solution. This shiR to higher q values would even be larger for a C, value in the range of the concentrations studied in the other samples (Table 2).15 The Y range of existence of the threadlike micelles is consistent with the results obtained previously by cryoTEM experiments' that have shown that the threadlike micelles disappear in the range between Y = 0.14 and Y = 0.30 when the total surfactant concentration is kept constant (1.7%(w/w)). Below that range, the meanlength of the micelles has been observed to decrease progressively.' At that point, it was not clear which mechanism was responsible for that behavior. The present experiments favor an even distribution of the DTAB surfactant in the micelles. Thus the relevant mechanism seems to be an overall decrease of the rigidity of the micelles due to the intimate mixing of the surfactants rather than a lowering of the free energy needed to build the end-caps, associated with a preferential location of the DTAB in these end-caps. This is probably due to the entropic penalty implied by such a confinement of the DTAB molecules in a very small volume fraction of the long cylinders. An additional information obtained through the SANS experiments concerns the evolution, upon addition of DTAB, of the correlations between the mixed micelles. The amplitude of the strong peak observed in the pure 12-2-12 micellar solution is decreasing smoothly with increasing molar fraction of DTAB. Recent SANS experimentsl2on 12-2-12 micelles in a shear flow have supported the view that the correlations between the rigid threadlike micelles may be at least partly orientational rather than purely electrostatic. The same conclusion was reached
3350 Langmuir, Vol. 11, No. 9, 1995
also for rigid polyelectrolyte solutions.16J7 The present results are consistent with that picture since they show a weakening of the correlations with the decrease of the overall rigidity. That effect is observed even when the micelles remain long enough to be entangled and would then be associated with a transition from a solution with some nematic order to a disordered system. The transition would then be related to a change of micelle rigidity at nearly constant concentration instead of being induced (16)Barrat, J. L.; Joanny, J. F.; J.Phys. IZ 1994,4, 1089. (17)Witten, T. A.; Pincus, P. J.Phys. 22 1994,4, 1103.
Schosseler et al. by a change in concentration at fixed rigidity as originally proposed for polyelectrolyte s01utions.l~
Acknowledgment. It is a pleasure to thank S. J. Candau for the stimulating discussion that started this study and for helpful comments. The authors acknowledge the help of Mrs. H. Levy in the purification of the 12-2-12 sample. The local contact at Saclay was J. P. Cotton, and his help at the starting up of the SANS experiments is gratefully acknowledged. LA950102G
