Formation of Silver Chloride Nanoparticles in Microemulsions by

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Langmuir 2003, 19, 8467-8474

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Formation of Silver Chloride Nanoparticles in Microemulsions by Direct Precipitation with the Surfactant Counterion Maen Husein,†,§ Eva Rodil,†,‡ and Juan Vera*,† Department of Chemical Engineering, McGill University, Montreal, Canada, H3A 2B2, and Department of Chemical Engineering, University of Santiago de Compostela, Santiago de Compostela, Spain Received February 7, 2003. In Final Form: May 22, 2003 Nanoparticles of silver chloride were prepared by direct precipitation of silver ions with the surfactant counterion in the water pools of microemulsions formed by dioctyldimethylammonium chloride in an organic n-decanol/isooctane phase. This work represents a new concept to form nanoprecipitates using a single reverse micellar system. The net result is a fast reaction with less dependency on the intermicellar exchange of solubilizate. The effects of the surfactant and cosurfactant concentrations, of the mole ratio of water to surfactant, R, and of the loading of silver nitrate were evaluated. Increasing the surfactant concentration at fixed values of R and moles of silver nitrate resulted in a higher dependency on the reverse micellar exchange dynamics and increased the particle size. At high n-decanol concentration, the particle size increased due to decreasing the interaction between the nanoparticles and the stabilizing surfactant layer. Similar results were found at high values of R. Increasing the amount of silver nitrate resulted in the formation of more nuclei, and hence in the production of smaller particles. The trends in the particle size and the size distribution were followed using UV spectrophotometry and transmission electron microscope photographs.

Introduction Nanoparticles are clusters of 10-1000 atoms with sizedependent physical, chemical, electronic, and optical properties.1-4 For some technological applications, these properties are considered to be superior to those of the bulk solid-state materials. Hence, nanoparticles find applications in solar cells, microelectronics, display devices, and catalysis.5 The technological importance of nanoparticles has promoted the development of many new techniques for their preparation, including gas-phase,6 liquid-phase,7-9 and vacuum synthesis10 techniques. Waterin-oil microemulsions or reverse micelles provide a convenient medium for preparation of a wide variety of nanoparticles with the advantage of an easy manipulation of the particle size and provide a highly homogeneous product due to the efficient mixing at the molecular level. Onion structure nanoparticles can be created via sequential synthesis.3,11 * Corresponding author. E-mail: [email protected]. † McGill University. ‡ University of Santiago de Compostela. § Current address: Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Canada T2N 1N4. (1) Pileni, M. P. In Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; Wiley-VCH: New York, 1998; p 71. (2) Switzer, J. A. In Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; Wiley-VCH: New York, 1998; p 53. (3) Pillai, V.; Kumar, P.; Hou, M. J.; Ayyub, P.; Shah, D. O. Adv. Colloid Interface Sci. 1995, 55, 241-269. (4) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmueller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665-7673. (5) Kamat, P. V. In Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; Wiley-VCH: New York, 1998; p 207. (6) Reents, W. D., Jr.; Mujsce, A. M.; Bondybey, V. E.; Mandich, M. L. J. Chem. Phys. 1987, 86, 5568-5578. (7) Kirchnerova, J.; Klvana, D.; Chaouki, J. Appl. Catal., A 2000, 196, 191-198. (8) Kirchnerova, J.; Klvana, D. Solid State Ionics 1999, 123, 307317. (9) Komarneni, S.; Abothu, I. R.; Rao, A. V. P. J. Sol.-Gel Sci. Technol. 1999, 15, 263-270. (10) Bishop, M. B.; LaiHing, K.; Cheng, P. Y.; Peschke, M.; Duncan, M. A J. Phys. Chem. 1989, 93, 1566-1569.

