4296
J . Phys. Chem. 1990, 94, 4296-4298
Electron Spin Echo Modulation Study of Sodium Dodecyl Sulfate and Dodecyltrimethylammonium Bromide Micellar Solutions in the Presence of Urea: Evidence for Urea Interaction at the Micellar Surface Piero Baglioni,* Department of Chemistry, University of Udine, 331 00 Udine, Italy
Enzo Ferroni, Department of Chemistry, University of Florence, 501 21 Florence, Italy
and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: November 6, 1989)
Electron spin echo studies have been carried out for a series of x-doxylstearic acid (x-DSA, x = 5 , 7 , IO, 12, 16) and 4-octanoyl-2,2,6,6-tetramethylpiperidine1-oxy (C,-TEMPO) spin probes in micellar solutions of anionic sodium dodecyl sulfate (SDS) and cationic dodecyltrimethylammonium bromide (DTAB) in D 2 0 and in the presence of 2 or 6 M urea or urea-d4. Modulation effects due to the interaction of the unpaired electron with urea and water deuterium show that urea does not affect the bent conformation of the x-DSA probe in the micelle. The analysis of the deuterium modulation depth and the Fourier transformation of the two-pulse electron spin echo spectra show that urea interacts with the surfactant polar headgroups at the micelle surface. It is also found that nondeuterated urea exchanges ‘H with *H in SDS/D20and DTAB/D,O and that the interaction strength of urea is greater with DTAB than SDS surfactant. These results support recent molecular dynamics and Monte Carlo calculations of micellar systems and are in agreement with direct interaction of urea at micellar surfaces in which it replaces some water molecules in the surface region.
Introduction
A common method used to study hydrophobic interactions in biopolymer systems and micellar solutions is the modification of the aqueous solvent by addition of electrolyte or nonelectrolyte substances. Urea and its derivatives are well-known denaturants of proteins’s2 and increase the critical micellar concentration3” of ionic and nonionic surfactant solutions. Two different mechanisms have been proposed by which urea perturbs the mixing state of hydrophobic solutes in aqueous solutions. One is an indirect mechanism in which urea changes the “structure” of the interfacial water to facilitate the solvation of a hydrocarbon chain’s8 and the other is a direct mechanism in which urea participates in the solvation of hydrophobic solutes in water by replacing some water molecules that solvate the hydrophobic chain and the polar headgroup of the amphiphile.g-l’ The indirect mechanism has received most of the attention and it is the most widely accepted. Many experimental results seem to support that the addition of urea to water does destroy the solvent structure.’s12-20 However most of the experimental ( I ) Franks, F. Water, A Comprehensioe Treatise; Plenum: New York, 1978; Vol. 4. (2) Schellman, J. A,; Schellman, C. In The Proreins, Neurath, H., Ed.; Academic Press: New York, 1974; Vol. 11, p 1. ( 3 ) Burning, W.; Holtzer, A. J . A m . Chem. SOC.1961, 83, 4865. (4) Mukerjee, P.; Ray, A. J . Phys. Chem. 1963, 67, 190. (5) Schick, M. J. J . Phys. Chem. 1964, 68, 3585. (6) Emerson, M. F.; Holtzer, A . J . Phys. Chem. 1967, 71, 3320. (7) Frank, H. S.; Franks, F. J . Chem. Phys. 1968, 48, 4746. (8) Wetlaufer, D. B.; Malik, S.; Stoller, L.; Coffin, R. I. J . Am. Chem. SOC. 1977, 99, 2898. (9) Nozaki, Y.; Tanford, C. J . Biol. Chem. 1963, 238, 4074. ( I O ) Enea, 0.;Jolicoeur, C. J . Phys. Chem. 1982, 86, 3370. ( 1 1 ) Roseman, M.; Jencks, W. P. J . Am. Chem. SOC.1975, 97, 631. (12) Kresheck, G . C.; Scheraga, H. A. J . Phys. Chem. 1965, 69, 1704. (13) Finer, E. G.: Franks, F.; Tait, M. J . Am. Chem. SOC.1972, 94, 4424. (14) MacDonald, J. C.; Serpillis, J.; Guerreva, J. J . J . Phys. Chem. 1973, 77, 370. ( 1 5 ) Bonner, 0. D.; Dednarek, J. M.; Arisman, R. K. J . A m . Chem. SOC. 1977, 99, 2848. (16) Herskovits, T. T.; Kelly, T. M. J . Phys. Chem. 1973, 77, 381. (17) Lang, J.; Tondre, C.; Zana, R. J . Phys. Chem. 1971, 75, 374. (18) Manabe, M.; Koda. M.; Shirahama, K. J . Colloid Interface Sci. 1980, 77, 189. (19) Philip, P. R.; Perron, G.; Desnoyer, J. E. Can. J . Chem. 1974, 52, 1079.
