Langmuir 1988,4, 1039-1044 Otherwise, the intensity of the asymmetric breathing vibration mode of the ring (1038-cm-l band) changes in a rather complex way with the potential. This dependence probably corresponds to two overlapping effects, one with its maximum value at ca. -0.2 V and another one with a wide maximum at about -0.5 V. The first function can be related to the potential region where the local OH- ion concentration at the interface is rather high whereas the second one develops around the pzc. The bands at 1215 and 625 cm-l both present maxima in the band intensity vs potential plot (Figure 2): the first one in the -0.7 to -0.8 V range and the second one at ca. -0.6 V, that is, in the vicinity of the pzc. Therefore, as far as the influence of the potential on the band intensities is concerned, one concludes that the symmetric modes behave rather similarly in contrast with the asymmetric modes. This difference presumably implies a configuration change in the adsorbate depending on the solution composition, which would reflect the influence of chemical contributions to SERS through the charge transfer between Ag and adsorbate sites of atomic-scale roughness.4143 However, to verify this suggestion further data on the pH influence on Py spectra of Ag are required, including more detailed information about the electrochemistry of the Ag/Py interface. These matters will be dealt with in a forthcoming publication. (41)Burstein, E.; Chen, Y.J.; Chen, C. Y.; Lundquist, S.; Tossatti, E. Solid State Comrnun. 1979,29, 567. (42)Gersten, J. I.; Birke, R. L.; Lombardi, J. R. Phys. Rev. Lett. 1979, 43, 147. (43)Otto, A. Appl. Surf.Sci. 1980, 6, 309
1039
Conclusion SERS of Py on Ag electrodes can be produced in alkaline solutions free of halide ions. SEW activation of Ag implies a certain metal roughness resulting from ORC treatment covering the potential range between the Ag/AgOH/OHand Ag/AgO/OH- redox equilibrium potentials. The electroreduced SERS-active Ag surface presents a rather uniform globular structure as revealed through SEM. Voltammetric data also indicate an appreciable degree of preferred crystallographic orientation at the initially reduced Ag surface with development of (111)faces. SERS band intensities change with the applied potential. A maximum absorption for the symmetric mode can be observed in the vicinity of the pzc. SERS appears to be related to a rather complex interaction between Py, HzO, and OH- ions with active Ag surface sites. In general, at potentials lower than -1.0 V the SERS effect is inhibited irreversibly in the base solution due to loss of atomic-scale roughness. SERS inhibition occurs parallel with the increase in the efficiency for the HER, resulting through the potential holding of Ag in the alkaline solution as reported previously."
Acknowledgment. This work was in part supported by FAPESP, CNPq, and FINEP (Brazil) and by CONICET, CIC (Pcia. Bs. As.), and CONICOR (Argentina). G.I.L. is indebted to CNPq/CONICET for the fellowship granted. Registry NO.Py, 110-86-1;Ag, 7440-22-4;H20,7732-18-5;OH-, 14280-30-9;H2, 1333-74-0;NaOH, 1310-73-2.
NMR Relaxation in Micelles Formed by a Long Zwitterionic Surfactant 0. Soderman,**tG. Carlstrom,? M. Monduzzi,* and U. Olssont Department of Physical Chemistry 1, University of Lund, P.O.B. 124, S-221 00 Lund, Sweden, and Department of Chemistry, University of Cagliari, Via Ospedale 72, Cagliari, Italy Received October 30, 1987. I n Final Form: March 15, 1988 A frequency-dependent14Nand 13C NMR relaxation study of the micellar region formed by a very long zwitterionic surfactant, viz., 6-(dimethyleicosylammonio)hexanoate(CAH), is presented. As a starting point the binary phase diagram of C&/D20 is determined. The phase diagram is similar to those found for shorter chain ionic surfactants with bulky head groups. The NMR relaxation data show a dependence upon the magnetic field strength and are discussed in terms of the two-step model of relaxation. The frequency dependence found at low frequencies in the 14Ndata is assumed to be caused by the rotational tumbling of the micelle, while it is argued that the frequency dependence found at high frequencies in the 13C relaxation data is caused by a local motion. The analysis of the data yields a radius of the micelle which is slightly shorter than an extended Czochain, pointing to a situation where the methylene groups in the head-group dipole are embedded in the micelle. Moreover, the local motions in the head-group region are slow as compared to the corresponding motions in single-chain ionic surfactants.
