Synthesis, Optical Properties, and Surface Enhanced Raman

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J. Phys. Chem. C 2007, 111, 3901-3907

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Synthesis, Optical Properties, and Surface Enhanced Raman Scattering of Silver Nanoparticles in Nonaqueous Methanol Reverse Micelles Palash Setua,† Anjan Chakraborty,† Debabrata Seth,† M. Umananda Bhatta,‡ P. V. Satyam,‡ and Nilmoni Sarkar*,† Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India, and Institute of Physics, Bhubaneswar, India ReceiVed: NoVember 11, 2006; In Final Form: January 7, 2007

In the present work, we have reported the in situ synthesis of metal nanoparticles in the polar core of nonaqueous methanol reverse micelles. We have obtained both monodisperse and partially polydisperse silver nanoparticles at different condition and time in the polar core of methanol reverse micelles using sodium borohydride as a reducing agent and the confined cavity and the charged surfactant (sodium bis(2-ethylhexyl)sulfosuccinate(AOT)) as a stabilizing agent in the presence of air. In the initial stage of the process, the average particle size is ∼4 nm and independent of the w values of the reverse micelles. The distribution has been gradually changed with time, and at lower w value (w ) 2), the relative abundance of larger particles is increasing and the size distribution becomes partially polydisperse.

1. Introduction In recent years, synthesis of nanoparticles is an emerging area of research due to the wide applications of nanosized materials.1-12 They have interesting applications in electronics,3 photonics,4-5 catalysis,10 drug delivery,11-12 and many other diverse fields.13-14 The stabilization of the nanoparticle in different solvents is of paramount importance for their utilization as a basic unit from both fundamental and applied considerations.13-14 The synthesized nanoparticles are often soluble in either the aqueous phase or organic phase but not in both. A great amount of work has been done to affect the phase transfer of particles from aqueous to organic or the more difficult organic to aqueous phases and most of the processes have their own disadvantages.13-17 Reverse micelles18 as nanosized aqueous droplets encapsulated by surfactant molecules existing at certain compositions of water-in-oil microemulsions are widely used today in the synthesis of many types of nanoparticles. It is possible to control the sizes of the reverse micelles by controlling the parameter w, defined as the molar ratio of polar solventto-surfactant. The size of the water pool is increasing with the increase in w value in water reverse micelles.18 Normally, the synthesized particle diameter is increasing with an increase in the w value.6-8,20-21,23 The first synthesis of silver nanoparticles in a microemulsion was done by Bernickel et al.19 A systematic study of silver nanoparticle formation in the water “pool” of the reverse micelles was investigated by Pileni and coworkers6-8 using silver sulfosuccinate (Ag(AOT)) as a surfactant. Synthesis of copper nanoparticles in reverse micelles was also done by Pileni and co-workers.6-8 The effect of the intermicellar exchange rate and cations on the size of silver chloride nanoparticles formed in reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate(AOT) was investigated by Bagwe et al.20 Kitchens et al.21 investigated the solvent effects on the growth and steric stabilization of copper metallic nanoparticles * To whom correspondence should be addressed. E-mail: nilmoni@ chem.iitkgp.ernet.in. Fax : 91-3222-255303. † Indian Institute of Technology, Kharagpur. ‡ Institute of Physics, Bhubaneswar, India.

Figure 1. Surface plasmon absorption spectra of silver nanoparticles in the different w values of methanol reverse micelles, (A) after 49 h and (B) after 380 h, where (i) the dash line is w ) 2; (ii) the dotted line is w ) 3; (iii) the dash-dot line is w ) 4; and (iv) the dashdot-dot line is w ) 5.

in AOT reverse micelle systems. Recently Raveendran et al.22 synthesized silver nanoparticles in a completely “green” chemical procedure. There are number of studies devoted in the past decades to the synthesis the metal nanoparticles only in the aqueous core of water reverse micelles and reviewed in recent years.6-8,19-21,23 Our aim is to use other polar organic solvents

10.1021/jp067475i CCC: $37.00 © 2007 American Chemical Society Published on Web 02/22/2007

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Figure 2. Curves representing the variation of absorption spectra of the nanoparticle formed with time (A) at w ) 2, (B) at w ) 3, (C) at w ) 4, and (D) at w ) 5, where (i) after 1 h (solid line), (ii) after 7 h (dash line), (iii) after 18 h (dotted line), (iv) after 49 h (dash dot line), (v) after 72 h (dash dot dot line), (vi) after 114 h (black sphere line), and (vii) after 380 h (plus symbol line).

