4030
J . Phys. Chem. 1987, 91, 4030-4036
Dynamics and Order of Nonionic Surfactants in Neat Liquid and Micellar Solution from NMR Relaxation and 13C NMR Chemical Shifts Multifleld T. Ahlnas, G. Karlstrom, and B. Lindman* Physical Chemistry 1, Chemical Center, University of Lund, S-221 00 Lund. Sweden (Received: November 25, 1986)
The I3C NMR chemical shifts and spin-lattice relaxation times have been determined for nonionic oligo(oxyethy1ene)dcdecyl ether surfactants (CI2E,) in water (C12E5) and as neat liquids (Ci2E3.CI2E4,C12E5, and CI2E6). For the ethylene oxide chains the chemical shifts suggest that a trans conformer around the C-C bonds becomes more stable when the temperature is increased or if the environment is made more apolar. This agrees with the predictions of a recent theoretical model for the conformation of ethylene oxide chains. The ordinary upfield shift corresponding to an increased population of gauche conformers was observed for the alkyl chains at increased temperature. The spin-lattice relaxation for micellar and neat nonionic surfactants was found to be magnetic field dependent. Using the so-called two-step relaxation model of Wennerstrom we have obtained order parameter and fast correlation time profiles in addition to a slow correlation time. These parameters provide novel information on the nonionic surfactant aggregates in water and on the structure of neat nonionic surfactant liquids. I3C NMR spectra of CIZE5in anisotropic phases show unusually narrow lines, an observation providing information on chain order.
Introduction
TABLE I: I3C NMR Chemical Shifts for CI2ESat Two Different Temperatures" chemical shift, chemical shift, PPm PPm
Nonionic surfactants of the ethylene oxide variety encounter a rapidly increasing use in many applications. This is one reason for the increased interest in basic and applied research on nonionic surfactant systems. Another reason is their complex and peculiar 6OC 27OC 6OC 27 O C physicochemical properties. Important information on areas of c 21 57.99 58.16 C5,7-10 16.21 16.11 application can be obtained from studies of phase behavior in 16.03 c12 57.13 57.20 C5,7-10 three-component systems of surfactant, oil, and water. This phase C13 56.04 56.25 C5,7-10 15.95 15.78 15.77 C14-18 55.93 56.10 C4,6 behavior is unusually complex and sensitive to small changes in C19 55.85 56.00 C4,6 15.68 15.68 temperature.'V2 Even two-component systems of surfactant and 12.33 12.36 55.65 c20 55.83 c11 water show a complex pattern with isotropic solution phases of c22 46.44 46.65 c2 8.84 8.81 at least three types and with a number of liquid-crystalline phases c3 18.21 0.00 0.00 18.17 c1 as well as a lower consolute temperat~re.~"In aqueous solutions, micelle size is very sensitive to the length of the oligo(ethy1ene "The average values for concentrations between 2.34 and 34.2 wt % are given. Differences between different concentrations are approxioxide) group, as well as to temperature and con~entration.~-~' As mately 0.01 ppm. an example, CI2ES(subscripts in C,E, are x = number of carbon atoms in a n-alkyl chain and y = number of ethylene oxide groups) the geometry of the surfactant molecule. Micellar growth increases micelles grow very strongly with increasing temperature, while with decreasing effective size of the polar head group. Water C,*Es micelles show only a minor g r o ~ t h . ~ We , ' ~ note for comself-diffusion studiesi7 give evidence for a decreased hydration parison that ionic surfactant micelles decrease in size as the with increasing temperature which would explain the temperatemperature is i n ~ r e a s e d . ' ~ - ' ~ Phase behavior and micelle growth can be r a t i ~ n a l i z e dfrom ~ ~ ~ ~ ~ ~ture-dependent growth. While an increased understanding of the complex self-associa model16 where aggregate size and shape is deduced mainly from ation pattern of ethylene oxide surfactants has undoubtedly been obtained in recent years we are still far from understanding the basic molecular mechanisms. For example, we have essentially (1) Kunieda, H.; Shincda, K. J . Dispersion Sci. Technol. 