Introduction Nuclear magnetic resonance (NMR) techniques have proved to be very valuable in providing information on the structure and dynamic behavior of surfactants in micellar aggregates as well as in cubic, hexagonal, or lamellar In particular, the use of different nuclei such as 2H, 13C, and 14Nprovides complementary details concerning the possible kinds of motions that may occur in 'University of Lund. University of Cagliari.
*
different phases. A general observation in the study of isotropic surfactant systems is that the spin-lattice relaxation times depend on the field strength and that they (1)Tiddy, G. In Nuclear Magnetic Resonance; Webb, G. A., Ed.; Speciahst Periodical Reports, Royal Society of Chemistry London, 1981; Vol. 10,p 267. (2)Soderman, 0. In Nuclear Magnetic Resonance; Webb, G. A., Ed.; Specialist Periodical Reports, Royal Society of Chemistry: London, 1985; Vol. 12,p 350. (3)Khan, A. In Nuclear Magnetic Resonance; Webb, G. A., Ed.; Specialist Periodical Reports, Royal Society of Chemistry: London, 1987; Vol. 16,p 414.
0743-7463/88/2404-lO39$01.50/00 1988 American Chemical Society
1040 Langmuir, Vol. 4 , No. 4, 1988 are longer than the spin-spin relaxation times. Moreover, the spin-lattice relaxation times are rather long. These observations imply that more than one motion causes the NMR relaxation in surfactant system^.^-^ In previous ~ o r k ~we* have ~ J ~shown that NMR relaxation data for several different spherical micellar systems were well described with the so-called “two-step” model of relaxation.11J2 In short, this model divides the motions causing the relaxation into one fast, slightly anisotropic, component and a slower isotropic component. For micelles formed by single-chain surfactants with chain lengths up to 16 carbons the slower motion is very well described by a single exponential correlation function with a correlation time in agreement with the one expected from rotational tumbling of the micelle and surfactant diffusion over the curved micellar surface, while the fast motion is in the extreme narrowing regime.l0 As a natural extension to our previous work on micelles it would seem interesting to perform NMR relaxation studies on a surfactant system where one would expect very large micelles to be formed. A surfactant that would potentially form very large micelles is 6-(dimethyleicosy1ammonio)hexanoate (abbreviated as C2&H):
CH,-(-CH2-)
CH3 I ,9-N+-(-CH,),-COOI
CH3 It should be remarked that there is a considerable amount of inherent interest in a surfactant like C2&H, due to the zwitterionic head group, which resembles the one found in phospholipids, making C2,AH a suitable model compound for these systems. In what follows we will present a variable-field 13Cand 14NNMR relaxation study of the binary C2,AH/water system.
Experimental Section Materials. 6-(Dimethyleicosylammonio)hexanoate (C2&H) was synthesized as described in ref 13. D20 (atomic enrichment 99.7%) was purchased from Norsk Hydro. The isotropic samples (5-37 wt % surfactant) in heavy water were prepared directly in 10-mm NMR tubes. T h e samples with a concentration greater than 40 wt % were prepared by weighing appropriate amounts of surfactant and heavy water into glass tubes that were flame sealed. These samples were homogenized by repeated heating and centrifugation and then left 7 days to equilibrate a t 25 “C before being used. Methods. Phase Diagram. In order to determine the phase diagram we made 19 samples evenly spaced in the concentration interval 5-95 wt %. Samples with concentrations greater than 35% were investigated as a function of temperature, in that 2H spectra from the heavy water contained in the samples were recorded every 10 “C between 25 and 95 “C. NMR Experiments. 2H NMR spectra were recorded at 6.00 T on a home-built spectrometer equipped with an Oxford (4) Canet, D.;Marchal, J. P.; Nery, H.; Robin-Lherbier, B. J. Colloid Interface Sci. 1983, 93, 241. ( 5 ) Marchal, J. P.; Canet, D.; Nery, H.; Robin-Lherbier, B.; Cases, J. M. J . Colloid Interface Sci. 1984, 99, 349. ( 6 ) Walderhaug, H.; Soderman, 0.;Stilbs, P. J. Phys. Chem. 1984,88, 1655.
(7) Tricot, Y.; Kiwi, J.; Niederberger, W.; Gratzel, M. J.Phys. Chem.
1981, 85,862. (8) Brown, M. F.; Ribeiro, A. A.; Williams, G. D. Proc. Nutl. Acad. Sci. U.S.A. 1983,80, 5325.