as the polar core of the reverse micelles, e.g., methanol, acetonitrile, glycerol, and formamide commonly known as nonaqueous reverse micelles etc. for the synthesis of nanoparticles. These reverse micelles are recently characterized by spectroscopic techniques and as well by solvent relaxation.24-25 The size of the methanol reverse micelle was determined by Levinger et al.24 and Shirota et al.25 The size of methanol reverse micelles is close to 3-4 nm from dynamic light scattering experiments, and it is almost w independent. This w independent nature of these reverse micelle is one of the most important differences from the aqueous reverse micelle and it raises questions about the real existence of these reverse micelle and, if they exist, then about their stability. In this regard, our work will give not only direct evidence of their existence but it will also prove their high stability and their capablity of forming and stabilizing nanoparticles within their core. It is also important to synthesize nanoparticles in a nonaqueous reverse micelle because most of the organic reactions take place in nonaqueous solvents and the nanoparticles have a wide application in catalysis which is verified from recent works.1,9,10,13 Recently Prasad et al.27 used the aqueous core of aerosol OT [sodium bis (2-ethylhexyl)-sulfosuccinate,AOT] to synthesize silver nanoparticles that can be dispersed in organic solvents. Moreover, in the last year, Andersson et al.28 investigated silver nanoparticle formation in microemulsions acting both as a template and a reducing agent. In this work, we are going to report the in situ synthesis of silver nanopartciles in the polar core of methanol reverse micelles. 2. Experimental Section Materials Used. AOT (dioctylsulfosuccinate, sodium salt, Aldrich) was purified by standard procedure.36 Silver nitrate (A.R. grade) was obtained from Qualigens Fine Chemicals

(Glaxo) and sodium borohydride (A.R. grade) was purchased from SRL (India) and used without further purification. We used n-heptane (distilled) and methanol (as received) both HPLC grade, Spectrochem (India). Apparatus. TEM measurements were performed using a JEOL 2010 (UHR version) transmission electron microscope operating at 200 keV with a LaB6 filament, see Figure 3, panels A and B. The other two (Figure 3, panels C and D) are taken using a (JEOL) JEM-2100 transmission electron microscope operating at 200 keV. Samples for TEM were prepared by blotting a carbon coated (50 nm carbon film) Cu grid (300 mesh, Electron Microscopy Science), with a drop of the Ag-micro emulsion and allowed to dry. Histograms are constructed manually by measuring the individual particle diameter with the help of Image-J software. In the TEM pictures, we got some large agglomerates and neglected them during diameter calculation. We have measured 240-250 particles for the calculation of the size distribution histogram at each w value. We have recorded the IR spectra using a FT-IR spectrometer, Spectrum RX1 (Perkin-Elmer). To record the absorption spectra, we have used a Shimadzu absorption spectrophotometer (model no: UV 1601). SERS spectra were obtained with a Renishaw Raman spectrometer, equipped with a He-Ne laser excitation source emitting at a wavelength of 633 nm, and a Peltier cooled (-70 °C) charge coupled device (CCD) camera. A Leica microscope was attached and was fitted with three objectives (5×, 20×, and 50×). For these experiments, the 20× objective was used, giving a spot size on the surface of about 5 mm. Laser power at the sample was 20 mW, and the data acquisition time was 60 s. The holographic grating (1800 grooves/mm) and the slit enabled the spectral resolution of 1 cm-1. The Raman band of silicon water at 520 cm-1 was used to calibrate the spectrometer,

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Figure 3. TEM images of silver nanoparticles in the polar core of methanol reverse micelles (A) at w ) 2 and (B) at w ) 4 taken after 40 h. (C) At w ) 2 and (D) at w ) 4 taken after 16 days. The scale bar corresponds to 20 nm.

