0 Copyright 1990 American Chemical Society
The ACS Joumal of
Surfaces and Colloids JULY 1990 VOLUME 6, NUMBER 7
Articles 2HNuclear Magnetic Relaxation of [1,l -2H]HexadecyltrimethylammoniumBromide in Micellar
Solutions of Nonaqueous Polar Solvents and Their Mixtures with Water Marie Sjoberg,**tvJUlf Henriksson,t and Torbjorn WarnheimJ Department of Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden, and Institute for Surface Chemistry, P.O. Box 5607, S-114 86 Stockholm, Sweden Received October 3, 1989. In Final Form: January 19, 1990 The *Hspin-lattice and spin-spin nuclear magnetic relaxation rates were measured at differentfrequencies for a-deuterated hexadecyltrimethylammonium bromide in micellar solutions of nonaqueous and mixed solvent systems. Three nonaqueous polar solvents, viz., formamide, ethylene glycol, and N-methylformamide, and their mixtures with water were used. The two-step model for magnetic relaxation of surfactant aggregates was fitted to the frequency-dependent relaxation rates for the surfactant, yielding a local order parameter and correlation times for a fast local motion and for a slow aggregate motion. The measurements show that the surfactant aggregates in formamide, ethylene glycol, and their mixtures with water. No aggregation occurs in N-methylformamide at water contents lower than 50 w t 5%. Approximate aggregate radii, obtained from the slow correlation times, show that the aggregates formed in formamide and in ethylene glycol are smaller than in water. The local order parameter, obtained from the relaxation measurements as well as from the quadrupolar splittings in the hexagonal phases, increases with increasing water content both in the formamide and in the ethylene glycol systems.
Introduction Most aspects on the aggregation of amphiphiles in water have been investigated extensively during the years, but only recently have reliable studies of amphiphilic systems with nonaqueous polar solvents been reported. Micelles or liquid crystals have been reported to form in solvents such as hydrazine,ls2 g l y ~ e r o l ,formamide,5-'4 ~?~ and
* To whom correspondence should be addressed. + Royal
Institute of Technology.
* Institute for Surface Chemistry.
(1) Ramadan, M.; Evans, D.F.; Lumry, R. J. Phys. Chem. 1983,87, 4538. (2) Ramadan, M.; Evans, D. F.; Lumry, R.; Philion, S.J. Phys. Chem. 1985,89, 3405. (3) Friberg, S. E.; Liang, Y. C. Colloid Surf. 1987,24, 325. (4) Evans, D.F.; Yamauchi, A.; Wei, G. J.; Bloomfield, V. A. J . Phys. Chem. 1983,87, 3537. (5) Lattes, A.; Rim, I. Colloid Surf. 1989,35, 221. (6)Almgren, M.; Swarup, S.; Ufroth, J. E. J. Phys. Chem. 1985,89, 4621.
0143-1463/90/ 2406-1205$02.50/0
different g l y ~ o l s . ~These ~ J ~ solvents have three physical properties in common: they have high cohesive energies and high dielectric constants and they are hydrogen bonding. Indeed, Evans et al. have proposed that the hydrogen-bonding ability of a solvent is a prerequisite for amphiphilic aggregation to occur.17 (7) Des, K. P.; Ceglie, A,; Monduzzi, M.; SBderman, 0.; Lindman, B. h o g . Colloid Polym. Sci. 1987, 73, 167. (8)Das, K. P.; Ceglie, A.; Lindman, B. J. Phys. Chem. 1987,91,2938. (9) Rico, I.; Lattes, A. J . Phys. Chem. 1986,90, 5870. (10) Belmajdoub, A.; Marchal, J. P.; Canet, D.; Rico, I.; Lattes, A. N o w . J . Chim. 1987,11,415. (11) Auvray, X.; Anthore, R.; Petipas, C.; 3im, I.; Lattes, A. C. R.Acad. Sci. Paris 2 1988, 306, 695. (12) Auvray, X.; Petipas, C.; Anthore, R.; Rico, I.; Lattes, A.; AhmahZadeh Samii, A.; de Savignac, A. Colloid Polym. Sci. 1987,265,925. (13) Belmajdoub, A.; ElBayed, K.; Brondeau, J.; Canet, D.;Rico, I.; Lattes, A. J. Phys. Chem. 1988,92,3569. (14) Binana-Limbele, W.; h a , R. Colloid Polym. Sci. 1989,267,440. (15) Ray, A. J. Am. Chem. SOC.1969,91, 6511. (16) Ray, A. Nature 1971,231, 313.