techniques used in these studies did not provide information at a molecular level and conflicting interpretations of the urea effect have been proposed.21+22Recent computer simulation of the urea effect in aqueous media showed negligible influence of the urea on the water structure while urea does weaken the water-water interaction by displacing several water molecules from an apolar solvation ~ h e 1 1 . ~ ~ - ~ * In a previous study on the effect of urea addition to micellar solutions of anionic sodium dodecyl sulfate and cationic dodecyltrimethylammonium bromide, it was shown, by monitoring the electron spin resonance of nitroxide radicals,29that urea slightly decreases the polarity of the micellar interface and increases the microviscosity of the micellar interface from 20% to loo%, depending on the surfactant and on the urea concentration. These results were interpreted as a direct mechanism of urea interaction with the micelle surface and are in agreement with a picture in which urea solubilizes at the micellar interface by displacing several water molecules that solvate the polar headgroup of the amphiphile. Electron spin echo (ESE) spectroscopy is a pulsed version of electron spin resonance (ESR) and enables the detection of very weak electron-nuclear dipolar hyperfine interactions. These interactions can be detected up to about 5-6 A from the unpaired electron and can be analyzed in terms of the number and distance of the magnetic nuclei interacting with the unpaired electron. This technique has been successfully employed to study the micellar (20) Rupley, J. A. J . Phys. Chem. 1964, 68, 2002. (21) Subramanian, S.; Sarma, T. S . ; Balasubramanian, D.; Ahuwalia, J. C . J . Phys. Chem. 1971, 75, 815. (22) Swenson, C. A. Arch. Biochem. Biophys. 1966, 117, 494. (23) Kuharski, R. A,; Rossky, P. J. J . A m . Chem. SOC.1984, 106, 5786. (24) Kuharski, R. A.; Rossky, P. J. J . Am. Chem. Soc. 1984, 106, 5794. (25) Tanaka, H.; Nakanishi, K.; Touhara, H. J . Chem. Phys. 1985, 82, 5. 1..R4.
(26) Tanaka, H.; Touhara, H.; Nakanishi, K.; Watanabe, N . J . Chem. Phjs. 1984, 80, 5170. (27) Marchese, R. A.; Mehrota, P. K.; Beveridge, D. L. J . Phys. Chem. 1984, 88, 5692. (28) Cristinziano, P.; Lelj, F.; Amodeo, P.; Barone, G.; Barone, V . J . Chem. SOC.,Faraday Trans. 1 1989, 85, 621. (29) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J . Phys. Chem., submitted for publication. (30) Narayana, P. A,; Kevan, L. Magn. Reson. Reti. 1983, I , 234
0 1990 American Chemical Society
ESEM Studies of SDS and DTAB Micellar Solutions
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4291 SDS/UREA/D,O
SDS/SDSA/UREA I
DTAB/SOSP, JREA
0.7
=f
5
g
7
10
12
16
SDS/UREA-d,/D,O
g0.71 6M Figure 1. Two-pulse electron spin echo decay envelopes at 4.2 K for 5-DSA in DTAB/ D20/urea-d4 and S D S / D 2 0 / u r e a - d 4 micellar solutions. The spectral baselines have been offset vertically to avoid overlap.