(9) Soderman, 0.; Walderhaug, H.; Henriksson, U.; Stilbs, P. J.Phys. Chem. 1985,89, 3693 and references therein. (10) Sijderman, 0.; Henriksson, U.; Olsson, U. J . Phys. Chem. 1987,
Soderman et al. wide-bore superconducting magnet. 13Crelaxation measurements were performed at 8.48 and 1.41 T on a Nicolet 360 spectrometer and on a Jeol FX-60 FT NMR spectrometer, respectively. I4N NMR relaxation measurements were carried out at 6.00 and 8.48 T with the same spectrometers as above. Additional measurements were performed at 1.88T on a Varian FT 80A spectrometer, equipped with a broad-band probe. Unless otherwise stated, the temperature in all measurements was 26 f 0.5 “C. Spin-lattice relaxation times were measured with the standard inversion recovery method. Spin-spin relaxation times (T2)for I4N were deduced from the bandwidths of the 14NNMR signals taken a t half-height after suitable corrections for the magnetic field inhomogeneities. Computational Details. All calculations on the raw experimental data were performed as described in ref 6.
Theoretical Considerations For a 14Nnucleus (spin I = 1)the quadrupolar mechanism dominates the NMR relaxation, and the spin-lattice (R,) and the spin-spin (R,) relaxation rates are given by14
Here x is the quadrupolar coupling constant. The main contribution to the spin-lattice relaxation rates of a protonated 13Cnucleus arises from the dipoledipole interaction with its directly bound proton(s):15 R, =
(3)
Here po is the vacuum permeability, yH and yc are the magnetogyric ratios of the proton and the carbon, rC-His the carbon-proton bond distance, and N is the nugber of protons directly bound to the carbon. In eq 1-3, J(w)are the various reduced spectral density functions, while wH, oc,and wN are the Larmor frequencies of proton, carbon, and nitrogen, respectively. Within the so-called two-step model, the spectral density for the 14N nucleus is given byl1J2
J(u)= (1 + v 2 / 3 - A2)jf(w) + A2Sg(o)
(4)
where “f” and “5” indicate the spectral densities for the fast and slow motions, respectively. A is the residual quadrupolar anisotropy, often called the order parameter, the definition of which can be found in ref 12, and q is the asymmetry parameter of the electric field gradient tensor. In anisotropic phases, e.g., in a hexagonal phase, A is obtained from the quadrupolar splitting A through the relation A = ( 3 / 8 ) x A . For a 13Cnucleus, eq 4 is still valid if q is set equal to zero. In this case A is usually denoted S and is given by the expression S = ( 1 / 2 ) ( 3cos2 0 - l ) f , where the average is taken over a time that is long enough to average the fast motion but not the slow motion. 0 is the angle between the C-H bond and the local director.
Results and Discussions Phase Diagram. When any kind of study is performed on surfactant systems it is imperative that the phase di-
91, 116.
(11) Wennerstrom, H.; Lindman, B.; Soderman, 0.;Drakenberg, T.; Rosenholm, J. B. J . Am. Chem. SOC.1981, 101, 6860. (12) Halle, B.; Wennerstrom, H. J. Chem. Phys. 1981, 75, 1928. (13) McGrady, J.; Laughlin, R. G. Synthesis 1984, 5, 426.
(14) Abragam, A. The Principles of Nuclear Magnetism; Clarendon: Oxford, 1961. (15) Doddrell, D.; Glusko, V.; Allerhand, A. J . Chem. Phys. 1972,56, 3683.
Langmuir, Vol. 4 , No. 4, 1988 1041
NMR Relaxation in Zwitterionic Micelles
Table I. "N NMR Relaxation Data at Three Magnetic Field Strengths for Micellar Solutions of C2,AH at 26 "C 1.88 T (s-') 6.00 T (9-l) 8.48 T (8-l) concn, wt % R1 R2 R1 R2 R1 R2 5
50.0 f 2.0 51.0 k 1.0 51.3 0.5 49.5 f 0.5 50.9 f 0.5
10
65.4 f 0.4 70.7 f 0.9
20 30
69.0 f 3.0 68.6 f 3.0 76.2 f 3.0 81.0 f 3.0 87.3 f 3.0
*
88.6 f 1.6 96.1 f 1.6
37
44.2 f 1.8 43.7 f 1.8 41.2 f 1.7 45.2 f 1.9 47.2 f 2.0
59.5 f 3.0 62.5 f 3.1 71.4 f 3.5 75.8 i 3.8 75.2 f 3.8
Table 11. Results of Fitting Eq 1,2, and 5 t o the Data in Table In concn. wt % A T,! ns 72. ns 5 14 f 1.3 10 15 f 3.7 20 0.126 f 0.017 0.175 f 0.020 28 f 9 30 0.136 f 0.010 0.182 f 0.012 28 f 2.9 37 31 f 5.3
~
~
~
~~~
See text for details.
