J. Phys. Chem. 1082, 86, 335-340
relatively low ratio of inside to outside molecules.
Conclusions The absorption spectroscopic measurements on annealed
thin films of bimanes yield well-defined optical absorption spectra similar to those for crystals dispersed in a KBr matrix.2 The present article suggests that increased electrostatic stabilization of the excited state can account for a major part of the spectroscopic shift to longer wavelengths resulting from the change from a random arrangement of the molecules of syn-(H,Cl)B to the crystalline form. Transformation of the bent into a planar molecule may be a second, but less important, factor contributing to the shift. A simple model is useful for estimating the electrostatic stabilization of the ground and
335
excited states using the distances from an accurate crystal structure. Further work should involve the preparation of synbimanes in which the electronegative chlorine has been replaced with a first-row element (e.g., F); crystals of such bimanea could have smaller intermolecular separations and thus augmented intermolecular electrostatic interactions. In addition, the electrostatic stabilization calculations should be reconsidered when more accurate functions of molecular charge distributions in the ground and excited states become available from theoretical calculations. Supplementary Material Available: Structure factor tables (6 pages). Ordering information is given on any current masthead page.
Molecular Motion of Micellar Solutes: A I3C NMR Relaxation Study R. E. Stark,' M. L. Karakevlch, and J. W. Grangert Department of Chemlstty, Amherst College, Amherst, Massachusetts 0 1002 (Recelved:July 29, 198 1)
A series of simple NMR relaxation experiments have been performed on nitrobenzene and aniline dissolved in the ionic detergents SDS and CTAB. Using 13C relaxation rates at various molecular sites, and comparing data obtained in organic media with those for micellar solutions, we have estimated the viscosity at the solubilizationsite and derived a detailed picture of motional restrictions imposed by the micellar environment. Viscosities of 8-17 CPindicate a rather fluid environment for solubilized nitrobenzene; both additives exhibit altered motional preferences in CTAB solutions only. As an aid in interpretation of the NMR data, quasi-elastic light scattering and other physical techniques have been used to evaluate the influence of organic solutes on micellar size and shape. The NMR methods are examined critically in terms of their general usefulness for studies of solubilization in detergent micelles.
Introduction Micelles are aggregates formed by amphipathic molecules in aqueous (or organic) solutions, and they have attracted particular interest as model membranes and moderately effective catalysts.' Often at the center of their physiological function is the ability to solubilize organic compounds which dissolve only slightly in water, yet many structural and dynamic details of the solubilization process remain unclear. We have been particularly interested in the motional behavior of small organic molecules in the micellar environment, and our approach has been to focus on the physical characteristics of additives which may be introduced at concentrations low enough to avoid disruption of the micellar aggregate. In the present study, we demonstrate the usefulness of simple NMR relaxation measurements to obtain dynamic information on monosubstituted benzenes in sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium bromide (CTAB) micella. We critically compare micellar viscosities and solute reorientation rates derived from 13Cand from combined 13C/14Nstudies, in terms of their reliability, informational content, and general applicability for the study of solubilization. Theoretical Section The connection between nuclear magnetic relaxation and molecular reorientation has long been recognized by both theoreticians and experimentalists in the field.2J NSF-URP participant, 1980. 0022-3654/82/2086-0335$01.25/0
Briefly, 13C nuclear dipoles undergo transitions between spin states in order to return to equilibrium after a perturbation; these transitions often result from fluctuating magnetic fields induced by motion of 13C-lH dipoles in the molecule, which is more or less efficient in reestablishing equilibrium depending on the overall molecular tumbling time Teff. It is hardly surprising, then, that the characteristic NMR relaxation rate depends on such factors as temperature and viscosity. Alternatively, nuclei with an electric quadrupole moment (14N,%) interact with electric field gradients (efg's) of the surrounding molecular environment; again transitions may occur to the extent that the efg's fluctuate at a rate matching the NMR resonance frequency. As shown previously in studies of 50% (v/v) nitrobenzene in organic s0lvents,4*~ it is useful to employ the formalism developed by Huntress,G Woessner,' and Hubbard8 to express measured I3C and 14Nrelaxation rates in (1)(a) J. H. Fendler and E. J. Fendler, "Catalysis in Micellar and Macromolecular Systems", Academic Press, New York, 1975; (b) B. Lindman and H. Wennerstr6m, Top. Curr. Chem., 87,1 (1980). (2)A. Abragam, 'Principles of Nuclear Magnetism", Oxford University Press, London, 1961. (3) T. C. Farrar and E. D. Becker, "Pulse and Fourier Transform NMR", Academic Press, New York, 1971. (4)D. R. Bauer, J. I. Brauman, and R. Pecora, J.Am. Chem. SOC.,96, 6840 (1974),and references therein. ( 5 ) R. E. Stark, R. L. Vold, and R. R. Vold, Chem. Phys., 20, 337 (1977). ~ - -.,..
