1516
J. Phys. Chem. 1987, 91, 1516-1518
Structural Effects of Alcohol Addition to Sodium Dodecyi Sulfate Micelles Studied by Electron Spin-Echo Modulation of 5-Doxyistearic Acid Spin Probe Piero Baglionif and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: August 11, 1986; In Final Form: November 4, 1986)
Electron spin-echo modulation studies with a nitroxide probe, 5-doxylstearic acid, have been carried out for sodium dodecyl sulfate frozen micellar solution, containing 2-propano1, 1-propanol, I-pentanol, I-octanol, 2-propanol-d,, and I-octanol-d17 in D 2 0 and HzO.Modulation effects due to nitroxide interactions with deuteriums in H 2 0 and D20 have been measured as a function of the concentration of the alcohol added. Alcohol partition coefficients between the micelle and bulk water have been obtained for the various alcohols from the modulation depths vs. alcohol concentration which agree with other methods. Alcohol addition leads to an increase in water penetration into the micellar interface that depends on the alcohol chain length. For high alcohol to sodium dodecyl sulfate mole ratios greater than two for longer chain alcohols (1-pentanol and I-octanol), large values of deuterium modulation were found which may indicate breakdown of the micellar structure.
Introduction
Increasing attention is being devoted to the study of the “incorporation” or solubilization of neutral molecules into micelles in aqueous solution. Some of the most studied solubilizates are alcohols, because of the important role they have in the preparation of microemulsions.’ It is generally accepted that the alcohols bind to the micelle in the surface region, leading to three principal effects: (a) The alcohol molecules intercalate between the surfactant ionic head groups to decrease the micelle surface area per headgroup and increase the degree of ionization.2 This effect is correlated with modification of the growth and shape of the micelle^.^ It seems to be a function of the mole fraction of alcohol at the micellar interface but is independent of the type of alcohoL4 (b) The dielectric constant at the micellar interface decreases, probably due to the replacement of water molecules in the micellar interface region by alcohol molecules.5 (c) The molecular order of the interface region of the micelle changes. The structural nature of the change is unclear since interpretations of both increased disorder6 and increased order in terms of a separation of hydrophobic and hydrophilic regions’ have been advanced. For high ratios of alcohol to surfactant (>2) micellar breakdown probably occurs.* The electron spin-echo modulation (ESEM) techniques, recently applied by Kevan and co-workers9-’l to frozen micellar systems, can determine the location of nitroxide spin probes and of solubilized 1-butanol in micelles by monitoring modulation effects due to interaction of the nitroxide group with deuterium in the surfactant molecules or in the solubilized In this paper we follow a similar approach in a systematic study of the location and interaction of a series of aliphatic alcohols in sodium dodecyl sulfate (SDS) micelles. We find that the addition of alcohol leads to a significant increase in water penetration into the micellar interface region and, for a high molar ratio of alcohol to surfactant, probably leads to micellar breakdown. These effects depend on the length and on the branching of the aliphatic chain in the alcohol. These results are in agreement with a picture in which the alcohol addition has a “disordering” role on the micellar interface structure which gradually decreases the separation between hydrophobic and hydrophilic domains.8 Experimental Section SDS was purchased from Eastman Kodak Co. It was recrystallized three times from ethanol, washed with diethyl ether, and dried under moderate vacuum. 5-Doxylstearic acid spin probe (SDSA) was purchased from Molecular Probes Inc. and used as received. 1-0ctano1, 1-pentanol, 1-propanol, and 2-propanol (HPLC products, purity >99.9%) were obtained from Aldrich ‘Permanent address: Department of Chemistry, University of Florence, Florence, Italy.
