7624
J. Phys. Chem. 1994,98, 7624-7627
Interaction of Poly(ethylene oxide) with the Sodium Dodecyl Sulfate Micelle Interface Studied with Nitroxide Spin Probes Young So0 Kang and Larry Kevan. Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: March 24, 19948
Electron spin resonance (ESR) line widths of 5-, 7-, 12-, and 16-doxylstearic acid (x-DSA) and tempo nitroxides versus the concentration of poly(ethy1ene oxide) (PEO) in sodium dodecyl sulfate (SDS) micelles show different trends. The ESR line widths of 5-, 7-,and 16-DSA increase with increasing concentration of PEO, which is interpreted as due to increasing viscosity in the environment of the nitroxide spin probe. The tempo and 12-DSA line widths were independent of the concentration of PEO. The line width showed the highest value for 5-DSA and the lowest value for tempo. The line width of x-DSA decreases from 5-DSA to a minimum value for 12-DSA and then increases somewhat for 16-DSA. This is interpreted as bending of the alkyl chain to provide different locations for the nitroxide moiety relative to the micelle interface. The relative distances of the nitroxide moiety of x-DSA from deuterated water at the S D S micelle interface was measured by deuterium electron spin echo modulation. The distances increased from 5-DSA to 12-DSA and then decreased for 16DSA. The interpretation of the ESR line width trend is supported by the deuterium modulation depth trend. The deuterium modulation depths of x-DSA also increase with increasing concentration of PEO. They are largest for 5-DSA, less for 12-DSA, and then larger again for 16-DSA. These changes of the ESR line widths and deuterium modulation depths for x-DSA indicate alkyl chain bending of x-DSA. The alkyl chain of x-DSA is bent near the 12-carbon position which corresponds to the deepest nitroxide location into the hydrocarbon region of SDS micelles. The data also show that the interaction site of PEO with SDS micelles is near the interface region.
Recently, attention has been focused on the interaction of polymers with surfactants in organic and aqueous solutions.1J The polymer-surfactant interaction results from a relatively weak dipolar interaction. In a polymer-micelle complexthe properties of both the micelles and the polymers can be modified.’~~The important aspects for industrial applicationsare the solubilization power and viscosity of an aqueous solution of polymer-bound micelles which have applications in paints, coatings, cosmetics, and detergents.3 Polymer-micelle interactions also play a role in tertiary oil recovery.3 In spite of various applications of polymer-micelle complexes, little is known about the details of the interaction mechanism. Nuclear magnetic resonance has been used to study the micellization process of sodium w-phenyldecanoateand the interaction between this surfactant and poly(ethy1ene oxide) (PEO).” In the presence of PEO, large chemical shifts of surfactant protons near the terminal phenyl group, large shifts of PEO protons by aromatic ring currents, and small shifts of surfactant protons near the headgroup suggest that the terminal phenyl group and the PEO are in close proximity. The degree of solubilization of PEO in the sodium dodecyl sulfate (SDS) micelles and sodium w-phenyldecanoate surfactant solutions was also determined by monitoring the change in the lH spin-lattice relaxation rate of PEO upon addition of paramagnetic ions to the aqueous phase. Winnik et al. have studied the interaction of SDS containing PEO with pyrene at both ends of the PEO chain by measuring the fluorescent properties of the bound pyrene.* Electron spin resonance (ESR) has been used to investigate the microenvironment of nitroxide spin probes in micelles and vesicles by measuring the nitrogen hyperfine coupling constant and the ESR line widthP The coupling constant (AN) is affected by the local polarity of the nitroxide. A more polar environment gives larger values of ANdue to greater electrondensity at nitrogen. *Abstract published in Advance ACS Absrrucrs, July 1, 1994.
