J. Phys. Chem. 1988, 92, 3569-3573
3569
Comparative Investigation of Cetyltrimethylammonium Bromide Micelles in Water and Formamide by Nuclear Magnetic Relaxation A. Belmajdoub, K. EIBayed, J. Brondeau, D. Canet,* Laboratoire de MPthodologie R M N , U.A. CNRS No. 406-LESOC, UniversitP de Nancy I , BP 239, 54506 Vandoeuvre-les- Nancy (cedex). France
I. Rico, and A. Lattes Laboratoire des IMRCP, U.A. CNRS No. 470, Universite Paul Sabatier, 118 route de Narbonne. 31062 Toulouse (cedexj, France (Received: September 22, 1987; In Final Form: January 2, 1988)
Molecular motions in the system cetyltrimethylammonium bromide/formamide (CTAB/FA) have been studied by proton, nitrogen-14, and carbon-13 longitudinal relaxation at different frequencies, with reference to the well-known system CTAB/D20, reinvestigated here in conditions allowing a straightforward comparison. These measurements demonstrate the presence of spherical micelles at 60 "C (above the Krafft point of the CTAB/FA system). Nitrogen-14 measurements of the surfactant polar head yield a correlation time T~ (of ca. 7.5 ns in CTAB/FA) associated with the slow motion affecting a micellized surfactant molecule. With the assumption of a micelle radius identical with the extended chain length, this leads to a value of 5 X lo-' cm2 s-l for the lateral diffusion coefficient. Two important parameters can be derived from carbon-13 relaxation data by making use of an appropriate T~ value deduced from proton data: a correlation time characteristic of a fast local motion and an order parameter at the relevant position in the hydrocarbon chain; similar parameters can be deduced from the frequency dependence of 14Nlongitudinal relaxation rates. From all those parameters the following structural and dynamical picture of micelles in formamide emerges: micelles are spherical, as in water and very likely of the same size. Micelle interior is slightly more organized in formamide than in water, whereas significantly less structure is found at the micelle surface.
Introduction
Micelle formation in nonaqueous solvents has attracted only little attention as compared with the large number of extensive N M R studies dealing with micellar systems in water.',* Aggregation in nonaqueous medium has been studied at relatively surface tension,6s7 low temperature (ca. 25 "C) by cond~ctance,~-~ and refractive i n d e ~ . ~In. spite ~ of breaks appearing in plots of these quantities versus concentration, Almgren et ai. recently proved the absence of micelles in a sodium dodecyl sulfate/formamide system.* Das et al. later confirmed this feature by self-diffusion measurements on systems where polar solvents, particularly formamide, replaces water.9 Since then, it has been suggested that those studies were carried out at temperatures below the Krafft point,I0 which is the lowest temperature compatible with the formation of micellar aggregates. We present here a nuclear magnetic relaxation investigation of CTAB/FA and CTAB/D20 systems, studied at the same temperature (60 "C) chosen well above their Krafft points, which is about 43 "C for CTAB/FA.'O Proton, nitrogen-14, and carbon-1 3 longitudinal relaxation times TI have been determined at different frequencies, thus allowing a probe of slow motions occurring outside the extreme narrowing region. Multifield proton magnetic relaxation measurements provide correlation time(s) associated with the slow motion(s) affecting a surfactant molecule engaged in micellar aggregates," whereas carbon- 13 spin-lattice relaxation time(s) (1) Siiderman, 0.; Canet, D.; Carnali, J.; Henriksson, U.; Nery, H.; Walderhaug, H.; Warnheim, T. In Microemulsion Sysrems; Rosano, H. L., Clausse, M., Eds.; Marcel Dekker: New York, 1987; Vol. 4, p 145. (2) Chachaty, C. Prog. N M R Spectrosc. 1987, 19, 183. (3) Gopal, R.; Singh, J. R. J. Phys. Chem. 1973, 74, 554. (4) Gopal, R.; Singh, J. R. Kolloid Z. Z. Polym. 1970, 239, 699. (5) Singh, H. N.; Saleem, S. M.; Singh, R. P.; Birdi, K. S.J. Phys. Chem. 1980, 84, 2191. (6) Ray, A. J. A m . Chem. Soc. 1969, 91, 6511. (7) Ray, A. Nature (London) 1971, 231, 313. (8) Almgren, M.; Swarup, S.; Lofroth, J. E. J . Phys. Chem. 1985,89,4621. (9) (a) Das, K. P.; Ceglie, A.; Monduzzi, M.; SMerman, 0.;Lindman, B. Prog. Colloid Polym. Sci. 1987, 73, 167. (b) Das, K. P.; Ceglie, A.; Lindman, B. J. Phys. Chem. 1987, 91, 2938. (IO) Rico, I.; Lattes, A. J . Phys. Chem. 1986, 90, 5870.
