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Structural Modifications of Aqueous Ionic Micelles in the Presence of Denaturants as Studied by DLS and Viscometry Sanjeev Kumar,† Deepti Sharma,† Goutam Ghosh,‡ and Kabir-ud-Din*,† Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India, and UGC-DAEF CSR, Mumbai Centre, B. A. R. C., Trombay, Mumbai 400 085, India Received June 12, 2005. In Final Form: August 13, 2005 Hydrophobic interactions control the morphologies of both surfactant aggregates and proteins. Globular proteins “denature” upon addition of excess amounts of denaturants such as urea. Understanding the microscopic basis of the urea effect on proteins or supramolecular aggregates such as micelles has always been a debated issue. Inspired by this need, the effect of urea (U), thiourea (TU), monomethylurea (MMU), dimethylurea(DMU), tetramethylurea (TMU), dimethylthiourea (DMTU), and tetramethylthiourea (TMTU) on the structural transition (spherical micelles to rod-shaped micelles, s f r) in the sodium dodecylbenzenesulfonate (SDBS)-1-pentanol system has been investigated through dynamic light scattering(DLS) and viscosity measurements at 25 °C. 1-Pentanol, at 0.14 M, is found to promote s f r in this system (0.2 M SDBS). The presence of the additives causes, in almost all cases, a decrease and increase in this 1-pentanol concentration depending upon the concentration and nature of the additive. These effects are explained in terms of an increased dielectric constant of the solvent medium due to the presence of additives and increased micellar hydration due to the repulsion of charged monomers caused by adsorption of the additives. Taken together, the data signal the exposure of biological assemblies to water at higher [additive], which causes a decrease in hydrophobic interactions responsible for compact structure formation (i.e., native protein).
Introduction Hydrophobic interactions are the main driving force for amphiphilic association and for native structure of globular proteins.1,2 A general method adopted to study hydrophobic interactions in such systems is to explore structural variation in aqueous solvents. This can be achieved in several ways (i.e., electrolyte3,4 or nonelectrolyte addition, change in the solvent, or change in the “structure” of the solvent itself5-10). The transition of proteins from an unfolded state to the folded one has some resemblance to micelle formation because both processes are governed by the same basic intermolecular and ionic forces. It is established that globular proteins unfold (i.e., denaturation) upon addition of excess amounts of denaturants such as urea or guanidine hydrochloride. Therefore, several attempts have been made using urea as an additive to check its effect on the properties of micellar solutions10-15 and on the denaturation of proteins.7,9,16-18 † ‡
Aligarh Muslim University. UGC-DAEF CSR.
(1) Kauzmann, W. Adv. Protein Chem. 1959, 14, 1. (2) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1980. (3) Corti, M.; Degiorgio, V. J. Phys. Chem. 1981, 85, 711. (4) Dorshow, R. B.; Bunton, C. A.; Nicoli, D. F. J. Phys. Chem. 1983, 87, 1409. (5) Abu-Hamdiyyah, M.; Kumari, K. J. Phys. Chem. 1990, 94, 6445. (6) Florenzano, F..H.; Politi, M. J. Braz. J. Med. Biol. Res. 1997, 30, 179. (7) Wallquist, A.; Covell, D. G.; Thirumalai, D. J. Am. Chem. Soc. 1998, 120, 427. (8) Carnero Ruiz, C. Colloids Surf., A 1999, 147, 349. (9) Bhuyan, A. K. Biochemistry 2002, 41, 13386. (10) Romsted, L. S.; Zhang, J.; Cuccovia, I. M.; Politi, M. J.; Chaimovich, H. Langmuir 2003, 19, 9179. (11) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J. Phys. Chem. 1990, 94, 8218. (12) Briganti, G.; Puvvada, S.; Blankschtein, D. J. Phys. Chem. 1991, 95, 8989. (13) Ambrosome, L.; Ragone, R. J. Colloid Interface Sci. 1998, 205, 454.