0022-3654f 94/2098-7624304.50/0
The line widths are controlled by the rotational and lateral diffusion of the spin probes which is affected by the viscosity, orderedness, and temperature of the spin probe environment in micelles and The larger ordering parameters of x-doxylstearic acids (x-DSA) at the interface regions of micelles or vesicles compared to the hydrocarbon regions indicate oriented x-DSA with its polar headgroup at the interface. This results in a broader line width and a slower tumbling rate for the spin probe consistent with a longer relaxation time. The relative interaction distance of a spin probe with respect to interface water (DzO) can be measured by deuterium electron spin echo modulation (ESEM).13J4ESEM can be measured only in the frozen state because the weakdipolar interactions measured are averaged out in liquids. However, numerous studies have demonstrated that micellar and vesicular structure is retained in rapidly frozen aqueous solutions and that meaningful ESEM studies are possible.I3 In the present study, 5-, 7-, 12-, and 16-doxylstearicacid and tempo nitroxide spin probes (see Figure 1) are solubilized into SDS micelles with different concentrations of PEO. The ESR line widths of the low and high magnetic field lines of the nitroxides are measured and compared to determine the relative viscosity changes of thenitroxideenvironmentas a functionof the nitroxide. The nitrogen coupling constants (AN) are also determined to obtain related information. These data are correlated with the normalized deuterium modulation depths of the nitroxides interacting with deuterated water at the micelle interface as a function of PEO concentration. ExperimentalSection
SampIePreparation. PEO (averagemolecularweightof 10 000 and average number to weight average molecular weight M,,/M, = 1.3) was obtained from Aldrich and used without further purification. SDS was obtained from Aldrich and used after recrystallization from ethanol three times, followed by washing with diethyl ester and drying at 50 OC under a moderate vacuum. Q 1994 American Chemical Society
Interaction of PEO with SDS Micelles
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7625
n .
I-
O
Tempo (2,2,6,6 - tetramelhyi-1-piperidinyloxy, free radical)
x - Doxylstearic Acid Figure 1. Structure of tempo and x-doxylstearic acids used.
Stock solutions of 0.1 M SDS surfactant were prepared in deuterium oxide (Aldrich, 99.9 atom % D) and deoxygenated by bubbling with dry nitrogen gas for 15 min. Stock solutions of 13mM for each X-DSAand tempo nitroxide from Sigma Chemical Co. were prepared in chloroform. A 17p L quantity of each spin probe stock solution was transferred into a 1.6-cm (0.d.) by 1.25-cm (i.d,) test tube, and then the chloroform was evaporated by bubbling nitrogen gas into the surface of the solution. Then 1 mL of a 0.1 M stock SDS micellar solution was added to the test tube, and 0, 0.005,0.01, 0.02, 0.04,andO.l gofPEO (O,OS,1,2,4,and 10mM) weredissolved into the 1-mL x-DSA/SDS/D*O micellar solutions. These solutions were used for up to 2 days. The spin probe concentration was 2.2 X 10-4 M. The samples were purged with nitrogen and introduced to into 1.O-mm (i.d,) by 1.l-mm (0.d.) Pyrexdisposable micropipets (CMS Co.) for ESR and 2-mm (i.d.) by 3-mm (0.d.) Suprasil quartz tubes for ESE and flame-sealed. Electron Magnetic ResonanceExperiments. ESR spectra were recorded at room temperature on a Bruker ESP 300 ESR X-band spectrometer with 100-kHz magnetic field modulation and 1.96mW microwave power to avoid power saturation. The magnetic fields were measured with a Varian E-501 gaussmeter, and frequencies were directly measured with a Hewlet-Packard 5350B microwave frequency counter. Each ESR spectrum was averaged for seven scans. The ESR line widths for the low and high magnetic field lines were measured from the peak-to-peakdistance of the first-derivative ESR spectra. The nitrogen coupling constant (AN) was determined from half of the peak-to-peak distance of the low and high magnetic field lines. The mean value was obtained from triplicate measurements. Two-pulse ESEM signals were recorded at 4.2 K at X-band on a home-built ESE spectrometer using 40- and 80-11s excitation pulses.l5J6 The microwave pulse sequences and data acquisition were controlled by a Nicolet 1280 minicomputer which was interfaced to the ESEM spectrometer. Each signal was scanned eight times. Once obtained, the ESEM data were transferred to an IBM-compatible 486-based microcomputer for analysis. The modulation showed about 0.5-ps periodicity characteristic of deuterium. Normalized modulation depths were determined by dividing the depth of the first modulation minimum from an extrapolated unmodulated echo decay by the depth to the base line at the same interpulse time.13J4 The reported values are an average of three separate experiments. Results The structure of the spin probes tempo and x-DSA used in this study are shown in Figure 1. Representativefirst-derivative ESR spectra of 5-DSA in SDS micelles with different concentrations of PEO are shown in Figure 2. The first-derivative ESR line widths of the low and high magnetic field lines of the spin probes are shown in Table 1. The line widths of 5-, 7-, and 16-DSA spin probes increase with increasing concentration of added PEO.