0022-3654/87/2091-15 16$01SO10
and used without further purification. 1-Propanol-d7-OH and 1-octanol-d17-OH were obtained from Merck Isotopes and used as received. Stock solutions of 0.1 M SDS were prepared in triply distilled and deoxygenated water and in deuteriated water. A stock M 5DSA was prepared in chloroform. solution of 1 X The samples were prepared in a nitrogen atmosphere by adding the SDS solution to a film of the spin probe, generated by evaporating the chloroform, to dissolve the film and then adding the alcohol. The concentrations were as follows: SDS, 100 mM; 5DSA, 0.1 mM; and alcohol, 0-200 mM. The samples were sealed in 2 mm X 3 mm (i.d. X 0.d.) Suprasil quartz tubes and frozen rapidly by plunging into liquid nitrogen. Two-pulse electron spin-echo signals were recorded at 4.2 K from the M I = 0 14N hyperfine transition of the nitroxide. Measurements of the spin-echo amplitudes as a function of interpulse time were made on a home-built ~pectrometer’~ with 50-11s exciting pulses. ESR measurements were made on a Varian E-4 spectrometer. Results
Figure 1 shows the two-pulse ESEM spectra obtained for 5doxylstearic acid spin probe in SDS micelles in D 2 0 as a function of the alcohol aliphatic chain length for an alcohol concentration of 25 mM. The maximum of each curve has been normalized to the same value. The normalized modulation depths were computed as described p r e v i o ~ s l y . ~ - ’ ~ (1) DeGennes, P. G.; Taupin, C. J . Phys. Chem. 1982,86,2294. Lindman, B.; Wennerstrom, H. In Topics in Current Chemistry; Springer-Verlag: Heidelberg, 1980; Vol. 87, pp 1-83. (2) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J . Colloid Interface Sci. 1981, 80, 208. Lianos, P.; Lang, J.; Strazielle, C.; Zana, R. J . Phys. Chem. 1982, 86, 1019. Almgren, M.; Lofroth, J. E. J. Colloid Interface Sci. 1981, 81, 486. Almgren, M.; Swarup, S. J . Colloid Interface Sci. 1983, 91, 256. (3) Israelchvilli, J. N.; Mitchell, D.; Ninham, B. W. J . Chem. SOC.,Faraday Tram. 2 1976, 72, 1525. Mitchell, D. J.; Ninham, B. W. J . Chem. SOC., Faraday Trans. 2 1981, 77,601. (4) Almgren, M.; Swarup, S. J . Colloid Interface Sci. 1983, 91, 1983; J . Phys. Chem. 1982, 86, 4212. ( 5 ) Zana, R.; Yiv, S.; Strazielle, C.; Lianas, P. J . Colloid Inferface Sci. 1981, 80,208. Mukerjee, P.; Cardinal, J. J . Phys. Chem. 1978, 82, 1620. (6) Maelstaf, P.; Bothorel, P., C.R. Seances Acad. Sci., Ser. C 1979, 288, 13. (7) 1319. (8) (9)
Russell, J . C.; Wild, U. P.; Whitten, D. G. J . Phys. Chem. 1986, 90, Russell, J. C.: Whitten, D. G. J . Am. Chem. SOC.1982, 104, 5937. Stilbs, P. J . Colloid Interface Sci. 1982, 87, 383; 1982, 89, 547. Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M.; Coleman, M. J. J . Am. Chem. SOC.1985, 107, 784. (IO) Jones, R. R. M.; Maldonado, R.; Szajdzinska-Pietek, E.; Kevan, L. J . Phys. Chem. 1986, 90, 1126. [ 11) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Berr, S . S.; Jones, R. R. M. J . Phys. Chem. 1985, 89, 1547. (12) Szajdzinska-Pietek, E.: Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Colloid Interface Sci. 1986, 1 I O , 514. (1 3) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Am. Chem. SOC.1985, 107, 6467. (14) Ichikawa, T.; Kevan, L.; Narayana, P. A. J . Phys. Chem. 1979,83, 3378.
