Mechanism of formation of inorganic and organic nanoparticles from microemulsions

Transcription

1 Advances in Colloid and Interface Science (2006) Mechanism of formation of inorganic and organic nanoparticles from microemulsions C. Destrée, J. B.Nagy Laboratoire de RMN, Facultés Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, 5000 Namur, Belgium Available online 24 July 2006 Abstract This chapter essentially deals with the preparation of nanoparticles using microemulsions. The preparation of inorganic nanoparticles Ni 2 B, Pt, Au, Pt Au, AgX and the synthesis of organic nanoparticles cholesterol, rhovanil, rhodiarome are systematically studied as a function of the concentration of the precursor molecules, the size of the inner water cores, and the manner of mixing the various solutions. Two different behaviors are observed in the various systems. The first case shows a dependence of the nanoparticle size on the various physicochemical parameters. Either a monotonous increase of the size or the presence of a minimum is observed as a function of the concentration of the precursor molecules. This case can be easily explained following the LaMer diagram, where the nucleation of the nanoparticles is separated from the particle growth. The second case does not show any dependence of the nanoparticle size on the physicochemical parameters. The size remains constant in all experimental conditions. The constant character of the size can be explained only by thermodynamic stabilization, where particles with a certain size are better stabilized. It should be emphasized that the size distribution is small in all the cases studied. Finally, the aging of the nanoparticles was also checked, especially for the organic nanoparticles. It is concluded that these particles remain stable for months in the microemulsion Elsevier B.V. All rights reserved. Contents 1. Introduction Preparation of nanoparticles using microemulsions Description of a microemulsion Mechanism of synthesis of nanoparticles in microemulsions Preparation of monodisperse inorganic colloidal particles Size of metal boride particles Quantitative aspects of the formation of monodisperse colloidal particles Characterization of the Ni 2 B nanoparticles Sizes of platinum and gold particles Characterization of the silver halide nanoparticles in microemulsions Synthesis of organic particles General considerations Nanoparticles of cholesterol prepared in different microemulsions Nanoparticles of rhodiarome (or Rhovanil) prepared in the AOT/heptane/water microemulsion Influence of the factor R and the concentration of the active principal on the nanoparticle size Recovery of the nanoparticles stabilized by surfactants in a microemulsion and their transfer into an aqueous medium Summary and conclusions References Corresponding author. address: janos.bnagy@fundp.ac.be (J. B.Nagy) /$ - see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.cis

2 354 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) Introduction In the mid-seventies S. Friberg and the late F. Gault proposed an original method using microemulsions to prepare monodisperse nanosized particles. These ideas were followed by a rapid increase in both original and review papers [1 5]. A previous review dealt with a rather comprehensive literature survey up to 1985 [3] involving microemulsions, vesicles, polymer solutions, surfactants in water, sodium citrate in water and general aqueous solutions as reaction media. It was followed by another one which was based only on the use of microemulsions [4]. Indeed, the above-mentioned reaction media represent only a small part of a large variety of systems for colloidal particle preparation. In particular, one could cite, among others, physical vapor deposition, chemical vapor deposition, Langmuir Blodgett films, polymer films [6 8], zeolite entrapped nanoparticles [9,10], supported catalysts [11]. For a quantitative evaluation of the properties of colloidal dispersions, the monodisperse nature uniform in size and shape is a prerequisite. The quantum size effects are particularly studied, since they lead to interesting mechanical, chemical, electrical, optical, magnetic, electro-optical and magneto-optical properties, which are quite different from those reported for bulk materials [7,9,12,13]. The nanoparticles not only are of basic scientific interest but have also resulted in important technological applications, such as catalysts, high-performance ceramic materials, microelectronic devices, and high-density magnetic recording [14 16]. The synthesis of nanoparticles in microemulsions allows one to obtain monodisperse size of the particles and in some cases to control the size of the particles by variation of the size of the microemulsion droplet radius and of the precursors concentrations. Although the synthesis of inorganic particles in microemulsions is already widespread, only polymeric nanoparticles have been synthesized in microemulsion media as far as the organic particles are concerned. In this chapter, it will be shown that it is also possible to synthesize organic particles by a direct precipitation reaction in the microemulsions. We emphasize some of the fundamental aspects of monodisperse nanoparticle formation. Two models are proposed for the formation of the particles: the first is based on the LaMer diagram, and the second is based on the thermodynamic stabilization of the particles. In the first case, the particle size varies as a function of either the size of the inner water cores or the precursor concentration; in the second case, the particle size is independent of these parameters. The monodisperse nanoparticles are characterized directly in the microemulsion or after transferring them in another medium. First, the size of the nanoparticles is determined as a function of various parameters. Their composition is analyzed by X-ray photoelectron spectroscopy (XPS) or energy dispersive X-ray analysis (EDX). The specific surface area is Fig. 1. Microemulsion regions L2 in various ternary systems (see text for explanation of the abbreviations).