1982, 3, 233. no insight into the conformation of the surfactant molecules, in (2) Shinoda, K. Progr. Colloid Polym. Sci. 1983, 68, 1. particular effects of temperature, concentration, and solubilization (3) Shinoda, K. J. Colloid Interface Sci. 1970, 34, 278. of a hydrocarbon. A recent theoryiE suggests a temperature(4) Lang, J. C.; Morgan, R. G.1980, 73, 5849. (5) Andersson, B.; Olofsson, G. Colloid Polym. Sci., in press. dependent conformation of the ethylene oxide chains, with an (6) Mitchell, D. J.; Tiddy, G . J. T.; Warning, L.; Bostock, T.; McDonald, increased fraction of trans conformers around the C-C bonds at M. P. J . Chem. Sor., Faraday Trans. I , 1983, 79, 975. higher temperature. The resulting reduced dipole moment would (7) Nilsson, P. G.; Wennerstrom, H.; Lindman, B. J . Phys. Chem. 1983, lead to a less favorable interaction with water and thus qualitatively 87, 1377. (8) Liifroth, J. E.; Almgren, M. In Surfactants in Solution, Vol. 1, Mittal, rationalize the appearance of a lower consolute temperature. K. L., Lindman, B., Eds.; Plenum: New York, 1984; p 627. The aim of the present work has been to start an examination (9) Brown, W.; Johnsen, R.; Stilbs, P.; Lindman, B. J . Phys. Chem. 1983, in some detail of the conformation of ethylene oxide surfactants 87, 4548. under a range of conditions, in particular the effect of temperature (10) Kato, T.; Seimiya, T. J . Phys. Chem. 1986, 90, 3159. for micellar and neat surfactant, the effect of the number of (11) Zana, R.; Weill, C. J . Phys. Lett. 1985, 46, L-953. (12) Nilsson, P. G.; Wennerstrom, H.; Lindman, B. Chem. Scr. 1985, 25, ethylene oxide groups, and the effect of concentration for micellar 67. surfactant. To do so, we have used two different N M R ap(13) Mazer, N. A.; Benedek, G. B.; Carey, M. C. J . Phys. Chem. 1976, proaches, 13Cshielding and multifield I3C spin-lattice relaxation, 80, 1075. which have recently been applied for a number of ionic surfactant (14) Turro, N. J.; Yekta, A. J. Am. Chem. SOC.1978, ZOO, 5951. (15) Malliaris, A.; LeMoigne, J.; Sturm, J.; Zana, R. J . Phys. Chem. 1985, 89, 2709. (16) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J . Chem. Sor., Faraday Trans. 2, 1916, 72, 1525.
0022-3654/87/2091-4030$01.50/0
(17) Nilsson, P. G.; Lindman, B. J . Phys. Chem. 1983, 87, 4756. (18) Karlstrom, G.J . Phys. Chem. 1985, 89, 4962.
0 1987 American Chemical Society
I3C N M R of Nonionic Surfactants
The Journal of Physical Chemistry, Vol. 91, No. 15. 1987 4031
.
PPM
I 0.6
I
t
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_i
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- 0.3
1
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.
20
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Figure 2. Temperature dependence of the I3Cchemical shifts of micellized CI2ES.The shifts are relative to those at I O OC. The temperature dependence of the reference (lock) signal is not included, but it would closely follow the CI2signal. 12
18
22
cx
Figure 1. I3CNMR chemical shift differences along the molecular chain between micellized CI2E5at 45 and I O OC ( 0 ) and between neat and micellized CI2E5at 27 "C ( 0 ) .The upper dashed line indicates the zero level for the former case after correcting for the reference (lock) signal temperature dependence. The lower dashed line indicates the zero-level for the latter case taking the methyl group as an internal reference. Carbon number one is the methyl group.
~ y s t e m s . ' ~ - ~ From l such studies it is possible to characterize different parts of the aggregated surfactant molecules in a number of ways, Le. trans/gauche ratio, degree of orientation (order parameter), and mobility.