Most studies available on the utilization of microemulsion systems for nanoprecipitate preparation have been based on mixing two microemulsion systems, each containing one of the ions forming the precipitate.12-22 Using this method, the intermicellar exchange of solubilizate is essential for product formation. Intermicellar exchange of solubilizate is achieved through Brownian diffusion of the reverse micelles, surfactant layer opening upon coalescence, diffusion of the solubilizate molecules, and finally decoalescence to return back to reverse micelles.23-25 For fast reaction kinetics, the opening of the surfactant layer is the rate-determining step,26 and thus it governs the particle specifications. For slow reactions, the precipitate formation is dependent upon the statistical (11) Tan, W.; Santra, S.; Zhang, P.; Tapec, R.; Dobson, J. U.S. Patent 2002. (12) Curri, M. L.; Agostiano, A.; Manna, L.; Monica, M. D.; Catalano, M.; Chiavarone, L.; Spagnolo, V.; Lugara, M. J. Phys. Chem. B 2000, 104, 8391-8397. (13) Debuigne, F.; Jeunieau, L.; Wiame, M.; Nagy, J. B. Langmuir 2000, 16, 7605-7611. (14) Jeunieau, L.; Verbouwe, W.; Rousseau, E.; Van der Auweraer, M.; Nagy, J. B. Langmuir 2000, 16, 1602-1611. (15) Ohde, H.; Ye, X.-R.; Wai, C. M.; Rodriguez, J. M. Chem. Commun. 2000, 23, 2353-2354. (16) Jeunieau, L.; Nagy, J. B. Colloids Surf., A 1999, 151, 419-434. (17) Liu, C.-Y.; Zhang, Z.-Y.; Wang, C.-Y. J. Imaging Sci. Technol. 1999, 43, 492-497. (18) Palla, B. J.; Shah, D. O.; Garcia-Casillas, P.; Matutes-Aquino, J. J. Nanopart. Res. 1999, 1, 215-221. (19) Schmidt, J.; Guesdon, C.; Schoma¨cker, R. J. Nanopart. Res. 1999, 1, 267-276. (20) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950959. (21) Monnoyer, Ph.; Fonseca, A.; Nagy, J. B. Colloids Surf., A 1995, 100, 233-243. (22) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1994, 33, 3262-3266. (23) Bagwe, R. P.; Khilar, K. C. Langmuir 2000, 16, 905-910. (24) Bagwe, R. P.; Khilar, K. C. Langmuir 1997, 13, 6432-6438. (25) Chew, C. H.; Gan, L. M.; Shah, D. O. J. Dispersion Sci. Technol. 1990, 11, 593-609. (26) Bommarius, A. S.; Holzwarth, J. F.; Wang, D. I. C.; Hatton, T. A. J. Phys. Chem. 1990, 94, 7232-7239.

10.1021/la0342159 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/26/2003

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Figure 1. TEM photograph and the corresponding particle size distribution histogram for the mixing of two microemulsions based on AOT. 0.1 M AOT, nAgNO3 ) nNaCl ) 0.054 mmol, R ) 10.