(6) W. T. Huntress, Jr., Adu. Magn. Reson., 4, 1 (1970);J. Chem. Phys., 48,3524 (1968). (7)D.E.Woessner, J. Chem. Phys., 37,647 (1962).
0 1982 American Chemical Society
336 The Journal of Physical Chemistv, Vol. 86,No. 3, 1982
Stark et ai.
terms of tumbling times, or diffusion coefficients, about each of three molecular axes: where reff(13C,para)= [4D, T,E(
+ D, + D,1/4Dr
(2a)
13C,ortho/meta) =
R1Q(14N)= (3r2/2)[e2qxQ/h]27eff(14N)
(3)
where reff(14N)= [4D,
+ (7 - 112D, + (7 + 1)2D,]/4Dr
(4)
Q and DD indicate NMR relaxation which proceeds via quadrupolar interactions (dominant for the spin-1 14N nucleus) or by a dipole-dipole mechanism (predominant for carbons bound directly to protons, and under conditions where the latter spins are ~ a t u r a t e d ) . ~yc and yH are gyromagnetic ratios (in rad/s), ( rCH)represents a vibrationally averaged C-H bond distance,1° e2q,Q/h is the principal x component (in Hz) of the quadrupole coupling (QCC) tensor, and 7 is the asymmetry parameter (9, q z ) / q x . Since the molecules under study are planar and possess CZvsymmetry, the principal axis orientations for both rotational diffusion and QCC tensors are known (see Figure 2). We also define the quantities D, = (D, D, + D,)/3 and D, = 3(D,D, + DyD, + D,Dx), and note the relation 7, = 1/6Dj (i= x , y, z ) between the diffusion constants and the time required for reorientation by 1rad. If NMR relaxation rates are measured for 14N of the substituent and for the two distinct 13Cring sites in, e.g., nitrobenzene or aniline, it should be possible to solve eq 1-4 to yield values of 7,, 7,,, and 7,. In practice, the success of this method depends on (a) prior knowledge of relevant bond lengths and quadrupole coupling parameters and (b) the presence at the quadrupolar nucleus of a large electric field gradient perpendicular to the molecular plane. If no quadrupolar species are present in the molecule, it is sometimes possible to unravel the details of molecular motion by evaluating cross-correlation terms between different pairwise dipolar interactions" or by combining NMR information with the results of depolarized Rayleigh scattering4 or Raman line-shape12experiments. When using 13C relaxation times to investigate solute dynamics in micelles, an important consideration is whether the organic additive causes significant perturbations of the aggregate structure. Thus, whether we choose to examine structurally simple solubilized species as tractable probes of more complex micellar assemblies or to study the solubilization process itself, we require independent confirmation of how the probe may affect the integrity of the micelle. While the effects of additives on critical micelle concentration (cmc) are well-known,' the present studies were conducted at concentrations well above the cmc's of SDS and CTAB detergents.13 Of greater concern, then, are
+
(8) P. S.Hubbard, J . Chem. Phys., 52, 563 (1970). (9) As described in ref 5, we aeaume for nitrobenzene and aniline that protonated carbons relax entirely by a dipolar mechanism and that nitrogen relaxation is unaffected by internal rotation about the C-N bond. (10) P. Diehl and W. Niederberg, J. Magn. Reson., 9, 495 (1973). (11) (a) L. G. Werbelow and D. M. Grant, Adu. Magn. Reson., 9,1 (1977); R. L. Vold and R. R. Vold, B o g . Nucl. Magn. Reson. Spectrosc., 12, 79 (1978). (12) K.T.Gillen and J. E. Griffiths, Chem. Phys. Lett., 17,359 (1972).