Experimental Section All samples were made by weighing into N M R tubes which then were provided with vortex plugs, closed with c a p , and secured with tape. The oligo(oxyethy1ene)dodecyl ether surfactants C12E3, CI2E4, ClzES,and C&6 were of the highest quality and obtained from Nikko Chemical Co. Ltd., Tokyo. The heavy water was obtained from Ciba-Geigy and the isotropic purity was better than 99.7 at. %. The relaxation measurements were done at three magnetic field strengths: 1.4 T with a Jeol FX-60,2.34T with a Varian XL-100 or a Jeol FX-100, and 8.5 T with a Nicolet NIC-360 spectrometer. Typical parameters in the experiments were 10 to 12 pulse interval times from 0.05 to 6.0 s. The time between the pulse sequences was a t least 6 s. The relaxation times were calculated through a three-parameter fit. The estimated error in the relaxation times was 10%a t the lowest fields and 5% a t the highest field. The uncertainty in temperature was f 1 OC. For further details of the relaxation experiments, see ref 22. The chemical shifts were measured at the highest field strength, 8.5 T, with the Nicolet spectrometer. The nonioic surfactant spectra were assigned according to Ribeiro and Dennisz3(cf. Table 1). The chemical shift reference was the terminal methyl group for the concentration-dependent measurements and the lock signal (D20) for the temperature-dependent measurements. For details see below. (19) Lindman, B.; SMerman, 0.; Wennerstrcm, H. In Surfactant Solutions: New Methods of Investigation, Zana, R., Ed.; Marcel Dekker: New
York. 1986: Chabter 6. D 263. (20) SBdermak O.;'WGderhaug, H.; Henriksson, U.; Stilbs, P. J . Phys. Chem. 1985,89, 3693. (21) Nery, H.; Siiderman, 0.;Canet, D.; Walderhaug, H.; Lindman, B J . Phys. Chem., in press. (22) Walderhaug, H.; Siiderman, 0.;Stilbs, P. J . Phys. Chem. 1984, 88, 1655. (23) Ribeiro, A. A.; Dennis, E. A. J. Phys. Chem. 1977, 81, 957
Downfield shifts are denoted positive, and the carbon atoms are numbered starting with the methyl carbon on the hydrocarbon chain.
Results 1 . I3CChemical Shifts. Micellar Phase. The chemical shifts are nearly constant at 27 and 6 OC as the surfactant concentration is increased. In Table I we show the average values for the shift together with a number indicating the spread in the shift between different concentrations. All measured concentrations are well above the critical micelle concentration. From Table I, the assignment of the carbon signals can be obtained. Neat Compared with Micellar Surfactant. In Figure 1, the shift difference between neat and micellized C12E5 is shown. The y axis shows the chemical shift difference with respect to the lock signal, but since the neat sample is locked on an outside coaxially placed water tube and the micellar sample is locked on internal water, a slight difference results due to the locking. Therefore, we have chosen the terminal methyl signal as a reference as indicated by the dashed line in Figure 1. We find that the ethylene oxide carbons of neat CI2E5are shifted downfield compared with the micellized CI2E5. For the alkyl chain part neat C12E5 carbons are shifted upfield from the micellized C12E5 carbons. Temperature Dependence in the Micellar Phase. In Figure 1, we show the chemical shift difference of micellized CI2ES between 45 and 10 OC. The D 2 0 lock signal is used as a reference. The dashed line a t 0.38 ppm in Figure 1 indicates the new zero-level after correcting for the temperature dependence of the lock signaLZ4 Thus, we find at 10 "C a downfield shift of the alkyl chain carbons and an upfield shift of the ethylene oxide carbons when compared with the shifts at 45 OC. No shift changes are seen at the CI2carbon. A more detailed temperature dependence of the chemical shifts of some carbons is shown in Figure 2. Again, t h e y axis shows the chemical shift difference of micellized Cl2E5 between the experimental temperature and 10 "C. No correction is made for the lock signal temperature dependence. 2. I3CSpin-Lattice Relaxation. Concentration and Temperature Dependence. In Figure 3, a and b, we show the relaxation rates of some alkyl chain and some ethylene oxide carbons as a function of surfactant concentration at two temperatures. All samples investigated are above the critical micelle concentration (cmc). We observe only slight changes in the relaxation rates when the concentration is changed. This is what we generally find for spin-lattice relaxation (both I3C and 'H) in micellar solutions of ionicz5and nonionic surfactants26well above the cmc. Comparing (24) Franks, F. Water-A Comprehensive Treatise; Plenum: New York, 1973; Vol. 1 , p 228. (25) AhlnBs, T.; Lindman, B., to be submitted for publication.
