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Electron Spin Resonance Study of Effect of Urea on Microenvironmental Properties of Alkylbenzenesulfonate Micellar Solutions Jingcheng Hao, Taotao Wang, Shuo Shi, Runhua Lu, and Hanqing Wang* Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, People’s Republic of China Received May 6, 1996. In Final Form: October 16, 1996X The effect of urea on micelle formation and the microenvironmental properties of sodium dodecylbenzenesulfonate micellar solutions has been investigated by using electron spin resonance spectroscopy and surface tension measurement at the air/water interface. Two different nonionic spin probes, 5-doxylstearic acid and the piperidinyl-1-oxy with long hydrocarbon chains (Tempo, C6-Tempo, C12-Tempo, C16-Tempo), have been used for studying sodium dodecylbenzenesulfonate solutions as a function of surfactant and urea concentrations. The surface tension results show that the addition of urea increases the critical micelle concentration values of the surfactant. The analysis of the nitrogen hyperfine coupling constant (AN) and the correlation time (τ) for the probe motion indicates that urea molecules interact with the polar headgroups of the sodium dodecylbenzenesulfonate micelles and penetrates below the polar headgroups. The addition of urea slightly decreases the micropolarity and strongly increases the microviscosity of the micellar interface. These results are in agreement with the recently reported mechanism where urea molecules replace some water molecules that solvate the hydrophobic chain and the polar headgroups of the surfactant.
Introduction
* Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, March 1, 1997.
can be solubilized into the micelle, change of the solvent, or change of the structure of the solvent itself. Urea has been used as an additive to check on the properties of micellar solutions, and two different mechanisms for urea action have been proposed.7,8 (i) Urea breaks the “structure” of water to facilitate the solvation of a hydrocarbon chain. (ii) Urea molecules replace some water molecules that solvate the hydrophobic chain and the polar headgroup of the amphiphile. Linear alkylbenzenesulfonate (LABS) is one of the commercially important surfactants finding a wide range of industrial applications.9 It is the largest tonnage anionic after soap. In addition, the presence of a number of isomers and the aromatic ring in the hydrophobic part make the study of this molecule extremely interesting. In recent years, the micellar structure of LABS has been investigated by using a variety of techniques, such as SAXS,10 fluorescence spectroscopy,11 and NMR.12 Some of the important structural features and physicochemical properties of organization of LABS in micelles and comicellization have been well obtained. In a recent study,13 Wang Jing-He reported the effect of inorganic salts and urea on the viscosity of sodium linear alkylbenzenesulfonate solution with high concentrations. Urea markedly diminishes the viscosity values of concentrated LABS solutions, and this result is interpreted in terms of forming the adducts between urea and the hydrated individual surfactant and diminishing LABS actual concentration of the equilibria between the hydrated individual and micelles. However, the effect of urea on the microenvironmental physicochemical properties of LABS, the properties of the comicellization of LABS with other anionic surfactants, such as sodium oleate (NaOL) and sodium laurate (NaL), and the effect of the linear
(1) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J. Phys. Chem. 1990, 94, 8218. (2) Krishnakumar, S.; Somasundarn, P. J. Colloid Interface Sci. 1994, 162, 425. (3) Ottaviani, F. M.; Baglioni, P.; Martini, G. J. Phys. Chem. 1983, 87, 3146. (4) Berliner, L. J., Ed. Spin labeling. Theory and Applications; Academic Press: New York, 1976. (5) Barelli, A.; Eicke, H. F. Langmuir 1986, 2, 780. (6) Marsh, D. In Membrane spectroscopy; Grell, E., Ed.; Spinger Verlag: Berlin, 1981.
(7) Wetlaufer, D. B.; Malik, S. K.; Stoller, L.; Coffin, R. I. J. Am. Chem. Soc. 1977, 99, 2898. (8) Enea, O.; Jolicoeur, C. J. Phys. Chem. 1982, 86, 3370. (9) Berth, P.; Jeschke, P. Tenside 1989, 2, 75. (10) Caponetti, E.; Triolo, R.; Patience, C. H. O.; Johnson, J. S.; Magid, L. J.; Butler, P.; Payne, K. A. J. Colloid Interface Sci. 1987, 116, 200. (11) Binanana, L. W.; Van Os, N. M.; Rumert, L. A. M.; Zana, R. J. Colloid Interface Sci. 1991, 141, 157. (12) Balasubramanian, D.; Shoba, J. J. Phys. Chem. 1986, 90, 2800. (13) Wang, Jing-He Acta Chim. Sin. 1995, 53, 237.
