1228
Langmuir 1993,9, 1228-1231
NMR Study of the Transformation of Sodium Dodecyl Sulfate Micelles Jing Zhao and B. M. Fung' Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019-0370 Received August 31,1992. I n Final Form: February 5,1993 Sodium dodecyl sulfate can form three types of micelles: spherical, ellipsoidal, and rodlike. When the concentration of the surfactant is above the critical micelle concentration (cmc)with no added electrolyte, spherical micelles are formed. At higher surfactant concentrations, spherical micelles can transform into larger micelles, most likely ellipsoidal. In the presence of added salt above a threshold concentration, both spherical and ellipsoidal micelles can change into rodlike micelles. A study of the transformation between different types of sodium dodecyl sulfate micelles by NMR is described, and the threshold values of NaCl have been determined.
Introduction A general characteristic of surfactants is the formation of micelles above a certain concentration in aqueous solution. This concentration is called the critical micelle concentration (cmc). When certain physical properties such as conductivity, surface tension, osmotic pressure, and chemical shift are plotted against the surfactant concentration, each shows a break point a t the cmc.' The major factors that affect the value of the cmc and the size of the micelles are the nature of the polar group, the surfactant counterion, the length and the structure of the hydrophobic chain, and the concentration of added ~ a l t s For . ~ solutions ~~ of ionic surfactants, the micelle size and shape may show abrupt changes when the concentration increases to a value much higher than the cmc or when the concentration of added salt has reached a certain threshold value. For solutions of sodium dodecyl sulfate (SDS)without added salt or with a low salt concentration, the volume of micelles increases abruptly at a certain concentration of the surfactant,= which is sometimes called the cmc(I1). However, the aggregation number of surfactants in water increases only slightly with the increase of concentration above the cmc(II),7 and it has been suggested that spherical or quasi-spherical micelles are the only micellar forms existing in the solution^.^ When extra salt is addedand its concentration reaches a threshold value, rodlike micelles forms12 because the presence of salt ions near the polar heads of the surfactant molecules decreases the repulsion force between the head groups. A reduction in the repulsion makes it possible for the surfactant molecules to approach each other more closely and form larger aggregates, which requires much more space for the hydrophobic chains. Because a spherical (1) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley Interscience: New York, 1978. (2) Stigter, D.; Mysels, K. J. J . Phys. Chem. 1955,55, 45. (3) Miura, M.; Kodama, M. Bull. Chem. SOC.Jpn. 1972,45,428. (4) Kokama, M.; Miura, M. Bull. Chem. SOC.Jpn. 1972,45,2265. (5) Kodama, M.; Kubota, Y.; Miura, M. Bull. Chem. SOC.Jpn. 1972, 45, 2953. (6) Kubota, Y.: Kodama, M.: Miura, M. Bull. Chem. SOC.J m . 1973, 46,100. (7) Lianos, P.; Zana, R. J . Colloid Interface Sci. 1981, 84, 100. (8) Hayashi, S.; Ikeda, S. J . Phys. Chem. 1980,84, 744. (9) Ikeda, S.; Ozeki, S.;Tsunoda, M. J. Colloid Interface Sci. 1980,73, 27. (10) Ozeki, S.; Ikeda, S. J. Colloid Interface Sci. 1981, 87, 424. (11) Imae, T.; Kamiya, R.;Ikeda, S.J . Colloid Interface Sci. 1985,108, 215. (12) Imae, T.; Kamiya, R.; Ikeda, S. J . Colloid Interface Sci. 1984,99, 300.