interface of pure ionic and nonionic surfactants and the effect of added aliphatic alcohols and crown ether^.^^"^ This study reports an analysis of the ESE deuterium modulation depth, which is related to the strength of the electron-deuterium dipolar interaction, for the sodium dodecyl sulfate and dodecyltrimethylammonium bromide micellar solutions in the presence of urea and urea-d,. The x-doxylstearic acids (x-DSA) and 4-octanoyl2,2,6$-tetramethylpiperidine- 1-oxy (C,-TEMPO) were used as probes of the micelle surface.
Experimental Section Sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) were obtained from Eastman Kodak. SDS was recrystallized three times from ethanol, washed with ethyl ether, and dried at 40 OC under moderate vacuum. DTAB was recrystallized three times from acetone and dried under moderate vacuum. Urea and urea-d4 were obtained from Aldrich and used without further purification. x-Doxylstearic acid spin probes (x-DSA), with x = 5, 7 , 10, 12, and 16, and 4-octanoyl-2,2,6,6tetramethylpiperidine- 1-oxy (C,-TEMPO) were obtained from Molecular Probes, Eugene, OR, and used as received. Stock solutions of x-DSA and CB-TEMPOwere prepared in chlorofarm. Stock solutions of 0.1 M surfactant were prepared in D20(Aldrich, 99+ atom %), D,O/urea, or DzO/urea-d4 and were deoxygenated by nitrogen bubbling. Films of the probes generated in vials by evaporating the chloroform were dissolved in the surfactant solution in a nitrogen atmosphere. The final probe concentration was 1 X lo4 M. All the samples were sealed in 2 mm i.d. X 3 mm 0.d. or 1 mm i.d. X 2 mm 0.d. Suprasil quartz tubes and frozen rapidly in liquid nitrogen. Two-pulse electron spin echo spectra were recorded at 4.2 K on a home-built spectrometer by using 40-11s exciting pulses.30 Results In Figure 1 two-pulse ESE spectra for the 5-DSA probe in SDS/D,O and DTAB/D20 micellar solutions as a function of urea-d, concentrations are reported. The echo decay curves show detectable modulation with periods of 0.5 and 0.08 bs corresponding respectively to electron-deuterium and electron-proton interactions. The Fourier transformation of the two-pulse spectra shows the presence of absorptions at 2.28 and 1.07 M H z corresponding to ,H and I4N nuclei. Figures 2 and 3 show the variation of the normalized deuterium modulation depth^^"'^ as a function of the doxyl positon, x, along the aliphatic chain of the stearic (31) Kevan, L. In Photoinduced Electron Transfer, Parr B; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; pp 329-394. (32) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M. J. Colloid Interface Sei. 1986, 110, 514. (33) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M.; Coleman, M. J. J . Am. Chem. SOC.1985, 107, 784. (34) Baglioni, P.; Rivara-Minten, E.; Kevan, L. J . Phys. Chem. 1988, 92, 4776 ..
(35) Baglioni, P.; Bongiovanni, R.; Rivara-Minten, E.; Kevan, L. J . Phys. Chem. 1989, 93, 5574. (36) Baglioni, P.; Kevan, L. J . Phys. Chem. 1987, 91, 1850.
2M g0.3
e
0.1 5
7
1 0 12 DOXYL POSITION ,X
16
Figure 2. Dependence of the normalized deuterium modulation depth on the position of the doxyl group, x, in x-doxylstearic acid spin probes in SDS/D,O/urea and S D S / D 2 0 / u r e a - d 4 micellar solutions.
