Comparison of Microenvironments of Aqueous Sodium Dodecyl

Microenvironments of aqueous sodium dodecyl sulfate (SDS) micelles was examined in the presence of additives such as sodium chloride and p-toluidine ...
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Langmuir 2005, 21, 10391-10397

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Comparison of Microenvironments of Aqueous Sodium Dodecyl Sulfate Micelles in the Presence of Inorganic and Organic Salts: A Time-Resolved Fluorescence Anisotropy Approach G. B. Dutt* Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Received June 2, 2005. In Final Form: August 9, 2005 Microenvironments of aqueous sodium dodecyl sulfate (SDS) micelles was examined in the presence of additives such as sodium chloride and p-toluidine hydrochloride (PTHC) by monitoring the fluorescence anisotropy decays of two hydrophobic probes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and coumarin 6 (C6). It has been well-established that SDS micelles undergo a sphere-to-rod transition and that their mean hydrodynamic radius increases from 19 to 100 Å upon the addition of 0.0-0.7 M NaCl at 298 K. A similar size and shape transition is induced by PTHC at concentrations that are 20 times lower compared to that of NaCl. This study was undertaken to find out how the microviscosity of the micelles is influenced under these circumstances. It was noticed that the microviscosity of the SDS/NaCl system increased by ∼45%, whereas there was a less than 10% variation in the microviscosity of the SDS/PTHC system. The large increase in the microviscosity of the former system with salt concentration has been rationalized on the basis of the high concentration of sodium ions in the headgroup region of the micelles and their ability to strongly coordinate with the water present in this region, which decreases the mobility of the probe molecules.

1. Introduction The ability to solubilize a broad spectrum of substances in one-phase formulation has enabled the use of micelles and microemulsions as suitable media to carry out a wide range of chemical reactions.1-3 The rates of bimolecular reactions depend on the microenvironment in which the reactants are solubilized, and, in this context, microviscosity along with micropolarity is one of the important internal properties of a micelle. As a consequence, numerous studies have been performed to experimentally determine this quantity for different types of micelles.4-29 * E-mail: [email protected]. (1) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (2) Holmberg, K.; Jo¨nsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; John Wiley & Sons: Chichester, England, 2003. (3) Quina, F. H.; Lissi, E. A. Acc. Chem. Res. 2004, 37, 703. (4) Shinitzky, M.; Dianoux, A.-C.; Gitler, C.; Weber, G. Biochemistry 1971, 10, 2106. (5) Pownall, H. J.; Smith, L. C. J. Am. Chem. Soc. 1973, 95, 3136. (6) Gra¨tzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1973, 95, 6885. (7) Kalyanasundaram, K.; Gra¨tzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1975, 97, 3915. (8) Rodgers, M. A. J.; Da Silva, E.; Wheller, M. E. Chem. Phys. Lett. 1976, 43, 587. (9) Zachariasse, K. A. Chem. Phys. Lett. 1978, 57, 429. (10) Turro, N. J.; Aikawa, M.; Yekta, A. J. Am. Chem. Soc. 1979, 101, 772. (11) Zinsli, P. E. J. Phys. Chem. 1979, 83, 3223. (12) Henderson, C. N.; Selinger, B. K.; Watkins, A. R. J. Photochem. 1981, 16, 215. (13) Turley, W. D.; Offen, H. W. J. Phys. Chem. 1985, 89, 2933. (14) Miyagishi, S.; Asakawa, T.; Nishida, M. J. Colloid Interface Sci. 1987, 115, 199. (15) Miyagishi, S.; Kurimoto, H.; Asakawa, T. Bull. Chem. Soc. Jpn. 1995, 68, 135. (16) Miyagishi, S.; Kurimoto, H.; Asakawa, T. Langmuir 1995, 11, 2951. (17) Miyagishi, S.; Suzuki, H.; Asakawa, T. Langmuir 1996, 12, 2900. (18) Miyagishi, S.; Akasohu, W.; Hashimoto, T.; Asakawa, T. J. Colloid Interface Sci. 1996, 184, 527. (19) Pistolis, G.; Malliaris, A. Langmuir 1997, 13, 1457.

Spectroscopic techniques, such as fluorescence,3-24 electron spin resonance (ESR),25-28 and NMR,29 are usually employed to get an estimate of this parameter. Among the fluorescence methods, excimer formation and fluorescence depolarization are commonly used for the measurement of microviscosity. In a recent study,24 a nondipolar probe, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP), and a dipolar probe, coumarin 6 (C6), were chosen, and their rotational relaxation was measured in a number of small ionic micelles. From the measured average reorientation times of the probes and by employing realistic hydrodynamic volumes calculated from the hydrodynamic models, the microviscosities of these micelles were obtained. It was established from the study that both DMDPP and C6 are solubilized in the headgroup region of the micelles and sense almost identical microviscosities for a given micelle. The present study was undertaken to compare the microviscosities of aqueous sodium dodecyl sulfate (SDS) micelles in the presence of inorganic and organic salts obtained using the same methodology.24 These anionic surfactants in water aggregate above their critical micelle concentration (CMC) and form small spherical micelles at low ionic strengths. The addition of inorganic salts, (20) Zana, R.; In, M.; Le´vy, H.; Duportail, G. Langmuir 1997, 13, 5552. (21) Zana, R. J. Phys. Chem. B 1999, 103, 9117. (22) Ruiz, C. C.; Molina-Bolı´var, J. A.; Aguiar, J.; MacIsaac, G.; Moroze, S.; Palepu, R. Langmuir 2001, 17, 6831. (23) Dutt, G. B. J. Phys. Chem. B 2003, 107, 10546. (24) Dutt, G. B. J. Phys. Chem. B 2004, 108, 3651. (25) Bales, B. L.; Stenland, C. J. Phys. Chem. 1993, 97, 3418. (26) Bales, B. L.; Ranganathan, R.; Griffiths, P. C. J. Phys. Chem. B 2001, 105, 7465. (27) Ranganathan, R.; Peric, M.; Medina, R.; Garcia, U.; Bales, B. L.; Almgren, M. Langmuir 2001, 17, 6765. (28) Bales, B. L.; Zana, R. J. Phys. Chem. B 2002, 106, 1926. (29) Lindman, B.; So¨dreman, O.; Wennerstro¨m, H. In Surfactant Solutions. New Methods of Investigation; Zana, R., Ed.; M. Dekker, Inc.: New York, 1987; Chapter 6.