0022-3654/88/2092-3569$01.50/0
T I of individual carbons can be used to obtain a detailed picture of local motions along the alkyl chain.I2 Finally, we can take advantage of nitrogen-14 for probing the mobility at the polar head 1 e ~ e l . I ~ In principle the correlation time associated with the slow motion permits us to compare micellar size and shape in formamide and in water. On the other hand, local parameters, Le., order parameters and correlation times describing fast motions, should yield comparative information about compactness and hindered motions inside the micelle, thus providing some insight into its organization. Moreover, the major differences between the two systems are expected to appear at the micelle surface, since the two solvents do not presumably interact in the same way with surfactant polar heads and counterions. Local dynamical parameters extracted from nitrogen-14 data should shed some light on this latter point. Experimental Section
Cetyltrimethylammonium bromide (CTAB) was purchased from Fluka (98% purity) and was recrystallized several times from ethanol. Formamide (Merck puriss grade) was used after further water elimination (to less than 0.5%). Ionic impurities were removed from DzO (CEA Saclay, France) by strongly basic anion-exchanger and strongly acidic cation-exchanger resins. At the outset, some N M R measurements were carried out at room temperature ( 2 5 "C) to ensure that no micelle is formed in these conditions.14 All measurements concerning micellar media were performed at 60 "C and at two different concentrations, 0.2 and 0.4 M for the CTAB/D20 system, 0.5 and 1 M for the CTAB/FA system. The following instruments were used: a highly modified Bruker HX-90 with multifield and multifrequency capabilities allowing us to investigate the range 3.4-90 (11) Belmajdoub, A,; Diter, B.; Canet, D. Chem. Phys. Lett. 1986, 131, 426. (12) Walderhaug, H.; Siiderman, 0.;Stilbs, P. J . Phys. Chem. 1984,88, 1655. (13) Henriksson, U.; Odberg, L.; Eriksson, J. C.; Westman, L. J . Phys. Chem. 1977, 81, 76. (14) Belmajdoub, A.; Marchal, J. P.; Canet, D.; Rico, I.; Lattes, A. New J . Chem. 1987, 11, 415.
0 1988 American Chemical Society
3570
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988
Belmajdoub et al.
TABLE I: Parameters, Associated with Micelle Overall Motion and/or Surfactant Polar Head Dynamics, Extracted from Frequency-Dependent Proton, Nitrogen-14, and Carbon-13 Longitudinal Relaxation Times in CTAB/D*O and in CTAB/FA Systems at 60 O C concn, M T.,” ns T , , ~ns Sb ns S‘ ns CTAB/FA CTAB/FA CTAB/D,O CTAB/D,O
0.5 1 0.2 0.4
2.44 f 0.04 2.05 f 0.05 2.60 f 0.04 2.51 f 0.04
7.6 f 0.7 7.5 f 0.6 4.8 f 0.5 5.3 f 0.5
0.089 0.160 0.180 0.195
f 0.006
0.069 f 0.009 0.068 f 0.008 0.060 f 0.004 0.058 f 0.004
f 0.008 f 0.008 f 0.005
0.15 f 0.009 0.15 f 0.009 0.13 f 0.005 0.12 f 0.005
0.012 f 0.0002 0.014 f 0.0002 0.013 f 0.0005 0.012 f 0.0005
“From proton data. *From nitrogen-I4 data. ‘From carbon-I3 data (polar head group N(CH,),).