3 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) determined by the BET technique. The direct solvation is analyzed by multinuclear magnetic resonance spectroscopy. 2. Preparation of nanoparticles using microemulsions 2.1. Description of a microemulsion A water-in-oil microemulsion is a thermodynamically stable, optically transparent dispersion of two immiscible liquids stabilized by a surfactant. The important properties are governed mainly by the water surfactant molar ratio (R = [H 2 O]/[surfactant]). This factor is linearly correlated with the size of the water droplets. The nanoparticles have been synthesized in different microemulsion systems. Some of them are shown in Fig. 1. The anionic Aerosol-OT (AOT)/heptane/water system is one of the best characterized microemulsions [17,18]. The cationic cetyltrimethylammonium bromide (CTAB)/hexanol/water system contains hexanol, which forms the organic phase and plays the role of cosurfactant [19]. The nonionic penta(ethylene glycol)-dodecylether (PEGDE)/hexane/water system was studied by Friberg and Lapczynska [20]. The reverse micellar droplets have a cylindrical shape in which the surfactant molecules are parallel to each other, forming a bilayer impregnated with water. Triton X-100 [p-(1,1,3,3-tetramethylbutyl)phenyl-polyethoxyethanol)/decanol/water system has been characterized by Ekwall and coworkers [21] Mechanism of synthesis of nanoparticles in microemulsions The aqueous droplets continuously collide, coalesce, and break apart, resulting in a continuous exchange of solution content. In fact, the half-life of the exchange reaction between the droplets is of the order of s [22,23]. Two models have been proposed to explain the variation of the size of the particles with the precursor concentration and with the size of the aqueous droplets. The first is based on the LaMer diagram [24,25], which has been proposed to explain the precipitation in an aqueous medium and thus is not specific to the microemulsion. This diagram (Fig. 2) illustrates Fig. 2. LaMer diagram. Fig. 3. Methods for preparation of monodisperse particles (X, Y, and Z are in wt. % of the various components). the variation of the concentration with time during a precipitation reaction and is based on the principle that the nucleation is the limiting step in the precipitation reaction. In the first step, the concentration increases continuously with increasing time. As the concentration reaches the critical supersaturation value, nucleation occurs. This leads to a decrease of the concentration. Between the concentrations C max and C min the nucleation occurs. Later, the decrease of the concentration is due to the growth of the particles by diffusion. This growth occurs until the concentration reaches the solubility value. This model has been applied to the microemulsion medium, i.e., that nucleation occurs in the first part of the reaction and later only growth of the particles occurs. If this model is followed, the size of the particles will increase continuously with the concentration of the precursor or a minimum in the variation of the size with the concentration can also be expected. This stems from the fact that the number of nuclei is constant and the increase of concentration leads to an increase of the size of the particles. The second model is based on the thermodynamic stabilization of the particles. In this model the particles are thermodynamically stabilized by the surfactant. The size of the particles stays constant when the precursor concentration and

4 356 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) the size of the aqueous droplets vary. The nucleation occurs continuously during the nanoparticle formation. These two models are limiting models: the LaMer diagram does not take into account the stabilization of the particles by the surfactant, and the thermodynamic stabilization model does not take into account that the nucleation of the particles is more difficult than the growth by diffusion Preparation of monodisperse inorganic colloidal particles The different monodisperse nanoparticles were prepared following either Scheme I or Scheme II of Fig. 3. We first discuss the mechanism of formation of particles following Scheme I, where small amounts of aqueous solutions are added to the initial microemulsion Size of metal boride particles Monodisperse colloidal nickel boride and cobalt boride particles were synthesized by reducing, with NaBH 4, the metallic ions solubilized in the water cores of the microemulsions. The NaBH 4 /MeCl 2 ratio was held equal to 3 because larger particles were obtained for a lower value, and the particle size remained constant above this ratio [2 4]. The composition of the particles was determined by XPS to be, respectively, Ni 2 B and Co 2 B. In each case, the size of particles ( nm) was much smaller than that obtained by reduction of Ni(II) or Co(II) in water ( nm) or in ethanol ( nm), and the size distribution was quite narrow (±0.5 nm). Fig. 4 shows the dependence of the nickel boride particle size on the water content in the microemulsion as well as on the Ni (II) ion concentration. The average size of the particles decreases with decreasing size of the inner water core (decreasing water content), and a complex behavior is observed as a function of the Ni(II) ion concentration; a minimum is detected at approximately molal concentration. These observations can be understood if one analyzes the nucleation and the growth processes of the particles. To form a stable nucleus, a minimum number of atoms are required [26]. Thus, for nucleation several atoms must collide at the same time, and the probability of this phenomenon is much lower than the probability of collision between a single atom and an already formed nucleus. The latter phenomenon is called the growth process. At the very beginning of the reduction, nucleation occurs only in water cores that contain enough ions to form a nucleus. At this moment, the micellar aggregates act as reaction cages where the nuclei are formed. On the other hand, the microemulsion being dynamic, the water cores rapidly rearrange. The other ions brought into contact with the existing nuclei essentially participate in their growth process. The latter Fig. 4. Variation of the average diameter (in nm) of the nickel boride particles as a function of water content and Ni(II) ion molal concentration.