0 1987 American Chemical Society
ESEM Study of Sodium Dodecyl Sulfate Micelles
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987 1517
I A
h
1
m
I\
5DSA/SDS
k
f
6-
t
a
a
Em 5 K 4
-4t
c
cn
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5
P 2LY
l-
'L
Figure 1. Two-pulse electron spin-echo decay envelopes at 4.2 K for 5DSA probe in S D S / D 2 0 frozen micellar solutions with and without the
various alcohols indicated. The alcohol concentrations were 25 mM. The spectral base lines have been offset vertically to avoid overlap. S
Figure 4. Two-pulse electron spin-echo decay envelopes at 4.2 K for 5DSA in S D S micellar solution in D 2 0 and H 2 0 in the presence of 25
m M Zpropanol-d7and 1-octan01-d~~. The spectral base lines have been offset vertically to avoid overlap.
5
5DSA/SDS/Z-PROPANOL-d,
06
5DSA/SDS/D20
z 0
5 0.6
I-OCTANOL
J- I-PENTANOL
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I-PROPANOL
W
5 0.2
2
0
2
[
0
, 5.0
, 100
,
oL
200 C&fn(mM)
Figure 2. The dependence of normalized deuterium modulation depths as a function of the indicated alcohol concentration, C,, in frozen 5 D S A / S D S / D 2 0 micellar solutions.
W
I
=
50
100
1
I
150
200
CA(mM)
Figure 5. The dependence of normalized deuterium modulation depths as a function of 2-propanol-d, concentration in frozen 5DSA/SDS micellar solutions in D 2 0 and H20.
,
150
is
5DSA/SDS/I -0CTANOL-di7
g
0.8
5DSA/SDS/D20
0
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Figure 6. The dependence of normalized deuterium modulation depths as a function of 1-octanol-d17concentration in frozen 5DSA/SDS micellar solutions in D 2 0 and H20. Figure 3. The dependence of normalized deuterium modulation depths as a function of 2-propanol concentration in frozen 5 D S A / S D S / D 2 0 micellar solution.
Figures 2 and 3 show the variation of the normalized modulation depths as a function of the alcohol concentration and of the alcohol aliphatic chain. Figure 4 reports the two-pulse electron spin-echo decay envelopes at 4.2 K for deuteriated 2-propanol-d, and 1octanol-d17in D 2 0 and H 2 0 a t an alcohol concentration of 25 mM. Figures 5 and 6 report the variation of the normalized modulation depths as a function of the alcohol concentration in H20and D 2 0 for the two deuteriated alcohols.
Discussion The ESEM experiments were conducted in frozen micellar solutions in order to prevent molecular averaging of the anisotropic electron-nuclear dipolar interactions responsible for the modulation effects.I5 At 4.2 K there is also the advantage of enhanced ( 1 5 ) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1973; Chapter 8 .
sensitivity and slower relaxation rates. In recent papers we have shown that micellar structure is retained in these rapidly frozen aqueous solution^.^^^^^^^ The cation radical of N,N,N',N'tetramethylbenzidine is readily photogenerated in anionic micellar solutions and is stable for tens of minutes.I6 In homogeneous solutions it has a lifetime of only a few microseconds. It is possible to generate TMB' in frozen micellar solutions, thaw the solution and, still observe TMB+ by electron spin resonance, thus indicating that the micellar structure was retained upon freezing.I6 Hashimoto and Thomas" have recently used luminescence quenching to measure micellar aggregation numbers and have found similar aggregation numbers for SDS micelles in liquid and in 77 K frozen ethylene glycol-water solutions. This also supports the retention of micellar structure in rapidly frozen solutions. The results reported in this study are obtained 'from an analysis of the deuterium modulation depths that reflect the magnitude of the dipolar interaction between the unpaired electron in the (16) Narayana, P. A.; Li, A. S. W.; Kevan, L. J . Am. Chem. SOC.1981, 103, 3603. (17) Hashimoto, S.;Thomas, J. K. J . Am. Chem. SOC.1983, f0.5,5230.