i
60
L1
S % 0
20
40
60
80
1
back to the rather large head group found both in CI2TACl and C,&H, which makes the formation of rod-shaped micelles difficult on account of the large repulsion between the bulky head groups (see below). NlMR Relaxation Study. Presented in Table I are the results of the 14NNMR relaxation study. There is a frequency dependence in the relaxation rates. Moreover, R1 does not equal R2 at any of the frequencies used. With a functional form for #(o) and P(o) in eq 4, it is now possible to fit eq 1, 2, and 4 to the data in Table I. For spherical micelles formed by ion& surfactants, we have shown that a Lorentzian form for Js(w)and a constant for #(a) describe frequency-dependent NMR relaxation data very well, and this is a natural first choice in the present study. Thus we write
C20AH
Figure 1. Phase diagram for the C2&H/heavy water system. L1denotes a micellar phase, I1 and 11,denote cubic phases, E is a hexagonal phase, and S denote hydrated crystals. Note that no two-phase regions are included in the phase diagram. agram is known. To this end we first present a partial schematic phase diagram of the C2&I/heavy water system in Figure 1. The diagram was drawn by observing samples of different concentrations through crossed polarizing glasses at different temperatures and by recording the 2H NMR signal from the heavy water contained in the samples. These signals gave a single Lorentzian band for micellar and cubic phases and a Pake doublet for the hexagonal phase. On inspection the phase diagram in Figure 1shows the following features. There is a large micellar region followed by a cubic phase of type I1in Ekwalls notation (please note that no two-phase regions are included in the phase diagram). The I, phase is succedded by a region with hexagonal phase, and finally at higher surfactant concentrations and higher temperatures a cubic phase of type 11,can be found. The phase diagram in Figure 1 resembles the phase diagram found for the ionic surfactant dodecyltrimethylammonium chloride, C12TAC1,16save for the fact that the latter system shows a lamellar phase at high surfactant concentration. Typical is the presence of the cubic phase 11,which provides strong evidence in favor of a situation where the micelles in the L1 region stay spherical in shape (or very nearly so) right up to the phase boundary." This feature can, at least partly, be traced (16)Balmbra, R. R.; Clunk, J. S.; Goodman, J. F. Nature (London)
1969, 222, 1159.
where ~ ,and f 7: are the correlation times for the fast and the slow motions, respectively. At this point it is necessary to digress somewhat and discuss the parameters x and q of eq 1, 2, and 4, since values for these parameters are required when extracting the correlation times and the residual anisotropy from the relaxation data. In the Appendix we present a simple argument in favor of a situation where q for C2JH is equal to 1while x for C 2 a H is equal to the value of x found in C12TAC1. Thus, x is assigned the value 116 kHz, which was the value used in ref 9 for CI2TAC1. It is interesting to note that the 14Nquadrupolar splitting for EI sample (concentration 67 wt TOC2,AH) in the hexagonal phase is 4.90 kHz at 26 "C, while it is 12.1 kHz in the hexagonal phase of C12TAC1.18 Following the reasoning in the Appendix, this difference can be ascribed to the fact that the principal axis systems do not coincide in the two molecules and that q differs in the two cases. Clearly, one needs at least three independent measurements to obtain the three parameters in eq 5. The frequency difference between 6.00 and 8.48 T is rather small, and with only four data points (two each at 6.00 and 8.48 T), the extraction of the three parameters may be less meaningful. We have therefore chosen to proceed in the following manner. First the data at 20% and 30% surfactant, where there are six independent data pairs, are used in the fit. Then the data at 570, l o % , and 37% are used to extract 7: at these concentrations, using fixed (17) Johansson,L. B.-A.;Soderman, 0. J. Phys. Chem. 1987,91,5275. (18) Eriksson,P.-0.; Khan, A.; Lindblom, G. J.Phys. Chem. 1982,56, 387.