5 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) Table 1 Important parameters for the formation of Ni 2 B colloidal particles [Ni(II)] 10 2 (molal) r a,b M (nm) N b,c M a,d n Ni(II) d e (nm) W t f (g) W g (g) N n h Nn N b;c M P lk¼2 p k F i 10 3 CTAB 24%/hexanol 60%/water 16% CTAB 30%/hexanol 50%/water 20% a Radius of water core. b Values given for the system containing three fourths of the total amount of water. c Number of water cores in 1 kg of solution. d Number of Ni(II) ions per water core. e Diameter of Ni 2 B particles. f W t is calculated with M(Ni 2 B)= g/mol. g W is calculated with M v (Ni 2 B)=7.9 g/cm 3. h Number of nuclei in 1 kg of solution. i Correction factor from N n ¼ FN M P l k¼2 p k (see text). being faster than nucleation, so no new nuclei are formed at this moment. As all the nuclei are formed at the same time and grow at the same rate, monodisperse particles are obtained. In summary, the particle size depends on the number of nuclei formed at the very beginning of the reduction, and this number is a function of the number of water cores, containing enough ions to form stable nuclei, that are reached by the reducing agent before the rearrangement of the system. However, the stabilization of the nuclei by surfactants is probably one of the most important factors in explaining the monodispersity of the particles Quantitative aspects of the formation of monodisperse colloidal particles The first step in the determination of the essential parameters that control particle size is a study of the distribution of the ions in the microemulsion water cores. By knowing the average radii of the microemulsion water cores (r M ) and the total volume of water (V T ) per kilogram of microemulsion, one can calculate the number of water cores per kilogram of reverse micelles (N M ), neglecting the solubility of water in the organic phase: N M ¼ V T ð1þ 4 3 kr3 M The parameter N M and the initial concentration of metal ions expressed in molality allow one to determine the average number of ions per water core (n ions ): ½ionsŠ6: n ions ¼ ð2þ N M The ions are statistically distributed in the aggregates. To calculate this distribution, Poisson statistics is quite adequate [27]. This gives the probability (p k ) of having k ions per water core (k is an integer taking the values 0, 1, 2, 3, ), provided the average number of ions per water core (λ=n ions ) is known: p k ¼ kk e k k! ð3þ The number of nuclei formed (N n ) when the ions solubilized in 1 kg of solution are reduced is proportional to the number of aggregates containing enough ions for nucleation. If the minimum number of ions required to obtain a nucleus is i, then N n can be calculated from the relation X l N n ¼ FN M ð4þ k¼i p k where Pl p k is the probability of having i or more ions per k¼i aggregate; hence N P l M p k is the number of water cores k¼i containing i or more ions. The F is a proportionality factor taking into account the proportion of aggregates reached by the reducing agent before rearrangement of the system can occur. In Eq. (4) we do not know the values of i and F but we can calculate all the other parameters. Indeed, the number of nuclei (N n ) is the number of particles prepared, and it is given by N n ¼ W t ð5þ W where W t is the total weight of the particles prepared per kilogram of micellar solution, W is the weight of one particle, and W t ¼ ½ionsŠM particle ð6þ x where M particle is the molecular weight of the particle and x is the number of metal atoms per particle. The weight of one particle is given by W ¼ 4 3 k d 3 M v;particle ð7þ 2 where d is the diameter of the particle measured by electron microscopy and M v,particle is the volumetric mass of the particle. All the experimental and computed data are reported in Table 1 for Ni 2 B particles.

6 358 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) The diameter of the particles is systematically higher than the diameter of the inner water cores. For all the particles synthesized, we calculated the proportionality factor F by systematically varying the value of the minimum number of ions required to form a nucleus (i). If i=1 or i>2, the values of factor F vary considerably (not shown). However, if i=2, its values are reasonably constant (see Table 1). The order of magnitude of the factor F is always This means that at the very beginning of the reduction, i.e., when the nuclei are formed, only one aggregate per thousand leads to the formation of metal boride particles. There is another indication that the nucleation occurs at the very beginning of the reduction. Indeed, the average radii of the water cores used for the calculation of the formation parameters of colloidal particles are measured for the system containing only three fourths of the total amount of water, which is the composition of the solution before the addition of the reducing agent. If the final composition is used, however, no clear-cut correlation can be obtained between the number of nanoparticles and either the Ni(II) concentration or the water content. The order of magnitude of the factor F is constant, but its value decreases with increasing water content in the microemulsion (see Table 1). This phenomenon can be easily understood because the rearrangement rate of the microemulsion decreases with the water amount and hence the number of aggregates reached by the reducing agent before rearrangement decreases. As the number of nuclei formed decreases at a constant concentration of precursor ions, the particle size increases with the water content in the system. The diameter of the particles is plotted as a function of micellar droplet concentration in Fig. 5. The values of the particle size are those obtained by interpolation of previous results in the presence of molal aqueous metal ion for the CTAB/hexanol/water systems. Particles prepared in the AOT/ Fig. 6. Variation in the probability of having k Ni(II) ions per aggregate for the CTAB 18%/hexanol 70%/H 2 O 12% microemulsion [Ni(II)] (molal): (a) , (b) , (c) heptane/water system at a much lower droplet concentration are included for comparison. The Co 2 B particles were also obtained in Triton X-100/decanol/water systems. The size of the particles decreases linearly with the micellar droplet concentration. This is a strong indication that the final size obtained for the particles is governed by the presence of reverse micellar aggregates. Indeed, if initial nucleation takes place in the water cores, then nucleation should be related to the micellar droplet concentration of the system. Further, the greater the number of micellar droplets, the greater the number of nucleation sites possible (the aqueous metal ion concentration being obviously maintained constant). The results of Table 1 also allow us to explain the minimum in the particle size as a function of the concentration of ions (see Fig. 4). For a constant microemulsion composition, at low ion concentration, only a few water cores contain the minimum number of ions (two) required to form a nucleus; hence, only a few nuclei are formed at the very beginning of the reduction, and the metal boride particles are relatively large. When the ion Fig. 5. Size of nanoparticles prepared in various microemulsions as a function of micellar droplet concentration. Fig. 7. Variation in ( ) the number of nuclei formed per aggregate (N n : number of nuclei; N M : number of water cores) and ( ) the probability of having two or more ions per aggregate as a function of Ni(II) concentration in the microemulsion CTAB 18%/hexanol 70%/water 12%.