distribution of the precipitate monomers among the reverse micelles.19,27 This work investigates the direct formation of nanoprecipitates utilizing the counterion of a cationic surfactant. Reactions with the surfactant counterion proceed at very high rates due to the compartmentalization effect.28-30 Moreover, since the surfactant counterion is available in every reverse micelle, nuclei can form directly inside the reverse micelles while accommodating the added salt. This reduces the effect that the opening of the surfactant layer has on the rate of nucleation, and hence the particle size. This fast reaction, that is less sensitive to the intermicellar exchange of solubilizate, represents a new concept that has not been discussed in the literature before. In addition to the product value, this study provides an insight on the stages of microemulsion formation.31 In this study, nanoprecipitates of silver chloride were prepared by titrating an aqueous solution of silver nitrate into dioctyldimethylammonium chloride/n-decanol/isooctane microemulsions. The silver cation reacted with the chloride counterion of the surfactant to form nanoprecipitates of silver chloride, which were contained in the water pools of the microemulsion. Nanoparticles of silver chloride are important for the photographic and the electronic industries.14,16,17,24 Experimental Procedure Materials. Bardac (Lonza Corp., Fair Lawn, NJ) containing 80 wt % dioctyldimethylammonium chloride (R2(Me)2N+Cl-), 10 wt % ethanol, and 10 wt % water was used as the surfactant. N-Decanol (Fisher Scientific, Montreal, QC, Canada) was used as the cosurfactant, and isooctane (Fisher Scientific) was used as the continuous organic phase. An aqueous solution of 12% (w/v) AgNO3 (A&C, Montreal, QC, Canada) was used as the source of silver. For the study of the two-microemulsion mixing technique, dioctylsulfosuccinate sodium salt, AOT (Sigma, St. Louis, MO), was used as the surfactant and isooctane as the continuous phase. Sodium chloride (Fisher Scientific) was used as the source of chloride ions. Particle Preparation. The experiments were carried out by changing one of the following variables at a time: the concentration of R2(Me)2N+Cl-; the concentration of the n-decanol cosurfactant; the moles of silver nitrate, nAgNO3; and the mole ratio (27) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1993, 32, 3014-3019. (28) Husein, M. M.; Weber, M. E.; Vera, J. H. Can. J. Chem. Eng. 2001, 79, 744-750. (29) Husein, M. M.; Vera, J. H.; Weber, M. E. Colloids Surf., A 2001, 191, 241-252. (30) Husein, M. M.; Weber, M. E.; Vera, J. H. Langmuir 2000, 16, 9159-9167. (31) Bulavchenko, A. I.; Batishcheva, E. K.; Podlipskaya, T. Yu.; Torgov, V. G. J. Anal. Chem. 2000, 366, 59-63.

Figure 2. Absorption spectrum of AgCl nanoparticles prepared by the mixing of two microemulsions based on AOT. 0.1 M AOT, nAgNO3 ) nNaCl ) 0.054 mmol, R ) 10. of water to R2(Me)2N+Cl-, R. Isooctane was always used as the continuous phase. A constant volume of 10 mL of the Bardac/ n-decanol/isooctane phase was used throughout the experiments. A calculated volume of water was titrated into a microemulsion system containing specified concentrations of R2(Me)2N+Cl- and n-decanol. The amount of water already existing in Bardac was taken into account in the calculation of R. A certain volume of 12% (w/v) AgNO3 was added to the microemulsion system to achieve the specified nAgNO3. The added volume of AgNO3 was accounted for in the calculation of R, assuming the density of the 12% (w/v) AgNO3 solution equals that of water. The samples were mixed for 2 min using a vortex mixer. In all the experiments, except the one at 0.5 M n-decanol, the concentrations were chosen so to avoid the appearance of cloudiness in the microemulsions after 2 min of mixing. After mixing for 2 min, the samples were then placed in an ultrasonic bath for 1 min and then left to stand at room temperature for 20 min. The UV-spectroscopy analysis was taken using a Cary Varian 1/3 UV/visible spectrophotometer (Varian Techtron Pty Ltd., Australia). A range of wavelengths between 190 and 500 nm was covered. Blank solutions containing only the surfactant and n-decanol in isooctane, at the same concentration as the sample, were used for the UV measurements. A thiophenol (Aldrich, Milwaukee, WI) capping agent was added to the samples, 1 µL thiophenol/1 mL sample. Upon addition of the thiophenol, cloudiness appeared in all the samples except the one at low moles of AgNO3, nAgNO3 ) 0.007 mmol. To help recover the nanoparticle-capping agent precipitate for this sample, water was added at a ratio of 1 mL water/10 mL microemulsion. The addition of thiophenol to a microemulsion