10.1021/la051444h CCC: $30.25 © 2005 American Chemical Society Published on Web 09/24/2005

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such as NaCl, screens the electrostatic headgroup repulsion between amphiphiles within a micelle, allowing the surface area per molecule to be reduced. This gives rise to a new packing condition for the surfactant molecules with a lower surface-to-volume ratio.30 As a consequence, these micelles grow in size and undergo a sphere-to-rod shape transition with an increase in the salt concentration. The size, shape transition, and polydispersity of SDS micelles as functions of surfactant concentration, electrolyte concentration, and temperature have been thoroughly investigated in a series of classic studies carried out by Mazer et al.,31-35 employing quasielastic light scattering methods. Since then, numerous reports have appeared in the literature36-41 that address the abovementioned aspects of SDS micelles using different techniques and methodologies. Recently, it was demonstrated that organic salts known as hydrotropes induce the sphereto-rod transition in SDS micelles at much lower concentrations.42-45 These organic counterions interact with the micelle-forming surfactant electrostatically as well as hydrophobically, and hence are efficient in inducing the growth of the micelles. Typically, the mean hydrodynamic radius rh of SDS micelles increases from 19 to 100 Å upon the addition of 0.0-0.7 M NaCl at 298 K.31-35 On the other hand, similar changes in size and shape have been brought about in SDS micelles by the presence of only 0.0-35.0 mM p-toluidine hydrochloride (PTHC). In other words, PTHC induces more or less the same growth in SDS micelles at concentrations that are 20 times lower compared to that of NaCl. It is therefore of interest to find out how an internal property such as microviscosity is affected in both the cases. The microviscosity of aqueous SDS micelles in the presence of NaCl was recently determined using the ESR technique.27 As already mentioned, the aim of this study is to compare this parameter for SDS micelles in the presence of NaCl and PTHC using time-resolved fluorescence anisotropy approach. It is also of interest to find out how the microviscosities determined using the fluorescence technique measure up to that determined by the ESR methodology. To this effect, the rotational relaxation of DMDPP and C6 (see Figure 1 for the molecular structures of these probes) was measured by monitoring the fluorescence anisotropy decays of the probes with the aid of a timecorrelated single-photon counting technique. It would be (30) Isaraelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (31) Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1976, 80, 1075. (32) Young, C. Y.; Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1978, 82, 1375. (33) Missel, P. J.; Mazer, N. A.; Young, C. Y.; Carey, M. C. J. Phys. Chem. 1980, 84, 1044. (34) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1983, 87, 1264. (35) Missel, P. J.; Mazer, N. A.; Carey, M. C.; Benedek, G. B. J. Phys. Chem. 1989, 93, 8354. (36) Bales, B. L.; Almgren, M. J. Phys. Chem. 1995, 99, 15153. (37) Quina, F. H.; Nassar, P. M.; Bonilha, J. B. S.; Bales, B. L. J. Phys. Chem. 1995, 99, 17028. (38) Almgren M.; Lo¨froth, J.-E. J. Colloid Interface Sci. 1981, 81, 486. (39) Siemiarczuk, A.; Ware, W. R.; Liu, Y. S. J. Phys. Chem. 1993, 97, 8082. (40) Dutt, G. B.; van Stam, J.; De Schryver, F. C. Langmuir 1997, 13, 1957. (41) Hall, D. G. Langmuir 1999, 15, 3843. (42) Hassan, P. A.; Raghavan, S. R.; Kaler, E. W. Langmuir 2002, 18, 2543. (43) Hassan, P. A.; Fritz, G.; Kaler, E. W. J. Colloid Interface Sci. 2003, 257, 154. (44) Hassan, P. A.; Sawant, S. N.; Bagkar, N. C.; Yakhmi, J. V. Langmuir 2004, 20, 4874. (45) Garg, G.; Hassan, P. A.; Aswal, V. K.; Kulshreshtha, S. K. J. Phys. Chem. B 2005, 109, 1340.