4032 The Journal of Physical Chemistry, Vol. 91, No. 15, 1987 1/T,, s
Ahlnas et al. eeee
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Figure 3. Spin-lattice relaxation rates (at 8.5 T) of some methylene groups of C,2E5in water as a function of C12E5concentration (percent by weight): (a, top) results at 27 'C; (b, bottom) results at 6 'C.
'/T1, s
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Figure 4. I3C spin-lattice relaxation rates of 43 wt % C12E5in heavy water as a function of field strength: results at 8.5 T ( O ) , at 2.34 T (A), and at 1.4 T ( 0 ) . The temperature was 27 O C .
Figure 3, a and b, reveals a considerable temperature dependence in the relaxation rates.
(26) Heatley, F.; Teo, H. H.; Booth, C. J. Chem. SOC.,Faraday Trans. I , 1984, 80, 981.
The Journal of Physical Chemistry, Vol. 91, No. 15, 1987 4033
I3C N M R of Nonionic Surfactants
'
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c12-20
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system. This phenomenon gives many further possibilities to obtain novel information on the anisotropic phases through I3C relaxation and chemical shift studies.
Discussion
0.5 kHz c12-20
c3-11
0.5 kHz Figure 8. "C NMR spectra of 5 5 wt % C12E5in heavy water as (a) lamellar phase at 27 OC and as (b) hexagonal phase at 6 OC.
Neat Surfactant Relaxation. In Figure 6 , relaxation rates of neat CI2El4and C12ESat different field strengths are displayed. W e have additional measurements of C12E3 and that also show a considerable field dependence. In Figure 7,relaxation rate profiles for neat nonionic surfactants, C12E3, C12E4, Cl2E5and C12E6, a t constant field strength (1.4 T ) are displayed. Despite the great difference in ethylene oxide chain length there are small differences between the relaxation rate profiles. It is a striking feature that the relaxation rate of the terminal ethylene oxide and methyl carbons are independent of the molecular size and magnetic field strength. The main differences in the relaxation rates are located in the middle of the surfactant molecule. 3. I3C N M R Spectra of Anisotropic Phases. An interesting observation is that lamellar and hexagonal phase samples of C12E5 in water give 13CN M R spectra with quite narrow lines (see Figure 8, a and b) which is contrary to what is found for ionic surfactants?' We expect this to be a farily general behavior of nonionic surfactants since this has also been observed in the C12E6-water (27) Henriksson, U.;Klason, T.; Finn. Chem. Lett. 1982, 139.
1. I3C N M R Chemical Shifts. Surfactant self-diffusion and proton N M R line widths in the micellar phase of CI2E5and water give evidence for a marked increase in aggregate size when the temperature or concentration is increased.8J2 The micellar hydrodynamic radius can be several hundred angstroms. The CI2E5 micelles at low temperatures become rodlike when approaching the hexagonal phase6*28(see Figure 9) but at higher temperatures on approaching the cloud point the situation is less clear, but even here rodlike micelles are probable.I2 In a recent theory on the oligo(ethy1ene oxide)-water phase behavior18 the miscibility gap is interpreted as caused by the different nature in the high- (less polar) and low-temperature (polar) states of the ethylene oxide chain. For the ethylene oxide segment, two conformations of 27 possible were taken as a lowtemperature state. The rest, excluding two sterically hindered conformations, were taken as the high-temperature state. The low-temperature state consists of the two gauche conformations around the C-C bond and trans for the C-0 bond. These states are shown by theory and experiment^'^^^^-^' to be more stable in a polar environment than the other conformations, due to their polar character. I3C chemical shifts in alkyl chains have been shown to depend on the gauche/trans ratio32and less on the environment (see ref 33 and references therein). In the ethylene oxide segment we have the same gauche/trans isomerism fo the carbon-carbon bond, but we also have the two carbon-oxygen bonds that will complicate the situation. It should, however, be noticed that the energy difference between gauche, and trans conformations around a C-0 bond is =12 kJ/mol (with the trans conformer being the lower energy form) whereas this energy difference is only =2 kJ/mol for the C-C bond (with the gauche conformer being the lowenergy form in polar solventsI8). This indicates that the larger part of the changes in the shifts could be expected from the changes in conformation around the C-C bonds. It is likely that medium effects will influence the shifts of the ethylene oxide carbons. However, as seen later in the discussion, hydrogen
(28) Nilsson, P.-G. Thesis, Lund, 1984. (29) Viti, V.; Xampetti, P.; Chem. Phys. 1973, 2, 233. (30) Podo, F.;NCmethy, G.; Indovina, P. L.; Radics, L.; Viti, V. Mol. Phys. 1974, 27, 521. (31) Viti, V.; Indovina, P. L.; Podo, F.; Radics, L. Mol. Phys. 1974, 27, 541. (32) Cheney, B. V.; Grant, D. M. J . Am. Chem. SOC.1967, 89, 5319. (33) Rosenholm, J. B.;Drakenberg, T.; Lindman, B. J . Colloid Interface Sci. 1978, 63, 538.