To understand the microscopic basis of urea action, two different mechanisms have been proposed:11,12,16,19,20 (i) urea changes the structure of water to facilitate the solvation of a hydrocarbon chain; (ii) urea replaces some water molecules that solvate the hydrophobic chain and the polar group of the amphiphile. Another dimension has been added to this debate by Politi et al.14 by proposing a third alternative for the formation of more “polar water” as a consequence of which better solvation of polar or ionic headgroups takes place. In a separate study, it has been reported that urea partitions distinctly less near the micellar interfacial region in comparison of its stoichiometric concentration in solution and destabilizes the headgroup-counterion pair with a concomitant effect on the micelle properties.10 The addition of urea to aqueous micellar solutions has a number of effects on micellar parameters, such as increasing the critical micelle concentration, cmc,5,8,12,21 or decreasing the aggregation number.12,22,23 It is also reported that urea stabilizes both the native and the denatured protein conformations.7,9,24 Recently, we have (14) Dias, L. G.; Florenzano, F. H.; Reed, W.; Baptista, M. S.; Souza, S. M. B.; Alvarez, E. B.; Chaimovich, H.; Cuccovia, I. M.; Amaral, C. L. C.; Brasil, C. R.; Romsted, L. S.; Politi, M. Langmuir 2002, 18, 319. (15) Kumar. S.; Parveen, N.; Kabir-ud-Din J. Phys. Chem. B 2004, 108, 9588. (16) Neri, D.; Billeter, M.; Wider, G.; Wiithrich, K. Science 1992, 227, 1559. (17) Liepinsh, E.; Otting, G. J. Am. Chem. Soc. 1994, 116, 9670. (18) Yelamos, B.; Nunez, E.; Gomez-Gutierrez, J.; Delgado, C.; Pacheco, B.; Peterson, D. L.; Gavilanes, F. Biochim. Biophys. Acta 2001, 87, 1546. (19) Wetlaufer, D. B.; Malik, S. K.; Stoller, L.; Coffin, R. I. J. Am. Chem. Soc. 1997, 99, 2898. (20) Enea, O.; Jollcoeur, C. J. Phys. Chem. 1982, 86, 3870. (21) Sarkar, N.; Bhattacharya, K. Chem. Phys. Lett. 1991, 180, 283. (22) Almgren, M.; Swarup, S. J. Colloid Interface Sci. 1983, 91, 256. (23) Caponetti, E.; Causi, S.; De Lisi, R.; Floriano, M. A.; Triolo, R. J. Phys. Chem. 1992, 96, 4950. (24) Matouschek, A.; Kellis, J. T., Jr.; Serrano, L.; Bycrott, M.; Fersht, A. R. Nature 1990, 3462, 440.
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found that urea can facilitate amphiphilic association if added in the low concentration range.15,25 The above facts and other observations of fairly recent theoretical analyses7,26-29 suggest that the interpretation of the urea effect in surfactant/protein research is still a controversy requiring further investigations. This is the reason that continuous research efforts are going on to answer the debated urea effect.7,8,10,12,14,15,25 In our earlier work,15 a model system30 (3.5% sodiumdodecyl sulfate (SDS) + 0.28 M NaCl + 1-pentanol) was chosen to see the effect of urea on micellar structural changes. Looking at the complexity of the system, we have now chosen a relatively simple sodium dodecylbenzene sulfonate (SDBS)-1-pentanol system, and the effects of adding urea as well as other related compounds such as