MHz for proton resonance, a modified Bruker WP-200 spectrometer interfaced to a Nicolet 1180 computer, and a Bruker AM-400 spectrometer (Service commun de R M N de 1’UniversitC de Nancy I). Spin-lattice relaxation times T I were measured from 3.4 to 200 MHz for proton, from 2 to 28 M H z for nitrogen-14, and at two frequencies, 50 and 100 MHz, for carbon-13 N M R . Classical inversion-recovery (IH, I4N) or fast inversion-recovery (I3C) was employed throughout. All T l values have been deduced from a three-parameter fit. Because CTAB I4N resonances are rather sharp and consequently are expected to be affected by inhomogeneity of the main magnetic field, no attempt was made to extract T2from their line widths. Analysis of Experimental Data
All longitudinal relaxation rates to be reported in this section will be handled according to the “two-step” model of Wennerstrom et a l l 5 This model, from which ar_e derived simple expressions for the reduced spectral densities J(w) entering relaxation parameters, has been found to be efficient in all investigations of micellar systems.I6-’* It consists of a separation of two types of motions: (1) a slow motion with correlation time(s) of the order of nanoseconds (or more in the case of elongated aggregates), corresponding to the reorientation of a local director; (2) a faster motion that takes place about this local director and whose correlation time is in the picosecond range; this motion is due to bond isomerization, torsion, libration, and possibly other deformations. In the case of spherical micelles the following expression holds for reduced spectral densities:
J ( w ) = (1 - s2)(27f) + s2[27,/(1
+ u’T,’)]
I
,
,
,
, , , ,
,
10 wo ‘h $ t ( M l h ) Figure 1. CTAB proton longitudinal relaxation rates as a function of measurement frequency, at 25 OC: (A) CTAB/FA (0.1 M); (A) CTAB/D,O (0.1 M); dotted curve, recalculated from a two-step fit; solid
2
”
curve, obtained from the three-step model.
(1)
2 10 d & %n,i where 7f is the correlation time associated with the fast local Figure 2. Proton longitudinal relaxation rates of the CTAB/D,O system motion, 7,is the correlation time for the slow motion, and S is as a function of measurement frequency, at 60 OC: (A)0.4 M; (A)0.2 an order parameter describing the orientational restriction of the M. Theoretical curves have been recalculated from the two-step model. relaxation vector with respect to the local director. The relaxation vector is meant as (1) the C-H bond for carbon relaxation and where x is the nitrogen- 14 quadrupolar coupling constant taken (2) the major direction of the field gradient tensor, assumed to as 116 KHz,” and wN/(2a) is the nitrogen Larmor frequency. be of axial symmetry, for nitrogen-14 relaxation. R,(’H) = A1 + Bl[1/(1 + wH27,2) 4/(1 + WH27,*)] (4) Expression of the relaxation rates relevant to the present investigation is given as a function of reduced spectral d e n s i t i e ~ : ’ ~ . ~ ~A i and B1contain contributions from the different dipolar in-
+
where N is the number of protons directly bonded to the considered carbon, and wH/(2a) and o c / ( 2 r ) are the Larmor frequencies of proton and carbon-13, respectively. The other symbols have their usual meaning.
(15) Wennerstrbm, H.; Lindman, B.; SGderman, 0.;Drankenberg, T.: Rosenholm, J. B. J . A m . Chem. Soc. 1979, 101, 6860. (16) Nery, H.; SGderman, 0.; Canet, D.; Walderhaug, H.; Lindman, B. J . Phys. Chem. 1986, 90, 5802, and references therein. (17) Siiderman, 0.; Henriksson, U.; Olsson, U. J . Phys. Chem. 1987, 91, 116. ....
(18) Ahlnas, T.; Sijderman, 0.;Hjelm, C.; Lindman, B. J . Phys. Chem. 1983, 87, 822. (19) Doddrell, D.; Glushko, V.; Allerhand, A. J . Chem. Phys. 1972, 56,
3683. (20) Abragam, A. The Principles of Nuclear Magnezism: Clarendon: Oxford, 1961.
teractions governing proton relaxation (inter- and intramolecular). Each of them is associated with a specific set of local parameters ( T ~ and S) that would appear in A I and B,. As a consequence 7, is the sole parameter that can be deduced from a frequency analysis of proton data.” CTAB proton T Idata pertaining to the main methylene signal and obtained at room temperature (25 “C) are shown in Figure 1 as a function of the measurement frequency. Theoretical curves are recalculated according to eq 4. For the CTAB/FA system, no measurement was made below 9.12 MHz due to the vicinity of the weak CTAB signal and of the large FA resonance. As far as the CTAB/D20 system is concerned, the existence of micelles is undoubted from the overall variation with frequency. As is evident from Figure 1, a better fit of the data is obtained with the “three-step” model. The latter is an extension of the “two-step” model suitable for nonspherical objects.21bIt consequently involves two correlation times for describing the slow motion: 7s N 1.2 ~~
~
(21) (a) Pratum, T. K.; Klein, M. P. J . Magn. Reson. 1983, 53, 473. (b)
Soderman, 0.; Walderhaug, H.; Henriksson, U.; Stilbs, P. J . Phys. Chem. 1985, 89, 3693.
