Forming Nano Particles From Micro Emulsions Destree 2006

Science 123–126 (2006) 353–367www.elsevier.com/locate/cis

Advances in Colloid and Interface

Mechanism of formation of inorganic and organic nanoparticlesfrom 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 — Ni2B,Pt, Au, Pt–Au, AgX— and the synthesis of organic nanoparticles— cholesterol, rhovanil, rhodiarome — are systematically studied as a functionof the concentration of the precursor molecules, the size of the inner water cores, and the manner of mixing the various solutions. Two differentbehaviors are observed in the various systems. The first case shows a dependence of the nanoparticle size on the various physicochemicalparameters. Either a monotonous increase of the size or the presence of a minimum is observed as a function of the concentration of the precursormolecules. This case can be easily explained following the LaMer diagram, where the nucleation of the nanoparticles is separated from the particlegrowth. The second case does not show any dependence of the nanoparticle size on the physicochemical parameters. The size remains constant inall experimental conditions. The constant character of the size can be explained only by thermodynamic stabilization, where particles with acertain size are better stabilized. It should be emphasized that the size distribution is small in all the cases studied. Finally, the aging of thenanoparticles was also checked, especially for the organic nanoparticles. It is concluded that these particles remain stable for months in themicroemulsion.© 2006 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3542. Preparation of nanoparticles using microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

2.1. Description of a microemulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3552.2. Mechanism of synthesis of nanoparticles in microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3552.3. Preparation of monodisperse inorganic colloidal particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

2.3.1. Size of metal boride particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3562.3.2. Quantitative aspects of the formation of monodisperse colloidal particles . . . . . . . . . . . . . . . . . . . . . . 3572.3.3. Characterization of the Ni2B nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3602.3.4. Sizes of platinum and gold particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3602.3.5. Characterization of the silver halide nanoparticles in microemulsions . . . . . . . . . . . . . . . . . . . . . . . . 361

3. Synthesis of organic particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3643.1. General considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3643.2. Nanoparticles of cholesterol prepared in different microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3653.3. Nanoparticles of rhodiarome (or Rhovanil) prepared in the AOT/heptane/water microemulsion . . . . . . . . . . . . . . . 366

3.3.1. Influence of the factor R and the concentration of the active principal on the nanoparticle size. . . . . . . . . . . 3663.3.2. Recovery of the nanoparticles stabilized by surfactants in a microemulsion and their transfer into an aqueous

medium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3664. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

⁎ Corresponding author.E-mail address: [email protected] (J. B.Nagy).

0001-8686/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.cis.2006.05.022

mailto:[email protected]

http://dx.doi.org/10.1016/j.cis.2006.05.022

354 C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science 123–126 (2006) 353–367

1. Introduction

In the mid-seventies S. Friberg and the late F. Gault proposedan original method using microemulsions to prepare monodis-perse nanosized particles. These ideas were followed by a rapidincrease in both original and review papers [1–5].

A previous review dealt with a rather comprehensiveliterature survey up to 1985 [3] involving microemulsions,vesicles, polymer solutions, surfactants in water, sodium citratein water and general aqueous solutions as reaction media. It wasfollowed by another one which was based only on the use ofmicroemulsions [4].

Indeed, the above-mentioned reaction media represent only asmall part of a large variety of systems for colloidal particlepreparation. In particular, one could cite, among others, physicalvapor deposition, chemical vapor deposition, Langmuir–Blodgett films, polymer films [6–8], zeolite entrapped nano-particles [9,10], supported catalysts [11].

For a quantitative evaluation of the properties of colloidaldispersions, the monodisperse nature — uniform in size andshape — is a prerequisite. The quantum size effects areparticularly studied, since they lead to interesting mechanical,chemical, electrical, optical, magnetic, electro-optical andmagneto-optical properties, which are quite different fromthose reported for bulk materials [7,9,12,13].

The nanoparticles not only are of basic scientific interestbut have also resulted in important technological applications,such as catalysts, high-performance ceramic materials, micro-

Fig. 1. Microemulsion regions L2 in various ternary sys

electronic devices, and high-density magnetic recording [14–16].

The synthesis of nanoparticles in microemulsions allowsone to obtain monodisperse size of the particles and in somecases to control the size of the particles by variation of thesize of the microemulsion droplet radius and of the precursorsconcentrations.

Although the synthesis of inorganic particles in microemul-sions is already widespread, only polymeric nanoparticles havebeen synthesized in microemulsion media as far as the organicparticles are concerned. In this chapter, it will be shown that it isalso possible to synthesize organic particles by a directprecipitation reaction in the microemulsions.

We emphasize some of the fundamental aspects ofmonodisperse nanoparticle formation. Two models are pro-posed for the formation of the particles: the first is based onthe LaMer diagram, and the second is based on thethermodynamic stabilization of the particles. In the firstcase, the particle size varies as a function of either the size ofthe inner water cores or the precursor concentration; in thesecond case, the particle size is independent of theseparameters.