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Figure 3. TEM photographs and the corresponding particle size distribution histograms for different concentrations of R2(Me)2N+Cl-: (a) 0.1 M; (b) 0.2 M; (c) 0.3 M. 0.3 M n-decanol, nAgNO3 ) 0.0175 mmol, R ) 3.5. containing no silver chloride nanoparticles did not result in any precipitation or in any observable changes. The sample with the capping agent was left in the ultrasonic bath for 4 h to allow for more precipitation. The sample was then centrifuged to remove the nanoparticle-capping agent precipitate. A volume of 20 mL of methanol was added to the precipitate, and the methanol solution was left for 4 h in the ultrasonic bath in order to disperse the nanoparticle-capping agent precipitate. The methanol solution was then centrifuged for 1 min at 1000 rpm. To investigate whether ultrasonic bath and centrifugation affect the particle size, in some samples, the methanol addition stage was only followed by 1 min of mixing using the vortex mixer. One drop of the methanol solution was deposited on a copper grid covered with Formvar and was left to evaporate for 24 h. The copper grid was manually shaken for a few seconds so that only a thin layer of methanol solution was present on the grid, hence minimizing coagulation during evaporation. Photographs of the nanoparticles were taken at different degrees of magnification from different spots on the grid using a JEOL 2000FX transmis-

sion electron microscope, TEM (JEOL USA Inc., MA), equipped with a Gatan 792 Bioscan 1k × 1k wide-angle multiscan CCD camera. Histograms showing the particle size distribution were constructed based on the different photographs. An average of 950 particles was used per histogram. For the two-microemulsion mixing method, two microemulsions, 30 mL each, of 0.1 M AOT in isooctane, containing 0.54 mL of aqueous solution of 0.1 M AgNO3 or 0.54 mL of aqueous solution of 0.1 M NaCl, at R ) 10 were mixed in a vortex mixer for 2 min, placed in an ultrasonic bath for 1 min, and left to sit at room temperature for 20 min. To ensure good distribution of the salts within each microemulsion, the individual microemulsions containing the AgNO3 and the NaCl were left for 20 min in the ultrasonic bath prior to mixing. UV spectroscopy of the microemulsion was taken after the 20 min sitting period. Twenty drops from the microemulsion were added to 2 g of isooctane. One drop of the isooctane solution was placed on a copper grid covered with Formvar and left to evaporate before taking the TEM photographs.

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Table 1. Mean Particle Diameter and Standard Deviation for the Different Concentrations of R2(Me)2N+Cl- a [R2(Me)2N+Cl-] (M)

mean particle diameter, D h (nm)