V
’14 G. Figure 2. ESR spectra at room temperature of 5-DSA in SDS/D2O micelles with 0, 0.5, 1, 2, 4, and 10 mM concentrations of PEO.
TABLE 1: First-Derivative ESR Line Widths to h0.02 C for the Low and High Magnetic Field L i e of Tempo and x-Doxylstearic Acids in 0.1 M SDS/DzO Micelles with Different Concentrations of PEO at Room Temperature [PEO],mM ESRline tempo 5-DSA 7-DSA 12-DSA 16-DSA 0 low 1.56 2.51 2.51 1.69 1.81 high 2.21 3.42 3.41 2.25 3.32 0.5 low 1.57 2.81 2.62 171 2.01 high 2.22 4.67 3.99 2.31 3.43 1 .o low 1.57 3.12 3.22 1.78 2.41 high 2.22 5.93 6.25 2.39 4.01 2.0 low 1.58 3.43 3.43 1.79 2.53 high 2.23 6.25 6.47 2.39 4.32 4.0 low 1.59 4.48 4.24 1.80 2.72 high 2.24 6.37 6.45 2.41 3.75 10 low 4.50 1.59 3.95 1.81 2.73 high 2.25 6.38 6.28 2.43 3.76
TABLE 2 Nitrogen Coupling Constant to fO.O1 G of Tempo and x-DSA Nitroxidm in 0.1 M SDS/DzO Micelle with Different Concentrations of PEO at Room Temperature [PEO], mM 0 0.5 1 .o 2.0 4.0 10
tempo 15.22 15.22 15.22 15.22 15.21 15.21
5-DSA 15.62 15.62 15.54 15.50 15.48 15.44
7-DSA
12-DSA
16-DSA
15.59 15.59 15.54 15.48 15.47 15.43
15.31 15.30 15.29 15.29 15.29 15.28
15.41 15.40 15.38 15.38 15.37 15.36
The line widths of tempo and 12-DSA spin probe are not affected by the concentration of PEO. The smallest line width is obtained from tempo. The largest line width is obtained from 5-DSA, which decreases for 12-DSA and then increases slightly for 16DSA. The nitrogen coupling constants (AN) are shown in Table 2. The coupling constants of 5- and 7-DSA slightly decreased with increasing concentration of PEO while those of tempo, 12-DSA and 16-DSA were constant. The AN of tempo in bulk water is 17.1 G, which is larger than in SDS micelles. The ESEM signals of 5-DSA in SDS/D20 micelles in the presence of different concentrations of PEO are shown in Figure 3. The normalized deuterium modulation depths of x-DSA versus the concentration of PEO are shown in Figure 4. The deuterium modulation depths increase with increasing concentration of added PEO to 4 mM PEO and then plateau at higher concentrations.