The monodisperse nanoparticles are characterized directlyin the microemulsion or after transferring them in anothermedium. First, the size of the nanoparticles is determined as afunction of various parameters. Their composition is analyzedby X-ray photoelectron spectroscopy (XPS) or energydispersive X-ray analysis (EDX). The specific surface area is

tems (see text for explanation of the abbreviations).

Fig. 3. Methods for preparation of monodisperse particles (X, Y, and Z are in wt.% of the various components).

355C. Destrée, J. B.Nagy / Advances in Colloid and Interface Science 123–126 (2006) 353–367

determined by the BET technique. The direct solvation isanalyzed by multinuclear magnetic resonance spectroscopy.

2. Preparation of nanoparticles using microemulsions

2.1. Description of a microemulsion

Awater-in-oil microemulsion is a thermodynamically stable,optically transparent dispersion of two immiscible liquidsstabilized by a surfactant. The important properties aregoverned mainly by the water–surfactant molar ratio (R=[H2O]/[surfactant]). This factor is linearly correlated with thesize of the water droplets.

The nanoparticles have been synthesized in differentmicroemulsion systems. Some of them are shown in Fig. 1.The anionic Aerosol-OT (AOT)/heptane/water system is one ofthe best characterized microemulsions [17,18]. The cationiccetyltrimethylammonium bromide (CTAB)/hexanol/water sys-tem contains hexanol, which forms the organic phase and playsthe role of cosurfactant [19]. The nonionic penta(ethyleneglycol)-dodecylether (PEGDE)/hexane/water system was stud-ied by Friberg and Lapczynska [20]. The reverse micellardroplets have a cylindrical shape in which the surfactantmolecules are parallel to each other, forming a bilayerimpregnated with water.

Triton X-100 [p-(1,1,3,3-tetramethylbutyl)phenyl-poly-ethoxyethanol)/decanol/water system has been characterizedby Ekwall and coworkers [21].

2.2. Mechanism of synthesis of nanoparticles in microemul-sions

The aqueous droplets continuously collide, coalesce, andbreak apart, resulting in a continuous exchange of solutioncontent. In fact, the half-life of the exchange reaction betweenthe droplets is of the order of 10−3–10−2 s [22,23].

Two models have been proposed to explain the variation ofthe size of the particles with the precursor concentration andwith the size of the aqueous droplets. The first is based on theLaMer diagram [24,25], which has been proposed to explainthe precipitation in an aqueous medium and thus is notspecific to the microemulsion. This diagram (Fig. 2) illustrates

Fig. 2. LaMer diagram.

the variation of the concentration with time during aprecipitation reaction and is based on the principle that thenucleation is the limiting step in the precipitation reaction. Inthe first step, the concentration increases continuously withincreasing time. As the concentration reaches the criticalsupersaturation value, nucleation occurs. This leads to adecrease of the concentration. Between the concentrationsCmax⁎ and Cmin⁎ the nucleation occurs. Later, the decrease ofthe concentration is due to the growth of the particles bydiffusion. This growth occurs until the concentration reachesthe solubility value.

This model has been applied to the microemulsion medium,i.e., that nucleation occurs in the first part of the reaction andlater only growth of the particles occurs. If this model isfollowed, the size of the particles will increase continuouslywith the concentration of the precursor or a minimum in thevariation of the size with the concentration can also be expected.This stems from the fact that the number of nuclei is constantand the increase of concentration leads to an increase of the sizeof the particles.

The second model is based on the thermodynamicstabilization of the particles. In this model the particles arethermodynamically stabilized by the surfactant. The size of theparticles stays constant when the precursor concentration and


the size of the aqueous droplets vary. The nucleation occurscontinuously during the nanoparticle formation.

These two models are limiting models: the LaMer diagramdoes not take into account the stabilization of the particles bythe surfactant, and the thermodynamic stabilization model doesnot take into account that the nucleation of the particles is moredifficult than the growth by diffusion.

2.3. Preparation of monodisperse inorganic colloidal particles

The different monodisperse nanoparticles were preparedfollowing either Scheme I or Scheme II of Fig. 3. We firstdiscuss the mechanism of formation of particles followingScheme I, where small amounts of aqueous solutions are addedto the initial microemulsion.

2.3.1. Size of metal boride particlesMonodisperse colloidal nickel boride and cobalt boride

particles were synthesized by reducing, with NaBH4, themetallic ions solubilized in the water cores of the microemul-sions. The NaBH4/MeCl2 ratio was held equal to 3 becauselarger particles were obtained for a lower value, and the particlesize remained constant above this ratio [2–4].

The composition of the particles was determined by XPS tobe, respectively, Ni2B and Co2B. In each case, the size of

Fig. 4. Variation of the average diameter (in nm) of the nickel boride parti

particles (2.5–7.0 nm) was much smaller than that obtained byreduction of Ni(II) or Co(II) in water (300–400 nm) or in ethanol(250–300 nm), and the size distribution was quite narrow(±0.5 nm).

