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Adsorbed Anthranilic Acid Molecules Cause Charge Reversal of Nonionic Micelles Gunjan Verma,† V. K. Aswal,‡ S. K. Kulshreshtha,† P. A. Hassan,*,† and Eric W. Kaler§ Chemistry DiVision and Solid State Physics DiVision, Bhabha Atomic Research Centre, Mumbai - 400 085, India, and Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed July 11, 2007. In Final Form: October 29, 2007 The effect of anthranilic acid, an aromatic amino acid, on the structural characteristics of nonionic micelles of Triton X-100 at different pH values was investigated by light scattering and small-angle neutron scattering (SANS) measurements. The scattered light intensity decreases as pH is increased or decreased on either side of the isoelectric point (IEP ) 3.4) of the amino acid. Analysis of the SANS data using a sticky hard-sphere model shows that the micelles are oblate ellipsoids with an axial ratio of approximately 2.3. No significant change could be observed in the size of the micelles with a change in the pH, while the stickiness parameter (τ), which is related to the interaction potential (u0) increases on either side of the IEP. As τ increases, uo becomes less negative, indicating a decrease in the attractive interaction between nonionic micelles. This can be explained in terms of the changes in the surface charge of the micelles resulting from a shift in the acid-base equilibrium of the adsorbed amino acid. The variation of the intermicellar interaction as calculated from the stickiness parameter is consistent with the picture of reversal of charge of amino acids with pH. This is further supported by the observed variation of the cloud point of the solutions at different pH values. The change in the interparticle interaction is also reflected in the diffusion coefficient of the micelles measured by dynamic light scattering.
Introduction Micelles are formed by the self-aggregation of surfactant molecules. Surfactants and their aggregates are an important component in a wide variety of applications such as detergency, solubilization, materials synthesis, pharmaceutical formulations, and biomedical applications.1-4 The effectiveness of a surfactant for such applications depends on the structural parameters of the micelles as well as their effect on solution properties.5-8 Micellar solutions of nonionic surfactants exhibit phase separation when the temperature is raised above the so-called cloud point temperature. The cloud point temperature varies with the concentration of surfactant and the presence of salt and other additives.9-12 The phase separation of a nonionic micellar solution above its cloud point has been exploited in processes for the effective separations of proteins and other macromolecules.13 Triton X-100 is used in separations of proteins from cell membranes,14,15 and it serves as an effective substrate for studying * Corresponding author. E-mail:
[email protected]. Tel: + 91- 22 25592327. † Chemistry Division, Bhabha Atomic Research Centre. ‡ Solid State Physics Division, Bhabha Atomic Research Centre. § University of Delaware. (1) Scarzello, M.; Klijn, J. E.; Wagenaar, A.; Stuart, M. C. A.; Hulst, R.; Engberts, J. B. F. N. Langmuir 2006, 22, 2558. (2) Li., L.; Nandi, I.; Kim, K. H. Int. J. Pharm. 2002, 237, 77. (3) Hamley, I. W. Angew. Chem., Int. Ed. 2003, 42, 1692. (4) Jayakumar, O. D.; Gopalakrishnan, I. K.; Kulshreshtha, S. K. AdV. Mater. 2006, 18, 1857. (5) Nakamura, K.; Shikata, T. Langmuir 2006, 22, 9853. (6) Siddiqui, U. S.; Ghosh, G.; Kabirud, D. In Langmuir 2006, 22, 9874. (7) Kumar, S.; Sharma, D.; Ghosh, G.; Kabiru, D. Langmuir 2005, 21, 9446. (8) Morini, M. A.; Messina, P. V.; Schulz, P. C. Colloid. Polym. Sci. 2005, 283, 1206. (9) Sharma, K. S.; Patil, S. R.; Rakshit, A. K. Colloids Surf. A: Physicochem. Eng. Aspects 2003, 219, 67. (10) Panchal, K.; Desai, A.; Nagar, T. J. Dispersion Sci. Technol. 2006, 27, 33. (11) Chai, J. L.; Mu, J. H. Colloid J. 2002, 64, 550. (12) Chen, W. J.; Gu, Q.; Yao, F. Y.; Li, G. Z. Acta Chim. Sin. 2002, 60, 810. (13) Fernandes, S.; Hatti-Kaul, R.; Mattiasson, B. Biotechnol. Bioeng. 2002, 79, 472. (14) Saitoh, T.; Hattori, N.; Hiraide, M. J. Chromatogr., A 2004, 1028, 149.