7626 The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 ,,
5-DSAISDSID20IPEO
I’ v)
0.0
0
2
4
6
0
1012
[PEOI “1 Figure 4. Normalized deuterium modulation depths at 4.2 K of 5-DSA (0),7-DSA (A), 12-DSA (0),(0) 16-DSA, and tempo (A)
in SDS/
DzO micelles versus the concentration of PEO.
The normalized deuterium modulation depths of spin probes in SDS/D20 micelles versus the kind of spin probe can also be in Figure 4. The smallest deuterium modulation depth is obtained from tempo, and the largest deuterium modulation depth is obtained from 5-DSA. The deuterium modulation depths decrease from 5-DSA to 12-DSA and then slightly increase for 16-DSA. Because of the relatively large number of interacting deuteriums and the unknown micellar interface structure, a more quantitative treatment of the ESEM data does not seem justified. Discussion The location and the degree of interaction of polymer chains with surfactant assemblies like micelles, vesicles, and reverse micelles are only partially understood. PEO containing pyrene groups at both ends of its chain has been used as a fluorescent probe.8.1’ A sharp onset of enhanced excimer emission and a correspondingly sharp chnge in the vibrational structure of the locally excited pyrene emission indicated that the hydrophobic end groups interact cooperatively
Kang and Kevan and that the polymer promotes surfactant aggregation at lower concentrations than the critical micelle concentration. The polymer tends to cyclize so that two polymer end groups interact with a single micelle. The interaction of PEO with SDS micelles also has been studied by the measurement of conductance and surface tension.18 The plots of surface tension and conductance as a function of the SDS concentration showed two transition points. These were interpreted in terms of a polymer-surfactant complex at the first point and micelle formation with binding between the surfactant ionic head group and the polymer at the second transition point. The interaction of PEO and vesicles has also been studied for dipalmitoylphosphatidylcholinevesicles by viscosity changes.19 It was reported that polymer addition increased the vesicle surface viscosity. The PEO/SDS system has also been studied by I3C, ‘H, and *3N a NMR.Zo Only the first three carbons of SDS, counted from the SO4- headgroup, exhibit 13C chemical shifts by the presence of PEO at the micelle interface. Since only the first three carbons atoms are influenced by the polymer, it suggests that PEO binds at the micellar surface. The NMR signals of the polymer were barely influenced by complexation with SDS micelles. This was interpreted as indicating that only a small fraction of the polymer is absorbed near the micellar surface, whereas the rest protrudes as loops into the aqueous surroundings. In contrast, and NMR study on PEO interacting with w-phenyldecanoate micelles concluded that the PEO resides in the interior of the micelle.&17 This conclusion was based on the 1H aromatic ring current-induced shifts of the PEO protons. However, the argument hinges on the assumption that the phenyl moieties do not bend back toward the surface of the micelles. Effect of PEO Concentration. Micelles containing polymer at their surface or interior have higher viscosity than the micelle or polymer solutions themselves.l-’ So an increasing concentration of PEO in the micellar solution increases the viscosity due to PEO interaction with the micelle. The increasing viscosity can be probed by a paramagnetic nitroxide group located near the micellar interface. The correlation time of nitroxide spin probes in micelles is related to the order parameter of the interface region and the rotational diffusion tensor.llJ* Previous studies9a.b have shown that detailed line shape simulations can give the order parameter and rotational diffusion tensor of nitroxides in vesicles, but this time-consuming analysis is not necessary to establish the trends with PEO concentration investigated here. Some representative correlationtimes in ammonium pentadecafluorooctanatemicelles are-1.1 X 10-19sfor5-DSAand-2.3 X 10-lOsfor16-DSA.* These values indicate that the x-DSA nitroxides in micelles are typically in the fast tumbling range (rC I s) in micelles. As PEO is added to the micellar solutions, the local viscosity of the nitroxide environment increasesas shown by increasing ESR line widths in Table l.z1-23 Baglioni et a1.10 confirmed a relationship between the rotational correlation time and the viscosity in such surfactant systems. Thus, the increasing ESR line widths of 5-, 7-, and 16-DSA nitroxide spin probes with increasing concentration of PEO as shown in Table 1 indicate that the PEO is located near the micelle interfacenear which location thenitroxidemoietiesofthesex-DSA spin probes are found. On the other hand, the constant ESR line widths of tempo and 12-DSA with increasing PEO concentration indicate that the location of the nitroxide moiety of these spin probes is farther from the PEO location near the micellar interface. The differences among the various x-DSA with PEO concentration is also supported by a small decrease in the nitrogen coupling constant for 5- and 7-DSA in contrast to an almost constant value for tempo, 12-DSA and 16-DSA with increasing PEO concentration (Table 2). This indicates that the polarity of the micellar interface near the 5- and 7-DSA positions of the