1518 The Journal of Physical Chemistry, Vol. 91, No. 6,1987
5-doxylstearic acid spin probe and the water or alcohol deuteriums. The modulation depth changes inversely with the average distance between the unpaired electron and the density of surrounding deuteriums and is detectable to about 0.6 nm for realistic deuterium d e n s i t i e ~ . ' ~ It is well demonstrated that the 5DSA spin probe is comicellized with the surfactant molecules with the hydrophilic nitroxide group in the Stern layer of the micelle near the interface and the hydrocarbon chain is the micelle i n t e r i ~ r . ~ " ~Therefore, . ' ~ ~ ' ~ changes in the deuterium modulation depth reflect mainly the structural variation of the interfacial region of the micelle. In other work we find some differences for the values of deuterium modulation depth with a N,N,N',N'-tetramethylbenzidine cation probe that is located deeper in the micelle.20 The spectra obtained for all the alcohols in D 2 0 and for the two perdeuteriated alcohols in H 2 0 showed echo decay curves that exhibit modulation with a 0.5-ps period which is indicative of electron-deuterium dipolar interaction and an additional modulation with a 0.08-ps period due to the interaction with hydrogens on the surfactant and/or the alcohol. This is seen in the spectra in Figures 1 and 4. From an analysis of Figures 2 and 3 it is evident that the addition of alcohol causes an increase in the SDSA interactions with deuterium, indicating that the micellar surface of SDS is more hydrated in the presence of the alcohol. However, this effect is qualitatively and quantitatively different for the various alcohols. The normalized deuterium modulation depth for 1-propanol and 2-propanol increased gradually as a function of alcohol concentration and reached a plateau value only above an alcohol/surfactant mole ratio of about 0.75 for I-propanol and about 1.0 for 2-propanol. The increase in deuterium modulation was sharper for added I-pentanol and 1-octanol which showed plateaus a t alcohol/surfactant mole ratios of about 0.25 and 0.15, respectively. The different plateau concentrations reflect different degrees of perturbation of the micellar interfacial region dependent on the alcohol structure and can be interpreted as different degrees of alcohol solubilization within the micelle interface region. We assume that all of the 1-octanol is associated with or solubilized in the micelle interface since it is the most hydrophobic of the alcohols used. Then, from the value of the plateau ratio for 1-octanol over the value of the plateau ratios of the other alcohols, we can obtain the fraction or percent of the other alcohols that are associated with the micellar surface. The data correspond to 15% of 2-propanol, 20% of 1-propanol, and 60% 1-pentanol being associated with the micelle. These values compare reasonably well with similar values of 25%, 32%, and 77% reported for 2-propano1, 1-propanol, and 1-pentanol, respectively, by N M R chemical shifts2' and thermochemical analysis.22 This agreement further corroborates that the micellar structure is retained in the frozen solutions studied here. The plateau magnitudes of the normalized modulation depth increased in the order 1-propanol < 2-propanol < 1-pentanol < 1-octanol, indicating different extents of water penetration. For the I-pentanol and 1-octanol there also appears to be further increase in the deuterium modulation depth for high alcohol/ surfactant ratios near two. This could indicate additional water penetration into the hydrophobic region of the micellar surface, but because of the alcohol/surfactant ratio being significantly greater than unity, it seems more probable to us that these results indicate the onset of some micellar breakdown.2' It is clear from (18) Baglioni, P. In Surfactants in Solution; Mittal, K. L., Bothorel, P.,
Eds.;Plenum: New York, in press. Baglioni, P.; Ferroni, E.; Martini, G.; Ottaviani, M. F. J . Phys. Chem. 1984,88, 5107. Ottaviani, M. F.; Baglioni, P.; Martini, G. J . Phys. Chem. 1983, 87, 3146. Baglioni, P.; Ottaviani, M. F.; Martini, G . ;Ferroni, E. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 1 , p 541. (19) Ramachandran, C.; Pyter, R. A,; Mukerjee, P. J . Phys. Chem. 1982, 86, 3198. (20) Baglioni, P.; Kevan, L., accepted for publication in J. Phys. Chem. (21) Stilbs, P. J. Colloid Interface Sei. 1981, 80, 608; 1983, 94, 463. (22) De Lisi, R.; Turco Liveri, V.; Castagnolo. M.; Inglese, A. J. Solution Chem. 1986, 15, 23. De Lisi, R.; Genoa, C.; Turco Liveri, V. J . Colloid Interface Sei. 1983, 95, 428 and references therein.