Electron spin resonance (ESR) has been used to characterize the micropolarity and microviscosity of the micellar interface.1-3 Nitroxyl radicals have been widely used in the past for studying the microenvironmental properties and reactivities of assemblies such as micelles4 and microemulsions.5 By using suitable nitroxide labeled spin probes that partition into the surfactant aggregates, it is possible to study the micellization and the behavior of micellar aggregates in solutions. The rotational correlation times measured from the ESR spectrum reflect the probe mobility and can be used to study the formation and microenvironmental properties of aggregates in solutions. The change of the probe mobility can often yield useful information on the structure of organized molecular assembles.6 The values of the hyperfine splitting constants are dependent on the polarity of the probe environment4 and can be used to extract valuable information on the probe environment. So, the ESR technique has an obvious advantage over other techniques in studying the microenvironmental properties of micelles as well as other organized molecular assembles. The physicochemical properties of surfactant solutions are extremely interesting. The properties of micellar solutions, such as critical micelle concentration (cmc), aggregation number, micelle size and shape, etc., depend on the balance between “hydrophobic” and “hydrophilic” interactions. For ionic surfactants this balance can be modified in several ways, i.e., salt addition, counterion complexation, addition of alcohols or other substances that
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Figure 1. Surface tension for the probe 5-DSA as a function of SDBS and urea concentrations.
alkylbenzenesulfonate (LABS) chain length on the micellization properties in the presence of urea, with respect to AN and τ in ESR spectrum, were not reported. This paper reports our experimental results, i.e., the effect of urea on sodium dodecylbenzenesulfonate (SDBS) micellar solutions which has been studied by using ESR of nitroxide spin probes and surface tension measurement at the air/water interface. At a future date, we are going to report the extension of our study to comicellization in mixed surfactant systems and a large number of surfactants. The object of this paper is to determine the mechanism for urea action and the local polarity and microviscosity of the micellar interface, through the analysis of the surface tension measurement, the nitrogen coupling constants (AN), and the correlation times for probe motion (τ), respectively. Here we used two different nonionic spin probes for 5-doxylstearic acid (5-DSA) and the piperidinyl1-oxy with different long hydrocarbon chains (Tempo, C6Tempo, C12-Tempo, and C16-Tempo). The results from the surface tension measurement and the parameters of ESR, AN, and τ show that urea increases the critical micelle concentration (cmc) and the microviscosity of the micellar interface and decreases the micropolarity of the micellar interface. These phenomena can be rationalized in terms of the binding of urea molecules at the water/micelle interface and replacement of some water molecules in the interface region.
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ESR Spectra. All ESR spectra were recorded by using a Varain E-115 ESR spectrometer operating at the X-band and a modulation frequency of 100 kHz using a quartz capillary tube. All experiments were performed at room temperature. The mean error was (0.02 G for the nitrogen hyperfine coupling constants, AN, measured between the central and low-field lines of the ESR spectra, and about 5% for the rotational correlation times, τ, calculated by using a procedure similar to that reported in previous studies.15,16 In the eq 1, h0 is the height of the central (MN ) 0) line, h-1 is the height of high field (MN ) -1) line, h+1 is the height of the low field (MN ) +1) line, and W0 is the line width of central line. In eq 2, a is the hydrodynamic radios of the probe, η the viscosity, k the Boltzmann constant, and T the temperature.
τ ) (6.5 × 10-10)W0[(h0/h-1)1/2 + (h0/h+1)1/2 - 2] s (1) τ ) τB ) τC ) 4 π na3/3kT s
(2)
Experimental Section Materials. Sodium dodecylbenzenesulfonate (SDBS) was obtained from Aldrich, Gold Label (purity >99%). Urea was obtained from China (analytical grade) and used without further purification. The spin labels used were as follows: 5-doxylstearic acid (I, 5-DSA) was obtained from Sigma Chemical Co.; piperidinyl-1-oxy compounds with different long hydrocarbon chains (II, Tempo; III, C6-Tempo; IV, C12-Tempo; V, C16-Tempo) were obtained as gifts from the Department of Chemistry, Lanzhou University, People’s Republic of China. They prepared them as described in ref 14. The water used was bidistilled in an allglass apparatus and all other reagents were analytical grade or better. Methods. The Surface Tension Measurements. The surface tensions at the air/aqueous interface were determined by a FACE CBVP-A3 interface tensometer. The values were recorded after equilibrium had been attained, and they were the average value measured three times. The accuracy was about 0.1 mN‚m-1. The measurements were conducted at 60 ( 1 °C. (14) Liu, Yu-Cheng; Liu, Zhongli; Wang, Yukun Chem. J. Chin. Univ. 1981, 2, 477.