micelle has a small volume, it must change into the rodlike micelle to increase the volume/surface ratio. The existence of rodlike micelles was inferred from experiments of light scattering8-ll and confirmed by direct observation under the electron microscope for some systems.1s14 However, evidence for the existence of rodlike SDS micelles is not very strong.*
Experimental Section SDS was purchased from Aldrich Chemicals and was recrystallized twice from 1-butanol. Deuterium oxide (100%)was purchased from CIL Cambridge Isotope Laboratories and used to prepare stock solutions of SDS and NaCl in D20,which were mixed in appropriated volumes to give desired concentrationsof the surfactant and NaC1. A Varian XL-300 MHz NMR spectrometer was used in the NMR study. For the chemical shift measurements, dioxane (ca. 0.1%) was used as an internal reference, and ita chemical shift was taken as 3.61 ppm.I5 The relaxationtime measurements were made by using the inversionrecovery sequence for TIand the Carr-Percell-Meiboom-Gill (CPMG)sequence16for T2. Results and Discussions When NMR is used to study solutions of surfactants, the cmc can be readily obtained by plotting the chemical shift or the relaxation rate versus the reciprocal concentration.15J7 We have found that the cmc(I1) can be obtained in a similar way if the measurement is extended to higher concentration^.^^ The results of the chemical shift measurements for SDS are plotted in Figure 1,and the values of the cmc and the cmc(I1) thus obtained are listed in Table I. There are four peaks in the proton NMR spectrum of SDS, and the numbering of the peaks is given below. The chemical shifts of peaks 2 and 3 are less sensitive to concentration changes and therefore not included in Figure 1. 4
3
2
1
CH&CH2)B-CH&H&S03-Na* ~~
(13) Fung, B. M.; Mamrosh, D. L.; O'Rear, E. A.; Frech, C. B.; Afzal, J. J. Phys. Chem. 1988,92,4405. (14) Vinson, P. K. J. Colloid Interface Sci. 1989, 133, 288. (15) Jones, R. A. Y.; Katritzky, J. N. Murrel; Sheppard, N. J. Chem. SOC.1962, 2567. (16) Carr, H. Y.; Purcell, E. M.; Pound, R. V. Phys. Rev. 1954,94,630. Meiboom, S.; Gill, D. Reu. Sci. Instrum. 1958,29, 688. (17) Chachaty, C.; Ahlnas, T.; Lindstrom, B.;Nery, H.; Tistchenko, A. M. J . Colloid Interface Sci. 1988,122, 406. (18) Zhao, J. M.S. Thesis, 1992, University of Oklahoma.
0743-~46319312409-122a$o4.00io 0 1993 American Chemical Society
NMR Study of SDS Micelles
Langmuir,Vol. 9, No. 5, 1993 1229
I
:I\,
04 0.0
'
I
'
03
a4
'
1
I
.
0.6
0.8
Yc (mM.1)
- , ,~
~
Figure 2. Spin-spin relaxation rate R2 versus the reciprocal concentration of SDS at 40 OC: 0,peak 1; peak 2; +, peak 4.
+,
0.71
a70
0
l o o a a , " Yc aH.1)
Figure 1. Chemical shift (6) versus the reciprocal concentration (l/c) of SDS in D20 at 25 "C: 0,cu-CHz group; 0,CH3 group. Table I. Critical Micelle Concentration Values of SDS Determined from NMR cmc, cmc(II), mM mM peak no. 1 (a-CH2) 8.00 IO chem shift, peak no. 4 (CH3) 8.00 84 25 "C relaxation rate, peak no. 1 1.7" 50b 40 O C peak no. 4 7.7" 6Ib literature 8.W 65d These data are the averaged results from R1 and Rz. These data are the results only from R2; see text. Reference 1. Reference 3.