possible alterations in micelle size and shape, which we chose to evaluate by quasi-elastic light scattering (QLS) techniques.14 When laser light is scattered from a solution of macromolecules, the intensity of light scattered into a narrow range of solid angle fluctuates in a manner which depends on the translational diffusion coefficients and associated hydrodynamic radii of the particles. In practice, one obtains a mean diffusion coefficient D for solutes which are polydisperse, by measuring the average time constant (7J for decay of the autocorrelation function of the scattered-light intensity. D is inversely proportional to 7c,and the mean hydrodynamic radius (R) may be calculated by using the Stokes-Ein~tein'~relation (for spherical particles) D = kT/(6r7R) (5) where 7 is the viscosity of the solvent. QLS methods avoid the difficult intermolecular corrections required with conventional light scattering experiments, and they have been advanced by a number of investigators1618 as a rapid and noninvasive route to micelle size determination. It should be noted, however, that the validity of eq 5 in moderately concentrated detergent solutions requires that the particles interact as hard spheres. Since electrostatic interactions between charged micelles may be rather substantial, D is overestimated and R underestimated unless these effects are screened out by addition of an inert electrolyte (e.g., NaC1).16
Experimental Section Nitrobenzene and aniline were obtained from Eastman and Fisher, respectively. All organic solvents used in this study were spectral grade; 99.7% DzOwas obtained from Merck. The SDS and CTAB detergents were purchased in highly purified form from Bio-Radlg and Sigma, respectively. The viscosities (*2%) of nitrobenzene-organic solvent mixtures were measured at 25.0 f 0.5 "C with an Ostwald capillary viscosimeter. Detergent solutions containing nitrobenzene and aniline were lightly sonified for 1min with a Branson W140 ultrasound unit, in order to ensure thorough m i x i i and solubilization of the organic additives. The solutions remained homogeneous, even many months after preparation. Since both nitrobenzene and aniline are sparingly soluble in water,20we assume that the solutes reside completely in the micellar aggregates. The NMR samples consisted of -2.5 mL of solution in 10-mm 0.d. tubes. Organic mixtures contained 0.25 mL of cyclohexane-d12to provide an internal field-frequency lock signal; detergent solutions were prepared in DzO. The organic mixtures were degassed to lo4 torr with several freeze-pump-thaw cycles and torch sealed. 13CNMR measurements were carried out at 25.5 f 1.0 "C and 25.05 MHz on a JEOL FX-100 spectrometer equipped with a quadrature phase detection system.
-
-
(13) P. Mukerjee and K. J. Mysels, Eds., "Critical Micelle Concentrations in Aqueous Surfactant Systems", Natl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.) No. 36, 51, 107 (1971). (14) (a) B. Chu, "Laser Light Scattering", Academic Press, New York, 1974; (b) B. J. Berne and R. Pecora, "DynamicLight Scattering",Wiley, New York, 1976. (15) A. Einstein, "Investigation on the Theory of the Brownian Movement", Dover Publications, New York, 1956, pp 19-34. (16) (a) N. A. Mazer, G. B. Benedek, and M. C. Carey, J. Phys. Chem., 80, 1075 (1976); (b) P. J. Missel, N. A. Mazer, G. B. Benedek, C. Y. Young, and M. C. Carey, ibid.,84, 1044 (1980). (17) M. Corti and V. Degiorgio in "Solution Chemistry of Surfactants", Vol. 1, K. L. Mittal, Ed., Plenum Press, New York, 1977, pp 377-90. (18) G. Porte, J. Appell, and Y. Poggi, J.Phys. Chem., 84,3105 (1980). (19) For QLS determinations of micelle size, other workers have recommended the use of highly pure SDS procured from British Drug House; materials obtained from Bio-Rad Laboratories may consist of a mixture of chain lengths. See ref 16b for details. (20) J. H. Fendler and G. L. Gasowski, J.Org. Chem., 33, 1865 (1968).