The ESR samples consisted of ternary mixtures of H2O, urea, and variable amounts of sodium dodecylbenzenesulfonate. The samples also contained a minimal amount of nitroxide spin probes, and the final probe concentration was 2.0 × 10-4 M. A suitable amount of the samples was then transferred to the ESR cell, and the samples were deoxygenated by bubbling dry nitrogen gas through the solutions for 30 min.
Results and Discussion Critical Micelle Concentration (cmc) of Sodium Dodecylbenzenesulfonate in the Presence of Urea. Surface tensions of the SDBS systems without and with 2.0 and 6.0 M urea were measured as a function of SDBS concentrations. Representative plots of the surface tensions are given in Figure 1. From Figure 1, it can be seen (15) Stone, T. J.; Buckman, T.; Nordio, P. L.; McConnell, H. M. Proc. Natl. Acad. Sci. U.S.A. 1965, 54, 1010. (16) Kuharski, R. A.; Rossky, P. J. J. Am. Chem. Soc. 1984, 106, 5794.
Effect of Urea on Micellar Solutions
Figure 2. ESR spectra for 5-DSA/SDBS/6.0 M urea systems as a function of SDBS concentrations: (a) below the SDBS cmc (0.1 cmc); (b) near the SDBS cmc; (c) above the SDBS cmc (10 cmc).
that the addition of urea increases the cmc of SDBS and the surface tensions reached the cmc’s. Cmc values were determined from the sharp changes in the slope of the surface tensions versus log[SDBS] plots. For the SDBS systems without and with 2.0 and 6.0 M urea, the cmc values are 1.12 × 10-3, 1.58 × 10-3, and 1.82 × 10-3 M, respectively. The above surface tension results reflect the fact that the thermodynamic activity of SDBS is decreased by the addition of urea. These phenomena can be explained by different action mechanisms in which urea modifies the “iceberg” structure and/or replaces some water molecules that solvate the hydrophobic chain and the polar headgroup of the surfactant.7,8 In the most widely studied examples, the effect of the addition of urea on micellar solutions is attributed to the breaking of the water structure favoring the dissolution of hydrophobic solutes, the urea is not considered to penetrate the micellar interface. However, some recent studies suggest that urea in the solvation region of an apolar sphere weakens the water-water interactions, replacing some water molecules from the apolar solvation shell.17 Effect of Urea on the Microenvironmental Properties. The ESR spectra of the C12-Tempo in urea-SDBS solutions (a) below the cmc of SDBS (0.1 cmc), (b) at the cmc of SDBS, and (c) above the cmc of SDBS (10 cmc), are shown in Figure 2. A characteristic triplet broadening line is observed as a result of micellization progression. Below the cmc of SDBS, the spectra are composed of three narrow and equal high lines corresponding to free spin probe tumbling in solution. At and above the cmc, the lines are broader than that below the cmc. The broadening is evidently caused by spin-spin interaction which is higher in the micelles than that in the bulk phase, because the spin probe is so hydrophobic that it is exclusively concentrated in the micelles. So the spin probes are slower motion in the micelles than those in the bulk phase. The motion is the rotation of the micelles or the diffusion of the spin probe inside the curved interfacial film of the micelles. Figures 3 and 5 show the nitrogen hyperfine coupling constants as a function of surfactant and urea concentration for 5-DSA/SDBS, C12-Tempo/SDBS systems. Figures 4 and 6 show the correlation times as a function of surfactant and urea concentrations for 5-DSA/SDBS and C12-Tempo/SDBS systems. It has been shown that the location of a nitroxide spin probe in a micelle is dependent on the interactions that the spin probe experiences with the micelle.15 Therefore the spin probes can interact in different ways with the micelles and can report on the behavior of the unlabeled (17) Baglioni, P.; Cocciaro, R.; Dei, L. J. Phys. Chem. 1987, 97, 4020.
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Figure 3. Nitrogen coupling constant (AN) for the probe 5-DSA as a function of SDBS and urea concentrations.