and the accuracy of the spectral measurement was estimated to be better than 1 cm-1. For dynamic light scattering (DLS) measurements, we used a Nano ZS Malvern instrument employing a 4 mW He-Ne laser (λ ) 632.8 nm) and equipped with a thermostatted sample chamber. All experiments are carried out at 173° scattering angle at 298 K. Methods. The synthesis of silver nanoparticles in nonaqueous methanol reverse micelles has been achieved by the following procedure with the optimum choice of the concentration of AgNO3 and NaBH4, which is different from the concentration used by other groups.6-8,20-21,23 We have prepared two stock solutions, one was 0.1 M AgNO3 in methanol and the other was 0.1 M NaBH4. Using these, we prepared two individual AOT (0.1 M) reverse micellar solution of desired reactant concentration one containing AgNO3 and the other containing NaBH4 such that after mixing these two we can get the required w value and reactant concentration ratio (AgNO3:NaBH4 ) 3 × 10-4 M:4 × 10-4 M). We have kept the same concentrations of the reagents for all of the w values. 3. Results and Discussion The reduction of silver ions has been done by using sodium borohydride as a reducing agent. We have synthesized silver nanoparticles in methanol reverse micelles at four different w values namely w ) 2, 3, 4, and 5. We cannot use the higher w range as that of water reverse micelles because of the limited solubility of methanol in the AOT reverse miceller solution. The solution turned yellow after different time interval for different w values (6 h for w ) 2 and 20 h for w ) 5), indicating

the formation of silver nanoparticles with certanity. Petit et al.7 showed that a very clear plasmon absorption of Ag(0) in water reverse micelles can be obtaied within 1 h. At w ) 5 of methanol, we have got a significant absorption band around 420 nm only after 49 h. However, a shoulder in the visible region of the absorption spectrum is appearing after 1 h and there is a very faint appearance of yellow color in the solution indicating the formation of silver nanoparticles (Figure 2, for details see the Supporting Information). The almost same feature in the absorption spectrum is also present in other w values. The UVvis absorption spectra of the sample at different w values are shown at two extreme times in (Figure 1). The clear appearance of surface plasmon absorption indicates the formation of silver nanoparticles in the polar core of methanol reverse micelles. The peak position of the surface plasmon bands at time 49 h are different for w ) 2 (436 nm) and for w ) 3, 4, and 5 (at 421 nm). The same characteristic is also present in the curve at 380 h for w ) 2 (at 425 nm) and for w ) 3, 4, and 5 (at 416 nm). Figure 1 shows a broad absorption band centered at 420 nm. In general, there is a linear variation of the half-width of the absorption band with the reciprocal of the particle diameter.29 So here this broad band indicates the presence of small particles. We have measured the systematic variation of absorption spectra in methanol reverse micelles with time at different w values (Figure 2; for details see the Supporting Information (Figure 1)). It is clearly seen from the absorption spectra that a shoulder in the absorption band (see the Supporting Information (Figure 1)) is starting to appear after 1 h at all w values, and the plasmon band has a clear peak after 49 h except at w ) 2 where it is forming at a slightly faster rate. After getting the maximum

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Figure 4. Histogram showing the size distribution of silver nanoparticle. Panels A and B represent the histogram measured after 40 h and panels C and D represent the same after 16 days. (A) The average particle size is 4.3 nm and σ ) 0.42 at w ) 2 and (B) average particle size is 3.8 nm and σ ) 0.32 at w ) 4. (C) Average particle size is 8.2 nm and σ ) 0.41 at w ) 2. (D) Average particle size is 4.8 and σ ) 0.30 nm at w ) 4.

optical density, the absorption curve at w ) 3 remains stable, but for other w values (Figure 2), the optical density slightly decreases. This process indicates that the nanoparticle formation in the polar core of methanol is a rather slow process compared to that of water reverse micelle. It is clearly seen from the absorption spectra (Figure 1) at different w values that at higher w values the optical density of the plasmon band is decreasing. It may be possible that at a higher w value, methanol, which is slightly miscible with n-heptane, is mixing with n-heptane resulting in a tranfer of NaBH4 and AgNO3 to this phase making the reduction less feasible. Though the formation of silver nanoparticles is a slow process in the methanol reverse micelle, the particles are stable even after 2 months indicating that the particles are thermodynamically stable. The transmission electron microscope (TEM) images of the nanoparticles formed are presented in Figure 3. In general, the particles are isotropic in shape. The TEM images confirm the formation of spherical Ag(0) nanoparticles through reduction of Ag+ inside the polar core of methanol reverse micelles. This is also supported from the absence of multiple peaks in the UV curve. We did not obtain any nanoparticles in mixed solvents or in AOT solution. There is a finite possibility of the mixing of n-heptane and methanol. To check whether the nanopartciels are formed in the binary solution, we did the experiment in the same condition as like methanol reverse micelles, but we could not get any color change even after a few days, which indicates that nanoparticles did not form. We also did the same experiment with the addition of AOT without methanol, but we could not observe any color change after keeping the solution for a sufficient time, indicating that the silver nanopartciles are not formed. In genaral, nanoparticle synthesis in microemulsion consists of mixing two microemulsions carrying the appropriate reactants