7 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) Fig. 8. Model of the preparation of colloidal Ni 2 B particles from a water-in-oil microemulsion. concentration increases, the distribution of precursor ions in the microemulsion is very different (Fig. 6), and the number of nuclei obtained by reduction increases faster than the total number of ions (Fig. 7). This results in a decrease in the particle size. When more than 80% of the water cores contain two or more ions, the number of nuclei formed remains quasi-constant with increasing ion concentration. Hence the size of the particles increases again. Fig. 4 also shows the particle size as a function of water content in the microemulsion for different Ni(II) concentrations. An increase in the average diameter is observed with increasing proportion of water. The decrease in the number of micellar aggregates (N M ) with water (Table 1) is accompanied by an increase in their size. For the same Ni(II) concentration with respect to water (i.e., for the same probability of collision between the ions in the same water core), the total number of nuclei formed in the early stage of the reduction decreases with increasing water concentration, and more ions can participate in the growth process. This results in an increase in the particle size. One should keep in mind that the total number of Ni(II) ions also increases with increasing water content. This is shown if the size of the particles is plotted as a function of micellar droplet concentration (Fig. 5). For most of the systems studied, a monotonous decrease in the size with increasing N M is observed. These results reinforce the hypothesis leading to the computation of the number of nuclei and underline the importance of the water cores as reaction cages. The F value for nickel boride is equal to ca The essential parameters for the formation of monodisperse colloidal particles are thus quantified. We have shown that two metal atoms are required to form a stable nucleus and that nucleation occurs only in the aggregates that are reached by the reducing agent before rearrangement of the system can occur (1 per 1000 aggregates). Fig. 8 illustrates quite well the mechanism of reduction in a water-in-oil microemulsion. After fast diffusion of the reducing agent, nucleation occurs in the water droplets where the preceding conditions are satisfied. The nucleus is stabilized by the adsorbed surfactant molecules. The growth of the particles requires an exchange between different water cores. Finally, surfactant-protected monodisperse particles are formed that can be used directly or by being deposited on a support. In the treatment just discussed, the nucleation step could not yet be clearly described. The model is based on the presence of discrete water pools in the microemulsion, whereas conductivity measurements showed that percolation already occurred in these systems, favoring the exchange between water pool contents [28]. More experiments are needed to determine the formation of the first nuclei using fast kinetics measurements. Nevertheless, the stabilization of the nuclei by the surfactant molecules at the interface could play a definite role in controlling their number formed at the very beginning of the reduction. The method of addition of the reducing agent in the aqueous solution is indeed very important, because if higher amounts of microemulsion systems are used, larger particles are obtained. Table 2 Specific surface area (S BET ) of the Ni 2 B nanoparticles prepared in the 18.0% CTAB/70.0% hexanol/12.0% H 2 O microemulsion H 2 O a Before washing After washing hyd (%) d b (nm) c S part (m 2 ) d B ads (%) e B ads (%) f S BET (m 2 /g) S part g (m 2 ) S part / S part S BET (m 2 /g) S part (m 2 ) S part / S part a %H 2 O hydrating the Ni 2 B nanoparticles; [Ni(II)]= molal. b Diameter of the Ni 2 B nanoparticles determined by TEM. c Total surface of nanoparticles synthesized in 100 g of micellar solution determined from TEM (transmission electron microscopy) measurements; [Ni (II)]= molal. d Percent boron adsorbed on the particles before washing. e Percent boron adsorbed on the particles after washing. f S BET : specific surface area of nanoparticles determined by N 2 adsorption. g S part =WS BET where W is the total weight of the particles synthesized in 100 g of microemulsion.