thiourea and mono-, di-, tetramethylurea, di-, or tetramethylthiourea on the observance of the sphere-to-rod transition (s f r) has been studied by dynamic light scattering (DLS) and viscosity measurements at 25 ( 0.1 °C. Experimental Section Sodium dodecylbenzene sulfonate (SDBS, g99%, TCI, Japan), 1-pentanol (99%, Fluka), urea (U, 99%, BDH), thiourea (TU, 99%, s.d. fine chem), monomethylurea (MMU, >99%, Sigma), dimethylurea (DMU, ∼98%, Fluka), dimethylthiourea (DMTU, 98%, Lancaster), tetramethylurea (TMU, g99%, Fluka), and tetramethylthiourea (TMTU, 98%, Lancaster) were used as received. Second-stage milli-Q water with a specific resistance of 18.7 MΩ cm was used for solution preparation for DLS experiments, whereas demineralized double-distilled water was used for samples for viscosity measurements. A home-built set up was used for DLS experiments.31,32 The incident beam was generated from a vertically polarized 15 mW He-Ne laser source (λ ) 632.8 nm) fixed at one arm of the goniometer. The scattered beam was passed through a vertical polarizer (Glan-Thomson polarizer) and counted by a photomultiplier tube (PMT) at 90°, mounted on the other arm of the goniometer. Before measurement, each sample was centrifuged at 10 000 rpm for 30 min to remove “dust” particles. The sample was then loaded onto an optical-quality 3 mL quartz cell. The sample cell was placed inside a borosilicate cuvette consisting of an index matching liquid (e.g., decalene) and aligned with the axis of rotation of the goniometer. Scattered photons from dispersed aggregates were counted by the PMT detector, which was operated at 5 °C. The output current from the PMT was then suitably amplified and digitized through various electronics before it was fed to a channel digital correlator (Malvern, U.K.). The whole assembly was placed on a vibration-free table. All correlation spectra were recorded at 25 ( 0.1 °C. The correlation curves were analyzed using the CONTIN33 software provided by Malvern. Because the count rate was observed to be on the low side, the data collection time was increased for each solution to improve the statistics of the DLS spectrum. The errors in the measurements of micellar sizes from DLS spectra are within (3% around the mean value of three best measurements of each sample. (In fact, we have taken 10 measurements for a single sample.) (25) Kumar, S.; Parveen, N.; Kabir-ud-Din J. Surf. Deterg. 2005, 8, 109. (26) Cristinziano, P.; Lelj, F.; Amodeo, P.; Barone, G.; Barone, G.; Barone, V. J. Chem. Soc., Faraday Trans. I 1989, 85, 621. (27) Nakanishi, K. Chem. Soc. Rev. 1993, 22, 177. (28) Mountain, R. D.; Thirumalai, D. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 8436. (29) Klimov, D. K.; Straub, J. E.; Thirumalai, D. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 14760. (30) Lindemuth, P. M.; Bertrand, G. L. J. Phys. Chem. 1993, 97, 7769. (31) Mata, J.; Varade, D.; Ghosh, G.; Bahadur, P. Colloids Surf., A 2004, 245, 69. (32) Kumar, S.; Sharma, D.; Ghosh, G.; Kabir-ud-Din Colloids Surf., A 2005 (in press). (33) Provencher, S. W. Comput. Phys. Commun. 1979, 27, 227.