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3571
CTAB Micelles in Water and Formamide
TABLE 11: CTAB Carbon-13 Spin-Lattice Relaxation Times TI (seconds) Measured to Two Frequencies (50 and 100 MHz) and at 60 O C " CTAB/FA (0.5 M) CTAB/FA (1 M ) CTAB/DIO (0.2 M) CTAB/D,O (0.4 M ) 50 MHz 100 MHz 50 MHz 100 MHz 50 MHz 100 MHz 50 MHz 100 MHz 0.83 1.14 1.14 0.99 0.80 0.7 1 1.09 N(CH3)3 0.77
Cl c 2
c3
c4-12 c13 c 1 4
c 1 5
CH3
0.54 0.55 0.56 0.65 0.60 0.61 0.91 1.58 2.44 4.27
0.51 0.53 0.56 0.57 0.56 0.56 0.76 1.57 2.37 4.23
0.98 0.96 1.02 1.09 1.03 1.01 1.42 2.31 3.07 4.45
1.o 1.1 1.1 0.91 1.o 1.02 1.35 2.13 3.1 4.5
0.56 0.61 0.65 0.61 0.65 0.67 1.o 1.4 2.2 4.2
0.86 0.87 0.90 0.99 0.95 0.93 1.31 2.30 3.14 4.45
0.55 0.58 0.64 0.62 0.65 0.68 0.95 1.48 2.2 4.2
1.1 1.o 1.o 1
.o
1.01 1.o 1.5 2.3 3.32 4.5
Numbering, given by subscripts, starts from polar head.
2 id 1" lb l%H,I Figure 3. Proton longitudinal relaxation rates of the CTAB/FA system as a function of measurement frequency, at 60 OC: ( 0 ) 1 M; (0) 0.5 M. Theoretical curves have been recalculated from the two-step model; X = 0.2 M.
a3
Is
t zf
30
'4
1
(pr'
14
!?
i
*
'(L---L---I~I (3 li tf 1b I Figure 5. Fast correlation times as a function of carbon position, at 60 'C. Top: in the CTAB/D20 system; (A)0.4 M; ( A ) 0.2 M. Bottom: i
j
in the CTAB/FA system; ( 0 ) 1 M; (0)0.5 M.
I
ai
1
'I
Figure 4. Order parameter profiles along the alkyl chain of micellized CTAB as deduced from I3C relaxation data, at 60 OC. Top: in water; (A) 0.4 M; ( A ) 0.2 M. Bottom: in formamide; ( 0 ) 1 M; (0) 0.5 M.
ns and s' N 7.3 ns. Those results confirm that micelles in water become rod-shaped at relatively high surfactant c o n ~ e n t r a t i o n . ~ ~ , ~ ~ Although data of the CTAB/FA system can be fitted according to eq 4 with a 7,value of ca. 1 ns, the existence of micelles at 25 "C is ruled out by the weakness of R , variation. Rather, one would invoke the presence of small short-lived aggregates. We present in Figures 2 and 3 similar proton T I measurements but carried out at 60 OC for the main methylene signal of the surfactant alkyl chain. We notice that the latter signal does not include either the resonance due to N(CH,), or that of the a-CH2, and, as a matter of fact, these results will be retained for analyzing motion and order along the aliphatic chain, with the exception of a-CH2. The curves shown in Figures 2 and 3 were recalculated according to eq 4. As can be inferred from the agreement with experimental (22) Ekwall, P.; Mandell, L.; Solyom, P. J . Colloid Interface Sci. 1971,
35, 519.
(23) Husson, R F.; Luzzati, V. J . Phys. Chem. 1964, 68, 3504.
3572
t
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988
1
Belmajdoub et al. where mic refers to surfactant molecules engaged in micelles, whose proportion is denoted by P. (Rl)obsdand (R)monare the measured relaxation rates in micellar and monomeric systems, respectively. Assuming that above the critical micellar concentration the monomer concentration can be approximated by the cmc itself,14we evaluate ( 1 - P) by cmc/C, where C is the total surfactant concentration.
is-1)
l1
501 "i 40
20
4 I 2
IL
10
* .~(MH,)
Figure 6. Nitrogen-14 longitudinal relaxation rates of CTAB/D,O as a function of the measurement frequency, at 60 OC: (A)0.4 M; (A)0.2 M. Theoretical curves have been recalculated from the two-step model.