8 360 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) Table 3 Analysis of Pt particle size as a function of the number of nuclei N n [K 2 PtCl 4 ] (molal vs. H 2 O) 2 a, b n PtCl4 d c (nm) W t 10 3 W (g) d (g) d, e N n f ± ± ± ± ± a Number of PtCl 2 4 ions per inner water core. b N M (number of inner water cores)= per kg solution; r M =6.0 nm (Radius of water core). c Diameter of Pt particles determined by TEM. d Values given for 1 kg of solution. e Assuming volumetric mass of Pt=21.45 g/cm 3. f Number of nuclei per kg solution Characterization of the Ni 2 B nanoparticles The nature of the boride particles was determined by XPS and EDX. Table 2 shows the specific surface area of the nickel boride particles. The composition of the nanoparticles was determined by EDX measurements and it corresponds to Ni 2 B stoichiometry. The as-prepared boride nanoparticles adsorb large amounts of BO 2 and CTAB + ions. The amount of adsorbed BO 2 ions was measured using 11 B nuclear magnetic resonance (NMR). It was determined as the difference between the total amount of NaBH 4 added and the final concentration of boron in the microemulsion after precipitation of the nanoparticles. The amount of boron adsorbed on the Ni 2 B nanoparticles is about 85% of the total boron present in the system as BO 2 ions. The borate ions are eliminated by two successive washings with an aqueous solution of HCl and three successive washings with distilled water. After washing, the remaining adsorbed boron is ca. 15%. The precipitated particles adsorb a non-negligible amount of water from the microemulsion inner water core. This amount can rise to about 15% of the initial water present in the microemulsion (Table 2). The specific surface area of the particles was determined on both the as-prepared and the washed nanoparticles. In Table 2 the S BET values are compared, the total surface of the nanoparticles obtained in 100 g of microemulsion and determined from the diameter of the particles (S part ), and the total surface of the particles (S part ) obtained from BET measurements. It is seen that the unwashed particles present a surface 2.3 times smaller than the theoretical surfaces (S part / S part ). After washing, the surface is almost clean for the Ni 2 B particles, because the S part /S part ratio is close to 1. water) [29]. The aqueous solution of hydrazine containing a 10- fold molar excess of hydrazine with respect to H 2 PtCl 6 had an initial ph of 10. The metal particle precursor is soluble in both the dispersed inner water core and the continuous (or hexanol) phases. If it is assumed that the nucleation occurs in both phases, the particle size is dependent only on its stabilization by the adsorbed surfactant molecules [3,29,30]. It is interesting to note that particles of a similar size were obtained, independently of water and H 2 PtCl 6 concentrations, from the AOT/heptane/water microemulsion [31]. If K 2 PtCl 4 is used instead as the particle precursor (for the same hydrazine to K 2 PtCl 4 ratio), a complex behavior is observed as a function of ph. At low ph values (1<pH<4), no Pt particles could be obtained. At 5 < ph < 8, dispersed Pt particles were formed, but the reduction was not complete even after 24 h of reaction. For high ph values (ph>9), complete reduction of the Pt salt occurred, but the particles thus obtained were aggregated. It is thus clear that the surface charge does influence the aggregation of the metal particles. In addition, the adsorption of the surfactant molecules, also ph dependent, can greatly influence the particle aggregation PEGDE hexane water microemulsion. Colloidal Pt particles were prepared following both Schemes I and II of Fig. 3. To avoid particle aggregation, a neutral surfactant, PEGDE, was used to form a microemulsion of composition PEGDE 9.5%/hexane 90%/water 0.5%. Only K 2 PtCl 4 was tested as a precursor salt, however, because it is insoluble in the organic medium. Table 3 and Fig. 9 show the variation in the size of the Pt particles obtained following Scheme I as a function of initial K 2 PtCl 4 concentration. The particle diameter increases monotonously with increasing K 2 PtCl 4 concentration and approaches a plateau at high concentration. This behavior seems to be different from those previously observed for the Pt particles using H 2 PtCl 6 [31,32] and for the Ni 2 B particles. In the first case, a constant particle size was obtained irrespective of the initial H 2 PtCl 6 concentration, and in the second case a minimum was observed in the particle size (Ni 2 B) versus NiCl 2 concentration curve Sizes of platinum and gold particles CTAB hexanol water microemulsion. Colloidal particles of Pt and Au were prepared following Scheme I of Fig. 3. The monodisperse Pt particles prepared from H 2 PtCl 6 dissolved in the CTAB/hexanol/water microemulsion had an average diameter of 4.0 ±0.5 nm, and their size was not dependent on the H 2 PtCl 6 concentration ( molal with respect to Fig. 9. Variation of the average Pt particle diameter as a function of K 2 PtCl 4 concentration with respect to water prepared according to Scheme I of Fig. 3.