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7627
Interaction of PEO with SDS Micelles
W PEO
/V x-DSA
\
\
\'
Figure 5. Schematic relative location of the nitroxide group in x-DSA and tempo spin probes in SDS micelles interacting with PEO.The curved bold line indicates PEO. The outer and inner semicircles indicate the interface region of SDS micelles and the 0.6-nmdistance from interface DzO probed by ESEM measurements, respectively.
nitroxide moiety is slightly decreased with increasing PEO concentration. The relative interaction distance changes between x-DSA nitroxide spin probes and micellar interface water (D20) is determined by the deuterium modulation depth The increasing deuterium modulation of x-DSA and tempo as a function of increasing PEO concentration as shown in Figure 4 indicates that PEO interacts with the SDS micellar interface, probably by intercalation between the headgroups of the SDS surfactant. This allows more water (D20) penetration into the interface which decreases the interaction distance of the nitroxide moiety of x-DSA and tempo with interface D20. However, above 4 mM PEO the deuterium modulation depth remains constant, which suggests that the decreasing interaction distance between the nitroxide moiety and D2O is partially cancelled by the replacement of water molecules by PEO in the interface. This effect has also been observed for urea additives to the interface of micelles and vesicle^.^^,^* Effect of Nitroxide Spin Probes. Different locations of the nitroxidemoietiesof thespin probes relative to themicelleinterface should show different effects of PEO located a t the interface. The ESR line widths decrease from 5-DSA to 12-DSA and then increase for 16-DSA as shown in Table 1. This is interpreted as a change in the location of the nitroxide moiety to regions of lower viscosity in the micelle for 12-DSA and to regions of higher viscosity for 16-DSA. The change in location is confirmed by the changing deuterium modulation depths in Figure 4. This data shows that the nitroxide moiety has the weakest interaction with interface water for 12-DSA, which indicates a location of lower viscosity. Since PEO increases the local viscosity, it is located at the micellar interface. This interpretation is also supported by the decreased nitrogen hyperfine coupling constant of 5- and 7-DSA with PEO concentration. These data also indicate alkyl chain bending of x-DSA with the deepest location of the alkyl chain occurring near 12-DSA as shown in Figure 5 . The deeper locations of 12-DSA and tempo nitroxides within the micelle interior are not influenced by viscosity and polarity changes at the micellar interface due to PEO addition. Conclusions Additions of PEO into SDS micellar solution increases the viscosity near the micellar interface, suggesting a PEO location
along the interface as shown in Figure 5 . The viscosity increase increases the ESR line widths of nitroxide spin probes located near the interface such as 5-, 7-, and 16-DSA. The constant ESR line width of 12-DSA and tempo nitroxides with PEO concentration indicates that the location of the nitroxide moiety of 12-DSA and tempo is too far from the micellar interface to be affected by the viscosity change associated with the PEO. This interpretation is supported by the slight decrease of the nitrogen coupling constant of 5- and 7-DSA versus PEO concentration and the relative locations of the nitroxide moiety from interface water (D20) measured by ESEM from the deuteriummodulation depths. An increasing concentration of PEO increases the deuterium modulation depth of x-DSA and tempo up to 4 mM PEO. This supports PEO intercalation between the surfactant headgroups to allow more water penetration into the interface. 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. References and Notes (1) Goddard, E. D. Colloid Surf. 1986, 19, 225. (2) Goddard, E. D.; Hanna, K. B. In Micellization, Solubilization and Microemulsion; Mittal, K. L.; Ed.; Plenum: New York, 1977; Vol. 2. (3) Brackman, J. C.; Engberts, J. B. F. N. Chem. SOC.Reu. 1993, 85. (4) Gao, Z.; Kwak, J. C. T.; Labonte, R.; Marangoni, G.; Wasylishen, R. E. Colloid Surf. 1990, 45, 269. (5) Gao, Z.; Wasylishen, R. E.; Kwak, J. C. T. Macromolecules 1989, 22, 2544. (6) Gao, Z.; Wasylishen, R. E.; Kwak, J. C. T. J. Colloid. Interface Sci. 1990, 137, 137. (7) Gao, Z.; Waslyishen, R. E.; Kwak, J. C. T. J. Phys. Chem. 1991,95, 462.