Molecular Motion of Micellar Solutes
Relaxation rates (R, = l/TJwere determined under conditions of 'H noise decoupling by the fast inversionrecovery technique (FIRFTLZ1 The appropriate pulse sequence is [180°-~-900-acquire-~,,; we employed a waiting time T 0.5-1.OTl'S in order to achieve time savings of 2-4 factors over conventional IRFT methodsF2 while still maintaining reasonable dynamic range in each experiment. Relaxation measurements for 0.10 M nitrobenzene or aniline solutions typically required n = 300 transients and 5-24 h of spectrometer time for each run of 10-12 7 values. On the basis of the results of repeated runs, we estimate the reproducibility of the I3C R1 values to be 5 % . 14N NMR measurements on nitrobenzene were performed a t 25.6 f 0.1 "C and 4.33 MHz on a home-built spectrometer interfaced to a Nicolet 1080 Fourier transform system.B Relaxation rates (fl%) were derived from line widths a t half-height of each peak, corrected for magnetic field inhomogeneity contributions determined with a sample of NH4Cl in DzO. Each measurement typically required a few hours of signal averaging. The equality of R1 and Rz has been verified previously by means of a standard inversion-recovery experiment on neat nitr~benzene.~ Molecular reorientation rates and correlation times were obtained from the I3C and I4N relaxation data by an iterative procedure described previously6 and in the following section. Data fitting was carried out on a Digital Equipment Corp. VAX 11/780 computer. Quasi-elastic light scattering in the micellar solutions was performed at 25.0 f 0.5 "C on a spectrometer which has been described in more detail p r e v i o u ~ l y . ~Light ~ scattered at 90" from the incident beam (emanating from a 70-mW helium-neon laser) was detected by a photon counting system. The signal was processed by a digital correlator to obtain the intensity autocorrelation function. A second cumulant least-squares analysis16aof each correlation function yielded an average diffusion coefficient, D, and a parameter related to the width of the micellar size distribution. In this study we are concerned only with the average diffusion coefficient; no systematic behavior of the width parameter was found. After sonication and a resting period to remove bubbles, each solution was passed through a 0.1-pm Nucleopore filter and degassed under house vacuum. Our measurements of average diffusion constants D for 0.069 M solutions of SDS containing 0-0.6 M NaCl were in good agreement with the results of Benedek et al.16a For nitrobenzene and aniline in 0.33 M SDS, we determined apparent D's (without added salt); we estimate the reproducibility of these measurements as 3% for repeated runs on the same sample, or for nominally identical samples prepared independently.
The Journal of Physical Chemlstv, Vol. 86, No. 3, 1982 337
-
Results and Discussion Effects of Additives on Micelle Size and Shape. As noted in the Theoretical Section, it is desirable to ascertain whether the dynamic information provided by micellar solutes accurately reflects the aggregate behavior in the absence of organic additives. For instance, solute-induced alterations in micelle size should be apparent in light scattering (QLS) experiments, which can yield mean dif(21)D. Canet, G. C. Levy, and I. R. Peat, J. Magn. Reson., 18, 199 (1976). (22)R. L. Vold, J. S. Waugh, M. P. Klein, and D. E. Phelps, J. Chem. Phys., 48,3831 (1968). (23)R. R. Void and R. L. Vold, J. Magn. Reson., 19, 365 (1975). (24)N. C. Ford, Jr., R. Gabler, and F. E. Karaez, Adu. Chem. Ser., 125, 25 (1979).
I
1
0 IO
0 20
Additwe Concentration(M)
Flguro 1. Mean translational diffusion constants for micelles in 0.33 M SDS sdutions containing varying concentrations of organic additives: ( 0 )aniline: (0)nitrobenzene. Fitting procedures and interpretation of quasi4astic light scattering data are described in the text.