phospholipase metabolism.16,17 A cloud point extraction method is also used for the extraction and preconcentration of metal ions.18-20 The growth and hydration of Triton X-100 micelles in the presence of monovalent alkali metal chloride salts such as LiCl, NaBr, KCl, and so forth have been studied using light scattering and small-angle neutron scattering (SANS) techniques, voltammetry, and fluorescence probe techniques.21-24 However, the structural changes of nonionic micelles in the presence of hydrophobic salts or hydrotropes that are adsorbed on the surface of micelles have not been studied extensively. In the case of ionic surfactants, hydrophobic salts1,25-31 induce worm-like micellar structures at substantially lower concentrations of added salt than is the case for an inorganic salt. Micelle formation and structure of Triton X-100 in mixed solvents of water and (15) Bumgarner, G. W.; Zampell, J. C.; Nagarajan, S.; Poloso, N. J.; Dorn, A. S.; Dsouza, M. J.; Selvaraj, P. J. Biochem. Biophys. Methods 2005, 64, 99. (16) Wissing, J. B.; Kornak, B.; Funke, A.; Riedel, B. Plant Physiol. 1994, 105, 903. (17) Gimes, G.; Toth, M. Acta Physiol. Hung. 1993, 81, 101. (18) Afkhami, A.; Bahram, M. Microchim. Acta 2006, 155, 403. (19) Shemirani, F.; Kozani, R. R.; Jamali, M. R.; Assadi, Y.; Hosseini, M. R. M. Int. J. EnViron. Anal. Chem. 2006, 86, 1105. (20) Shemirani, F.; Jamali, M. R.; Kozani, R. R.; Salavati Niasari, M. Sep. Sci. Technol. 2006, 41, 3065. (21) Molina-Bolivar, J. A.; Aguiar, J.; Ruiz, C. C. J. Phys. Chem. B 2002, 106, 870. (22) Bulavin, L. A.; Garamus, V. M.; Karmazina, T. V.; Avdeev, M. V. Colloid J. 1997, 59, 13. (23) Molina-Bolivar, J. A.; Aguiar, J.; Peula-Garcia, J. M.; Ruiz, C. C. Mol. Phys. 2002, 100, 3259. (24) Charlton, I. D.; Doherty, A. P. J. Phys. Chem. B 2000, 104, 8327. (25) Rehage, H.; Hoffman, H. J. Phys. Chem. 1988, 92, 4712. (26) Buwalda, R. T.; Stuart, M. C. A.; Engberts, J. B. F. N. Langmuir 2000, 16, 6780. (27) Garg, G.; Hassan, P. A.; Kulshreshtha, S. K. Colloids Surf. A: Physicochem. Eng. Aspects 2006, 275, 161. (28) Hassan, P. A.; Sawant, S. N.; Bagkar, N. C.; Yakhmi, J. V. Langmuir 2004, 20, 4874. (29) Hassan, P. A.; Raghvan, S. R.; Kaler, E. W. Langmuir 2002, 18, 2543. (30) Garg, G.; Hassan, P. A.; Aswal, V. K.; Kulshreshtha, S. K. J. Phys. Chem. B 2005, 109, 1340. (31) Hassan, P. A.; Fritz, G.; Kaler, E. W. J. Colloid Interface Sci. 2003, 257, 154.