The cmc value of SDS agrees well with that of the literature value. The cmc(I1) determined from the proton chemical shift of the a-CH2 group is also close to the literature value. The value determined from the data for the CH3 group is somewhat larger, but its value is less accurate because the change in the slopes is not very obvious (Figure 1). In an aqueous solution, the motional behavior of the surfactant molecules depends on whether they exist as monomers or in micelles and on the size of the micelles. When the surfactant molecules aggregateto form micelles, their motions slow down and the correlation time becomes longer, and the relaxation rates become faster. Due to fast exchange between the monomers and micelles, the observed relaxation rate, Robs, is a weighted average between the monomers and micelles, which is expressed as (1) = Pm8mo + PmiRmi where pmois the mole fraction of the surfactant molecules existing in monomeric form and pmi is the mole fraction of molecules in the micelles. Rmo and Rmi are the relaxation rates of monomer and micelle, respectively. Since Rm, C RA, both the spin-lattice relaxation rate (R1) and the spinspin relaxation rate (Rz) become larger with the increase of the surfactant concentrations. If it is assumed that the concentration of monomers is a constant and equal to the cmc, the observed relaxation rate is expressed by Robs
(2) Rob = Rmi + (Rmo- Rmi)Cm/c where Cm is the value of the cmc and c is the total Concentration of the surfactant. Therefore, when the relaxation rate is plotted against the reciprocal concentration, a straight line would be obtained for the micellar
"1
02 0.0
L, 1 a4
Q2
0.6
0.6
(mM.1) Figure 3. Spin-lattice relaxation rate R1 versus the reciprocal concentration of SDS at 40 O C : 0,peak 1; peak 2; +, peak 4. Yc
+,
solution. For the monomeric solution, the relaxation rate is a constant and has a different slope. The interception point of the two lines gives the cmc. A similar argument can be made for the transformation of spherical micelles to larger micelles. The results of the spin-spin relaxation rate RPfor SDS are plotted in Figure 2. The relaxation studies were made at a temperature (40 OC) higher than that for chemical shiftmeasurements, because we want to extend the studies to include the effect of adding NaC1, which decreases the solubilityof SDS at 25 "C. Data for peak 3are not included in Figure 2 because this peak is a superposition of nine different CH:! groups, which all have different relaxation rates. Values of the cmc and the cmc(I1) obtained from the R2 plot are also listed in Table I, and they are comparable to those obtained from the chemical shift measurements at 25 OC. The spin-lattice relaxation rate (R1 = UT,) for each group is about 30-40% of the corresponding Rz values. However, plots of R1 versus l / c are not linear above the cmc, and there is no clear break point corresponding to the cmc(I1) (Figure 3). To explain this, let us consider the proton relaxation rates, which are expressed by19
R, = UT, = (1/5)1(1+ i)(r4h2/r6)[~(W)+ 4 4 2 4 3 (3) R, = 1/T, = (1/5)1(1+ l)(r4h2/r6)[(3/2)5(0) + (5/2)J(w) + J(2w)l (4) for intramolecular homonuclear dipole-dipole interaction and
R, = I/T, = ( 2 . r r / 5 ) w " ( ~
+ i ) r 4 ~ 2 [ ~ ( W +) 4 m W ) l (5)
(19)Harris, R. B.Nuclear Magnetic Resonume Spectroscopy; Long man Group (FE)Ltd.:London, 1986.
Zhao and Fung
1230 Langmuir, Vol. 9, No. 5, 1993 R, = 1/T2= (2?r/5)(N/aD)I(I+ l)y4h2[(3/2)J(0)+ (5/2)J(w) + J(2w)l (6) for intermolecular homonuclear dipole-dipole interaction. In eqs 3-6, y is the magnetogyric ratio, r is the protonproton distance, N is the concentration of spins, a is the distance of closest approach of two different spins, D is the diffusion coefficient, and the J's are spectral densities. Becausethe molecular motions of the surfactant molecules are complicated, involving segmental motions of the alkyl chains and anisotropic rotations of the whole molecule, quantitative treatment of the spectral densities will not be attempted here. It suffices to note that R1 is mainly determined by segmental motions of the aliphatic chain and R2 is mainly determined by the slower rotation of the whole surfactant molecule. The latter has a large contribution to the J, term in eq 4 and 6, especially when the surfactant molecules aggregate to form micelles. Therefore, R2 > R1 for all groups, and the CH3 group has the smallest relaxation rate because of its internal rotation. It has been suggested that at the cmc(I1) spherical micelles of SDS change shape to form larger micelles?