fusion constants (D)for nitrobenzene-SDS and anilineSDS mixtures (see Figure 1). Although the absence of added salt precludes quantitative interpretation of these results in terms of micellar radii, the implication of constant D values in each series is clear: neither solute produces significant perturbations in aggregate size under conditions of the NMR relaxation experiments. By contrast, dramatic changes in mean radius (R) and polydispersity have been observed in QLS studies of mixed micelle and of SDS solutions containing luminescent probe molecules.27 Possible changes in micelle size or shape are considerably more difficult to assess in nitrobenzene-CTAB and aniline-CTAB mixtures. Both detergent solutions exhibit a large increase in bulk viscosity upon addition of aromatic probe species, an effect which has been attributed to formation of rodlike aggregates (in 0.1 M CTAB-benzene mixtures, but only i f NaBr is present).28 The sample preparation procedures required for QLS studies become quite unwieldy under these circumstances; but, at least with larger aromatic additives, significant growth of myristoyltrimethylammonium bromide (MfI'AB) micelles has been deduced by light scattering methods (again in the presence of NaBrLz7 The evidence from prior NMR relaxation studies is somewhat conflicting in nature. 81Brline widths normally reflect changes in micelle shape because the degree of counterion binding is also altered, but such effects are induced by high concentrations of aromatic solubilizates only for CTAB concentrations in excess of -0.3 MSrn On the other hand, 14Nrelaxation rates in 0.17 M CTAB solutions exhibit a dependence on added benzene concentration which mimics the behavior (presumed micelle growth) of increasingly concentrated detergent preparations t h e m ~ e l v e s . ~ ~ We consider it likely that addition of substituted benzenes to 0.33 M CTAB does alter the micelle structure, though the nature of these changes remains an unsettled question. A more extensive investigation of both NMR and other physical properties seems warranted, at a variety of CTAB and additive concentrations. Rather than fo(25)N. A. Mazer, G. B. Benedek, and M. C. Carey, Biochemistry, 19, 601 (1980). (26)R. E. Stark,to be submitted for publication. (27)H.W. Offen, D. R. Dawson, and D. F. Nicoli, J . Colloid Interface Sci., 80, 118 (1981). (28)J. W. h e n , L. J. Magid, and V. Payton, Tetrahedron Lett., 29, 2663 (1973). (29)0. Lindblom, B. Lindman, and L. Mandell, J . Colloid Interface Sci., 42,400 (1973). (30)U. Henriksson, T. Klason, E. Florin, and J. C. Eriksson in "Magnetic Resonance in Colloid and Interface Science", J. P. Fraissard and H. A. Resing, Eds., Reidel, New York, 1980,pp 681-6.
338
Stark et al.
The Journal of Physical Chemistry, Vol. 86, No. 3, 7982
TABLE I: NMR Relaxation Rates for Nitrobenzene in Organic Media solve ntu cyclohexane n-tridecane cyclooctane n-pentadecane cis-decalin
viscosity, CP
0.77 1.49 1.71 2.09 2.11
~ , ( l ~ ~ , p a rs-l a ) ,R~, ( " C , ~ r t h o / m e t a ) ,s~- ' 0.0723 f 0.0036 0.112k 0.006 0.121 t: 0.006 0.138 f 0.007 0.148 t 0.007
0.0530 f 0.0755 % 0.0778 ?: 0.0880 2 0.0938 i:
R l (14N), 5 - l
0.0027 0.0037 0.0039 0.0043 0.0046
88.7 t 134.3 t 134.3 f 181.4t 167.3 k
R 1 (para)/ R,(ortho/meta)
0.9 1.3 1.3 1.8 1.7
1.36 f: 1.48 t 1.56 k 1.57 2 1.58 ?:
0.14 0.15 0.16 0.16 0.16
a All solutions are 0.10 M in nitrobenzene. I3C relaxation rates corrected for dipolar contributions from next-nearestneighbor protons: 3.5% for para and meta sites, 1.7% for ortho site.