4034
The Journal of Physical Chemistry, Vol. 91, No. 15, 1987
bonding would give opposite shifts to what is observed. This indicates that the possible medium effects on the shifts will be overruled by the gauche/trans shift change (estimated to be 4.8 PPm). Summarizing, we expect the I3C chemical shifts of the ethylene oxide chain to be sensitive to the gauche/trans ratio of the C-C bonds (as in alkyl chains) and, according to the theory above, indirectly also to the polarity of the environment, since altered polarity induces changes in the gauche/trans equilibrium. This interpretation of the experimental data is of course not unique. It is, however, comforting to notice the agreement between the theoretical predictions of the model and the experimentally observed data (vide infra). To test this prediction the chemical shifts of CI2E5as monomers, as micelles, and as neat liquid can be measured. Also for a micellar solution the temperature can be varied to change the polarity of the ethylene oxide environment (water). (The conformational entropy will also populate the nonpolar trans conformers, when the temperature is increased giving similar effects as a decrease in the polarity of the environment.) In general, downfield 13Cchemical shifts are observed for the alkyl chain carbons when surfactant monomers are aggregated into micelle^.^^-^^ This is interpreted as an increased amount of trans conformations in the chain. The maximum in the shift difference is located in the middle of the chain. In Figure 1, we display the shift differences between micellized and neat C12ESat 27 OC. The y-axis scale gives the shift difference with the lock signal as a reference. However, as indicated by the dotted line, we have taken the terminal methyl group as an internal reference, since it has been shown that the shift of the lock signal is changed upon addition of for example sodium dodecyl sulfate36 and that the shift of the terminal methyl is quite invariable. We observe a downfield shift for the alkyl chain carbons when going from neat liquid to micellized C12E5. This is interpreted as an increased amount of trans states in the alkyl chain part of micellized surfactant. The micellar structure would thus more restrict the available conformations for the alkyl chain compared to the structure in neat surfactant liquid. For the ethylene oxide carbons we find a change upfield for micellized surfactant compared with the neat surfactant. At first sight this seems to be peculiar. Regarding the water-ethylene oxide interaction, the hydrogen bonds would deshield the ethylene oxide carbons and thus cause a downfield shift. In neat surfactant liquid, only the terminal OH groups could participate in the hydrogen bonds with the ethylene oxide groups. Thus, the observed opposite shift change indicates that the downfield shift due to hydrogen bonding is overruled by a larger upfield shift. This upfield shift change can be explained by a change in the gauche/trans isomerism in the C-C bond and it would indicate that in neat surfactant liquid the trans state would be more stable than in a micelle. According to the theory for ethylene oxide chain conformations, the gauche state is more stable in more polar environment and this seems also to be indicated by the chemical shifts. Corno et al.37investigated I3C chemical shifts of C12E, ( x = 2, 3, .., 10) upon micellization in water. They concluded that in both the alkyl and ethylene oxide parts an increase in the trans states resulted. Thus, the alkyl chain follows the behavior of ionic surfactants and the ethylene oxide chain behavior is in accordance with the generally predicted increased trans state stability in more apolar environment. Above cmc the chemical shifts are constant within the micellar phase as seen in Table I. This would indicate that there is no change in the polarity of the ethylene oxide chain environment and indirectly indicate that the hydration is not changed. Further support for this idea comes from a study of Nilsson et al.,17who found from water diffusion measurements that hydration is (34) Maeda, H.; Ozeki, S.; Ikeda, S.; Okabayashi, H.; Matsushitu, K. J . Colloid Interface Sci. 1980, 76, 532. (35) Persson, B. 0.; Drakenberg, T.; Lindman, B. J . Phys. Chem. 1976, 80, 2 124. (36) Clifford, J.; Pethica, B. A. Trans. Faraday SOC.1964, 60, 1483. (37) Corno, C.; Platone, E.; Ghelli, S . Colloid Polym. Sci. 1984, 262, 667.