-' which are most likely ellipsoidal. These ellipsoidal micelles would have longer rotational correlation times than the spherical micelles, but the segmental motions of the chains probably do not change considerably. Since R1 is mainly determined by segmental motions, it is not surprising that the formation of ellipsoidal micelles causes the plots of RI versus l / c to be nonlinear (Figure 3) above the cmc rather than showing a break at the cmc(I1). On the other hand, R2 is affected more by the slow rotation of the micelle itself. In particular, the a-CH2 group has more restricted segmental motion than other groups in the chain. Therefore, a discernible break in the plot of Rz versus l/c can be observed for the a-CH2 group (peak 1 in Figure 2). When the sharpest linear segment is extrapolated to l/c = 0, the value of Rz is about twice that at the cmc(I1). This shows that the change in the micelle size is indeed not very pronounced, in agreement with a previous study7and the suggestion of "quasi-spherical", or ellipsoidal, micelle formation. When salt is added to surfactant solutions above the cmc and the salt concentration is above a threshold value, the spherical or ellipsoidal SDS micelles can be transformed into rodlike micellese8However, the results of light scattering are not very definitive: and when the viscosity and conductance of the solutions are plotted against the concentration of NaC1, they did not yield sharp breaks.20 The results of proton chemical shift study are also not very clear-cut.l* On the other hand, since rodlike micelles would have much slower rotational correlation times than spherical or ellipsoidal micelles, a study on the relaxation rates for aqueous solutions of SDS with different concentrations of NaCl may offer better information on the formation of rodlike micelles. For this purpose, relaxation studies were carried out for three sets of samples with SDS concentrations of 21, 42, and 104 mM and NaCl concentrations from 0 to 1.4 M. The reason we chose these three concentrations is that rodlike micelles form only in solutions with the surfactant concentrations above the cmc. The first two SDS concentrations are between the cmc (8 mM) and the cmc(I1) (65 mM), and the third SDS concentration is above the cmc(I1). If it is considered that small micelles aggregate to form rodlike micelles when counterion binding reaches a critical value, the process would be similar to the aggregation of monomers to form micelles. The rotational relaxation time (20)Ozeki, S.;Ikeda, S. J. Colloid Interface Sci. 1980, 77, 219.
0.8
0
5
10
P
15
uc*(M.1)
0
5
10 l/C*
m
15
(MI)
Figure 4. Relaxation rate R1 of SDS (21 mM) versus the reciprocal concentration of NaCl at 40 "C.
a, 10 0
0
5
10
15
a0
UC. mr1)
0
5
10
15
a,
uc* MI)
Figure 5. Relaxation rate Rz of SDS (21 mM) versus the reciprocal concentration of NaCl at 40 "C.
of the rodlike micelles would be different from those of the small micelles, and the chemical shifta and relaxation rates would be affected accordingly. If we consider the ratio of the head groups of the surfactant molecules to the tightly bound counterions to be a constant, the observed relaxation rates would follow a correlation similar to eq 2 after the rodlike micelles start to form Robs = R,
+ (R, - R,)C,/c*
(7)
where the subscripts r and s refer to rodlike micelles and small micelles, respectively, C, is the threshold value of added salt, and c* is the total salt concentration. The effect of activity coefficients is not considered in these expressions. When R1or R2 is plotted against the reciprocal
Langmuir, Vol. 9, No. 5, 1993 1231
NMR Study of SDS Micelles Table 11. Threshold Values of NaCl for the Formation of Rodlike Micelles in SDS Solutions at 40 OC SDS threshold value of NaCl concn (mM) peak1 peak2 peak3 peak4 average 21
R1
Rz 42
R1
104
Rz R1
Rz
0.60 0.69 0.61 0.66 0.54 0.46
0.85 0.71 0.70 0.66 0.56 0.47
0.69 0.66 0.52
0.78 0.72 0.71 0.69 0.60 0.52
0.73 f 0.13 0.70 f 0.02 0.67 f 0.06 0.67 f 0.02 0.57 f 0.03 0.49 f 0.03
Table 111. R1 and RZValues of SDS Aqueous Solutions at l/c* = 0 and 40 OC SDS concn
21mM
42mM
104 mM
R , peak4 1.23f0.06 1.26 f 0.06 1.30 f 0.07 R2 peak 3 (9.1 f 0.5) X lo2 (1.00 f 0.05) X lo2 (1.00 f 0.05) X lo2 (3.5 f 0.2) X 102 peak 4 (3.5 f 0.2) X lo2 (4.4 f 0.2) X 102