9 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) For low initial K 2 PtCl 4 concentration (up to 0.01 molal with respect to water), N n increases as a function of Pt concentration. This behavior was observed earlier for the case of Ni 2 B particles. However, for higher K 2 PtCl 4 concentrations, the N n value decreases, leading to larger particles. The probable nucleus is a surfactant-stabilized Pt atom that is able to form the final Pt particle [33]. If the particles are prepared following Scheme II, where the two microemulsions containing the precursor K 2 PtCl 4 and the reducing agent N 2 H 4, are mixed together, smaller sizes are obtained. Indeed, the Pt particles prepared from the microemulsion with [K 2 PtCl 4 ]=0.1 molal with respect to water have a diameter of 3.5±0.5 nm, whereas the diameter is much greater (9.0±1.0 nm) if Scheme I is used (Table 3). Fig. 9 illustrates the variation of the average diameter of the Pt particles as a function of the concentration of K 2 PtCl 4 prepared by Scheme I. The larger size of the Pt particles obtained by the method of Scheme I can be explained in a first approximation by the diffusion of the aqueous solution through the organic phase being slower than the exchange between the water cores. Although in the PEGDE/hexane/water microemulsion no separate spherical droplets are present, the water is probably the dispersed phase in the microemulsion. The structure of the microemulsion is better represented as a lamellar aggregate where the surfactant molecules are associated head to head along a cylinder DOBANOL hexane water microemulsion. Particles of Pt, Au, and Pt Au were prepared in a DOBANOL hexanol water microemulsion following Scheme II of Fig. 3. DOBA- NOL is a mixture of penta(ethylene glycol) undecyl (<1 wt.%), dodecyl (41 wt.%), tridecyl (58 wt.%), and tetradecyl (< 1 wt.%) ethers. The microemulsion region is smaller than for the PEGDE system [36]. Table 4 illustrates the influence of DOBANOL and PEGDE surfactants. Within experimental errors, the same diameter is obtained for both systems. The Au particles were obtained from the precursor AuCl 3. In the PEGDE/hexane/water microemulsion at low precursor concentrations (less than 0.05 molal with respect to water), only small particles (about 3.0 nm diameter) were formed (Table 5a and Fig. 10), whereas both small and large (about 10 nm diameter) particles were formed at higher precursor concentrations. Table 5 Average diameter, d, of Au particles showing the bidispersion in PEGDE/ hexane/water microemulsions [AuCl 3 ] (molal vs. H 2 O) d (nm) d (nm) (a) PEGDE 9.5%/hexane 90%/water 0.5% ± ± ± ± ± ± ± ±0.5 (b) DOBANOL 9.5%/hexane 90%/water 0.5% ± ± ± ± ±1.6 In the DOBANOL/hexane/water microemulsion only one type of particle is obtained, the size of which increases with increasing precursor concentration (Table 5b). The Pt Au particles were prepared in both microemulsion systems (Table 6). Both small (about 3.0 nm diameter) and large (about 12 nm diameter) particles were obtained in both systems. The size of the particles is not dependent on the composition of the precursor salts. The large particles are clearly formed by aggregation of the small particles. The nanoparticles are true mixed Pt Au particles, as was shown by scanning transmission electron microscopy (STEM)/EDX measurements Characterization of the silver halide nanoparticles in microemulsions The silver halide nanoparticles were prepared following Scheme II of Fig. 3, where the precursor salts AgNO 3 and NaX were dissolved in AOT/heptane/water microemulsions of similar compositions. In the numerous studies concerning the synthesis of nanoparticles in microemulsion media, the location of water after the nanoparticle synthesis has never been determined. Two models can be proposed (Fig. 11). In Table 4 Average diameter, d, of Pt particles synthesized from PEGDE 9.5%/hexane 90%/water 0.5% and DOBANOL 9.5%/hexane 90%/water 0.5% microemulsions [K 2 PtCl 4 ](molalvs.h 2 O) d (nm) PEGDE DOBANOL ± ± ± ± ± ± ± ± ± ±0.4 Fig. 10. Variation in gold particle diameter as a function of precursor AuCl 3 concentration versus water synthesized in PEGDE 9.5%/hexane 90%/water 0.5% microemulsion. The presence of larger particles shows particle aggregation.

10 362 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) Table 6 Average diameter, d, of Pt Au particles as a function of Pt mole fraction x showing the bidispersion in both systems a x d (nm) d (nm) (a) PEGDE 9.5%/hexane 90%/water 0.5% ± ± ± ± ± ± ± ± ± ±0.4 (b) DOBANOL 9.5%/hexane 90%/water 0.5% ± ± ± ± ± ± ± ± ± ±0.4 a [AuCl 3 ]+[K 2 PtCl 4 ]=0.1 molal vs. H 2 O. the first one the particles are surrounded by a layer of water, and in the second the surfactant molecules (the AOT) are directly adsorbed onto the particles and only a small amount of water molecules is present. In order to discriminate between these two models, 2 HNMR measurements on deuterated water in microemulsions have been carried out. Two NMR lines were observed in the 2 H NMR spectra (Fig. 12) for the various microemulsions without particles of silver bromide. Fig. 12. NMR spectra of the deuterated water in the AOT/heptane/water microemulsion for R=3.1. Fig. 11. Two models of the nanoparticles stabilized in the microemulsion media, (a) The particle is surrounded by a layer of water, (b) AOT is directly adsorbed onto the particle. If the same spectrum is taken for a very low R value, such as R=0.5 (Fig. 13), three NMR lines are observed. These lines are not due to the presence of impurities in fact, their intensity does not decrease as the amount of water decreases so these lines stem from different types of water molecules. This is illustrated by the measurements of their relaxation times T 1.In fact, for R=1 the following three relaxation times T 1 were obtained at 273 K: 321 ms for the broader line, 804 ms for the line situated at 3.50 ppm, and 1087 ms for the line situated at 3.95 ppm. As the variation of the relaxation time with temperature indicates that we are in a region where the relaxation time increases with the decrease of temperature, these two lines correspond to water molecules that are less mobile and, therefore, more in contact with the surfactant molecules. Generally, three kinds of water may exist in a microemulsion medium: bulk water in the center of the water core, bound water that interacts with the hydrophilic part of the surfactant molecule, and trapped water that is trapped at the interface in the form of monomers or dimers [34]. Bulk water molecules are normally not present for R values below 6 10, where all the water molecules are structured due to their interaction with Na + counterions and the strong dipole of the AOT polar group [35]. As in this case the ratio R=[H 2 O]/[AOT] is 3.1, only two kinds of water molecules would be expected. Therefore, it is assumed that the two NMR lines observed here correspond to bound water and trapped water. In order to check this assumption, the same experiment was carried out for higher R values. The