Ahlnas et al. proportional to ethylene oxide chain length and at low temperature 4-6 water molecules per ethylene oxide segment were perturbed. A slight concentration dependence (decrease) was deduced from the data. However, analyzing the data using an improved theory of water self-diffusion in colloid systems,38one expects the hydration to be concentration independent as long as full hydration can be maintained. Moreover, water I7O NMR relaxation studies39indicate that the interaction between water and ethylene oxide segments is independent of ethylene oxide chain length but is temperature dependent. From these observations one may deduce that the C12E5 micelle does not change its shape but remains rod-shapedI2 in the concentration range studied, since such changes imply changes in the water concentration and thus in the polarity of the environment of the E-O groups, which would lead to shift changes. Further, from the fact that carbon 14-18 have the same shifts one is tempted to say that they experience an equally polar environment and this probably implies that they form some sort of cover of the hydrocarbon part of the micelle. Carbon atoms 19 and 20 have a significantly lower shift indicating a more polar environment. This probably means that they have some tendency to stick out into the water. If we compare micellar chemical shifts for the alkyl chain part at two different temperatures, we observe downfield shifts with decreasing temperature (Figure 1). This indicates an increased amount of trans states in the chain when lowering the temperature. For the ethylene oxide part an opposite behavior is observed. At lower temperature the carbon signals are upfield from those at higher temperature. Again, the change in hydrogen bonding between water and the ethylene oxide segments would result in downfield shifts for the carbons when lowering the temperature. The observed upfield shift can be explained by a shift change, due to a change in the gauche/trans isomerism caused by entropy. Thus, the observed shift changes indicate that C-C gauche conformations in the ethylene oxide chain would have a larger statistical weight at a low than at a high temperature. This conclusion is in accordance with the prediction of the theory for ethylene oxide chain conformations discussed earlier. In Figure 2, a more detailed temperature dependence of some alkyl and ethylene oxide carbon signals is shown. The shifts are relative to those at 10 OC and the sample is locked at internal heavy water. We have chosen to measure our chemical shifts relative to the lock signal (D20), since the temperature dependence for the standard reference Me4Si (tetramethylsilane) seems to be debatable.a The change in the shifts thus includes the lock signal temperature dependence which is relative linear.24 If we make the correction for the temperature dependence of the lock signal, the methyl signal will at 45 OC be shifted less than 0.1-ppm upfield from the value at 10 OC. After the correction, carbon number 12 would have almost a temperature-independent shift. 2. I3CN M R Relaxation. As seen in Figure 4, there is a field dependence in the relaxation. From these data we can determine the order parameters and the fast and slow motion correlation times using the so-called two-step relaxation model.41 In eq 1
the spin-lattice relaxation rate is given as a function of these parameters. Here, is the permeability of vacuum, h is Planck's constant, yH and yc are the magnetogyric ratios of the proton and carbon, r is the carbon-proton distance, S is the order pa(38) Jonsson, B.; Wennerstrom, H.; Nilsson, P. G.; Linse, P. Colloid Pclym. Sci. 1986, 264, 77.
(39) Carlstrbm, G., private communication. (40) Jackowski, K.; Raynes, W. T. Org. Magn.Reson. 1980, 13, 438. Schneider, H. J.; Freitag, W.; Schommer, M. J . Magn. Reson. 1975, 18, 393 Drakenberg, T.; Ro(41) Wennerstrom, H.; Lindman, B.; SGderman, 0.; senholm, J. B. J . Am. Chem. SOC.1979, 101, 6860.
The Journal of Physical Chemistry, Vol. 91, No. 15, 1987 4035
I3C N M R of Nonionic Surfactants S
1
0.25
,
6-4-4-6-4-4
6,'
0.051
45 Figure 10. Order parameters as a function of position in the chain for micellar C12E5 at 27 (0)and at 6 OC (0). The position of the atoms at 6 O C have been displaced +0.5 units along the abscissa to avoid overlap. Error bars denote f one standard deviation. S
I 0 C12E4
0.25-
0.20
-
Figure 12. Fast motion correlation times as a function of carbon position in micellar C12E5at 27 (0) and at 6 OC (0). Error bars denote one
-
standard deviation.