of the salt concentration, one would expect to observe a break point correspondingto the criticalsalt concentration for the formation of rodlike micelles. Plots of R1 and RZfor the samples with 21 mM SDS versus the reciprocal concentration of NaCl are shown in Figures 4 and 5, respectively. Plots for other SDS concentrations are similar.l* A break point appears in each of the curves, indicating an abrupt change in the motional behavior of the surfactant molecules. Two straight lines are drawn, and the calculated values of the reciprocalconcentrationat the intersectionpoint are listed in Table 11. Because the solution is above the cmc, this point would correspond to the reciprocal threshold value of NaCl for the formation of rodlike micelles from spherical micelles. The average value of the threshold concentration of NaCl is 0.74 f 0.13 M from the R1 data and 0.70 f 0.02 M from the Rz data. Since the changes in Rz (Figure 5 ) are much more pronounced that those in R1 (Figure 41,we regard the latter value as more reliable. The reason for the larger change in Rz has been explained previously. Two more sets of samples with 42 and 104 mM SDS were measured, and results similar to those for the first set were acquired. The threshold values obtained from the plots are shown in Table 11,and they are slightlysmaller than that for the first set. The decrease in the threshold value of NaCl with the increase in the concentration of SDS indicatesthat rodlike micelles are formed more easily at higher concentrations of SDS. Peak 3 in the proton NMR spectrum of SDS is a superposition of the signals of nine CHZgroups. Because different CHz groups may have different TIvalues, the composite data are rather scattered and do not change as much as those for the other peaks to yield definite values for the cmc(I1).
From the plots of R1 and Rz vs l/c*, more information can be obtained when the curves are extropolated to l/c* = 0,or c* = c(NaC1) = 0 3 , Because of the fairly large uncertainties of the results of peaks 1and 2,only data for peaks 3 and 4 are listed in Table 111. The results show that the limiting relaxation rates are much larger than the relaxation rate of the small micelles. For example, in a solution of 42 mM SDS without added NaC1, Rz = 1.3 s-l for peak 4 (Figure 2), compared with Rz = 440 s-1 for the same peak at c* = (Table 111). Furthermore, at l/c* = 0,Rz >> R1 (Table111). These comparisons of the relaxation rates c o n f i i that the presence of NaCl abovethe threshold value induces a change in the micellar conformation to form much larger, most likely rodlike, micelles. It should also be noted that the values of R1 and Rz for peaks 3 and 4 at c* = 03 are independent of the SDS concentration within experimental error. In other words, under the limiting condition of infinite NaCl concentration,the sizes of the rodlike micelles with different SDS concentrations are similar. This implies that the spherical micelles and the ellipsoidal micelles both would reorganize to form the same kind of rodlike micelles when the concentration of NaCl is above its threshold value. Another interesting result is that the Rz values of peak 3 are approximately 3 times the RZvalues of peak 4. This can be explained by the fact that the methyl group (peak 3) is freely rotating about the 3-fold axis.lg In summary,the transformation between different types of SDS micelles has been studied by proton relaxation. In aqueous solutionsof SDSwithout added salt, the relaxation rate Rz at infinite surfactant concentration is about twice the Rz at the cmc(I1). This is consistentwith the suggestion that the spherical micelles change to larger "quasispherical" shape, most likely ellipsoidal, at the cmc(I1). In SDS samples withNaC1,the relaxation rate increases much more rapidly when the NaCl concentrationincreasesabove a threshold concentration. The values of Rz at infinite NaCl concentration are about 20-30 times the value of Rz at the threshold NaCl concentration. The large increase of Rz above the threshold NaCl concentration indicates a significant increase in the size of the micelles, and the most possible conformational change is from spherical or ellipsoidal to rodlike micelles. Values of the NaCl threshold concentration were determined, and they are smaller for higher SDS concentrations.
Acknowledgment. We acknowledge the financial support of the industrial sponsors of the Institute for Applied Surfactant Research which include Kerr-McGee Corporation,SandozChemicals Corporation, E. I. Du Pont De Nemours & Co., and Union Carbide Corporation.