Dutt

Figure 1. Molecular structures of the probes DMDPP and C6.

interesting to find out how the microviscosity of the SDS micelles varies when they undergo sphere-to-rod transition, and it also remains to be seen whether both of the micellar systems (SDS/NaCl/water and SDS/PTHC/water) have the same internal environment. The remainder of the paper is organized in the following manner: Section 2 describes the experimental methods used to measure the rotational relaxation of the probes in the micelles. The results are presented and analyzed in section 3. Section 4 deals with the discussion of these results, and the conclusions of this work are summarized in the final section. 2. Experimental Section The probes DMDPP and C6 were obtained from Ciba Specialty Chemicals Inc., and Aldrich, respectively. The surfactant SDS was purchased from Gibco Life Technologies and the salts NaCl and PTHC were obtained from Fluka. All of the chemicals were of the highest available purity and were used as such. Deionized water from Millipore A-10 was used in the preparation of the micellar samples. The concentration of NaCl was varied from 0 to 0.7 M, whereas that of PTHC was varied from 0 to 35.0 mM. The surfactant concentration was maintained at 50 mM, and all of the measurements were performed at 298 K. The probe molecules were dissolved in the micelle solutions by being gently heated in a water bath, and it was ensured that the concentrations of the probes were in the range of 10-5-10-6 M. Steady-state anisotropies (〈r〉) of the samples were measured using a Hitachi F-4010 spectrofluorometer, and the details were described in our earlier publication.46 The probes DMDPP and C6 were excited at 440 nm, and their emission was monitored in the range of 515-585 nm. Time-resolved fluorescence measurements were carried out using the time-correlated singlephoton counting47 facility at the Tata Institute of Fundamental Research, Mumbai; the details of the system have been described elsewhere.48 In brief, the frequency-doubled output of a picosecond Ti:sapphire laser (Tsunami, Spectra Physics) was used as the excitation source, and the probes DMDPP and C6 were excited at 440 nm with a vertically polarized pulse. Fluorescence decays were collected at parallel (I|(t)), perpendicular (I⊥(t)), and 54.7° (I(t)) orientations of the emission polarizer with respect to the polarization of the excitation radiation. The emission in all the three cases was monitored at 550 nm. For fluorescence lifetime (τf) measurements, 10 000 peak counts were collected; for anisotropy measurements, 20 000 peak counts were collected for I|(t), and I⊥(t) was corrected for the polarization bias or the G-factor of the spectrometer. The decays were collected in 512 channels with time increments of 20, 40, or 80 ps/ch. Each measurement was repeated at least 2-3 times, and the average (46) Dutt, G. B.; Srivatsavoy, V. J. P.; Sapre, A. V. J. Chem. Phys. 1999, 110, 9623. (47) O′Connor, D. V.; Phillips, D. Time-Correlated Single Photon Counting; Academic Press: London, 1984. (48) Dutt, G. B.; Srivatsavoy, V. J. P.; Sapre, A. V. J. Chem. Phys. 1999, 111, 9705.

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values were reported. The desired sample temperature was attained with the help of a temperature controller, Eurotherm. The decays measured in this manner were convoluted with the instrument response function (IRF), which was measured by replacing the sample with a solution that scatters light. The full width at the half-maximum of the IRF is ∼50 ps. The lifetimes of the probes DMDPP and C6 in SDS micelles were obtained from the measured fluorescence decays and the IRF by an iterative reconvolution method using the Marquardt algorithm as described by Bevington.49 Likewise, the anisotropy decay parameters were obtained by a simultaneous fit50,51 of the parallel I|(t) and perpendicular I⊥(t) components. The criteria for a good fit was judged by statistical parameters such as the reduced χ2 being close to unity and the random distribution of the weighted residuals. Details concerning the analysis of the fluorescence and anisotropy decays were mentioned in an earlier publication.52

3. Results The fluorescence decays of both DMDPP and C6 in SDS micelles could be adequately described by singleexponential functions at all concentrations of NaCl. The lifetimes of DMDPP are in the range of 8.26-8.37 ns, and apparently, there is no systematic variation in τf with [NaCl]. On the other hand, the lifetimes of C6 gradually increase from 2.64 to 2.84 ns with an enhancement in the NaCl concentration from 0 to 0.7 M. The fact that both of the probes used in this study are hydrophobic and the recovery of one lifetime from the fluorescence decay analysis are indications that they are solubilized at a single site in these micelles. In contrast, the lifetimes of both DMDPP and C6 decrease with an increase in the concentration of PTHC. The mean τf values of DMDPP decreased by a factor of 3 from 8.26 to 2.73 ns, and those of C6 decreased from 2.64 to 2.27 ns as the concentration of PTHC varied from 0 to 35.0 mM. It is evident from these measurements that PTHC quenches the fluorescence of both of the probes, and it has also been noticed that for [PTHC] greater than 10 mM, the fluorescence decays of both the probes can only be described by either two or three exponentials. This type of multiexponential fluorescence decay kinetics has been reported for typical probequencher systems, such as coumarin-amine, in SDS micelles.53 Steady-state anisotropies can essentially be employed as a gauge to monitor the mobility of the probe in systems such as the ones used in the present study, provided there is no change in its lifetime as a function of salt concentration. Variations of 〈r〉 versus [salt] for both DMDPP and C6 in SDS/NaCl and SDS/PTHC micellar systems are displayed in Figure 2. It has been observed that steadystate anisotropies of both the probes increase with salt concentration for the two micellar systems investigated. The values of 〈r〉 for DMDPP and C6 increase by factors of 1.7 and 1.4, respectively, as [NaCl] increases from 0 to 0.7 M, and the corresponding numbers are 2.8 and 1.4, respectively, for an increase in [PTHC] from 0 to 35.0 mM. However, only in the DMDPP/SDS/NaCl system, where there is almost no variation in fluorescence lifetime with salt concentration, can the increase in 〈r〉 be interpreted as a decrease in the mobility of the probe. Usually Perrin’s equation54,55 is employed to calculate the (49) Bevington, P. R. Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1969. (50) Cross, A. J.; Fleming, G. R. Biophys. J. 1984, 46, 45. (51) Knutson, J. R.; Beechem, J. M.; Brand, L. Chem. Phys. Lett. 1983, 102, 501. (52) Dutt, G. B. J. Phys. Chem. B 2002, 106, 7398. (53) Kumbhakar, M.; Nath, S.; Pal, H.; Sapre, A. V.; Mukherjee, T. J. Chem. Phys. 2003, 119, 388. (54) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1999.