Figure 1. Variation of hydrodynamic diameter (Dh) with added [1-pentanol] in 0.2 M SDBS solution containing different fixed amounts of urea (U) at 25 °C. The viscosity measurements under Newtonian flow conditions were made by using a Ubbelohde viscometer.34
Results and Discussion DLS experiments have been performed to measure the hydrodynamic diameter (Dh) of SDBS micelles in 0.2 M SDBS + 1-pentanol + urea aqueous solutions at 25 °C. Figure 1 shows the plots of Dh versus [1-pentanol] in the presence of various fixed amounts of urea. The Dh for the spherical SDBS micelle (0.2 M, no added urea) was measured to be 2.5 nm, which is lower than the value of 3.4 nm obtained earlier for a spherical SDS micelle31 although both of the surfactants have the same dodecyl chain in their monomers. This can be understood in light of the fact that the SDBS monomer has one extra benzene ring. As a result, the headgroups cannot come closer beyond a certain limit because of the repulsive interactions of the π-electron cloud of the benzene rings present near the micellar surface. To alleviate these unfavorable electrostatic consequences, the hydrocarbon chains in the micelles of SDBS take up folded conformations, and hence SDBS micelles would experience a relatively more wet environment. Thus, the Dh of SDBS micelles is expected to be less than that of SDS. This indeed is observed (Figure 1). Similar observations were drawn from an independent study by small-angle neutron scattering measurements on SDS and SDBS micelles.35 Variation of Dh with [1-pentanol] is sigmoid in nature. The same behavior was observed with different urea concentrations (Figure 1) as well as with other additives of urea/thiourea family (not shown). At low [1-pentanol], micelles remain roughly spherical and, therefore, no distinct change in Dh was observed. Similar results were observed with the sodium dodecyl sulfate + NaCl system in the presence of 1-pentanol.32 After a certain [1-pentanol], a sharp increase in Dh is indicative of structural changes in SDBS micelles (i.e., spherical micelles are changed to cylindrical micelles, s f r). This [1-pentanol] at which Dh increases sharply can be considered to be the concentration needed for an (34) Kabir-ud-Din; Kumar, S.; Aswal, V. K.; Goyal, P. S. J. Chem. Soc., Faraday Trans. 1996, 92, 2413. (35) Kumar, S.; Sharma, D.; Sharma, D.; Kabir-ud-Din J. Surf. Deterg., submitted for publication, 2005.
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s f r transition (i.e., [1-pentanol]sfr36). It has been reported that the site of solubilization of different compounds within micellar systems can be correlated with the structural organization of aggregates.37 The structural transition is accompanied by a distinct rise in viscosity,38 which can be correlated with the present Dh variation with [1-pentanol] (Figure 1). Zana39,40 has studied the s f r of the tetradecyltrimethylammonium bromide system in the presence of 1-pentanol. Continued addition of such alcohols to a normal spherical micellar solution is like pushing the system toward rods. The addition of longer alcohols has indeed produced rod-shaped micelles with increased aggregation numbers.41 Through a comparison of the enthalpic42 and light scattering studies,39,40 it has been concluded that the end of the break observed calorimetrically corresponded to the s f r observed with light scattering. One may argue that 1-pentanol forms larger spherical mixed micelles with SDBS, which could be responsible for the sigmoidal behavior observed in Figure 1. If this is the case, then 1-pentanol should solubilize in the micellar core, but this possibility in the present situation is remote on the basis of results published earlier.37-42 Another factor is that the Dh should have been increased regularly with higher [1-pentanol], which is not observed by the DLS data (Figure 1). These facts allow us to say that the present sigmoidal behavior of Dh versus [1-pentanol] is due to s f r and not from the formation of larger spherical mixed micelles in the solution. The most plausible explanation for s f r is the increase in hydrophobic forces due to the embedding of 1-pentanol between SDBS monomers in the micelles. Mukerjee43 showed that an additive, which is surface-active for a hydrocarbon-water interface, will be mainly solubilized near the micellar headgroup region and will facilitate the structural transition (e.g., s f r). These factors modify the effective packing parameter of the surfactant44,45 and are responsible for the micellar growth with a concomitant increase in Dh. A marginal increase in Dh with further higher [1-pentanol] (Figure 1) suggests that the s f r is complete and the additional 1-pentanol is being used only to increase the length of the micelle. Unlike other techniques, however, the DLS measurements show a welldefined beginning and end of the s f r transition. The data (Figure 1, no urea) allow us to say that the s f r is completed at ∼0.14 M 1-pentanol in the 0.2 M SDBS system. This value of [1-pentanol] is distinctly higher than for the system studied earlier (∼0.04 M).15,30,32,46 This is due to the fact that in all earlier studies a definite amount of salt was present in the system. The presence of salt counterions caused a decrease in electrostatic repulsions among the surfactant headgroups and was responsible for the need for less 1-pentanol in comparison to the (36) There is no doubt that the polydispersity index value would increase while going from s f r, causing a poorer resolution, but a corresponding increase in the scattering intensity (more than twice) was taken as a clear indication of the structural transition. (37) Kabir-ud-Din, Kumar, S.; Kirti; Goyal, P. S. Langmuir 1996, 12, 1490. (38) Kumar, S.; Aswal, V. K.; Singh, H. N.; Goyal, P. S.; Kabir-udDin Langmuir 1994, 10, 4069. (39) Candau, S.; Zana, R. J. Colloid Interface Sci. 1981, 84, 206. (40) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208. (41) Almgren, M.; Lofroth, J. E. J. Colloid Interface Sci. 1981, 81, 486. (42) Nguyen, D.; Bertrand, G. L. J. Phys. Chem. 1992, 96, 1994. (43) Mukerjee, P. Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, p 153. (44) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601. (45) Kabir-ud-Din; Bansal, D.; Kumar, S. Langmuir 1997, 13, 5071. (46) Kumar, S.; Khan, Z. A.; Parveen, N.; Kabir-ud-Din Colloids Surf., A 2005 (in press).