0 1990 American Chemical Society
1206 Langmuir, Vol. 6, No. 7, 1990
On the other hand, some studies give definitely contradictory results. The micellization and the microemulsion formation in formamide with different amphiphilic molecules have been investigated in a number of publication^.^-^^ Results indicate that no micellization of ionic surfactants takes place in solutions with formamide,6-s but later it has been pointed out that studies in some of these systems were performed at a temperature where the surfactant monomer solubility was below the critical micelle concentration, i.e., below the Krafft point.g For example, hexadecyltrimethylammonium bromide, CleTABr, aggregates in formamide at temperatures above about 43 0C.9 However, the size and shape of t h e aggregates are still controversial, and also it is not clear whether the aggregation is cooperative, i.e., a proper micelle formation.12J3 We have previously determined phase diagrams of different alkyltrimethylammonium surfactanh in different polar solvents.18 For the binary systems with CleTABr, liquid-crystalline phases were observed in glycerol, ethylene glycol, and formamide but not in N-methylformamide. The possible correlation between formation of liquid crystalline phases and surfactant aggregation to micelles in more dilute solutions is of fundamental interest. We have therefore chosen to study the possible aggregation of C16TABrin three of these solvents, formamide, ethylene glycol, and N-methylformamide, and their mixtures with water (at a water content up to 50% in the solvent). Micellization is most often demonstrated through the observations of discontinuities at certain concentrations of amphiphile in solution for different physical properties such as conductivity, surface tension, etc. However, these observed break points might have other origins than aggregation of the amphiphiles, and it is therefore desirable in ambiguous cases to use other methods giving more direct information on the surfactant systems. Recently, multifield nuclear magnetic resonance relaxation rate measurements have become a useful tool for t h e investigation of surfactant system^.^^-^^ From the frequency dependence of the relaxation rates, the correlation times for the local and global (aggregate) motions in the system can be derived. Since deuterium relaxation measurements have proved to be particularly convenient,21we have chosen to work with a-deuterated Cl6TABr. Materials and Methods Chemicals. Cl6TABr was obtained deuterated in the a-position by reacting hexadecanoic acid chloride (Fluka, 98%) with dimethylamine to produce the corresponding amide, which was reduced with LiAl(2H)d. The resulting a-deuterated hexadecyldimethylamine was then reacted with CH3Br to give hexadecyltrimethylammonium bromide. To produce C I ~ T A C ~ , CH&1 was used in the last step. No significant trace of protons was found in the a-position in the final product according to 1H NMR. Ethylene glycol (Riedel-de Haen, 99.5 % 1, formamide (Merck, 99.5%), and N-methylformamide (Aldrich, 99%) were used as (17) Beesley, A. H.; Evans,D. F.; Laughlin, R. G. J.Phys. Chem. 1988,
92, 791.
(18)WCnheim, T.;Jdnsson, A. J.Colloid Interface Sci. 1988,125,627. (19)WennerstrBm, H.; Lindman, B.; SBderman, 0.; Drakenberg, T.; Rosenholm, J. B. J.Am. Chem. SOC. 1979,101,6860. (20)Halle, B.; WennerstrBm, H. J . Chem. Phys. 1981, 75, 1928. (21) (a) Sbderman, 0.;Walderhaug, H.; Henriksson, U.; Stilbs, P. J. Phys. Chem. 1985,89, 3643. (b) Henriksson, U.; Stilbs, P.; Sdderman, 0.;Walderhaug, H. In Magnetic Resonance and Scattering m Surfactant Systems; Magid, L., Ed.; Plenum Press: New York, in press. (c) Soderman, 0.; Henriksson, U.; Olsson, U. J.Phys. Chem. 1987,91,116.