1042 Langmuir, Vol. 4, No. 4, 1988
Soderman et al.
motions are isotropic and statistically independent, the combined effect of these motions can be expressed as ( 7cs)-l = ( 7ce,rot)-l + ( 7cs,diff)-1 (8)
log(vol
Figure 2. 14Nrelaxation data at three magnetic field strengths for a 30 w t % sample of C&H in D20 plotted vs the logarithm of the frequency in Hz. The solid lines are the results of the fits of eq 1, 2, and 5 to the data (see text for details).
values for 7,f and A , obtained from the data a t 30%. The results are shown in Table 11,while the experimental and predicted values (from A , ~ c f and , 7:) are shown for the 30% sample in Figure 2. As can be seen in Figure 2, eq 5 gives a fair description of the data. The reason for using fixed valued for 7,f and A for the data at 5%, lo%, and 37% is that these parameters depend on the local properties of the surfactant in the micelle, and these properties are not expected to depend to any greater extent on the concentration. This is also borne out by the results at 20% and 30%. We note in passing that although the values of x and 77 to some extent are uncertain given the fact that the reasoning in the Appendix may be overly simplistic, the actual values for x and 9 do not inff uence the goodness of the fit to the relaxation data in the least-squares sense. Nor do they influence the value of 7: obtained. They do, however, influence the values for 7,f and A. A doubling of x halves the obtained value for A and makes 7,f slightly less than a factor of 4 smaller, while the value chosen for 77 does not influence the value for A but affects 7,f, so that if 77 is changed from 1 to 0, 7,f increases by slightly more than 33%. The following can be said about the values in Table 11. A is 0.13, which can be compared with the value in the hexagonal phase, viz., 0.113. The difference is hardly significant, pointing to a situation where the order of the nitrogen, as measured with A , is not very different in the micellar and the hexagonal phase. The value of 7,f differs significantly from the value found for C12TACl,viz., 50 ps. This is true for any reasonable value of x. We will return to this fact below. Finally, the value of 7,S spans the range from about 14 ns a t 5% to 31 ns a t 37% surfactant. For micelles formed by single-chained ionic surfactants, it has been possible to assign the slow motion to the combined effect of rotational tumbling and surfactant diffusion over the curved micellar surface. For the first of these two motions, the Debye-Stokes-Einstein relation holds
while for the second motion the diffusion equation is valid (7)
Here RM is the radius of the micelle, DL is the lateral surfactant diffusion coefficient in the aggregate, and the rest of the quantities have their usual meaning. Since both
Given values for DL and R M a value for 7: can be calculated. Now, DL can be measured in the cubic phase 11,,and this value can be taken as an estimate for the value of the micellar DLsince the value of DL is not expected to depend critically on the aggregate g e ~ m e t r y . ' ~Unfortunately, !~~ the 11,phase is only stable at high temperatures (see Figure l ) , and we have therefore assumed that the temperature dependence of DL in the present case follows the same temperature dependence as that found for hexadecyltrimethylammonium fluoride (C16TAF).19Using a simple activation process for the lateral surfactant diffusion with the activation energy taken from CI6TiiF,one then obtains DL = 5 X m2 s-' at 26 "C. With this value for DL it can be shown that (7:I-l will be dominated by 7:"Ot for reasonable values of R M (for instance, if R M = 30 A, then the diffusion term will only contribute 10% to (~:)-l). Thus we may neglect 7:rdiff in eq 8. Using the value of 7: obtained at 5%, where the effects of intermicellar interactions are smallest, and eq 6 one finally obtains R M = 25 A. It is interesting to note that this value is slightly smaller than the extended chain length for a Cz0chain, which is 27 Given the micellar radius and group contributions to the volume of a surfactant molecule, viz., v c H . 3 = 49 A3, 2+3,v = 28 A3, and Vcoo- = 35 A3,22923it is possible to compute the aggregation number, and the value arrived at is 75. It should be noted that this result deviates from that found by Faucompre et al., who found for some zwitterionic surfactants that the micellar aggregation number was indeed given approximately by the extended chain length.24 In that study both the hydrocarbon chain and the head-group dipole were shorter than in the present case, a fact that may explain the difference in the aggregation numbers between the two studies. It follows that the increase in 7: as the surfactant concentration is increased can be caused by either an increase in the aggregation number (the micelle can still grow while retaining its spherical shape) or an increased intermicellar interaction. Of course a combination of the two effects is also possible. An interesting question when dealing with zwitterionic surfactant of the present type concerns the arrangement of the head-group dipole. There are two conceivable extreme cases. In one the dipole is extended out from the micelle and in the other the dipole is folded back onto the micellar surface, so that the methylene groups in the dipole are embedded in the micelle. Clearly the micelle in these two cases would have different dimensions. In the first case the radius would be that of the CzOchain plus the contribution from the head-group dipole (i.e., for an alltrans molecule 37 A), while in the second case the micellar radius would be that of the Czo chain alone. The radius obtained from the slow correlation time at 5 w t % supports the latter of these two possibilities, i.e., that the head-group methylene groups are embedded in the micelle, thus minimizing the water/hydrocarbon contact. It follows that the area per head group, Ab, is relatively large for C,JH, ~
(19) Lindblorn, G.; Larason, K.; Johansson, L. B.-A.; ForsOn, S. J. Am. Chem. SOC.1979,101, 5465. (20) Nery, H.; Siiderman, 0.;Canet, D.; Walderhaug, H.; Lindman, B. J . Phys. Chem. 1986, 90, 5802. (21) Tanford, C. J. J . Phys. Chem. 1972, 76, 3020. (22) Jonsson, B. Ph.D. Thesis, University of Lund, Sweden, 1981. (23) Gallot, B.; Skoulios, A. Kolloid Z. Z. Polym. 1966, 208, 37. (24) Faucompre, B.; Lindman, B. J.Phys. Chem. 1987, 91, 383.