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present case. We have chosen the above 0.2 SDBS + 1-pentanol system to see the effect of added urea (Figure 1). Before interpreting the actual DLS results on the urea (and other members) effect, it is worthwhile to have an idea of the micellar interface, urea partitioning among micellar and aqueous pseudophases, and influence of urea addition on the partitioning of 1-pentanol itself. Surfactant monomer aggregation greatly enhances the ionic concentration at the micellar surface either as headgroups or as counterions. This provides the micellar surface some of the properties of a concentrated salt solution.47,48 In this situation, short-range specific interactions, for example, H-bonding with anions, solvated anion/cation interactions, induced dipoles (polarization), and partial desolvation to give solvent-separated and tight ion-pairs, may contribute to the overall balance of forces controlling micelle formation, ionization, size, and shape. It has been reported that the aqueous urea solution is in a single phase and urea molecules are spread homogeneously throughout.14 Recently, Romsted et al.10 reported that urea partitioning does not change much (within 10%) even in the presence of aqueous micellar solutions of the type of surfactant systems addressed in the present case. This indicates that the binding interaction of urea with the micellar surface (whether anionic or cationic) is overall weak and limits the specificity of the urea-micelle interaction. This also confirms the conclusions of an earlier study on urea addition to protein that urea-protein interactions are weak and short-lived.17 It has been reported that urea increases the dielectric constant of water.49 Singh et al.50 concluded that micelle formation could be a combined effect of the dielectric constant of the medium, the nature of H-bonding, and the dispersion forces among the alkyl chains of the surfactant monomer. Thus, the dielectric constant of the solvent medium seems to be an important factor (among others) affecting the micellar association of ionic surfactants. Because urea-water mixtures are more polar than water itself, specific and Coulombic interactions at micellar surfaces with their high local concentrations of charged headgroups and counterions are reduced.15 Both of these effects enhance the stability of free ions but also reduce the interheadgroup repulsion within the micellar surface. Therefore, urea would affect the two opposite forces responsible for micellization (i.e., (i) it enhances the stability of free ions (opposes micellar association) and (ii) it decreases interheadgroup repulsion (responsible for the predominance of hydrophobic interactions)). Hence urea addition may produce a barrier that depends on [urea] itself in the system. Figure 2 shows that the [1-pentanol]sfr first decreases and then increases with continuous increase in [urea]. The results indicate that the s f r is facilitated by low [urea] in the system, which supports the viewpoint that urea acts as an ameliorator for micellization15 and renaturant7 for proteins (if added in low concentrations). As the [urea] continuously increases, the micelle ionization also increases.12,23,51 Urea is also reported to contribute to the breakdown of alcohol-Na+-I- clusters in quaternary (47) Bostrom, M.; Williams, D. R. M.; Ninham, B. W. Langmuir 2002, 18, 6010. (48) Solidi, V.; Keiper, J.; Romsted, L. S.; Cuccovia, I. M.; Chaimovich, H. Langmuir 2000, 16, 59. (49) Carvalho, B. L.; Briganti, G.; Chen, S.-H. J. Phys. Chem. 1989, 93, 4282. (50) Singh, H. N.; Saleem, S. M.; Singh, R. P. J. Phys. Chem. 1980, 84, 2191. (51) Causi, S.; De Lisi, R.; Milito, S.; Tirone, N. J. Phys. Chem. 1991, 95, 5664.