standard deviation, σ

0.1 0.15 0.2 0.25 0.3

3.9 3.8 5.5 4.9 5.4

1.7 1.8 2.2 2.7 2.3

a

0.3 M n-decanol, nAgNO3 ) 0.0175 mmol, R ) 3.5.

Results and Discussion Observations on Microemulsion Formation. Upon addition of AgNO3 aqueous solution to the microemulsion, a very thin cloud of precipitate formed at the boundary between the microemulsion and the bulk aqueous solution. This cloud disappeared immediately with mixing, and the aqueous solution was completely incorporated into the microemulsion system. It is expected that most of the precipitation occurs after the AgNO3 solution is incorporated into the microemulsion due to the large surface that the system provides for the interfacial reaction. Dispersion of the Nanoparticle-Capping Agent Precipitate in Methanol. Treating the methanol solution in an ultrasonic bath and then a centrifuge versus only vigorous mixing before depositing on the copper grid did not have an effect on the particle size and the particle size distribution. These results are expected, since the capping agent preserves the identity of the particles and prevents them from aggregating. Moreover, coagulation of the capped particles on the copper grid was minimized by diluting with methanol and shaking, which left only a thin layer of methanol to be evaporated. AOT Microemulsions. As the mixing of two microemulsion systems based on AOT has been widely studied in the literature, it was used as a reference in this study. Figure 1 shows a typical TEM photograph and the particle size distribution histogram, and Figure 2 shows the UV spectroscopy for the sample prepared using 0.1 M AOT, nAgNO3 ) nNaCl ) 0.054 mmol, and a mole ratio of water to AOT, R, of 10. The UV peak pertaining to silver chloride nanoparticles was obtained at a wavelength of around 250 nm. This result is consistent with the UV spectra collected for AgCl nanoparticles in an AOT microemulsion.17 Bulavchenko et al.31 reported curves for UV absorbance of AgCl nanoparticles at higher wavelengths. This could have been due to a higher concentration of AgCl nanoparticles in the microemulsion or to larger AgCl particles.4,23,24 Effect of R2(Me)2N+Cl- Concentration. Figure 3a-c shows typical TEM photographs and the corresponding particle size distribution histograms obtained by varying R2(Me)2N+Cl- concentration between 0.1 and 0.3 M. The concentration of n-decanol was 0.3 M, the amount of AgNO3 was 0.0175 mmol, and R was 3.5. Table 1 shows the mean particle diameter and the standard deviation for the different concentrations of R2(Me)2N+Cl-. Figure 3 and Table 1 show that at a surfactant concentration of 0.2 M and higher, larger particles with a broader size distribution formed. This result can be explained in light of the effect of increasing the surfactant concentration on nuclei formation and particle aggregation. Larger particles form when particle growth takes place on fewer nuclei. The effect of the surfactant concentration on nucleation can be thought of as follows. As the surfactant concentration and the water content increase, the possibility of forming more reverse micelles

Figure 4. Absorption spectra of AgCl nanoparticles for different concentrations of R2(Me)2N+Cl-: (a) 0.1 M; (b) 0.2 M; (c) 0.3 M. 0.3 M n-decanol, nAgNO3 ) 0.0175 mmol, R ) 3.5.

increases.23,24 The Ag+ ion occupancy number, the number of Ag+ ions per reverse micelle, upon direct titration of AgNO3 decreased. Hence, the number of reverse micelles with AgCl monomer concentration above the critical nucleation concentration decreased, and nucleation became more dependent on the intermicellar exchange of solubilizate, intermicellar nucleation. Fewer nuclei and hence larger particles formed. The results obtained in this work are in agreement with those reported in the literature for the technique of mixing two microemulsions,32 where the intermicellar exchange of solubilizate controls the rate of nucleation. Figure 4 shows the UV absorption spectra for different R2(Me)2N+Cl- concentrations. The UV peaks were obtained at around 250 nm, consistent with AOT results. As can be seen in Figure 4, the absorbance decreased as R2(Me)2N+Cl- concentration increased. The absorbance is related to the quantity of formed particles.22 Since the total amount of silver chloride is constant, the absorbance is related to the particle size. At a constant amount of silver chloride, a smaller particle size gives a larger peak. In addition, there is a small red shift in the position of the absorption peak corresponding to an increase in the particle size as the R2(Me)2N+Cl- concentration increased.23 Effect of Increasing the Amount of AgNO3. Figure 5a-c shows typical TEM photographs with the corresponding size distribution histograms for the experiment where the number of moles of AgNO3 added to the microemulsion system was varied. This experiment was carried out at 0.3 M R2(Me)2N+Cl-, 0.4 M n-decanol, and R of 5. It can be seen from the histograms that the particle size decreased as the moles of AgNO3 increased from 0.007 to 0.035 mmol. Keeping in mind that smaller particles form when growth takes place on a larger number of nuclei, the effect of increasing the moles of AgNO3 can be explained as follows. At a fixed surfactant concentration and a fixed value of R, increasing the amount of AgNO3 increased the (32) Fang, X.; Yang, C. J. Colloid Interface Sci. 1999, 212, 242-251.

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Figure 5. TEM photographs and the corresponding particle size distribution histograms for different moles of AgNO3, nAgNO3: (a) 0.007 mmol; (b) 0.0175 mmol; (c) 0.035 mmol. 0.3 M R2(Me)2N+Cl-, 0.4 M n-decanol, R ) 5.