NMR Relaxation in Zwitterionic Micelles Table 111. lacSpin-Lattice Relaxation Rates at Two Field Strengths for Selected Carbons of a 30 wt % CmAH Sample at 26 “C Rl, s-l carbona 1.41 T 8.48 T 1,l’ 16.7 i 1.7 5.6 i 0.2 2.2 i 0.02 46 1.7 f 0.01 6.9 f 0.34 176 2.4 f 0.04 2OC 2.7 f 0.13 1.03 f 0.004 2’ 5.2 f 0.27 3.4 f 0.09 5’ 4.9 f 0.3 2.7 f 0.04 ”Numbering of carbons start at the carbons adjacent to the nitrogen. Carbons in the head-group dipole are designated with a prime. bThe main methylene peak. Consists of three bands at 8.48T and one band at 1.41 T. ‘For comparisons with methylene groups, the observed R1 values have been multiplied by 2/3.
being 105 A2, in comparison with single-chain ionic surfactants, i.e., sodium dodecyl sulfate, where Ahg is 60 A2. As pointed out above, this fact is one of the reasons why the micelles in the C2&H system, in spite of the long hydrocarbon chain which normally promotes the formation of rod-shaped micelles, remain spherical or very nearly so up to high concentrations. In order to shed some light on the somewhat atypical value of 7,‘ we have extended the relaxation measurements to higher frequencies by performing a 13C T1 relaxation study. In Table I11 we present 13C T1 at two different magnetic field strengths for some selected carbons. As for nitrogen, there is pronounced frequency dependence in the data. The relaxation rates are largest for the carbons adjacent to the nitrogen; they decrease toward the wmethyl group and toward the carboxylic group. The observed frequency dependence requires the presence of motions with correlation times around a nanosecond, and in the framework of the two-step model, as applied above with a single-exponential correlation time for the slow motion, they cannot be caused by the same motion that is responsible for the frequency dependence in the 14Ndata (note that ( 7 , “ ( 2 ~ ) u , takes )~ the value 7 if 7,“ is 28 ns and uc is 15 MHz, corresponding to the lowest frequency “sampled”by 13CR1 at 1.41 T). The frequency dependence could, however, be caused by one of the motions contained in .:7 This statement deserves a few comments. The fast motions, which are described by 7:, are presumably rather complex. There will be rotations around carbon-carbon bonds, torsions, rotation of the whole molecule around its long axis, and so on. For micelles formed by single-chain ionic surfactants with chain lengths up to 16 carbons, it has been shown that the rates of these motions are all in the extreme narrowing regime for accessible NMR field strengths. In such a case 7,f is an effective correlation time or, more formally, an integral over a correlation function. The 13Cdata in Table I11 cannot be explained with a single exponential correlation function; that is, there has to be several motional components causing the 13C relaxation. What we require in the present context is then simply that at least one of these motions has a correlation time around 500 ps. This is clearly compatible with the value for 7,f obtained from the 14Ndata. Moreover, the 13Crelaxation rate for the carbon adjacent to the nitrogen is about a factor of 3 larger than the corresponding value for C16TAC1 at 1.41 T, while the differences further down the chain are smaller, being for instance some 35% larger in Cz,,AHthan in CI6TAC1for the (0-2) carbon (13Cdata for Cl6TAC1from ref 6). One would a priori expect that the internal motions in the micelle would be slower in the CzoAHthan in c16TACl on account of the fact that the viscosity of a Czo
Langmuir, Vol. 4, No. 4, 1988 1043 chain is larger than that for a c16 chain. It thus appears as if the dynamics in the head-group region in the surfactant presently under study is slowed down as compared to simple ionic surfactants while further down the chain the methylene groups are “decoupled” from this effect. This effect can presumably be traced back to the size and conformation of the head-group dipole in CzoAH,as discussed above. The same trend has been found in a recent 13C relaxation of another zwitterionic surfactant (decyldimethylammonio)propanesulfonate,where the fast correlation time was found to be around 80 ps, i.e., about a factor of 2 larger than that for C12TAC1,for the carbon adjacent to the nitrogen. The zwitterion in this surfactant has a shorter dipole and thus a smaller head-group area than C,,,AH, which may explain the shorter value of 7,f for the former surfactant. In the analysis we have assumed that the slow motion is