Baglioni and Kevan the above that the alcohol "perturbation" of the micelles is correlated to the alcohol chain length and branching and probably reflects a different average location of the solubilizate in the micelle for the different alcohols. In general, short chain length alcohols are expected to be located near the micellar interface, whereas long chain length alcohols are expected to tend more toward the micellar core.2 It is interesting to note that although 2-propanol is less solubilized by the micelle than its linear counterpart, it produces a stronger micellar perturbation, as indicated by the modulation plateau magnitude which may be related to steric distortion of the micellar interface.21 It was recently reported23that headgroup repulsion between surfactant molecules of SDS as well as intermicellar interactions does not change when H,O is replaced by D 2 0 . This allows us to compare the deuterium ESEM effects obtained for two deuteriated alcohols (2-propanol-d, and 1 -0ctano1-dl,) in H 2 0and D 2 0 and for nondeuteriated alcohols in D,O. The variation of deuterium modulation depth as a function of deuteriated 1-octanol-d,, in H 2 0 in Figure 6 shows that there is little interaction between the probe and 1-octanol at low 1-octanol concentration. In contrast, the strong increase in the modulation depth found in D,O at low 1-octanol concentration directly indicates that the addition of 1-octanol leads to a strong perturbation of the micellar surface with an increase in water penetration into the interface region. This supports the interpretation previously advanced for 1-butanol addition to SDS micelle^.'^ These results also indicate that addition of a long-chain alcohol does not produce expulsion of water molecules from the micellar interface as has been suggested from fluorescence s t ~ d i e swhich ~ - ~ indicate decreases in polarity at the fluorescence probe. "Polarity" relates to the local solvent organization at the fluorescent probe and does not directly indicate the specific molecules involved. The deuterium modulation depths in Figure 5 for deuteriated 2-propanol-d, in H 2 0 and D 2 0 show, as found for l-octanol-d17, an increase in deuterium modulation with 2-propanol-d,. In this case too for low alcohol concentrations a lesser increase of deuterium modulation for the system in H 2 0 is seen relative to the increase in D 2 0 solvent. However, the effect is much less prominent than with l-octanol-dl,. This indicates an increase in water penetration into the micelle surface at low alcohol concentration even for short-chain alcohols but also shows that this effect is strongly dependent on the alkyl chain length of the alcohol. Results with a N,N,N',N'-tetramethylbenzidine cation probe, which is located deeper into the micelle, also show that the addition of long-chain alcohols produces greater perturbations of the micellar "structure" than the addition of short-chain alcohols.20
Conclusion The addition of alcohol significantly perturbs the interface of SDS micellar solutions by intercalation of the alcohol into the headgroup region and opening up the headgroup region to stronger water interaction. The intensity of this effect depends on the length and branching of the aliphatic alcohol chain, in the order 1propanol < 2-propanol < 1-pentanol < 1-octanol. The analysis of normalized deuterium modulation curves for deuteriated 2propanol and 1-octanol in H,O and D 2 0 shows that the alcohol interaction with the micelle leads to increased interaction of water molecules in the micellar interface region at low alcohol concentration and not to expulsion of water molecules as suggested by fluorescence probe studies. The fluorescence probes indicate a decrease in the micellar interface polarity which may be associated with an increase in the disorder of the interface. Acknowledgment. This research was supported by the Department of Energy, Office of Basic Energy Sciences. Thanks are due to Mike Colaneri for helpful discussions. P.B. thanks the Italian Ministry of Public Instruction for partial financial support. Registry No. SDS,151-21-3; SDSA,29545-48-0; I-propanol, 71-23-8; I-pentanol, 71-41-0; 1-octanol, 1 1 1-87-5; 2-propanol, 67-63-0. (23) Chang, N. J.; Kaler, E. W. J . Phys. Chem. 1985, 89, 2996.