Figure 2. Plots of the steady-state anisotropies of DMDPP and C6 in SDS micelles as a function of [NaCl] and [PTHC]. The smooth lines through the data points are drawn as visual aids.

rotational correlation times from the steady-state anisotropies and lifetimes to find out whether the observed changes in 〈r〉 values are due to the variation in the mobility of the probes molecules or to an increase or decrease in their lifetimes. In this scenario, however, a slightly modified approach had to be adopted because the anisotropy decays of the probes in micelles are rather complex; this aspect will be elaborated in due course. In addition to an increase in the microviscosity of the micelles, specific interactions such as hydrogen bonding between the probe and its surroundings can also contribute to the increase in the observed steady-state anisotropy of the probe. However, numerous studies carried out with DMDPP and C6 in organic solvents indicate that the interactions experienced by these probes are not strong enough to impede their rotation.56,57 Hence, it is logical to conclude that specific interactions do not contribute to the enhancement of steady-state anisotropies for the systems under investigation. To obtain incisive information pertinent to the mobility of the probe molecules in these micellar systems, timeresolved anisotropy studies were performed. The anisotropy decays of both DMDPP and C6 in the two micellar systems at all concentrations of NaCl and PTHC could be adequately fit by biexponential functions of the form described by eq 1.

[ ( )

r(t) ) r0 β exp

( )]

-t -t + (1 - β) exp τslow τfast

(1)

In the above equation, τslow and τfast are the two time constants associated with the rotation of the probe molecule in the micelle, and β represents the percentage contribution of τslow to the decay of the anisotropy. The value r0 is the inherent depolarization due to the noncollinear alignment between the absorption and emission transition dipoles and is characteristic for any given molecule. Typical anisotropy decays of DMDPP and C6 in SDS micelles at the highest salt concentrations (0.7 M and 35.0 mM for NaCl and PTHC, respectively) together with the fitted curves are displayed in Figure 3. The anisotropy decay parameters of DMDPP and C6 that were obtained from the analysis are given in Tables 1 and 2 for the SDS/NaCl and SDS/PTHC systems, respectively. The (55) Kelkar, D. A.; Chattopadhyay, A. J. Phys. Chem. B 2004, 108, 12151. (56) Dutt, G. B. ChemPhysChem 2005, 6, 413. (57) Dutt, G. B.; Raman, S. J. Chem. Phys. 2001, 114, 6702.

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Figure 3. Anisotropy decays of DMDPP (b) and C6 (O) in SDS micelles at [NaCl] ) 0.7 M (top) and [PTHC] ) 35.0 mM (bottom). The smooth lines passing through the data points are the fitted curves. The average reorientation times, defined by eq 2, are 0.78 and 1.74 ns for DMDPP and C6, respectively, in the SDS/ NaCl micellar system, and the corresponding numbers are 0.55 and 1.30 ns in the SDS/PTHC system.

average reorientation times, 〈τr〉, which were calculated using eq 2, are also given in these tables.

〈τr〉 ) βτslow + (1 - β)τfast

(2)

Inspection of these tables reveals that the average reorientation times of DMDPP and C6 increase by a factor of 1.6 and 1.7, respectively, for the SDS/NaCl system, and the corresponding numbers are 1.1 and 1.3 for the SDS/ PTHC system. From the time-resolved anisotropy parameters and mean fluorescence lifetimes (〈τf〉), steady-state anisotropies have been calculated from the following relation:52

[

〈r〉 ) r0

βτslow

(〈τf〉 + τslow)

+

(1 - β)τfast (〈τf〉 + τfast)

]

(3)

The above equation reduces to Perrin’s equation if there is only one anisotropy decay component. However, it must be noted that, when the decay of anisotropy has two components, rotational correlation times cannot be calculated from the steady-state anisotropies and fluorescence lifetimes using eq 3. The agreement between the experimentally measured steady-state anisotropies and those obtained from eq 3 is within 10% (results not shown), indicating that the anisotropy decay parameters have been recovered accurately. This exercise also reiterates the fact that, except in case of the DMDPP/SDS/NaCl system, the experimentally observed variations in the 〈r〉 values can be attributed to a combination of changes in the mobility of the probe molecules as well as to their lifetimes. 4. Discussion To comprehend the results presented in the preceding section, it is essential to have knowledge regarding the structure of the micelles and the location of the probes in these micelles. The structure (size and shape) of aqueous SDS micelles in the presence of NaCl and PTHC has been extensively investigated,31-43 and the salient features are mentioned in the introduction. Concerning the location of