in order to obtain the desired particles. It can be seen that, after mixing both microemulsions containing the reactants, the interchange of the metal salt and reducing agent takes place during the collisions of the methanol droplets. This interchange of reactants is fast so it occurs during the mixing process. The reaction then takes place within the nano pool of the reverse micelles, which control the final size of the particles. The dynamic exchange of reactants such as metallic salts and reducing agents between methanol droplets via the continuous oil phase is strongly depressed due to the restricted solubility of inorganic salts in the oil phase. Once the particle attains the final size, the surfactant molecules are attached to the surface of particles, thus stabilizing and protecting them against further growth. The histograms of the particle size distribution at w ) 2 and w ) 4 of reverse micelles are shown in Figure 4. At the initial stage, at w ) 2 of methanol reverse micelles, the average silver particle diameter is 4.3 nm and σ ) 0.43. At w ) 4 of methanol reverse micelles, the average silver particle diameter is 3.8 nm and σ ) 0.32. At the final stage, the value changes, at w ) 2, the average particle diameter becomes 8.2 nm and σ ) 0.41, and at w ) 4, the average particle diameter becomes 4.8 nm with σ ) 0.30. We used the lognormal fitting to the histogram using the equation y ) y0 + A/(σ × xxΠ) × e (-(ln(x/xc))2)/2 × σ2 to obtain the above values, where xc is the average particle size and σ is the shape parameter which is directly related to dispersity. This change is also supported by the change in absorption spectra with time. From this experiment, it is clear that this reverse micelle successfully plays the role of a microreactor and stabilizes the particle. The average particle size diameter and distribution obtained at w ) 2 and 4 are in good agreement within the experimental error with the previously reported value of methanol reverse micelle24 but at w ) 2 in the final stage. As we are getting smaller average

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Figure 5. IR spectra by plotting absorbance vs wavenumbers (A) showing the sulfonate group frequency and (B) showing the carbonyl group frequency region. The green and red curves represent methanol reverse micelle at w ) 4 and 2, respectively, having no silver nanoparticles. The blue and black curves represent the same having silver nanoparticles.

diameter at higher w value (w ) 4) compare to lower w (w ) 2), we have performed the DLS measurement (Supporting Information (Figure 2)) ourselves at two conditions, w ) 2 and 4, to get more information about the pure methanol reverse micelles size distribution and fitted the result using the same lognormal function. DLS measurement also gives us the comparable value and the same trend, higher size distribution, and higher average particle size diameter at w ) 2 compared to w ) 4 (Supporting Information (Figure 2)). However, we cannot explain the final size distribution of the synthesized particle at w ) 2 obtained from the TEM picture histogram, because the final average particle size is larger than the average size of the reverse micelle24 and also from the value obtained from the DLS measurement. We are also unable to interpret the result depending on the models that are built on water reverse micelles, assuming an inert nano templating role of reverse micelles, and normally based on the introduction of parameter w as the major influence on the product properties. Here the dynamic interaction among the reverse micelle may be the bigger influencing factor than the w parameter that influences the morphologies and properties of the final product.30 It is observed that the attraction between aggregates is found

to increase when the alcohol chain length decreases, the polar head area increases, and the molecular volume of oil increases.32 Here we use methanol, the smallest chain length alcohol as the core solvent, so attraction among the reverse micelle will be high compared to that of the water reverse micelle. It will favor the aggregation of the reverse micelle, dimer or trimer formation. At lower w value, this interaction may be stronger because of the incomplete formation of reverse micelle and dynamic interaction of free AOT molecule with the reverse micellar aggregates (anomalous result in DLS experiment, i.e., larger size of reverse micelles at lower w value (see the Supporting Information)). Our assumption is also supported by the faster

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Figure 6. SERS spectra of the AOT molecule of reverse micellar solution. (A) Containing silver nanoparticles (black at w ) 2, red at w ) 3, green at w ) 4, and blue at w ) 5). Sulfonate stretches (symmetric νSSO3 and asymmetric νaSO3) and the carbonyl stretch (νCO) of the AOT head group; the CH stretches (νCH), CH bends (δCH), and CC stretches (νCC) of the AOT alkyl tail. (B) Without silver nanoparticle (black at w ) 2 and red at w ) 4).