11 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) experiments is the presence of silver bromide particles, all observed differences must be due to the particles. In the presence of these particles, the quantity of trapped water is larger, as shown by a comparison of spectra in the presence and in the absence of nanoparticles (Fig. 15). The total intensity is also higher in presence of silver bromide particles, also stemming from the greater importance of the trapped water. In fact, this water freezes at a lower temperature. Furthermore, not all the water cores of the microemulsion are occupied by a particle, only 1 water core out of is occupied by a particle. Hence, if the microemulsion structure stayed the same, with the same number of water molecules in each water core, no Fig. 13. NMR spectrum of deuterated water in the AOT/heptane/water microemulsion for R=0.5 at T=297 K. chemical shift increases with the R value until it reaches approximately that of the pure deuterated water (used as reference) while the line width at half-height decreases with R (Fig. 14). Such variation has already been observed [35] and is the result of a fast exchange (faster than s 1 ) between the bulk water and the bound water. At low R values, the observed chemical shift comes from the variation of the number of hydrogen bonds in which the water molecules are involved. In fact, the water molecules adsorbed at the interface (or solvating the Na + ions) form fewer hydrogen bonds, provoking a highfield chemical shift. The smaller number of hydrogen bonds has previously been shown by Wong et al. [36] using 1 H NMR experiments. Furthermore, if the NMR spectra are recorded at lower temperatures, the NMR line corresponding to the bound water decreases due to the freezing of this kind of water (the bandwidth becomes too large to be detectable) (Fig. 12). In fact, the freezing point of bound water seems to be about 243 K inside the inverted micelles. This corresponds to the decrease of the freezing point of water with the size of the droplet; for example, the freezing point of water in a droplet corresponding to R = 4.5 in AOT/water/2,2,4-trimethylpentane is at around 241 K [37]. On the other hand, the line corresponding to the trapped water shows no freezing and its intensity remains quasiconstant. In order to distinguish between the two models of AgBr stabilization (see earlier), the NMR experiments mentioned have also been carried out in presence of silver bromide nanoparticles. As the only difference between the two Fig. 14. (a) Variation of the 2 H chemical shift as a function of the R factor, (b) Variation of the line width as a function of the R factor.

12 364 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) Fig. 15. NMR spectra of the deuterated water in the AOT/heptane/water microemulsion (full line) and in presence of AgBr particles (dotted line) at 263 K. influence on the NMR spectra could be observed upon addition of AgBr. The higher amount of trapped water is in favor of model (b), where the particles are in closer contact with the interfacial layer. However, the NMR line of the adsorbed water could overlap that of the trapped water. In order to check this hypothesis, the number of water molecules per AOT was calculated. The spectra in Fig. 15 have been decomposed in two bands corresponding, respectively, to the bound water and to the trapped water. The difference in intensities of the two NMR lines corresponding to the trapped water in the spectra without and with AgBr particles gives the amount of water trapped or adsorbed on the particles. The number of AOT molecules per particle has been calculated using a spherical surface of 4.6 nm diameter and a surface area of 0.41 nm 2 for the polar part of the AOT molecule [38]. It has been computed that if the whole line intensity corresponded to the trapped water there would be 2000 water molecules per AOT molecule. As the trapped water is considered to be in the form of a monomer or a dimer, this value is too high to correspond only to water molecules trapped in the interface. Hence, it has to be assumed that the additional water molecules so computed are adsorbed on the AgBr particles and the NMR lines of the trapped water and the adsorbed water overlap. If it is assumed that all these additional water molecules are adsorbed on the particles, the number of water monolayers can be calculated by using the van der Waals radius of a water molecule. Approximately 1000 monolayers of water can be formed around the nanoparticles. These two arguments, the observation of an NMR line corresponding to the adsorbed water molecules and the estimation of the number of water monolayers, are in favor of model (a). Hence, this model will be adopted. In order to quantify by another method the amount of water adsorbed on the nanoparticles, a microemulsion in which the particles had sedimented was also examined. This microemulsion was obtained by adsorption of pseudoisocyanine on the particles. This dye causes a rapid sedimentation of the particles [39], and a 2 H NMR spectrum was taken after sedimentation of all the particles. From this spectrum, it was established that 68% of the water was adsorbed on the particles. The number of water monolayers formed around the particles was calculated and a value of about 4600 monolayers of water was obtained. This value is too large and physically impossible; in fact, the radius of the corresponding water core should be 2.6 μm. These water cores should scatter the light, and as the colloidal suspension is limpid, the number of water molecules bound to the silver halide particles must be overestimated in this approach. Such a large amount of water in the precipitate can be explained only if the sedimented particles form a sort of gel where a large amount of water is required. This gelation was previously shown in the case of Ni 2 B nanoparticles prepared from CTAB/n-hexanol/ water [40] microemulsion. This high amount of adsorbed water molecules is also in favor of model (a). 3. Synthesis of organic particles 3.1. General considerations Different organic nanoparticles have been synthesized in certain microemulsions. The active compounds are cholesterol, rhodiarome, and rhovanil (Fig. 16). The microemulsions used are AOT/heptane/water, Triton/decanol/water, and CTABr/ hexanol/water. The general preparation of these organic nanoparticles consists of the direct precipitation of the active compound in the aqueous cores of the microemulsion. After their preparation, nanoparticles are revealed with iodine vapor and observed with a transmission electron microscope (Philips EM301) [41]. The mechanism of the formation of nanoparticles has been proposed previously [3,29,30]. This consists of several stages. The solution of the active compound in an appropriate solvent penetrates inside the aqueous cores by crossing the interfacial film. The solvent certainly plays a role in the transport of the active compound inside the aqueous cores. The active Fig. 16. Structures of the active compounds.