the probes, numerous studies carried out in the literature58-61 pertinent to the sites of solubilization of the aromatic molecules in micelles indicate that they are located at or near the micelle-water interface. For the systems used in This study, evidence for the location of the probes can also be obtained from fluorescence lifetime data. The cores of these micelles resemble an alkane-like environment, and in alkane solvents, the lifetime of the probe DMDPP is in the range of 5.5-6.7 ns and that of C6 is in the range of 2.3-2.4 ns.46,57,62-64 However, the lifetimes of DMDPP and C6 in these micelles are 8.3 ns and 2.6 ns, respectively, in the absence of additives, which is an indication that both DMDPP and C6 are not solubilized in the cores of the micelles. Because the probes used in this study are hydrophobic, the headgroup region is the only remaining location where they can get solubilized. It may be argued that the headgroup region of the micelle also contains some water. However, it has been established from recent solvation dynamics studies65-67 that the water present in the headgroup region of a micelle has different properties compared to the bulk water, which is present outside the micelle. As already mentioned, the addition of NaCl of did not alter the fluorescence decay profiles of DMDPP and C6. Nevertheless, in the presence of PTHC (>10 mM), the fluorescence decays of both of the probes follow two-exponential kinetics. This observation raises the possibility that the probe molecules may be solubilized in two distinct regions (the headgroup and the core region) of the micelles. For this argument to be valid, the magnitudes of the two decay components should not change with an increase in PTHC concentrationsonly their relative contributions (preexponential factors) to the decay of fluorescence. On the contrary, it was observed that there is a gradual decrease in the decay components and small fluctuations in the preexponential factors with an increase in [PTHC] for both of the probes. In addition, for [PTHC] > 20 mM, the fluorescence decays could only be described by three exponentials for DMDPP as well as for C6. Hence, the observed behavior is due to fluorescence quenching rather than the distribution of the probe molecules at two sites in a micelle. Once the structural details of the micelles have been obtained from the literature and the location of the probes, the observed biexponential anisotropy decays can be interpreted using a two-step model.68-71 A number of rotational relaxation studies involving organic solutes in micelles23,24,52,72-75 and reverse micelles62,63,76 have con(58) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 279. (59) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978, 82, 1620. (60) Mukerjee, P. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1. (61) Ganesh, K. N.; Mitra, P.; Balasubramanian, D. J. Phys. Chem. 1982, 86, 4291. (62) Dutt, G. B. J. Phys. Chem. B 2004, 108, 805. (63) Dutt, G. B. J. Phys. Chem. B 2004, 108, 7944. (64) Dutt, G. B.; Sachdeva, A. J. Chem. Phys. 2003, 118, 8307. (65) Bhattacharyya, K.; Bagchi, B. J. Phys. Chem. A 2000, 104, 10603. (66) Pal, S. K.; Peon, J.; Bagchi, B.; Zewail, A. H. J. Phys. Chem. B 2002, 106, 12376. (67) Bhattacharyya, K. Acc. Chem. Res. 2003, 36, 95. (68) Kinoshita, J.; Kawato, S.; Ikegami, A. Biophys. J. 1977, 20, 289. (69) Lipari, G.; Szabo, A. Biophys. J. 1980, 30, 489. (70) Wang, C. C.; Pecora, R. J. Chem. Phys. 1980, 72, 5333. (71) Lipari, G.; Szabo, A. J. Am. Chem. Soc. 1982, 104, 4546. (72) Quitevis, E. L.; Marcus, A. H.; Fayer, M. D. J. Phys. Chem. 1993, 97, 5762. (73) Maiti, N. C.; Krishna, M. M. G.; Britto, P. J.; Periasamy, N. J. Phys. Chem. B 1997, 101, 11051. (74) Kelepouris, L.; Blanchard, G. J. J. Phys. Chem. B 2003, 107, 1079. (75) Dutt, G. B. J. Phys. Chem. B 2003, 107, 3131. (76) Wittouck, N.; Negri, R. M.; Ameloot, M.; De Schryver, F. C. J. Am. Chem. Soc. 1994, 116, 10601.

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Table 1. Anisotropy Decay Parameters of DMDPP and C6 in SDS Micelles as a Function of [NaCl] at 298 K τslow/ns

β

a〈τ

τfast/ns

r〉/ns

[NaCl]/M

DMDPP

C6

DMDPP

C6

DMDPP

C6

DMDPP

C6

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

0.23 ( 0.02 0.24 ( 0.01 0.28 ( 0.03 0.26 ( 0.01 0.26 ( 0.01 0.32 ( 0.02 0.36 ( 0.02 0.35 ( 0.02

0.33 ( 0.02 0.38 ( 0.04 0.45 ( 0.01 0.47 ( 0.01 0.50 ( 0.04 0.55 ( 0.02 0.53 ( 0.03 0.58 ( 0.02

1.16 ( 0.02 1.20 ( 0.02 1.16 ( 0.04 1.32 ( 0.06 1.42 ( 0.04 1.40 ( 0.04 1.44 ( 0.02 1.46 ( 0.04

1.82 ( 0.07 1.90 ( 0.06 1.88 ( 0.03 2.05 ( 0.09 2.22 ( 0.06 2.24 ( 0.09 2.56 ( 0.09 2.52 ( 0.03