,
DTAB / UREA-d,/D,O
1
L
0.3
38 0.1f - f 2
5
7
10
12
16
5
2 E
l
DTAB/UREA/D,O
5 0.3 P
0.1 5
7
10
12
16
DOXYL POSITION, X
Figure 3. Dependence of the normalized deuterium modulation depth on the position of the doxyl group, x, in x-doxylstearic acid spin probes in DTAB/D,O/urea and D T A B / D 2 0 / u r e a - d 4 micellar solutions. TABLE I: Normalized Deuterium Modulation Depths for the C,-TEMPO Probe Solubilized in 0.1 M Sodium Dodecyl Sulfate (SDS) and 0.1 M Dodecyltrimethylammonium Bromide (DTAB) Micellar Solutions in D20 as a Function of Urea or Urea-d, Concentrations normalized deuterium modulation depth urea OM 2M 6M urea-d, OM 2M 6M
SDS/D,O
DTAB/DzO
0.29 f 0.03
0.40 f 0.02 0.55 f 0.03
0.25 f 0.02 0.50 f 0.02 0.59 f 0.03
0.29 f 0.03 0.48 f 0.04 0.60 f 0.02
0.25 f 0.02 0.58 f 0.03 0.66 0.03
*
acid probe for SDS/D,O and DTAB/D20 micellar solutions in the presence of urea and urea-d,. Finally, Table I reports the deuterium modulation depths for S D S / D 2 0 and DTAB/D20 micellar solutions containing the C,-TEMPO probe as a function of urea and urea-d4 concentrations.
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The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
Discussion Previous ~ o r k s show ~ ~ that - ~ the ~ electron spin echo modulation (ESEM) technique can be applied semiquantitatively to obtain rather detailed structural information in complex systems such as micelles and vesicles. In these studies deuteration of the system is performed and the deuterium modulation is typically analyzed in terms of the average strength of interaction between a paramagnetic probe and deuterium located in specific positions such as the alkyl chain, the polar headgroup of the surfactant, the water solvent, or elsewhere. A typical example is reported in a recent paper35where three different deuterium locations have been used: (i) micelle deuterated in the polar headgroup, (ii) micelle deuterated in the core region, and (iii) non-deuterated micelle in deuterated water. The comparison of the deuterium modulation depths in the three different systems allows determination of the probe location and conformation in the micelle and the amount of water penetration into the micelle. The average probe location for several micellar systems has been r e p ~ r t e d . ~It~ has - ~ ~been demonstrated that the x-DSA probe is bent or U-shaped in lithium dodecyl sulfate, tetramethylammonium dodecyl sulfate, DTAB, and nonionic poly(ethy1ene oxide) micellar solution^.^^-^^ When the water solvent is deuterated, one expects a decrease in the normalized deuterium modulation depth as x increases if the probe is not bent and the doxyl group is moved further from the polar headgroup of the spin probe and therefore from the micelle surface and the deuterated solvent. This is indeed observed from x = 5 to 10 but, at x = 12, the modulation depth increases again. This indicates that the doxyl probe bends near x = 10-12 and that its tail (i.e., near x = 16) can probe the micelle surface. This implies that the doxyl probe has some gauche links and is roughly U-shaped in these micellar systems. Figures 2 and 3 report the variation of the normalized modulation depth as a function of the doxyl position, x, for S D S / D 2 0 and DTAB/D20 micellar solutions as a function of urea and urea-d, concentration. Several points can be deduced from the analysis of these figures. The trend of the modulation depth as a function of the doxyl position is similar for the systems with and without urea, meaning that the urea addition does not change the probe bending. Another consideration is that urea dramatically increases the deuterium modulation depth and that this increase is dependent on the surfactant and on the urea concentration as well as on the amount of the 2H isotope present in the solution. This increase can be explained in two different ways: (i) urea produces a strong increase of water penetration into the micelle so that the micellar structure is partially destroyed, or (ii) protons on urea exchange with 2H of the deuterated water and the urea interacts directly with the micellar surface. This latter model would produce a strong increase in the deuterium concentration at the micellar surface. The Fourier analysis of the ESE spectra in the presence of urea shows an absorption at 1.07 MHz that corresponds to the I4N frequency. Since this absorption is not seen