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Figure 2. Amount of 1-pentanol needed to bring about s f r, [1-pentanol]sfr, with additive concentrations at 25 °C.
Figure 3. Comparison of the behavior [1-pentanol]sfr vs [additive] for U and TU.
solutions of alcohol/water/urea/NaI.52 Therefore, the charged/uncharged fraction of the micelle also has a role to play in the urea effect. Compared with uncharged solute, urea is preferentially adsorbed by the charged solute pair.7 Solvation of the strongly interacting solute by urea destabilizes the contacts between the solutes (surfactant monomer ions in the present case). The adsorption of urea on charged surfactant monomers inside the micelles leads to a repulsion between them at the micellar surface, which exposes the hydrophobic portion of the monomers to water. The onset of water into the micellar interior leads to the destabilization of the rod-shaped micelles, resulting in spherical ones. This explains the higher value of [1-pentanol]sfr needed at the increased concentration of urea (Figure 1). A similar picture appears to be implied in the unfolding simulations of barnase in the presence of urea.53 It is not out of context to mention that by electrostatic binding to the peptide groups urea can effectively unfold (denature) a protein.54 The picture provided here clarifies the origin of urea-adsorbed interactions and their possible influence on micellar association (i.e., hydrophobic interactions). It should be mentioned here that the coaggregation tendency of 1-pentanol in SDBS micelles may decrease in going from H2O to urea solutions of different concentrations (0-5 M).5 However, in the study no distinct change in the transfer free energy of 1-butanol from SDS micelles in water to 1 M aqueous urea solution was observed (Figure 7 of ref 5). In the present study, because the maximum [urea] used was 0.8 M, it can safely be assumed that no significant effect on 1-pentanol partitioning is expected in the presence of such small concentrations of urea (0-0.3 M). The present urea effects very closely parallel the effects of urea on the cmc of SDS and cetyltrimethylammonium bromide.15,46 It is worth noting that the urea effect of micelle destabilization (increase in [1-pentanol]sfr at higher [urea]) shows up at comparatively
much lower [urea] than in the case of proteins, which may be due to the size difference involved in the two types of systems. Figure 2 also depicts the variation of [1-pentanol]sfr for other alkylureas (MMU, DMU, and TMU). For each urea analogue, there exists a minimum in [1-pentanol]sfr that is dependent upon the nature and number of methyl groups in the additive. However, the decrease in [1-pentanol]sfr is less marked (even absent in the case of TMU). Taking the analogy of the urea effect as discussed above, one can say that as we replace H- with a CH3- group in a particular urea its adsorption effect on the charged surfactant monomer is expected to increase. MMU almost works as urea, but with DMU the advancement of s f r is observed comparatively up to lower [MMU]. This picture is almost changed in the case of TMU, and only a delay of the s f r transition is observed. Higher [TMU] causes micelles to remain spherical up to fairly high [1-pentanol]. This means that urea with a higher number of methyl groups destabilizes the rod-shaped micelles and, therefore, can be used as a better denaturant in place of urea in protein chemistry. Similar effects of alkylureas were observed in increasing the cmc of nonionic surfactants.55,56 Figure 3 shows the comparative effect of the addition of U/TU on [1-pentanol]sfr. It can be seen that TU causes a larger decrease in [1-pentanol]sfr than U, but the increasing ability of [1-pentanol]sfr also starts from a low concentration of TU in comparison to that of U. This means that TU can cause an advancement of s f r even at very low concentration. Keeping in view the molecular structure of U and TU,57,58 we observe that (i) U has more H-bonding sites; (ii) sulfur in TU, being larger in size, has a stronger local dipole moment as compared to oxygen in U; (iii) the crystal structures of U and TU are different; and (iv) U is less basic than TU. The interactions with the micelles
(52) Hawlicka, E.; Tomasz, L. Z. Naturforsch. 1994, 490, 623. (53) Tirado-Rives, J.; Jorgensen, W. L.; Orozco, M. D. Biochemistry 1997, 36, 7313. (54) Nandi, P. K.; Rubinson, D. R. Biochemistry 1984, 23, 6661.