Ag+ ion occupancy number. More reverse micelles with AgCl monomer concentration higher than the critical nucleation concentration formed, and the rate of nucleation became less dependent on the intermicellar exchange of solubilizate. The larger number of nuclei provides more seeds for particle growth and results in particles with smaller diameter. This result is different from the results reported in the literature for the technique of mixing two microemulsions.22 In the latter case, larger particles were obtained upon increasing the reactant concentrations due to the generation of more rigid surfaces, which, by causing a lower solubilizate exchange dynamics, resulted in fewer nuclei. Figure 6 shows that the UV absorption increased as the amount of AgNO3 increased. This increase represents both the increase of the amount of AgCl nanoparticles17 and the decrease in the particle size. However, no correspondence between the shift in the position of the absorption peak and the particle size could be found.23

This mismatch is probably due to the broad spread in the particle size. Effect of n-Decanol Concentration. Figure 7a-c shows typical TEM photographs and the corresponding size distribution histograms showing the effect of increasing the cosurfactant concentration. The concentration of R2(Me)2N+Cl- was 0.3 M, the moles of AgNO3 was 0.035 mmol, and R was 5. The percentage of particles with diameter larger than 11 nm increased at high n-decanol concentration. When the n-decanol concentration in the microemulsion increased, parts of it may have remained in the bulk oil phase, rather than penetrating to the interface, since its chain length is relatively high.33 Wang et al.34 reported a decrease in the water uptake of Winsor type II micro(33) Zabaloy, M. S.; Vera, J. H. J. Chem. Eng. Data 1996, 41, 14991504. (34) Wang, W.; Weber, M. E.; Vera, J. H. J. Colloid Interface Sci. 1994, 168, 422-427.

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Figure 6. Absorption spectra of AgCl nanoparticles for different moles of AgNO3, nAgNO3: (a) 0.007 mmol; (b) 0.0175 mmol; (c) 0.035 mmol. 0.3 M R2(Me)2N+Cl-, 0.4 M n-decanol, R ) 5.

emulsions at high cosurfactant concentrations. At high concentration, n-decanol acted as a cosolvent in addition to its role as a cosurfactant. Preliminary tests on

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R2(Me)2N+Cl-/n-decanol/isooctane reverse micellar systems showed a decrease in water uptake at high n-decanol concentration, 0.6 M. The abundance of n-decanol in the oil continuous phase increased its polarity, and its ability to accommodate the surfactant became higher. As a result, the interaction between the surfactant protective layer and the nanoparticles in the water pool decreased, allowing flocculation to take place. This explanation is supported by the appearance of cloudiness in the sample at an n-decanol concentration of 0.5 M. Curri et al.12 studied the preparation of CdS nanoparticles by mixing two microemulsions based on cetyltrimethylammonium bromide (CTAB)/n-pentanol/nhexane/water and reported a decrease in the particle size as n-pentanol concentration increased. They explained the decrease in the particle size by the increase in the solubilizate exchange dynamics due to the presence of more n-pentanol at the interface that reduces its rigidity. N-Pentanol has a relatively small chain length, and its ability to penetrate to the interface is higher than that of n-decanol. Contrary to our observation, decreasing the n-pentanol concentration induced sedimentation. This observation was explained by the stabilizing effect npentanol has on the CdS nanoparticles. Figure 8 shows only the UV absorption spectra for the samples at 0.3 and 0.4 M n-decanol. The UV spectrum for the sample at 0.5 M n-decanol could not be taken due to

Figure 7. TEM photographs and the corresponding particle size distribution histograms for different concentrations of n-decanol: (a) 0.3 M; (b) 0.4 M; (c) 0.5 M. 0.3 M R2(Me)2N+Cl-, nAgNO3 ) 0.035 mmol, R ) 5.