Sjoberg et al. received. The water content in the solvents was less than 0.5 wt 7%. Water was twice distilled. Samples were prepared by weighing the surfactant and solvent into NMR tubes that subsequently were flame sealed. All samples for the relaxation measurements contained 20 wt 5% C16TABr, and the water content of the solvent was 0, 25, or 50 wt 7% . Measurements were also performed on the Cl6TAC1/ water system for comparison with the aggregates formed in water. (The clsTABr/water system was avoided since rodshaped micelles may form in this system, which complicates the comparison.) NMR Measurements. 2H relaxation rate measurements were performed on a Bruker AM 400 spectrometer operating at 61.4 MHz and a Bruker MSL 200/90 spectrometer equipped with a 4.7-T cryomagnet, i.e., 30.7-MHz 2H frequency, and an iron magnet which was used for 2H frequencies between 2 and 13.8 MHz. The measurements were carried out at 60 f 0.5 O C . The spin-lattice relaxation times (2'1) were measured at nine frequencies by the standard inversion recovery method, while the spin-spin relaxation times (2'2) were measured a t two frequencies, 30.7 and 13.8 MHz, with the Carr-Purcell-MeibomGill method. The measured relaxation rates for the surfactant in pure formamide and in pure ethylene glycol were corrected for the relaxation rates of the monomers in the solution, assuming that the concentration and the relaxation rates of monomers are constant above the cmc. The relaxation rates measured in the water system have not been corrected, since t h e monomer concentration is lower and the estimated corrections are negligible. The monomer concentration in the mixed solvent systems should not be of any large importance, since the cmc will increase very slowly when the solvent continuously is changed from water to another solvent.22 We have therefore not made any corrections in these systems. However, if such corrections should be carried out, it would only influence the fast correlation time and not our main interest; the slow correlation time. The 2H NMR spectra from the hexagonal phases were recorded at 30.7 MHz by using the quadrupolar echo method. 1H NMR self-diffusion measurements in the cubic phases of C16TABr in different solvents were performed with the FTNMR PGSE technique,23 using the Bruker MSL 200 spectrometer equipped with a microimaging probe. Surface Tension Measurements. Measurements of surface tension were performed with the du Nouy ring method. The samples were kept at 60 1 "C in a thermostated bath during the measurements. Viscosity Measurements. The kinematic viscosity of the pure solvents and their mixtures with water was determined with KPG Ubbelohde viscometers. The viscometers were placed in a thermostated bath during the measurements, and the temperature was kept at 60 f 0.1 OC.
*
Results Measurements of the surface tension were made a t varying concentration of C16TABr in ethylene glycol and formamide. Reasonably sharp break points occurred both in the formamide and ethylene glycol systems, and we tentatively assume the concentration a t the break point to be a cmc. The data for Cl6TABr/ethylene glycol give a more distinct break point than literature data on ClrTABr/ethylene glyc01.'~ The data for the formamide system are in reasonable agreement with previously published materialg The derived cmc values (Table I) are 2 orders of magnitude lower in water than in the other solvents. The 2H relaxation rates for C16TABr in the different solvent systems including mixtures with water are shown in Figures 1-3. In addition, the relaxation rates for Cl6TACl in pure water are shown in Figure 4. There is a (22) Magid, L. In Solution Chemistry of Surfactants; Mittal, K . L., Ed.; Plenum Press: New York 1979; Vol. 1, p 427. (23) Stilbs, P. Progr. NMR spectrosc. 1987, 19, 1.
Langmuir, Vol. 6, No. 7, 1990 1207
Micellar Dynamics in Nonaqueous and Mixed Solvents Table I. Concentrations at Break Point for Surface Tension Curves for the ClrTABr/Solvent Systems surfactant solvent" temp, "C cmc, w t % cmc, mM 1.32 HzO 55 0.048 CleTABrb 60 4.5 f 0.2 144 i 6 CleTABr EG FA 60 4.0 * 0.2 130 f 6 ClsTABr CleTABrC FA 60 2.9 90 FA = formamide, EG = ethylene glycol. cmc from ref 33.
70.
cmc from ref 9. 70.
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