cusing on micellar structure, NMR relaxation studies in CTAB mixtures can nevertheless provide a detailed picture of changes in solute dynamics which occur during the solubilization process. Micellar Viscosities. In order to determine a calibration curve for the dependence of reorientation times (rj) on solution viscosity ( q ) , 13C and 14N relaxation rates were measured for 0.10 M nitrobenzene in a variety of hydrocarbon solvents (see Table I). As demonstrated in a previous study of 50% (v/v) nitrobenzene solutions, this combination of carbon and nitrogen Rl's suffices to determine reorientation times r,, ry, and T, about each principal diffusion axis of the molecule-provided that all relevant bond distances and quadrupole coupling parameters are known inde~endently.~ In practice, we used a generalized Newton-Raphson meth0d3l to solve eq 1-4 for the diffusion coefficients Dj, finally obtaining T, (i= x , y , z ) from the relation rj = 1/60? These results are plotted in Figure 2 as a function of solution viscosity. As expected, we find that, for media of comparable viscosity, nitrobenzene generally reorients more rapidly in dilute (-1% v/v) rather than concentrated (50% v / v ) ~ solutions, probably because of the absence of specific solute-solute interactions.* In agreement with the earlier NMR study, we obtain a reasonably linear dependence of 7;s on q , where the relative reorientation rates about various axes may be rationalized largely on the basis of molecular shape. In-plane motion about z and the outof-plane motion about x are both facile modes which are rather insensitive to changes in viscosity, presumably because they do not require that nitrobenzene "push too much solvent out of the way". ry is largest and depends quite heavily on q, because this tumbling motion requires the greatest displacement of surrounding molecules. In neither of these nitrobenzene studies (nor with a combination of 13C NMR and depolarized light scattering data)5*32are the variations of rj's with 9 in accord with hydrodynamic "slip" models which describe the reorientation of solutes whose size is comparable to the surrounding solvent molecule^.^^ Although NMR relaxation methods can yield a wealth of detail regarding the overall rate and anisotropy of molecular motion, there are several difficulties with this sort of approach. First, it should be emphasized that the complete motional analysis requires 14Nas well as 13CRl's, and it relies on prior knowledge of both magnitude and orientation of the 14Nquadrupole coupling tensor. Such information is often scarce and/or difficult to obtain, particularly for molecules in a mobile liquid.% Secondly, (31) C. L. Perrin, 'Mathematics for Chemists", Interscience, New
York, 1970, p 35. (32) In a comparison of tumbling times obtained by combined l%/14N
NMR or '% NMR/depolarii light scattering (LS)techniques, we have previously suggested6 that discrepancies for 50% (v/v) nitrobenzene solutions arise in large part from difficulties in correcting LS data for the effects of pair correlations. Results of the current NMR study allow us to predict 7~ at close to infinite dilution in any solvent; we remain at odds with the empirical values by close to a factor of 2. (33) C.-M. Hu and R. Zwanzig, J . Chem. Phys., 60,4354 (1974).
Ti
( psec:
20
o f 0
&:
N ''
15
IO
5
I
0.5
I
I
I.o
II
77kP)
1.5
1
- I ir 2.o -.
Flgure 2. Reorientational correlation times for 0.10 M nitrobenzene in organic solvents. T* ( O ) , T~ (O),and T~ (0)represent tumbling times about each of the molecular axes and are determined from a combination of 13C and "N relaxation rates, as described in the text.
the nonlinear character of eq 1-4 produces rather large errors in the fitted values of ry. And most discouraging in the present context, the structural requirement of a properly placed quadrupolar nucleus in an asymmetric electronic environment conflicts with our desire to examine the solubilization process for a diverse selection of micellar solutes. As an alternative strategy, we chose to investigate solute-micelle interactions by relying entirely on 13CNMR relaxation behavior. Such a procedure is experimentally straightforward, but it involves a loss of motional information (it becomes impossible in many cases35to determine all three principal reorientation times); we hoped to obtain nevertheless a good physical understanding of the solubilization process. Figure 3 is a plot of 13C relaxation rates for 0.10 M nitrobenzene in organic solvents of varying viscosity (from Table I). As expected from a number of prior NMR s t ~ d i e s , ~we* ~find ' that both &(para) and R,(ortho/meta) (34) J. A. B.Lohman, C. A. delange, and C. MacLean, Chern. Phys. Lett.,55, 29 (1978). (35) However, see ref 11 for counterexamples in which cross-correlation effects are used to perform a full motional analysis.
Molecular Motion of Micellar Solutes
The Journal of Physical Chemistty, Vol. 86, No. 3, 1982 339
Rl(sec-9 /
/
R,(sed
/
0.15 -
IG3cFy
84cPp /
04 -
para
/ / /
/
/
/
/
/
/
/
/
/
O.'O
/
/
/
/
/
t
0.5
I.o
I .5
2.o
7) (CP)
Flgure 3. '% spin-lattice relaxation rates for 0.10 M nitrobenzene in organic solvents.