formation rate of nanoparticles at w ) 2 than the other w values, evident from UV measurements (Figure 1). For explaining the final product morphology at w ) 2 depending on the dynamic interaction and finally attachment of the surfactant molecule (AOT) of the reverse micelle, we are considering the following scheme.35 In process 2, we do not show the intermediate dimer state, but it forms before fragmentation or coagulation. We have no direct evidence in favor of the above scheme, but we got a few unusual particle assemblies in the TEM pictures (Figure 7) that gave us the idea regarding a temporary dimmer and its breaking. Now if we consider this model depending attraction among the reverse micelle as a major controlling influence, then temporary dimer formation is the crucial point because it is the key factor for the exchange of intramiceller material like reagent molecules or initially formed very small particles. Though the lifetime of such a dimer will be very small, in some instances, it may be slightly higher, which may lead to uneven fragmentation and increased polydispersity. A similar observation was reported by Mehra et al.33 They explain the result depending upon the slowness of the process and concluded that a low reaction rate may lead to a large particle irrespective of the exchange protocol of the micellar content. Higher polydispersity at lower w value may be the reason that at lower w only the interfacial layer of the polar solvent is formed, not the internal pool of the polar solvent inside the reverse micelle. A similar observation was reported by Arriagada et al.34 Within a synthesis of silica particles, as the parameter w increases from 0.7 to 2.3, the particle size decreased and the size distribution

Figure 7. Unusual particle assembly in TEM picture at w ) 2 which leads to the idea of an intermediate dimer formation and its fragmentation scheme. Both of the particles (inside the mark) are remaining in such a state that they have just originated by unequal division of a single body.

became narrower. Another important consideration related to the above scheme is that, after achieving the final size, it is stabilized by the direct attachment of the AOT molecule. To verify our assumption, we have recorded IR spectra (Figure 5) of these samples. In reverse micelles, the polar head group

Silver Nanoparticles and Micelles remains directed to the polar core, so it has the maximum possibility of attachment with the nanoparticle. Figure 5, panels A and B, represents the sulfonate and carbonyl frequency region of the AOT molecule. There is no change in the sulfonate region (Figure 5A) but an appreciable change in the carbonyl group frequency (Figure 5B) for the solution containing silver nanoparticles. For further confirmation, we have measured the SERS spectra (Figure 6) of these samples and get a large enhancement of Raman peak of the AOT molecule in the silver nanoparticle solution than the blank one at the same w value. This peak enhancement is the direct evidence of surface attachment of the AOT molecule. We get maximum enhancement at w ) 3. That means at this condition AOT molecules are best attached to the surface. We have assigned the important bands in the SERS spectra considering the work of Nagasoe et al.37 and Deak et al.38 As surface attachment is directly related to nanoparticle stability, silver nanoparticles at w ) 3 will have the maximum stability, which is also supported by the time depended UV spectra. This work indicates that the polar core diameter is the major influence for controlling size and distribution at higher w values but not the soul parameter to control the final size distribution of the nanoparticles at lower w in nonaqueous methanol reverse micelles; their initial and final size and morphology are different. Further studies are needed to understand clearly the structural effect of nonaqueous reverse micelles on the nanomaterial product morphology. 3. Conclusion In this paper, we have shown for the first time that silver nanoparticles can be synthesized in the polar core of methanol reverse micelles using NaBH4 as a reducing agent. It confirms the existence of really stable reverse micelle in AOT/methanol/ n-heptane system. The nanoparticle formation in the core of the methanol reverse micelles is a very slow process and it takes several days to stabilize the particles. In the initial stage, the average particle size is ∼4 nm, comparable to the dimension of the reverse micelles and independent of the w value. However, in the final state at lower w value, the size of the particle increases and their size distribution changes which can be explained by considering the attractive interaction among the reverse micelle. This method may also be applied to synthesize nanoparticles in other nonaqueous polar solvents containing reverse micelles, which may be important for catalysis. Acknowledgment. N.S. is thankful to Council of Scientific and Industrial Research (CSIR), Govt. of India for a generous research grant. P.S., A.C., and D. S. are thankful to CSIR (Government of India) for research fellowships. We are thankful to Prof. T. Pal of our department and Dr. S. Pal of SNBNCBS Kolkata, for allowing us to use the Raman and DLS instruments, respectively. Supporting Information Available: Absorption spectra at different times (Figure 1) and distribution histograms of pure