Figure 4. Correlation time (τ) for the probe 5-DSA as a function of SDBS and urea concentrations.
surfactant molecules in different regions of the micellar interface. The nitrogen hyperfine coupling constants (AN) are sensitive to the local polarity for the microenvironment. At the surfactant cmc, a sharp decrease of AN is usually present and corresponds to the transfer of the spin probe from the continuous bulk phase to the less polar interface of the micelles. From the analysis of the AN trend as a function of surfactant concentrations, it is possible to determine the cmc and the local polarity of the micellar interface. Thus the cmc is obtained from the inflection point in the AN vs the surfactant concentration curve, which is related to the interaction which the probe experiences with the micelles. It follows that if this interaction is hindered, the cmc deduced by using the above method will be higher than the “true” cmc. This can be used to investigate the effect of the additives to micellar solutions. Figures 3 and 5 show the nitrogen coupling constants trend as a function of urea and DSBS concentrations for two different spin probes for 5-DSA and C12-Tempo. We can deduce several main results from the analysis of Figures 3 and 5. (i) From the same AN vs the surfactant concentration curve for 5-DSA and C12-Tempo spin probes, it can be concluded that the two spin probes have the same interaction ways with the micelles. 5-DSA and C12-Tempo are of surfactant-like molecules and mainly interact with the micelles via hydrophobic interactions and are solubilized in the surfactant micelles with the nitroxide group located in the micellar interface. (ii) From Figures 3 and 5, when the surfactant concentrations are below the cmc of SDBS, it can be seen that the AN values decrease as the urea concentrations increase. This shows that urea molecules interact with 5-DSA or
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Figure 5. Nitrogen coupling constant (AN) for the probe C12Tempo as a function of SDBS and urea concentrations.
C12-Tempo by replacing some water molecules, which is in agreement with the results obtained by Baglioni et al.1 Above the cmc of SDBS, the AN values decrease as urea concentrations increase, demonstrating that the addition of urea decreases the polarity and supporting the view that urea molecules interact with the SDBS micelles by replacing some water molecules that solvate the hydrophobic chain and the polar headgroups of the surfactants. (iii) The cmc values of the surfactant increase as the addition of urea concentrations increase. The cmc values can be computed from an AN trend as a function of the surfactant and urea concentrations. With 5-DSA spin probes, the cmc values are 1.19 × 10-3 M, without urea, and 1.33 × 10-3 and 1.78 × 10-3 M with 2.0 and 6.0 M urea, respectively. The cmc values obtained with C12Tempo spin probes are 1.26 × 10-3 M, without urea, and 1.41 × 10-3 and 1.91 × 10-3 M with 2.0 and 6.0 M, respectively. The cmc values obtained using 5-DAS and C12-Tempo agree and are also in agreement with the values measured by surface tensions. This shows that the addition of urea does not affect the spin probes (5-DAS and C12-Tempo) solubilization in the SDBS micelles. The above findings can also be strongly supported by analyzing the correlation times. In Figures 4 and 6, when the surfactant concentrations are below the cmc of SDBS, the addition of urea slightly increases the correlation times, demonstrating that the interaction of urea molecules with spin molecules leads to a slight decrease of the microviscosity sensed by the spin probes. Above the cmc of SDBS, the addition of urea increases the correlation times and shows an increase of the microviscosity of the micellar interface. As reported previously in literature, the results can be explained by considering that urea molecules interact with the surfactant polar headgroups
Figure 6. Correlation time (τ) for the probe C12-Tempo as a function of SDBS and urea concentrations. Table 1. Nitrogen Coupling Constants, AN, and Correlation Time, τ, for Tempo, C6-Tempo, and C16-Tempo Probes in 5.0 × 10-2 M SDBS as a Function of Urea Concentrations AN, G
τ, ns
C16C6C16C6urea (mol‚dm-1) Tempo Tempo Tempo Tempo Tempo Tempo 0.00 2.00 6.00
16.5 16.5 16.5
16.5 16.5 16.5
16.5 16.5 16.5
1.03 1.85 2.29
1.15 1.94 3.72
1.48 2.47 5.87
at the micellar interface. Further evidence comes from the results obtained with Tempo and C6- and C16-Tempo spin probes. AN and τ parameters are listed in Table 1. The AN values in Table 1 are almost insensitive to the addition of urea while the correlation times increase with urea concentrations. The results in Table 1 from the analysis of AN and τ all show that the addition of urea to surfactant micellar solutions decreases the polarity and increases the microviscosity of the micellar interface. This demonstrates that urea molecules solubilize at the micellar interface and penetrate below the surfactant polar headgroups by replacing some water molecules that solvate the hydrophobic chain and the polar headgroup of the surfactants. Acknowledgment. This work was supported by National Science Foundation of China. LA9604442