TABLE 11: NMR Relaxation Behavior for Additives in Organic and Micellar Media R,('3C,yira),bR,("C,ortho/ Smeta).b ,. s-l
R ,(para)/
R,(ortho/ meta)
mixture' nitrobenzene1.51' organics nitrobenzene- 0.476 t 0.024 0.256 f 0.013 1.86 i 0.19 SDS nitrobenzene- 0.651 i 0.033 0.506 % 0.025 1.29 % 0.13 CTAB aniline-
0.219 f 0.016 0.182 f 0.014 1.20 f 0.18
aniline-
0.523 f 0.026 0.357
benzened
i
0.018 1.47 t 0.15
SDS aniline1.682 t 0.084 0.762 f 0.038 2.21 % 0.22 CTAB Solutions are 0.10 M in nitrobenzene or aniline and 0.33 M in detergents, unless otherwise noted. Valves corrected for near-neighbor contributions, as noted in Table I. An average of the entries in Table I. 20% (v/v) solution. See ref 41. are proportional to solution viscosity-in fact these plots are noticeably more linear than those of T~ vs. q (Figure 2). Thus, to the extent that the environment provided by a micellar aggregate may be modeled as a viscous organic solvent, a simple analysis of 13CR1's is the NMR method of choice for obtaining precise viscosities at the solubilization site. Turning to the micellar solutions themselves, it is clear from eq 1that the dramatic enhancements of 13C relaxation (shown in Table I1 for both nitrobenzene and aniline) result from larger values of T,S, i.e., molecular motion which requires a longer time period for a 1-rad reorientation of the relevant C-H vector. We can draw more quantitative conclusions by using the calibration plots of Figure 3 to derive micellar viscosities for nitrobenzene in two common ionic detergents (see Figure 4). Our values range from 8 (36)G. C.Levy,R. A. Komoroski, and J. A. Halstead, J. Am. Chem. SOC.,96,5456 (1974). (37)F. M. Menger and J. M. Jerkunica, J . A m . Chem. SOC.,100,688 (1978).
(38)S. Ohnishi, T.J. R. C y , and H. Fukushima, Bull. Chem. SOC. Jpn., 43,673 (1970).
(39)M. Shinitzky, A X . Dianoux, C. Gitler, and G. Weber, Biochemistry, 10,2106 (1971). (40) F. M. Menger, Acc. Chem. Res., 12,111 (1979).
340
The Journal of Physical Chemistry, Vol. 86,No. 3, 7982
Stark et at.
effects are involved in the solubilization process. the size of the “local” order parameter of the relevant methylene fragment. The importance of these effects for In the present study, we chose to examine the influence solutes dissolved in SDS and CTAB micelles is unclear, of micelle structure on nitrobenzene and aniline, two s h though evidence for solute ordering is available from ple organic molecules whose motional behavior in consplittings of C6D6and C& in the concenventional solvents is already well u n d e r ~ t o o d . ~ ~ quadrupole ~~~~ trated micellar phase (L,) of CTAB.47 Similar TlanomEquations 1-4 show that 13C (and 14N)relaxation rates depend in a complex manner on diffusional motion about alies and a slowly relaxing local structure model have been each principal molecular axis defined in Figure 2. A good noted recently for solutes in thermotropic liquid crystals just above the nematic-isotropic phase transition temdeal of physical insight is provided, however, by the simple ratios Rl(para)/Rl(ortho/meta), summarized for the two perature.@ We are currently investigating the possibility of field-dependent T1’s in SDS and CTAB solutions conaromatic solutes in the last column of Table 11. taining aromatic compounds which are thought to be First, consider the possibility that tumbling times in solubilized near the micelle surface. micellar solutions follow the trends delineated in organic solvents, as shown in Figure 2. Since ry increases much Conclusions more rapidly in viscous media than do T , or rZ,the relative This study illustrates the usefulness of 13C NMR recontributions of each mode of molecular motion will neclaxation techniques in studies of solute dynamics in deessarily be altered for solutes which reorient in micellar tergent micelles. First, 13CRl’s can provide a reasonable environments of 7 10 cP. In other words, even in the estimate of viscosity at the site of an organic additive, absence of specific solute-micelle interactions, we expect particularly if the micelle does not seriously alter the diffusional motions about the y axis to decrease in relative motional preferences of the solute molecules. Comparisons importance. Using eq 1 and 2, we calculate that R1of the relaxation behavior in organic vs. detergent media (para)/Rl(ortho/meta) for nitrobenzene