J. Phys. Chem. C, Vol. 111, No. 10, 2007 3907 methanol/AOT/n-heptane reverse micelles (Figure 2). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Hayward, R. C.; Saville, D. A.; Aksay, I. A. Nature 2000, 404, 56. (3) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (4) Law, M.; Donald, Sirbuly, J.; Johnson Justin, C.; Goldberger, J.; Richard, Saykally, J.; Peidong, Y. Science 2004, 305, 1269. (5) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. AdV. Mater. 2001, 13, 1501. (6) Pileni, M. P. Nat. Mater. 2003, 2, 145. (7) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (8) Pileni, M. P. Langmuir 1997, 13, 3266. (9) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (10) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (11) Salem, A. K.; Searson, P. C.; Leong, K. W. Nat. Mater. 2003, 2, 668. (12) Thachenko, A. G.; Xie, H.; Coleman, D.; Glomm, W.; Ryan, J.; Anderson, M. F.; Franzen, S.; Feldheim, D. L. J. Am. Chem. Soc. 2003, 125, 4700. (13) Coontz, R.; Szuromi, P. Science 2000, 290, 1523; Special Issue on Nanotechnology. (14) Henglein, A. Chem. ReV. 1989, 89, 1861. (15) Swami, A.; Kumar, A.; Sastry, M. Langmuir 2003, 19, 1168. (16) Sarathy, K. V.; Kulkarni, G. U.; Rao, C. N. R. Chem. Commun. 1997, 6, 537. (17) Sastry, M. Curr. Sci. 2003, 85, 1735. (18) Lusi, P. L., Straube, B. E., Eds.; ReVerse Micelles; Plenum Press: NewYork, 1984. (19) Barnickel, P.; Wokaun, A. Mol. Phys. 1990, 69, 1. (20) Bagwe, R. P.; Khilar, K. C. Langmuir 1997, 13, 6432. (21) Kitchens, C. L.; McLeod, M. C.; Roberts, C. B. J. Phys. Chem. B 2003, 107, 11331. (22) Raveendran, P.; Fu, J.; Wallen, S. L. J. Am. Chem. Soc. 2003, 125, 13940. (23) Capek, I. AdV. Colloid Int. Sci. 2004, 110, 49. (24) Riter, R. E.; Kimmel, J. R.; Undiks, E. P.; Levinger, N. E. J. Phys. Chem. B 1997, 101, 8292. (25) Shirota, H.; Horie, K. J. Phys. Chem. B 1999, 103, 1437. (26) Hzra, P.; Chakrabarty, D.; Sarkar, N. Langmuir 2002, 18, 7872. (27) Prasad, B. L. V.; Arumugam, S. K.; Bala, T.; Sastry, M. Langmuir 2005, 21, 822. (28) Andersson, M.; Pedersen, J. S.; Palmqvist, A. E. C. Langmuir 2005, 21, 11387. (29) Kawabata, A.; Kubo, R. J. Phys. Soc. Jpn. 1986, 90, 2929. (30) Natarajan, U.; Handique, K.; Mehra, A.; Bellare, J. R.; Khilar, K. C. Langmuir 1996, 12, 2670. (31) Melo, E. P.; Costa, S. M. B.; Cabral, J. M. S.; Fojan, P.; Petersen, S. B. Chem. Phys. Lipids 2003, 124, 47. (32) Brunetti, S.; Roux, D.; Bellocq, A. M.; Fourche, G.; Bothorel, P. J. Phys. Chem. 1983, 87, 1028. (33) Jain, R.; Mehra, A. Langmuir 2004, 20, 6507. (34) Arriagada, F. J.; Osseo-Asare, K. J. Colloid Interface Sci. 1999, 211, 210. (35) Uskokovic, V.; Drofenik, M. Surf. ReV. Lett. 2005, 12, 239. (36) Hazra, P.; Chakrabarty, D.; Sarkar, N. Chem. Phys. Lett. 2003, 371, 553 and references therein. (37) Nagasoe, Y.; Okabayashi, H.; Abe, M.; Eastoe, J.; O’Connor, C. J. Vib. Spectrosc. 2000, 23, 151. (38) Deak, J. C.; Pang, Y.; Sechler, T. D.; Wang, Z.; Dlott, D. D. Science 2004, 306, 473.