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formamide have been investigated by photophysical and light scattering methods.32 SANS studies on Triton X-100 micelles in the presence of sodium dodecyl sulfate (SDS) show that, as the SDS concentration increases, a correlation peak appears, indicating micellar ordering due to the increase of the micelle charge.33 The effect of salicylic acid on the structure of Triton X-100 micelles has been investigated by SANS,33,34 and the surface charge of salicylic acid-solubilized Triton X-100 micelles changes from neutral to negative with an increase in pH. The change in the surface charge of the micelles with pH alters the electrostatic interaction between the aggregates. This in turn influences several properties such as coagulation, ordering, and clouding.35,36 Amino acids are an important class of compounds that are susceptible to pH-induced charge reversal37 because they carry specific ionizable carboxyl and amine groups. They thus offer an alternate and novel way to form pH-sensitive assemblies.38,39 pH-induced changes in the surface charge of vesicles and consequent changes in aggregate morphologies formed from a sugar-based gemini surfactant have been reported.1,38 By changing pH, a gradual transition from micelles to vesicles to bilayers to precipitate has been reported in the aqueous mixtures of histidine and sodium dodecyl benzenesulphonate.40 The formation of thermoresponsive, pH-sensitive fibrous structures and gels in aqueous mixtures of the amino acid lysine with oppositely charged sodium alkyl sulfate surfactants has also been reported.41 Mixed catanionic surfactant systems based on amino acids are known to form liquid crystal dispersions.42 Thus, amino acids are attractive additives to surfactant systems that could have potential biochemical applications.43 In this context, we examine the behavior of Triton X-100 micelles in water at a fixed concentration (8% or 127 mM) in the presence of the hydrophobic amino acid, anthranilic acid (50 mM) at varying pH using light scattering and SANS techniques. This work is aimed to demonstrate the pH-induced changes in the surface charge of micelles caused by adsorbed hydrophobic molecules and is a step toward the study of the connection between micellar structural parameters (size, shape, and intermicellar interactions) and macroscopic properties such as the cloud point. This approach makes use of the hydrophobic interaction between surfactant and additive without involving any covalent bond formation. Experimental Section A. Chemicals. Triton X-100 (isooctylphenoxy polyethoxy ethanol) was obtained from Sisco Research Laboratories, Mumbai (India), and anthranilic acid was obtained from Spectrochem, Mumbai (India). Hydrochloric acid from Thomas Baker, Mumbai (India) and sodium hydroxide from E. Merck, Mumbai (India) were used to adjust the pH of the solution. Figure 1 shows the chemical structure of anthranilic (32) Molina-Bolivar, J. A.; Aguiar, J.; Ruiz, C. C. Mol. Phys. 2001, 99, 1729. (33) Kelkar, V. K.; Mishra, B. K.; Rao, K. S.; Goyal, P. S.; Manohar, C. Phys. ReV. A 1991, 44, 8421. (34) Manohar, C.; Kelkar, V. K.; Mishra, B. K.; Rao, K. S.; Goyal, P. S.; Dasannacharya, B. A. Chem. Phys. Lett. 1990, 171, 451. (35) Schaefer, D. W. J. Chem. Phys. 1977, 66, 3980. (36) Degiorgio, V.; Corti, M. In Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; North-Holland: Amsterdam, 1985; p 303. (37) Garrett, R. H.; Grisham, C. Biochemistry, 2nd ed.; Saunders College Publishing: Philadelphia, PA, 1999. (38) Johnsson, M.; Wagenaar, A.; Engberts, J. J. Am. Chem. Soc. 2003, 125, 757. (39) Wang, C. Z.; Gao, Q.; Huang, J. B. Langmuir 2003, 19, 3757. (40) Gonzalez, Y. I.; Nakanishi, H.; Stjerndahl, M.; Kaler, E. W. J. Phys. Chem. B 2005, 109, 11675. (41) Gonzalez, Y. I.; Kaler, E. W. Langmuir 2005, 21, 7191. (42) Rosa, M.; Infante, M. R.; Miguel, M. D.; Lindman, B. Langmuir 2006, 22, 5588. (43) Boettcher, C.; Schade, B.; Fuhrhop, J. H. Langmuir 2001, 17, 873.
Verma et al.