described by a single exponential correlation function. This is certainly true for the rotational tumbling of the micelle, a motion which is always present. This means that motions that are slower than the tumbling of the micelle do not contribute to the NMR relaxation. Motions that are as fast or faster will, however, contribute. One type of motions that has been discussed in connections with NMR studies of phospholipid vesicles is so-called order director fluctuations.26 If present, these would have motional components over a large frequency range and could conceivably be the origin of the frequency dependence in the 13C relaxation data. In the present case we feel that such motions do not contribute in any major extent to the relaxation. The reason for this is twofold. First, in a recent field-cycling experiment no evidence was found that such motions are present in the nanosecond time regime27for micellar and lyotropic liquid crystalline phases in the potassium dodecanoate/water system. The second argument is a bit more subtle. As pointed out above, the residual anisotropy is approximately equal in the micelles and in the hexagonal phase of C2,AH. Now, the value of A is a measure on how much of the quadrupolar interaction is averaged by the motions present. In the micelle motions with rates down to the inverse of the correlation time of the micellar rotational tumbling contribute to this averaging (i.e., faster than around 30 MHz) while in the hexagonal phase motions which are more rapid than the magnitude of the quadrupolar splitting contribute (i.e,, motions faster than 5 kHz). It follows that, since the residual quadrupolar interaction is more or less equal in the hexagonal phase and in the micelle, any motion(s) that occur with rates between 30 MHz and 5 kHz must be small in amplitude, since otherwise they would reduce the interaction further in the hexagonal phase. This would indicate that if collective fluctuations of the director are present, their amplitudes are small and, as a consequence, their contribution to the relaxation is small. In conclusion, we have shown that NMR relaxation data for micellar C2,,AHcan be well explained with the two-step model of relaxation, where the slow motion is dominated by the micellar rotational tumbling. The analysis of the data indicates that the head-group dipole has a bent conformation so that the methylene groups of the headgroup dipole are embedded in the micellar core and the area per polar head group is substantially larger than the area in single-chain ionic surfactants. Finally, the residual anisotropy for the nitrogen is approximately the same in (25) Jansson, M.; Li, P.; stilbs, P.; J. Phys. Chem. 1987, 91, 5279. (26) Brown, M. F. J. Chem. Phys. 1982, 77, 1576. (27) Kuhner, W.; Rommel, E.; Noack, F.; Meier, P. Z.Naturforsch. 1987, 428, 127.
Langmuir 1988, 4 , 1044-1048
1044
the micellar and hexagonal phases.
Acknowledgment‘ Thanks are due to R*G* and Gamble Company, Cincinnati, for a of the kind gift of the surfactant used in the present study and to G. Karlstrom and H. Wennerstrom for valuable discussions. The Swedish Natural Science Research Council and Comitate Teccnologico (C*N.R-,Italy) are thanked for financial support. Finally, we would like to thank Bruce Springsteen for providing suitable acoustical background in our NMR room. Appendix this Appendix we will presenta simple argument in favor of a situation where the main components of the electric field gradient, V,,, and thus the quadrupole coupling constants, x, in C12TACl and C2,AH are equal, whereas the asymmetry parameters, 7, are not equal at the
site of the nitrogen in the two surfactants. Let us assume that the contribution to the field gradient on the nitrogen from the hydrocarbon chain does not depend on the length. Moreover, let us arbitrarily set this contribution equal to zero. To the contribution from the methyl partial charges are positioned at the sites of these groups, and the contribution from one such charge to the field gradient on the nitrogen is arbitrarily set equal to It is now easy to show that for C12TACl vz,= while = o, and the z-axis of the principal axis system of the field gradient tensor coincides k t h the N-a-CH2 bond. In the C-&H case, V,, = Vowhile t = 1, and the z-axis of the Principal axis system is Perpendicular to the CH2-N-CH2 plane; the axis with the field gradient component equal toO‘- is Perpendicular to the CH3-N-CH3 plane.
vo. -vo
Registry No. C,&H, 72861-47-3.