Discussion Values of the slow correlation time 7, deduced from proton data strongly suggest the presence of spherical micelles in the CTAB/FA as well as in the CTAB/D,O systems, at all the concentrations investigated in this work. Because of its location close to the aggregate surface and because it stems from the sole quadrupolar mechanism, nitrogen-14 is a good candidate for probing dynamics at micellar interface. However, a large discrepancy can be noticed from Table I between the slow correlation times 7, deduced from 'H and I4N data, respectively. This discrepancy cannot arise from a misinterpretation of 14Ndata (due, for instance, to neglect of the asymmetry parameter) since P , is deduced from a frequency analysis. This latter analysis does not involve the magnitude of the considered relaxation mechanisms and only resorts to functions of the type 1 / ( 1 w27,*). This point has been considered in a previous article24and explained rather by the identification of the local direction: if it is assumed that the local director DL relative to the alkyl chain is tilted with respect to the normal N to the aggregate surface, it is reasonable to consider an additional motion of DL about N . As a consequence, 7 , deduced from proton data is shortened by this latter contribution. In assessment of the overall motion of a surfactant molecule, it is therefore recommended to rely on the 7 , value extracted from 14N relaxation rates. 1/7s can be decomposed into two contributions, one arising from the aggregate tumbling and one from lateral diffusion of the surfactant molecules around the micelle curved /'s /7tumbling + / 7 d ~ f f (6) The aggregate tumbling is described by the Debye-Stokes-Einstein equation:
+
I
di)
I
w "';do Figure 7. Nitrogen-14 longitudinal relaxation rates of CTAB/FA as a function of the measurement frequency, at 60 OC: ( 0 ) 1 M; (0)0.5 M. Theoretical curves have been recalculated from the two-step model. 2
data, the slow motion is indeed well described by a single exponential correlation function. This reveals the presence of spherical aggregates in both systems. The relevant values of the slow correlation time 7, in CTAB/D20 and in CTAB/FA systems are given in Table I. I3C relaxation data are gathered in Table 11. Provided that 7, is known, they enable us to derive the local parameters S and 7f values, presented in Figures 4 and 5 at each carbon site. The 7, value extracted from proton data has been used from /3-CH2 to the terminal methyl, whereas for N(CH3)3and a-CH2 we used 7, derived from 14N data. 14Nlongitudinal relaxation rates are shown in Figures 6 and 7. Again an excellent agreement is obtained between experimental points and the theoretical curve, recalculated according to eq 3 and 1. Relevant parameters are given in Table I: a slow correlation time 7, (different from the one deduced from proton data; , an order parameter see Discussion), a fast correlation time T ~ and S. It can be noticed that S and 7f, in contrast with 7,, depend on the value chosen for the quadrupole coupling constant and the asymmetry parameter. Before a discussion of the results, the question of the influence of monomers must be raised. The critical micelle concentration (cmc) for the CTAB/D20 system at 60 "C is around 2 mM.I4 Concentrations of the present study are well above the cmc; therefore, any contribution from CTAB monomer can be neglected. Conversely, the cmc of the CTAB/FA system at 60 "C is around 9.4 X low2M.'O9l4 This permits us to carry out a TI measurement in a monomer solution and subtract its contribution from relaxation parameters in micellar systems. This can be accomplished by using the relation (Rl)obsd = P(Rl)m~c+
-
p)(Rl)rnon
(5)
(7)
and the lateral diffusion correlation time is given by 7 d ~ f f = a2/(6Ddiff) (8) where 7 is the viscosity of the medium, a is the radius of a spherical micelle, assumed to be identical with the length of an all-trans A), and Ddlffis the lateral hydrocarbon ~ h a i n l (a ~ ,=~ 23.74 ~ diffusion coefficient. With a value of 9 equal to 1.02 and 0.52 CP measured at 60 "C for formamide and water, respectively, we obtain values for Ddlrfof ca. 5 X low7cm2 s-I (CTAB/FA) and 4X cm2 s-l (CTAB/D20). For the latter system, the value deduced for Ddlfffrom eq 8 is in good agreement with literature r e s ~ l t s . * ~Within ~ * ~ the hypothesis made, the value found for Ddlff in the CTAB/FA system appears quite reasonable and tends to reinforce the assumption of identical micellar radius for both systems (Le., the aliphatic chain length for an all-trans conformation). Therefore, one would conclude that micelle size and shape are very likely similar to formamide and in water. This is not necessarily in contradiction with the recent work of Das et al.,' who found at 25 "C very little organization, if any. This again emphasizes the role played by temperature in a system involving formamide as solvent. Conversely, Auvray et al. carried out X-ray scattering measurements on a CTAB/FA system at 50 "C and found that smaller spherical aggregates (with radius a E 9 %.) are formed in formamide at concentrations just above
(24) Belmajdoub, A,; Brondeau, J.; Boubel, J. C.; Canet, D. Chem. Phys. Lett. 1987, 140, 389. ( 2 5 ) Tandford, C. The Hydrophobic Effect; Wiley-Interscience: New York, 1973. (26) Hakemi, H.; Varanazi, P. P.; Tcheurekdjian, N. J . Phys. Chem. 1987, 91, '1 20. (27) Eriksson, P. 0.; Khan, A,; Lindblom, G . J . Phyr. Chem. 1982, 86, 387.