0.29 ( 0.01 0.31 ( 0.01 0.31 ( 0.01 0.33 ( 0.01 0.35 ( 0.01 0.33 ( 0.01 0.35 ( 0.03 0.41 ( 0.01

0.63 ( 0.01 0.61 ( 0.02 0.61 ( 0.02 0.66 ( 0.05 0.66 ( 0.02 0.61 ( 0.01 0.69 ( 0.04 0.69 ( 0.05

0.49 ( 0.03 0.52 ( 0.03 0.55 ( 0.04 0.59 ( 0.03 0.63 ( 0.03 0.67 ( 0.04 0.74 ( 0.05 0.78 ( 0.04

1.02 ( 0.06 1.10 ( 0.09 1.18 ( 0.04 1.31 ( 0.09 1.44 ( 0.10 1.51 ( 0.09 1.68 ( 0.12 1.74 ( 0.08

a

Calculated using eq 2. Table 2. Anisotropy Decay Parameters of DMDPP and C6 in SDS Micelles as a Function of [PTHC] at 298 K τslow/ns

β

a〈τ 〉/ns r

τfast/ns

[PTHC]/mM

DMDPP

C6

DMDPP

C6

DMDPP

C6

DMDPP

C6

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0

0.23 ( 0.02 0.26 ( 0.04 0.31 ( 0.03 0.31 ( 0.03 0.35 ( 0.02 0.45 ( 0.01 0.45 ( 0.02 0.49 ( 0.01

0.33 ( 0.02 0.35 ( 0.01 0.56 ( 0.02 0.49 ( 0.01 0.60 ( 0.01 0.58 ( 0.01 0.64 ( 0.02 0.56 ( 0.01

1.16 ( 0.02 1.07 ( 0.07 1.03 ( 0.05 1.02 ( 0.06 0.97 ( 0.01 0.94 ( 0.01 0.95 ( 0.01 0.89 ( 0.03

1.82 ( 0.07 1.87 ( 0.02 1.62 ( 0.02 1.70 ( 0.05 1.77 ( 0.03 1.78 ( 0.02 1.78 ( 0.02 1.94 ( 0.02

0.29 ( 0.01 0.28 ( 0.02 0.27 ( 0.01 0.27 ( 0.03 0.28 ( 0.01 0.26 ( 0.02 0.24 ( 0.02 0.22 ( 0.01

0.63 ( 0.01 0.63 ( 0.03 0.45 ( 0.04 0.65 ( 0.06 0.52 ( 0.01 0.66 ( 0.06 0.45 ( 0.01 0.49 ( 0.02

0.49 ( 0.03 0.49 ( 0.07 0.51 ( 0.04 0.50 ( 0.07 0.52 ( 0.03 0.57 ( 0.03 0.56 ( 0.03 0.55 ( 0.03

1.02 ( 0.06 1.06 ( 0.04 1.11 ( 0.05 1.16 ( 0.07 1.27 ( 0.03 1.31 ( 0.05 1.30 ( 0.04 1.30 ( 0.04

a

Calculated using eq 2.

clusively demonstrated that τslow and τfast arise as a result of the probe molecule undergoing different kinds of motion at or near the interface of the micelle. According to this model, τslow is a combination of the lateral diffusion of the probe on the spherical surface of the micelle and the rotation of the micelle as a whole. τfast has contributions from the fast wobbling motion of the probe in an imaginary cone, which is defined by semiangle θ and τslow. The respective relationships between these two experimentally measured quantities and the model parameters are given by the following relations:72

1 1 1 ) + τslow τL τM

(4)

1 1 1 1 ) + + τfast τW τL τM

(5)

In the above equations, τL, τW, and τM are the time constants for lateral diffusion, wobbling motion, and the overall rotation of the micelle, respectively. τM is calculated from the Stokes-Einstein-Debye (SED) hydrodynamic model with a stick boundary condition,77 which is given by

τM )

4π rh3η 3kT

(6)

where η is the viscosity of the medium in which the micellar rotation takes place, k is the Boltzmann constant, and T is the absolute temperature. Note that the overall rotation of the micelle contributes to τslow and τfast only at low salt concentrations. However, at high salt concentrations due to an increase in the size of the micelles, τM becomes much larger (typically by a few hundred nanoseconds, which was calculated using eq 6) compared to τslow and τfast, and as a consequence, the overall rotation of the micelle does not contribute to the decay of anisotropy. In other words, the term 1/τM can be neglected from eqs 4 and 5 at high salt concentrations. However, note that, even though the micelle size becomes very large, the decay (77) Debye, P. Polar Molecules; Dover: New York, 1929.

of anisotropy will contain the slow component (τslow) because the lateral diffusion of the probe persists. Now the question that needs to be addressed is whether the two-step model, which is usually employed to explain the rotational relaxation of organic solutes in spherical micelles, is adequate to comprehend the same in the case of large spherocylindrical micelles. The answer seems to be affirmative because lateral diffusion or translational diffusion on or inside any curved surface, in principle, can contribute to the depolarization of fluorescence. However, the diffusion coefficients (lateral and wobbling) calculated from the standard formulas in this scenario may not be realistic because they are sensitive to the radius of the surface and also to the orientation of the probe in the headgroup region of the micelle. In view of this, no attempt has been made to obtain these entities from the anisotropy decay parameters. An estimation of the microviscosities of the micelles from the anisotropy decay parameters, which is the central theme of this work, was carried out in the following manner. The microviscosities of the micelles were obtained from the average reorientation time of the probe, 〈τr〉p, under the assumption that it follows the SED relation. Even though τslow and τfast account for the rotational relaxation of the probe in micelles in a detailed manner, it is 〈τr〉p that essentially describes the overall mobility of the probe. In an ideal scenario, the microviscosity should have been determined independently from the lateral and wobbling diffusion coefficients by using the appropriate friction coefficients. Nonetheless, the difficulty involved in carrying out such an exercise is the accurate estimation of the respective friction coefficients from the hydrodynamic models. Moreover, for the purpose of comparing the microviscosities of two micellar systems, the methodology adopted here is reasonable. As already mentioned, 〈τr〉 has contribution from the overall rotation of the micelle, especially at low salt concentrations. Under these circumstances, the average reorientation time of the probe is given by eq 7.24