in the absence of urea, it can be attributed to the presence of urea molecules at the micelle surface. I n the DTAB/D20/urea system this absorption could come from interaction with I4N in the surfactant polar headgroup; however, no absorption at 1.07 MHz seems to be present for this system in the absence of urea. This result shows that urea interacts with the micellar surface. The exchange of ‘H in urea with ZHin water is consistent with a recent study on the fractionation of hydrogen and oxygen isotopes between hydrated and free water molecules in urea solutions. Kakiuchi and Matsuo3’ showed that deuterium is enriched in the (37) Kakiuchi, M.; Matsuo, S. J . Phys. Chem. 1985, 89, 4627
Baglioni et al. weaker hydrogen-bonding site (urea-water cluster) and it is depleted in free water. The analysis of Figures 2 and 3 also shows that the increase of the deuterium modulation depth, in passing from urea to urea& is very weak in agreement with the fairly large *H partitioning in the urea-water cluster.38 Furthermore, the deuterium modulation depth is greater for the cationic surfactant, suggesting the presence of higher deuterium concentration and, therefore, urea concentration at the DTAB micellar surface. These results provide an explanation at the molecular level of the different variation of the critical micelle concentration (cmc) in DTAB and SDS surfactant solutions upon the addition of urea. The cmc increases by about 40% and 300% for DTAB and by about 20% and 100% for SDS upon addition of 2 and 6 M urea, r e ~ p e c t i v e l y .Our ~ ~ ~results ~ ~ ~ ~support ~ that the main cause of this increase in the direct interaction of urea with the surfactant polar headgroups that probably leads to a looser packing of the headgroups. Thus, it is oversimplified to distinguish between two classes of organic substances that affect the critical micellar concentration of aqueous solution surfactants in terms of substances that decrease the cmc being incorporated into the micelle and substances that modify the “structure of water” at the micellar interface (see, for example, refs 1 and 38). The results obtained with the probe C8-TEMPO, reported in Table I, are in good agreement with the x-DSA results. Overall, the analysis of the normalized deuterium modulation depth and the Fourier transformation of the two-pulse ESE spectra clearly show that the non-deuterated urea exchanges ’ H with 2H in D 2 0 and that it interacts with the polar headgroups of SDS and DTAB micelles. Furthermore, it is found that the 2H concentration at the micellar surface is greater in DTAB than in SDS providing an explanation at molecular level of the different variations of the cmc in DTAB and SDS micellar solutions upon the addition of urea. Conclusions The results obtained from the analysis of the ESEM spectra of x-DSA and C8-TEMP0 probes in SDS and DTAB micellar solutions in D 2 0 in the presence of 2 and 6 M urea or urea-d, show that urea does not change the x-DSA probe conformation in the SDS or DTAB micelles. The addition of urea increases the deuterium modulation depth of x-DSA and C8-TEMPO probes, indicating that non-deuterated urea exchanges ’H with *H in D 2 0 and that urea directly interacts with the micellar surface. The comparative analysis of the deuterium modulation depths in SDS and DTAB micelles shows, for 2 and 6 M urea, the presence of higher deuterium concentration at the DTAB micellar surface. This provides an explanation at a molecular level of the different percentage variation of the cmc in DTAB and SDS surfactant solutions upon the addition of urea. These results are in good agreement with those reported from a recent ESR study,29 with recent molecular dynamics and Monte Carlo ~ a l c u l a t i o n s , ~ ~ - ~ ~ and with a Raman study on the urea/acetone/water ternary system39in which it is postulated that the main mechanism of urea action is a direct interaction with the organic molecules. Acknowledgment. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U S . Department of Energy, and the Italian Ministry of Public Instruction. R@tV NO. SDS, 151-21-3; DTAB, 11 19-94-4; 5-DSA, 29545-48-0; 7-DSA, 40951-82-4; 10-DSA, 50613-98-4; 12-DSA, 29545-47-9: 16DSA, 53034-38-1; C,-TEMPO, 126328-27-6; urea, 57-13-6. ( 38) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989. (39) Mizutani, Y.; Kamogawa, K.; Nakanishi, K. J . Phys. Chem. 1989, 93. 5650.