0.15
*
order parameters in Figure 10. The highest order is found at carbon numer 11 and the order is reduced when going to either the terminal methyl or the terminal ethylene oxide. According 0.05to eq 1 we obtain the largest contribution from the slow part for the highest order parameter. At lower fields this contribution is particularly large. The measured relaxation rate is influenced by the order parameter, the fast motion, the slow motion, and the field strength. Thus, the fast motion cannot be determined from single low-field measurements. Heatley et a1.26recently published a proton N M R relaxation study where they report frequency-dependent relaxation for, among others, CI2El2methyl ether in water. The proton relaxation process rameter of the C-H vector, 7: is the fast motion correlation time, is complicated by the presence of both intra- and intermolecular is the slow overall motion correlation time, wc and wH are the contributions but still some interesting observations can be made. carbon and proton Larmor angular frequencies at a given magnetic Thus, they found the highest order close to or at the alkyl/ethylene field, respectively. oxide conjunction depending on the nonionic surfactant. In the 2.1. Order Parameters. Using the two-step model we have ethylene oxide part very low order was found. The concentration determined the order parameter profiles for micellar C12E5at two dependence of TI and T2 was very weak for Cl4E27 methyl ether. temperatures, 27 and 6 OC, see Figure 10. Since there is no Many of these observations parallel our findings from I3C reconcentration dependence in the relaxation rates there will be no laxation of the C12E5 surfactant in water. concentration dependence in the order parameters or the fast For ionic surfactants no dramatic changes with concentration motion correlation times. A slightly higher order is found at the are seen for the fast local motions in the micellar s o l ~ t i o n s . ~ ~ ~ ~ ~ lower temperature, but the profile is not changed. This is also what we find for C12E5. As seen in Figure 3, a and For neat liquid nonionic surfactant the field-dependent relaxb, the relaxation rates at the highest field strength (8.5 T) are ation indicates the presence of anisotropy in the fast local motions quite constant when the concentration is changed. At the highest which suggests that the neat liquid is aggregated. In Figure 11, field strength we have only a small contribution from the slow the order parameter profile is shown for C12E4 and C12ES.The motion to the relaxation and essentially we then see the fast local order parameters are the same within experimental error for CI2E4 motions. Thus, this indicates that the fast motion correlation time and Cl2E5. is not very concentration dependent, supporting the idea that the 2.2. Fast Motion Correlation Times. In Figure 12 the deduced micelle structure is independent of concentration. Changing the fast motion correlation times along the alkyl and ethylene oxide temperature from 6 to 27 "C changes the fast motion correlation chains are shown for micellar C12E5 at 27 OC. We observe that times roughly by a factor of two (see Figure 12). The fast motion the maximum motional retardation is not at the alkyl/ethylene correlation time profile clearly shows that the molecule is not oxide chain connection but shifted more toward the ethylene oxide behaving as a free chain. part. This is probably due to the stronger inter- and intramolecular For the neat liquid nonionic surfactants C12E4 and CI2E5the interactions in this region. Single, low-field m e a s u r e m e n t ~ ~ ~ * ~ *fast , ~ ~motion correlation times seem to be fairly insensitive to have previously been interpreted to show a maximum in the ethylene oxide chain length as seen in Figure 13. motional retardation at the alkyl/ethylene oxide connection. The 2.3. Slow Motion Correlation Times. In principle, we have reason for this discrepancy can be explained by the results for the three unknown parameters per carbon to fit but one of them, the slow motion correlation time, should be common. This will
-
0.10
1
9
(42) Ribeiro, A. A,; Dennis, E. A. J . Phys. Chem. 1976, 80, 1746. (43) Mathis, G.; Boubel, J.-C.; Delpuech, J.-J.; Ravey, J. C.; Buzier, M. NATO Advance Study Institutes Series, Series C, Fraissard, J. P., Resing, H. A,, Reidel: Holland, 1980 p 597.
(44) Ahlnas, T.; Sderman, 0.;Walderhaug, H.; Lindman, B. In Surfactants in Solution, Vol. 1, Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; p 107.
4036 The Journal of Physical Chemistry, Vol. 91, No. 15, 1987 f
TABLE 11: Error Sums as a Function of Slow Motion Correlation
;TPs
Times"
sample micellized CI2ES(45 wt %) at 27
-I
45
Ahlnas et al.