Formation of Nanoparticles by Direct Precipitation

Figure 8. Absorption spectra of AgCl nanoparticles for different concentrations of n-decanol: (a) 0.3 M; (b) 0.4 M. 0.3 M R2(Me)2N+Cl-, nAgNO3 ) 0.035 mmol, R ) 5.

its cloudiness. The UV spectra correspond to the histograms, which show no significant difference between the particle size at the two concentrations.

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Effect of Water to R2(Me)2N+Cl- Mole Ratio, R. Figure 9a-c depicts the particle size distribution histograms and typical TEM photographs at different values of the water to R2(Me)2N+Cl- mole ratio, R. The R2(Me)2N+Cl- concentration was kept at 0.3 M, the cosurfactant concentration at 0.4 M, and the amount of AgNO3 at 0.007 mmol. The value of R was increased from 5 to 15. As can be seen from the histograms at a high R value, R ) 15, a higher percentage of large particles (>11 nm) was obtained. Increasing the water content of the reverse micelles introduced a dilution effect. Thus, the number of reverse micelles with AgCl monomer concentration above the critical nucleation concentration decreased, and nucleation became more dependent on the solubilizate exchange dynamics, intermicellar nucleation. Higher water content in the reverse micelles increases the solubilizate exchange dynamics, since it reduces the surface rigidity.24 However, the interaction between the surfactant headgroups and the nanoparticles in the water pools becomes weaker. As a result, the probability of particle aggregation upon coalescence of reverse micelles becomes higher, and larger particles form. No cloudiness appeared for the sample at R ) 15, probably due to the small content of AgCl in the microemulsion. Figure 10 shows a decrease in the UV absorption spectra with increasing R, which is consistent with having larger

Figure 9. TEM photographs and the corresponding particle size distribution histograms for different values of R: (a) 5; (b) 10; (c) 15. 0.3 M R2(Me)2N+Cl-, 0.4 M n-decanol, nAgNO3 ) 0.007 mmol.

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Figure 10. Absorption spectra of AgCl nanoparticles for different values of R: (a) 5; (b) 10; (c) 15. 0.3 M R2(Me)2N+Cl-, 0.4 M n-decanol, nAgNO3 ) 0.007 mmol.

particle size as R increased. The shift in the position of the absorption peak was not significant, however. Conclusions Nanoparticles of silver chloride were formed by direct precipitation of silver ions with the surfactant counterion, chloride. Since the surfactant counterion is available in every reverse micelle and reactions with the surfactant counterion proceed at very high rates, the effect of the intermicellar exchange of solubilizate, intermicellar nucle-

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ation, on the particle size is reduced. Nucleation became more dependent on the solubilizate exchange dynamics whenever the Ag+ ion occupancy number upon direct titration of AgNO3 decreased, and the concentration of silver chloride monomer in the water pool was below the minimum nucleation concentration. Trends similar to those found in the literature for the mixing of two microemulsions were obtained, whenever nucleation was dependent on the intermicellar exchange of solubilizate. Increasing the R2(Me)2N+Cl- concentration and the water content at a fixed amount of AgNO3 resulted in the formation of larger particles, since fewer nuclei formed directly upon titrating AgNO3 and nucleation and particle growth took place simultaneously through the intermicellar exchange of solubilizate. Increasing the moles of AgNO3 decreased the AgCl particle size. At fixed surfactant and water content, increasing the moles of AgNO3 increased the number of nuclei formed directly upon titrating AgNO3 and nucleation was less dependent on the solubilizate exchange dynamics. At high n-decanol concentration, the organic phase became more polar and its ability to accommodate the surfactant increased, leading to less interaction between the surfactant protective layer and the nanoparticles in the water pools, and hence induced flocculation. At high R, aggregation took place and larger particles formed due to decreasing the interaction between the surfactant headgroups and the nanoparticles in the water pools. Generally, the UV absorbance, which is related to the particle size and particle concentration, showed similar trends as the TEM photographs. Acknowledgment. The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support and to Xunta de Galicia, Spain, for the support for the project PGIDT00PXI20902PR and for the travel grant to Eva Rodil. LA0342159