1 1 1 ) 〈τr〉p 〈τr〉 τM

(7)

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Langmuir, Vol. 21, No. 23, 2005

Dutt Table 4. Microviscosities of SDS Micelles in the Presence of PTHC Obtained from the Average Reorientation Times of the Probes ηm/mPa s

Figure 4. Plots of the average reorientation times of the probes obtained from eq 6 for DMDPP (b) and C6 (O) in SDS micelles as a function of [NaCl] (top) and [PTHC] (bottom). The smooth lines through the data points were obtained by a linear leastsquares fit. Table 3. Microviscosities of SDS Micelles in the Presence of NaCl Obtained from the Average Reorientation Times of the Probes ηm/mPa s [NaCl]/M

DMDPP

C6

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

11.9 ( 0.9 12.1 ( 0.7 12.8 ( 1.1 13.5 ( 0.9 14.4 ( 0.7 15.1 ( 0.9 16.6 ( 1.1 17.5 ( 0.9

11.5 ( 0.9 11.2 ( 1.1 12.2 ( 0.5 13.1 ( 1.0 14.0 ( 1.1 14.3 ( 0.9 15.9 ( 1.2 16.4 ( 0.8

The calculated 〈τr〉p values of DMDPP are, on average, 2.3 times smaller than those of C6 in both SDS/NaCl and SDS/PTHC micellar systems. Even though DMDPP and C6 have almost identical van der Waals volumes, they experience significantly different hydrodynamic volumes (Vh) in simple liquids64 and micelles.24 The hydrodynamic volume of DMDPP is smaller by a factor of 2.4 compared to that of C6, which accounts for the observed differences in the 〈τr〉p values. The substantial disparity in the Vh values of two probes is not due to either of them experiencing specific interactions with their surroundings, but is rather a consequence of the distinct shapes of DMDPP and C6. In other words, the friction experienced by these probes is considerably different because of their respective shapes, the relevant aspects of which have been thoroughly discussed in our earlier work.64 The variation in the 〈τr〉p values of DMDPP and C6 as a function of [NaCl] and [PTHC] are displayed in Figure 4. 〈τr〉p values increase by ∼47 and 43% for DMDPP and C6, respectively, in the case of SDS/NaCl, whereas the corresponding numbers are less than 10% for the SDS/PTHC system. The microviscosities of SDS/NaCl and SDS/PTHC micellar systems were obtained from the 〈τr〉p values with the aid of eq 8 and are given in Tables 3 and 4, respectively.

ηm )

〈τr〉pkT Vh

(8)

As mentioned before, the main assumption involved in such an exercise is that the average reorientation time of the probe follows the SED77 relation. It was shown earlier46,57,64 that the shape of the probes DMDPP and C6 can be approximated as asymmetric ellipsoids, and the

[PTHC]/mM

DMDPP

C6

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0

11.9 ( 0.9 11.7 ( 1.8 12.1 ( 1.2 11.7 ( 1.8 11.9 ( 0.7 12.8 ( 0.7 12.6 ( 0.7 12.4 ( 0.7

11.5 ( 0.8 11.6 ( 0.6 11.9 ( 0.7 12.3 ( 0.8 12.4 ( 0.3 12.5 ( 0.5 12.3 ( 0.4 12.3 ( 0.4

shape factors (f) and boundary condition parameters (C) can be calculated with the aid of the hydrodynamic models. Using these parameters, the hydrodynamic volume for a given probe is obtained as the product of van der Waals volume V, shape factor f, and boundary condition parameter C (Vh ) VfC). Rotational dynamics studies involving DMDPP and C6 in alkane solvents indicate that the boundary condition parameter is sensitive to the size of the solvent.46,57,64 For DMDPP and C6, the boundary condition parameters are close to the ones predicted by the slip boundary condition78 in a nonpolar solvent such as hexadecane. As mentioned earlier, the rotational diffusion of these probes takes place in the headgroup region of the micelles, and because they do not experience specific interactions with their surroundings, it is not unreasonable to assume that the boundary condition parameter will be akin to that in hexadecane. However, it may be argued that the water present in the headgroup region of the micelles can interact with the probe molecules, which can affect the microviscosity sensed by them. Nevertheless, it has been demonstrated that the specific interactions experienced by the probes with a number of organic solvents such as n-alcohols are not strong enough to impede their rotation.46,57 Hence, the slip boundary condition has been used to model the rotation of DMDPP and C6 in these micelles. The calculated hydrodynamic volumes of DMDPP and C6 using the slip boundary condition are 183 and 436 Å3, respectively.46,57 Similar arguments were used while estimating the microviscosities of a number of small ionic micelles and the temperature-dependent microviscosities of large nonionic micelles, such as Brij-35, in our earlier studies.23,24 An inspection of Tables 3 and 4 reveals that the microviscosities sensed by both DMDPP and C6 for a given micellar system at a particular salt concentration are almost identical within the limits of experimental error. Figure 5 provides plots of the average microviscosities (average of ηm reported by DMDPP and C6) of SDS/NaCl and SDS/PTHC micellar systems as functions of the respective salt concentrations. The microviscosities of the SDS/NaCl system increase by ∼45% in response to an enhancement in NaCl concentration from 0 to 0.7 M, and, in contrast, the variation in ηm is less than 10% for the SDS/PTHC system. It must be emphasized that the microviscosities obtained in this manner are realistic for comparing the fluidity of different surfactant/salt systems. However, a certain amount of caution must be exercised while using their absolute values because the probe senses the microviscosity of the medium that is perturbed by its mere presence. This result is an indication that, even though the variation in the size and shape of both of the micellar systems (SDS/NaCl and SDS/PTHC) are more or less identical upon the addition of salts, the changes (78) Hu, C. M.; Zwanzig, R. J. Chem. Phys. 1974, 60, 4354.