Figure 1. Chemical structure of anthranilic acid. acid. All chemicals were used as received. Deionized water from a Millipore Milli-Q system (resistivity ∼18 MΩ cm) was used in all cases to prepare aqueous solutions. B. Light Scattering. Light scattering measurements were performed using a Malvern 4800 Autosizer employing a 7132 digital correlator. The light source was Ar-ion laser operated at 514.5 nm with a maximum power output of 2 W. All measurements were carried out at 25.0 ( 0.1 °C using a circulating water bath. Measurements were made at five different angles ranging from 50° to 130°. Cylindrical quartz cells of 10 mm diameter were used in all of the light scattering experiments. The samples of micellar solutions were filtered through 0.2-µm filters (Acrodisc) to avoid interference from dust particles. The intensity of scattered light was measured five times for each sample at different angles. Dynamic light scattering (DLS) measurements were carried out on the solution of Triton X-100 (8%) and anthranilic acid (50 mM) at varying pH. The pH of the solutions was varied by the addition of different concentrations of hydrochloric acid (HCl) or sodium hydroxide (NaOH) solutions. The concentration of Triton X-100 (8%) and anthranilic acid (50 mM) was kept constant for all the measurements. C. SANS. SANS experiments were carried out using the SANS diffractometer at Dhruva reactor, Bhabha Atomic Research Centre, Trombay.44 The diffractometer makes use of a beryllium oxidefiltered beam with a mean wavelength (λ) of 5.2 Å. The angular distribution of the scattered neutrons was recorded using a onedimensional position-sensitive detector (PSD). The accessible wave vector transfer (Q ) 4π sin θ/λ, where 2θ is the scattering angle) range of the diffractometer is 0.02-0.3 Å-1. The PSD allows simultaneous recording of data over the full Q range. The samples were held in a quartz sample holder of 0.5 cm thickness. The temperature was fixed at 30 °C for all measurements. The measured SANS data have been corrected and normalized to absolute units (as cross-section per unit volume), using standard procedures. D. Cloud Point. The cloud points of Triton X-100 (8%) micellar solution in the presence of 50 mM anthranilic acid at various pH were determined visually by noting the temperature at which the thermally equilibrated solution becomes turbid. The solutions were equilibrated in a water bath, and, close to the cloud point, the temperature was varied at intervals of 0.5 °C. E. SANS Analysis. For a system of monodisperse ellipsoidal micelles, the coherent differential scattering cross-section can be expressed as45 dΣ (Q) ) n[〈F(Q)2〉 + 〈F(Q)〉2(S(Q) -1)] + B dΩ
(1)
where n is the number density of micelles, F(Q) is the single particle form factor, and S(Q) is the structure factor. B is a constant term that represents the incoherent scattering background mainly from the hydrogen atoms present in the sample. The single-particle form factor for a core-shell ellipsoid of revolution, with semi-axes a1 ) b1 * c1 for the core and a2 ) b2 * c2 for the outer shell, is given by45 〈F(Q)2〉 )
∫
〈F(Q)〉2 ) (
1
0
∫
1
0
[V1(F1 - F2)F1(Q,µ) + V2(F2 - F3)F2(Q,µ)]2dµ (2) [V1(F1 - F2)F1(Q,µ) + V2(F2 - F3)F2(Q,µ)]dµ)2 (3)
(44) Aswal, V. K.; Goyal, P. S. Curr. Sci. 2001, 79, 947. (45) Berr, S. S. J. Phys. Chem. 1987, 91, 4760.
Charge ReVersal Caused by Absorbed Anthranilic Acid Fi(Q,µ) )
3 [sin(wi) - wi cos(wi)] wi3
wi ) Q[ci2µ2 + ai2(1 - µ2)]1/2, i ) 1, 2
Langmuir, Vol. 24, No. 3, 2008 685 (4)
(5)
where µ is the cosine of the angle between the axis of revolution and Q. The volume of the core is V1 and the total volume is V2. The scattering length densities of the core, outer shell, and solvent are F1, F2, and F3, respectively. In the following analysis, S(Q) is calculated by approximating the ellipsoid as an equivalent sphere of diameter σ ) 2(a22c2)1/3 and then using Baxter’s sticky hard-sphere model for S(Q).46 Menon et al.47 have shown that this model depicts the micellar system as a collection of (spherical) particles interacting via a thin attractive square-well potential of depth u0 (