(55) Costantino, L.; D’Errico, G.; Roscigno, P.; Vitagliano, V. J. Phys. Chem. B 2002, 104, 7326. (56) Barone, G.; Crescenzi, V.; Liquori, A. M.; Quadrifoglio, F. J. Phys. Chem. 1967, 71, 984. (57) Halvorson, H. N.; Halpern, J. J. Am. Chem. Soc. 1956, 78, 5562. (58) Masunov, A.; Dannenberg, J. J. J. Phys. Chem. B 2000, 104, 806.
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Figure 4. Variation of [1-pentanol]sfr, with additive concentrations at 25 °C.
and background solution and hence the s f r would undoubtedly be affected by these differences between U and TU. This could be the reason for the effectiveness of TU in decreasing and increasing [1-pentanol]sfr at lower and higher concentration ranges (Figure 3). As we increase the number of methylene groups in thiourea (Figure 4), the lower additive concentration effect (i.e., decrease in [1-pentanol]sfr) is diminished, which is like the effect of TMU (Figure 2) and can be explained in a similar fashion (supra vide). Surfactant solutions containing spherical micelles are isotropic and have low viscosity.59 The presence of anisotropic micelles (e.g., rod-shaped) in the solution causes a distinct rise in viscosity.60,61 Therefore, viscosity can be used to study structural transitions in the surfactant solutions (e.g., s f r).59,62,63 To provide additional evidence regarding the low concentration effect of the additives, viscosity measurements were carried out on the chosen system in the presence of different ureas/thioureas. The viscosity of 0.2 M SDBS solution increases at ∼0.14 M, which is the same value obtained by DLS at which the s f r is complete. The [1-pentanol]sfr at each additive (59) Kohler, H.-H.; Strnad, J. J. Phys. Chem. 1990, 94, 7628. (60) Gamboa, C.; Sepulveda, L. J. Colloid Interface Sci. 1986, 113, 566. (61) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (62) Kumar, S.; Naqvi, A. Z.; Kabir-ud-Din Langmuir 2001, 17, 4787. (63) Hoffmann, H.; Ebert, G. Angew. Chem., Int. Ed. 1988, 27, 902.
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Figure 5. Comparative data on [1-pentanol]sfr obtained from different methods.
concentration (U, MMU, DMU, TMU, TU, DMTU, or TMTU) was obtained from the relative viscosity (ηr) versus [1-pentanol] plots (not shown). The variation of [1-pentanol]sfr (obtained viscometrically) for U is compared with the [1-pentanol]sfr obtained by DLS measurements in Figure 5, which illustrates that the two types of measurements provide nearly the same value of [1-pentanol]sfr thus confirming the validity of the two data sets. It can be concluded that the s f r transition in the micellar solution is remarkably influenced by different ureas and their effects are dependent upon the nature and the concentration. At lower concentrations, a change in the dielectric constant of the solvent plays a dominant role and is responsible for the advancement of s f r. However, at higher additive concentrations, the effect is overshadowed by the adsorption of the additive on the charged amphiphilic species (monomers) of the micelle (or exposure of hydrophobic parts in proteins) with the concomitant shift of s f r in favor of the reverse direction. Hence, the exposure of the inner portions of micelles (or proteins) seems to be the prime cause of their destabilization. Acknowledgment. We thank Dr. P. S. Goyal, Centre Director, UGC-DAEF-CSR, Mumbai Centre, for his kind permission to carry out DLS experiments and for valuable comments. LA051553W