Microenvironments of Aqueous SDS Micelles

Figure 5. Plots of the microviscosities of SDS/NaCl and SDS/ PTHC micellar systems as a function of salt concentration. The symbols b and O are for the SDS/NaCl and SDS/PTHC systems, respectively. The smooth lines through the points were obtained by a linear least-squares fit. Notice that the increase in ηm is 45% for the SDS/NaCl micellar system, whereas it is less than 10% for the SDS/PTHC system.

induced in the respective microenvironments of the micelles by these additives are distinct. What is the reason for the differences in the variation of the observed microviscosities with salt concentration for both of the micellar systems? Ranganathan et al.27 pointed out that viscosity is a phenomenological quantity and lacks a microscopic description. However, the trend in its variations can be speculated. It is plausible that, at low concentrations of NaCl, the probes, which are located in the headgroup region of the micelles, can rotate with less hindrance because of the presence of a small number of sodium ions. Nevertheless, as the concentration of NaCl is increased, the sodium ions that are present in large numbers strongly coordinate with the water molecules because of their high charge-to-size ratio and thus render the microenvironment that is sensed by the probes less fluid. For the SDS/PTHC system, however, the increase in the salt concentration is only marginal, and the organic ions present in the system cannot coordinate with the water molecules in the same manner as the inorganic ions. Thus, it is probable that the microenvironment experienced by the probe molecules remains more or less the same. At this juncture, it is imperative to compare the somewhat similar results obtained by Ranganathan et al.,27 who reported the microviscosities of an aqueous SDS/ NaCl micellar system by measuring the rotational correlation times of the spin probe 5-doxylstearic acid methyl ester using ESR. In their measurements, the surfactant concentration was 100 mM, and the salt concentration was varied in the range of 0.0-0.5 M. It was found that ηm increases from 0 to 0.3 M and plateaus from 0.3 to 0.5 M. In contrast, the microviscosities of SDS/NaCl micelles obtained in this study increase linearly with the salt

Langmuir, Vol. 21, No. 23, 2005 10397

concentration. Moreover, the ηm values reported by us are higher than those reported by Ranganathan et al.27 The initial increase and subsequent saturation in the microviscosity that is experienced by the spin probe has been rationalized on the basis of a gradual decrease in the volume fraction of the water present in the polar shell of the micelle before the onset of the sphere-to-rod transition, which also reaches a saturation once the shape transition is complete. Even though the microenvironment sensed by the probes in this study is also affected with salt concentration, its variation appears to depend on how the water present in the headgroup region is coordinated by the sodium ions. Conclusions In this work, a comparison is made between the microviscosities of aqueous SDS micelles in the presence of inorganic and organic additives. Aqueous SDS micelles undergo sphere-to-rod transition in the presence of 0.00.7 M NaCl at 298 K, whereas a similar shape transition is induced in the micellar system upon the addition of an organic salt such as PTHC at ∼20 times lower concentration compared to NaCl. In an attempt to find out whether two micellar systems having almost identical external properties will have the same internal properties, this study was undertaken, and the important conclusions are as follows: Microviscosities estimated from the anisotropy decay parameters of two hydrophobic probes, DMDPP and C6, for a given micelle are more or less the same within the limits of experimental error. The microviscosity of the SDS/NaCl system increases by ∼45% in response to an enhancement in the salt concentration; on the other hand, the variation in ηm of the SDS/PTHC system is less than 10%. The probable reason for the observed result can be explained by taking into consideration the large number of sodium ions present in the headgroup region of the NaCl/SDS system and their propensity to strongly coordinate with the water present in this region, which makes the microenvironment less fluid and hence decreases the mobility of the probe molecules. This result clearly indicates that it is the nature of the additives that determines the microenvironments of the organized assemblies, even when they are formed with the same surfactant units and possess identical external properties. Acknowledgment. I thank P. A. Hassan and T. Goel for useful discussions. I am grateful to M. H. Kombrabail of the Tata Institute of Fundamental Research for her help with time-resolved fluorescence experiments. I thank P. N. Bajaj and T. Mukherjee for their encouragement throughout the course of this work. LA051444H