Mossbauer Spectroscopic Studies of Fe(I1)-Y, Fe(11)-Mordenite, and Fe(11)-ZSM-5 Zeolites Luis M. Aparicio and J. A. Dumesic” Chemical Engineering Department, University of Wisconsin-Madison, 1415 Johnson Drive, Madison, Wisconsin 53706 Received January 26, 1988. I n Final Form: April 12, 1988 Mossbauer spectroscopy was used to study the room temperature adsorption of dioxygen and carbon dioxide on a high %/A1 ratio Fe(I1)-Y zeolite and of carbon monoxide on Fe-mordenite and Fe-ZSM-5. All of these adsorbates were found to interact with cations that contribute Fe(II) doublets with small splittings to the spectra (i.e., inner doublets). In each case, the adsorption process increased the coordination of the cations, decreasing the spectral area of the Fe(I1) inner doublet and forming a new doublet with a larger quadrupole splitting. The adsorption of CO on Fe-mordenite and Fe-ZSM-5 removed the inner doublet completely, whereas the same adsorbate does this only partially for Fe(I1)-Y. The assignments made for the two doublets present in the spectra of these zeolites were tested theoretically by calculating temperature dependences of the quadrupole splitting for Fe(1I) cations in various exchange sites in Y-zeolite. The calculations suggested that the differences in Mossbauer spectroscopy parameters of cations giving the two doublets may arise from differences in the length and degree of covalency of Fe-0 bonds in the two types of coordination.
Introduction Mossbauer spectroscopy has been used in a number of studies to characterize divalent iron exchange into zeolites. The zeolites covered by these studies have included Y~ e o l i t e , l -m~~ r d e n i t e ,A-zeolite,6-12 ~ L-zeolite,13J4 and ZSM-5.15 In general, the Mossbauer spectra of these materials are composed of two Fe(I1) quadrupole doublets. One of these, generally denoted as the “outer doublet”, has a relatively high isomer shift (1.0-1.3 mm/s with respect to metallic Fe) and a large, temperature-dependent quadrupole splitting (2.1-2.4 mm/s at room temperature). The other doublet, generally denoted as the “inner doublet”, has a lower isomer shift (0.8-1.0 mm/s with respect to metallic Fe) and a small, temperature-indedpendent quadrupole splitting (0.5-1.0 mm/s). The outer doublet has been assigned to coordinatively saturated Fe cations, while the inner doublet has been assigned to coordinatively unsaturated cations. These assignments were originally made for Y-zeolite based on an empirical correlation between isomer shift and coordination number and
* Author to whom all correspondence
should be addressed.
0743-7463/88/2404-1044$01.50/0
on observed interactions between the cations responsible for the doublets and adsorbate gases of different ~ i z e . ~ . ~ Assignments in other zeolites have been made primarily from the similarities between their spectra and the spectrum of Fe(II)-Y.5,6J3 (1)Morice, J. A.;Rees, L. V. C. Trans. Faraday SOC.1968,64,1388. (2)Delgass, W.N.;Garten, R. L.; Boudart, M. J. Phys. Chem. 1969, 73,2970. (3)Garten, R. L.; Delgaas, W. N.; Boudart, M. J. Catal. 1970,18,90. (4)Aparicio, L. M.;Dumesic, J. A.; Fang, S.M.; Long, M. A.; Ulla, M. A.; Millman, W. S.; Hall, W. K. J. Catal. 1987,104,381. (5)Garten, R. L.; Gallard-Nechtschein, J.; Boudart, M. Ind. Eng. Chem. Fundam. 1973,12,299. ( 6 ) Dickson, B. L.; Rees, L. V. C. J. Chem. SOC.,Faraday Trans. 1 1974. 70. 2038. ( 7 ) Dickson, B. L.; Rees, L. V. C. J. Chem. SOC.,Faraday Trans. 1 1974,70,2051. (8)Dickson, B. L.; Rees, L. V. C. J. Chem. SOC., Faraday Trans. 1
-.
1974. 70. - -, 20130.
(9)Gao, Z.;Rees, L. V. C. Zeolites 1982,2, 72. (10)Gao, Z.;Rees, L. V. C. Zeolites 1982,2,79. (11)Gao, Z.;Rees, L. V. C. Zeolites 1982,2,205. (12)Gao, Z.;Rees, L. V. C. Zeolites 1982,2, 222. (13)Fitch, F. R.; Rees, L. V. C. Zeolites 1982,2, 33. (14)Fitch, F. R.; Rees, L. V. C. Zeolites 1982,2,279. (15)Petrera, M.; Gennaro, A.; Gherardi, P.; Gubitosa, G.; Pemicone, N. J. Chem. SOC.,Faraday Trans. 1 1984,80, 709.
0 1988 American Chemical Society