13 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) Fig. 19. Variation of the cholesterol nanoparticle size as a function of the concentration. Fig. 17. Variation of the cholesterol nanoparticle size as a function of R at different concentrations in the AOT/heptane/water microemulsions. compound precipitates in the aqueous cores because of its insolubility in water, and the nuclei are thus formed. The soformed nuclei can grow because of the exchange of the active compound between the aqueous cores. Finally, the nanoparticles are stabilized by the surfactants Nanoparticles of cholesterol prepared in different microemulsions Fig. 17 represents the evolution of nanoparticle size as a function of R at different concentrations of the cholesterol solution in chloroform in the AOT/heptane/water microemulsion. It should be noted that the total amount of cholesterol added increases with increasing R, as the volume of chloroform solution is equal to that of the water in the microemulsion. The mean particle size is 3 6 nm and a minimum is observed for a certain R value. A hypothesis is the participation of water as a reaction medium in the precipitation reaction. In this case, for low values of R, the amount of water is not enough to enable the formation of an optimal number of nuclei. As the concentration of water increases, the number of nuclei increases and the size of the particles decreases. For a large amount of water, the number of nuclei is already optimal, and the size of the particles remains constant. In this case, nanoparticles of a certain size are thermodynamically stabilized, hence this size remains constant. In this hypothesis, the LaMer diagram is followed for small R values in the synthesis of the cholesterol particles. Another hypothesis to explain the presence of minimum stems from the direct participation of chloroform in the stabilization of the cholesterol nanoparticles. Indeed, the amount of chloroform increases with R value and the relative amount of chloroform in the solvation sphere could depend on the size of the particles. In order to check the veracity of this hypothesis, another series of experiments were carried out: the same amount of chloroform solution of cholesterol (0.3 ml) was added to the various microemulsions with different R values (Fig. 18). Fig. 18. Variation of the cholesterol nanoparticle size as a function of R at a fixed concentration (50 g/l). Fig. 20. Variation of the rhovanil nanoparticle size as a function of R.

14 366 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science (2006) In this case, the particle size is constant as a function of R. The size of the particles is hence controlled by the thermodynamic stabilization of the particles. But in this experiment, the amount of cholesterol also stays constant. Hence, the statement that the variation of the chloroform concentration is responsible for the minimum observed in Fig. 17 cannot be accepted as a proof. Fig. 19 shows the variation of nanoparticle size as a function of the concentration of cholesterol in the same microemulsion system. Contrary to the previous graph (Fig. 17), no minimum appears. The size of the particles is thus controlled by thermodynamic stabilization with surfactant molecules. Nanoparticles of cholesterol have also been synthesized in two other microemulsion systems: Triton/decanol/water and CTABr/hexanol/water. Similar experiments have been carried out. In these two cases, the nanoparticle size was independent of both the factor R and the concentration of the cholesterol solution. The particles are thus thermodynamically stabilized by the surfactants at certain favored sizes. The nanoparticles were stable for months, no precipitate appeared, and the final solutions were still limpid Nanoparticles of rhodiarome (or Rhovanil) prepared in the AOT/heptane/water microemulsion Influence of the factor R and the concentration of the active principal on the nanoparticle size An example is presented of the formation of nanoparticles of rhovanil. A solution of rhovanil in acetone (50 g/l) was used. Fig. 20 presents the variation of the mean diameter as a function of R. The nanoparticle size is relatively constant as a function of R and is between 4.5 and 6.2 nm for the four concentrations studied. It is the same for rhodiarome, where the nanoparticle size is independent of the factor R. The second parameter studied is the concentration of the active principal in the solvent. Fig. 21 shows a constant size between 4.5 and 7.0 nm. Fig. 22. Variation of the rhodiarome (R) nanoparticle size as a function of R (or [H 2 O]/[surfactant]) before and after the recovery of the nanoparticles (A: acetone used as carrier). In the two cases, a hypothesis can be made: the nanoparticle size is essentially determined by thermodynamic stabilization by the surfactant molecules at a certain size as it is dependent neither on R nor on the concentration Recovery of the nanoparticles stabilized by surfactants in a microemulsion and their transfer into an aqueous medium Some potential pharmaceutical applications can be considered if less toxic solvents are used. Thus, the residual solvents (heptane, for example) are evaporated under vacuum and the nanoparticles stabilized by surfactants are recovered. These particles are suspended in distilled water under ultrasound energy in order to obtain a limpid and stable dispersion. Fig. 22 shows the variation of the nanoparticle size as a function of R. The size lies between 5.7 and 6.3 nm and does not change after the recovery. The nanoparticles are thus thermodynamically stabilized by the surfactants. The change of the medium does not influence the nanoparticle size. Biocompatible microemulsions have also been employed in order to allow their use in drug delivery [42]. 4. Summary and conclusions Fig. 21. Variation of the nanoparticle size as a function of the concentration of rhovanil in acetone. This chapter has placed emphasis on the mechanism of formation of the particles. Two models have been proposed: the LaMer diagram and the thermodynamic stabilization of the particles. These two models are relatively simplistic. The LaMer diagram is based on the separation between the nucleation and the growth of the particles. It is consistent with the mechanism proposed by Lopez-Quintela and Rivas [43] for Fe nanoparticles obtained in AOT microemulsions using a stopped-flow technique and measuring the time-resolved small-angle X-ray scattering (SAXS) with synchrotron radiation. Nucleation implies an increase in the number of scattering centers (number of particles) for a given observation window, and, therefore, it gives an increase in the scattered intensity. On the contrary, the growth of particles is associated with a decrease of the scattered