would increase offer a straightforward measure of micellar viscosity, poslightly to 1.70 and to -1.72 in SDS and CTAB, respectentially applicable to a wide variety of probe solutes and tively. catalytic substrates. Whenever possible, viscosities derived Does solubilization in a micellar aggregate actually imfrom 13CRl’s should be checked against solute behavior pose motional restrictions on organic solutes? Several in a comparable noninteracting organic solvent. investigat~rs~~v~l have claimed that an increase in R1By obtaining relaxation data at several sites on an aro(para)/Rl(ortho/meta) indicates an increasing preference matic ring of the solute molecule, it is possible to establish for out-of-plane motions about the x axis, but we have how the micelle alters the relative importance of various shown above that such arguments should be applied with modes of molecular tumbling. Although the changes are caution to micellar solutions. Moreover, quantitative inof modest size for nitrobenzene and aniline, a recent study terpretation of the l3C relaxation rates in terms of motional of w-phenylalkanoic acids in SDS exhibits more dramatic anisotropy is strictly applicable only for molecules with trends.% It is plausible, but as yet unproven, that motional axial symmetry.42 interpretations of solute Rl’s are complicated by fieldFor solutes such as nitrobenzene and aniline, we can dependent contributions from the aggregate structure. demonstrate by direct numerical calculation that changes Quasi-elastic light scattering provides independent in R1 ratios are largely (but not completely) determined evaluation of micellar perturbations in organic soluteby alterations in D,, the largest rotational diffusion consurfactant systems; QLS experiments are an essential stant. On the basis of the 13Crelaxation data of Table 11, companion to natural-abundance 13C NMR studies which we draw the following qualitative conclusions: motional are necessarily conducted at high solute concentrations. anisotropy is not affected appreciably for either solute in Addition of nitrobenzene and aniline has no effect on the SDS solutions; CTAB micelles decrease the importance size of SDS micelles, so that our NMR results appear to of x motions for nitxobenzene but enhance them for aniline. reflect the environment of unperturbed aggregates. Since both solutes are probably solubilized near aggregate Changes in size and/or shape seem likely in CTAB mixthe motional results and implications of tures, yet 13Crelaxation in these systems remains a senspecific solute-CTAB interactions are particularly intrisitive proba of alterations in solute dynamics which are guing. Additional determinations of solute location via 13C associated with the solubilization process. Both sorts of chemical shift or UV spectral studies would be useful in physical information can contribute significantly to our developing a fuller understanding of these trends. understanding of the role of micelles in catalytic and Wennerstrom et al. have recently proposed that the membrane environments. anomalously high R1 ratio in o-phenylalkanoate micelles3’ is attributable not to motional anisotropy effects but rather Acknowledgment. This work was supported in part by to differing degrees of short-term order for C-H vectors grants from the National Institutes of Health (RR-07110) at para and ortho/meta positions.46 They present a model and the National Science Foundation (SPI-7926547). We for 13C relaxation in sodium octanoate micelles, which wish to thank R. R. and R. L. Vold for their encouragement includes a field-dependent contribution from slow overall and for providing access to 14N NMR facilities at the motion of the aggregate and which depends critically on University of California, San Diego. We are also indebted to D. J. Hewitt and J. E. Kleiner for performing some of (41)G. C. Levy,J. D. Cargioli, and F. A. L. Anet, J.Am. Chem. SOC., the light scattering experiments and to K. H. Langley and 95, 1527 (1973). M. R. Pate1 at the University of Massachusetts for use of (42)A. Allerhand, D. Doddrell, and K. Komoroski. J . Chem. Phvs.. - 66. . the light scattering facilities supported by NSF grants 189 (1971). (43)J. C. Erikason and G. Gillbera. PCM-7815795 and PCM-8021136. -. Acta Chem. Scand.. 20. 2019
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