(20 wt %)
at 6 OC
0.8 1.o 6.0 10.0 15.0 7.66
0.039
1 2 5 8
0.122 0.166 0.184 0.224
neat CI2ESat 2 1 'C
0.8 1
0.048 0.054 0.077 0.093 0.123
2 5 8
15 T
0.774 0.068 0.123 0.206 0.238
neat C12E4 at 27 OC 30 1
correlation time, ns error sum O C
"The error sum is defined as the sum taken over all squares of differences between the observed and calculated relaxation rates divided by the square of the observed relaxation rate. O
I
1
5
10
15
20
cx
Figure 13. Fast motion correlation times as a function of carbon position in neat liqud CI2E4 (0)and CI2E5( 0 ) at 27 O C . Error bars denote t
one standard deviation. The position of the CI2ES atoms have been displaced +0.5 units along the abscissa to avoid overlap. substantially reduce the number of free parameters. However, to characterize the slow motion accurately it is required that a large enough part of the measured relaxation is due to this motion. Moreover, to verify that there are just two steps in the relaxation frequency dependence more than three magnetic field strengths would be needed. We have chosen to examine the relaxation data by evaluating the error sum of the multiparameter fit. The error sum is defined as the sum over all squares of differences between the observed and calculated values divided by the square of the observed value. For micellized C12E5(45 wt %, 27 "C) the best fit to the data was obtained for a slow motion correlation time of about 1 ns. However, when varying the slow motion correlation time a quite shallow minimum was found for the error sum (see Table 11). At low temperature a good fit to the data was found for the slow correlation time of 7.7 ns. At 27 O C the much shorter slow motion correlation time cannot be explained by aggregate rotation and/or lateral diffusion. Since the aggregates a t this temperature are expected to be quite large, probably flexible rods there might be intermediate motions such as fast changes in the aggregate curvature or exchange of monomers between aggregates that could explain the quite short effective slow motion correlation time deduced. The narrow line widths in the spectra of the anisotropic phases must be due to a motion which effectively reduces the line widths. To investigate the slow motions thoroughly, measurements should be extended to very low field strengths. For CI2E4and CI2ESneat liquids (at 27 "C) a slow motion correlation time of about 1 ns gave a good fit to the data as indicated in Table 11. However, varying this correlation time from 1 to 8 ns did not effect the error sum significantly.
3. Spectra of Anisotropic Phases. As seen in Figure 8a, the 13C spectrum of a lamellar phase sample at 27 O C shows very narrow lines and especially the terminal methyl and ethylene oxide I3Cline widths are comparable with those found in the isotropic phases. We find that the broadening of the lines correlate well with the order parameters deduced from the relaxation in the micellar phase. Thus, highest order and the broadest lines are found in the middle of the surfactant molecule and very low order and narrow lines are found at both ends of the molecule. For ionic surfactants, like sodium dodecyl sulfate, aI3Cspectrum of the lamellar phase shows very broad lines and nearly nothing is seen after just four accumulations as was used to obtain these nonionc spectra in Figure 8, a and b. As the signal-to-noise ratio is good, this rules out the possibility of observing small isotropic domains in the anisotropic sample. Furthermore, spinning the sample at about 10 H z ruled out the possibility of spontaneous orientation of the lamellae at the magic angle. The sample was very viscous and clearly birefringent when viewed between crossed Polaroid glasses. We suggest that the 13C spectra of the lamellar and the hexagonal phases indicate a substantial decrease of the long-range order when compared to 13Cspectra of anisotropic phases of ionic surfactants. The local molecular chain order is expected to be similar. Thus, to explain the narrow lines there should be motions present in the nonionic liquid crystals on the nanosecond time scale that are isotropic or almost isotropic. The lack of these motions in the ionic surfactant liquid crystals on the other hand could be due to the long-range electrostatic repulsion between the surfaces. Acknowledgment. This work was supported by grants from the Swedish Board for Technical Development. Prof. P. Stilbs is greatfully acknowledged for the use of the JEOL FX-100 spectrometer a d for putting at our disposition a computer program for the calculations. Registry No. CI2ES, 3055-95-6; CI2E3, 3055-94-5; C12E4, 5274-68-0; C,*Es, 3055-96-7.