( 8 ) Hu, Y. Z.; Wasylishen, R. E.; Zhao, C. L.; Winnik, M. A. Lungmuir 1990,6,880. (9) (a) Bratt, J. P.; Kevan, L. J. Phys. Chem. 1993,97,7371. (b) Bratt, J. P.; Kevan, L. J. Phys. Chem. 1992, 96, 6849. (c) Ristori, S.; Martini, G. Langmuir 1992,8,1937. (d) Bosoni, F.; Gabrielli, G.; Margheri, E.; Martini, G. Lungmuir 1990,6, 1769. Ristori, S.;Ottaviani, F.; Lenti, D.; Martini, G. Langmuir 1991, 7, 1958. (10) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J. Phys. Chem. 1990, 94, 8218. (1 1) Meirovitch, E.; Nayeem, A.; Freed, J. H. J. Phys. Chem. 1984,88, 3454. (12) Schreier, S.; Polnaszek, C. F.; Smith, I. C. P. Biochim. Biophys. Acta 1978, 515, 375. (13) Kevan, L. Inr. Reu. Phys. Chem. 1990, 9, 307. (14) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1979; Chapter 8. (15) Ichikawa, T.; Kevan, L.; Narayana, P. A. J. Phys. Chem. 1979,83, 3378. (16) Nayarana, P. A.; Kevan, L.; Szajdzinska-Pietek, E.; Jones, R. R. M. J. Chem. Phys. 1984,81, 3985. (17) Zhao, C. L.; Winnik, M. A. Lungmuir 1990, 6, 514. (18) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (19) de Gennes, P. G. J. Phys. Chem. 1990, 94, 8407. (20) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (21) Carrington, A.; McLachlan, A. D. In Introduction to Magnetic Resonance; Harper & Row: New York, 1967; pp 194-196. (22) Goldman, S.A.; Bruno, G. V.; Freed, J. H. J. Phys. Chem. 1972,76, 1858. (23) Freed, J. H. In Spin Lubeling Theory and Applications; Berliner, L. J., Ed.; Academic: New York, 1976; pp 567-571. (24) Szadzinska-Pietek,E.; Maldonaldo,R.; Kevan, L.; Jones, R. J.Colloid Interface Sci. 1986, 110, 514. (25) Szadzinska-Pietek, E.; Maldonaldo, R.; Kevan, L.; Berr, S.S.;Jones, R. J. Phys. Chem. 1985,89, 1547. (26) Bagloni, P.; Kevan, L. J. Phys. Chem. 1987, 91, 1516. (27) Kang, Y. S.;McManus, H. J. D.; Kevan, L. J. Phys. Chem. 1992, 96, 10049. (28) Kang, Y. S.; McManus, H. J. D.; Kevan, L. J. Phys. Chem. 1992, 96, 10055.