CTAB Micelles in Water and Formamide the cmc up to 2.5 times the cmc.28 At higher concentrations, relevant to the present work, interpretation of X-ray data becomes intractable, thus preventing a direct comparison with our results. It can be noticed that the TI (‘H) dispersion curve shown in Figure 3 (CTAB/FA system at 0.2 M, concentration in the range investigated by Auvray et al.) would corroborate their results: the weaker variation with frequency is indicative of smaller aggregates, although, because of experimental uncertainties, it did not prove possible to obtain a reliable value for T ~ . Order parameter and fast correlation time profiles along the hydrocarbon chain weakly depend on concentration but more significantly on the solvent. In all cases, as it is generally observed,’*l2J6those parameters decrease when going from the polar head toward the terminal methyl group according to an S-shaped curve, more or less accentuated. Order parameters are higher in FA than in DzO: this feature is more pronounced at the middle of the chain, and it almost disappears for the three terminal carbons. Surprisingly fast correlation times are lower in FA than in D 2 0 , although the difference is less marked than for order parameters. These latter observations, which would indicate more structure and more mobility for micelles in formamide, are apparently in contradiction; one would expect that an increased order results from more compactness or more hindrance, thus lowering local mobility. In fact, it must be realized that order parameters are rather structural parameters whereas correlation times are purely of dynamical nature. The present results can indeed be reconciled if we suppose that aliphatic chains are more rigid in FA micelles than in water micelles (different dielectric properties or solvent internal pressure29are capable of modifying conformational equilibrium), thus leading to higher order parameters. In the same time the stiffness of the chain would facilitate rotation of each surfactant molecule around its long axis. This latter motion, which is known to contribute to Tf, could explain a slightly lower value in FA than in D 2 0 . As stated above, nitrogen-14 data should lead to valuable information about micellar interface. The order parameter extracted (28) Auvray, X.;Petipas, C.; Anthore, R.; Rico, I.; Lattes, A.; Samii, A. A.; Savignac, A. Colloid Polym. Sci. 1987, 265, 925. (29) Dack, M. R. J . Chem. SOC.Reu. 1975, 4, 211.
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3513 from eq 3 describes the orientation of the C-N vector with respect to the normal to the aggregate surface since this direction is known to coincide with the major principal axis of the electric field gradient tensor.lg This order parameter is found to be much higher in water than in formamide at relatively small surfactant concentration, indicating an “hydrophobic-hydrophilic” interface less structured in formamide. However, it becomes similar when the amount of solvent decreases. These observations can be related to a larger bulkiness of formamide as compared with water. Considering the lateral diffusion coefficient, its higher value in formamide than in water is in agreement with a looser interface. As far as fast correlation times are concerned, they are seen to be identical, within experimental error, in water and in formamide. Therefore, local rotational mobility at the nitrogen level is solvent independent. Finally local parameters ( T and ~ S) extracted from I3C data of N(CH3)3are almost independent of both solvent and concentration. This means that they essentially reflect rotation of each methyl group around its symmetry axis. These rotations and their possible hindrance by gear effect are indeed expected to depend little on environment.
Conclusion This work provides consistent results concerning CTAB micelles; it can be postulated with some confidence that, at relatively high concentration, micelles in formamide are similar to micelles in water, spherical in shape with a radius close to the extended chain length. Only tiny differences are observed in the inner organization: for micelles in formamide, surfactant chains are slightly more organized and more mobile (presumably through rotation around their long axis). Larger differences appear concerning the “hydrophobic-hydrophilic” interface, that is, at the micelle surface: order parameter, sensed by the major direction of the field gradient tensor at the nitrogen level, is significantly weaker when formamide is used at relatively low surfactant concentration. Conversely, local mobility at the interface is not affected by the solvent. Acknowledgment. We thank Dr. C. Tondre for his help and advice. We are grateful to C N R S for financial support. Registry No. CTAB, 57-09-0; FA, 75-12-7