Fig. 4 shows the dependence of the nickel boride particle sizeon the water content in the microemulsion as well as on the Ni(II) ion concentration. The average size of the particlesdecreases with decreasing size of the inner water core(decreasing water content), and a complex behavior is observedas a function of the Ni(II) ion concentration; a minimum isdetected at approximately 5×10−2 molal concentration. Theseobservations can be understood if one analyzes the nucleationand the growth processes of the particles.

To form a stable nucleus, a minimum number of atoms arerequired [26]. Thus, for nucleation several atoms must collide atthe same time, and the probability of this phenomenon is muchlower than the probability of collision between a single atomand an already formed nucleus. The latter phenomenon is calledthe growth process. At the very beginning of the reduction,nucleation occurs only in water cores that contain enough ionsto form a nucleus. At this moment, the micellar aggregates act as“reaction cages” where the nuclei are formed. On the otherhand, the microemulsion being dynamic, the water cores rapidlyrearrange. The other ions brought into contact with the existingnuclei essentially participate in their growth process. The latter

cles as a function of water content and Ni(II) ion molal concentration.

Table 1Important parameters for the formation of Ni2B colloidal particles

[Ni(II)]×102 (molal) rMa,b (nm) NM

b,c×10−22 nNi(II)a,d de (nm) Wt

f (g) Wg×1019 (g) Nnh×10−18

Nn

Nb;cM

Plk¼2 pk F i ×103

CTAB 24%/hexanol 60%/water 16%1.00 1.17 1.86 0.32 4.5 0.64 3.77 1.70 9.14×10−5 0.0415 2.22.50 1.32 1.29 1.17 4.2 1.60 3.06 5.23 4.05×10−4 0.3265 1.27.50 1.54 0.81 5.58 4.0 4.81 2.65 18.15 2.24×10−3 0.9752 2.310.00 1.57 0.77 7.82 5.1 6.41 4.87 11.67 1.52×10−3 0.9964 1.8

CTAB 30%/hexanol 50%/water 20%1.00 1.34 1.54 0.39 6.7 0.64 12.44 0.51 3.31×10−5 0.0589 0.62.50 1.48 1.16 1.30 4.9 1.60 4.87 3.28 2.83×10−4 0.3732 0.87.50 1.68 0.79 5.72 4.6 4.81 4.03 11.93 1.51×10−3 0.9780 1.510.00 1.72 0.74 8.14 4.9 6.41 4.87 13.16 1.78×10−3 0.9973 1.8aRadius of water core. bValues given for the system containing three fourths of the total amount of water.cNumber of water cores in 1 kg of solution. dNumber of Ni(II) ions per water core. eDiameter of Ni2B particles.fWt is calculated with M(Ni2B)=128.23 g/mol. gW is calculated with Mv(Ni2B)=7.9 g/cm3. hNumber of nuclei in 1 kg of solution.iCorrection factor from Nn ¼ FNM

Plk¼2 pk (see text).


being faster than nucleation, so no new nuclei are formed at thismoment. As all the nuclei are formed at the same time and growat the same rate, monodisperse particles are obtained. Insummary, the particle size depends on the number of nucleiformed at the very beginning of the reduction, and this numberis a function of the number of water cores, containing enoughions to form stable nuclei, that are reached by the reducing agentbefore the rearrangement of the system. However, thestabilization of the nuclei by surfactants is probably one ofthe most important factors in explaining the monodispersity ofthe particles.

2.3.2. Quantitative aspects of the formation of monodispersecolloidal particles

The first step in the determination of the essential parametersthat control particle size is a study of the distribution of the ionsin the microemulsion water cores.

By knowing the average radii of the microemulsion watercores (rM) and the total volume of water (VT) per kilogram ofmicroemulsion, one can calculate the number of water cores perkilogram of reverse micelles (NM), neglecting the solubility ofwater in the organic phase:

NM ¼ VT43kr

3M

ð1Þ

The parameter NM and the initial concentration of metal ionsexpressed in molality allow one to determine the averagenumber of ions per water core (nions):

nions ¼ ½ions� � 6:023� 1023

NMð2Þ

The ions are statistically distributed in the aggregates. Tocalculate this distribution, Poisson statistics is quite adequate[27]. This gives the probability (pk) of having kions per watercore (k is an integer taking the values 0, 1, 2, 3,…), provided theaverage number of ions per water core (λ=nions) is known:

pk ¼ kke−k

k!ð3Þ

The number of nuclei formed (Nn) when the ions solubilizedin 1 kg of solution are reduced is proportional to the number ofaggregates containing enough ions for nucleation. If theminimum number of ions required to obtain a nucleus is i,then Nn can be calculated from the relation

Nn ¼ FNM

Xlk¼i

pk ð4Þ

wherePlk¼i

pk is the probability of having i or more ions peraggregate; hence NM

Plk¼i

pk is the number of water corescontaining i or more ions. The F is a proportionality factortaking into account the proportion of aggregates reached bythe reducing agent before rearrangement of the system canoccur. In Eq. (4) we do not know the values of i and F butwe can calculate all the other parameters. Indeed, the numberof nuclei (Nn) is the number of particles prepared, and it isgiven by

Nn ¼ Wt

Wð5Þ

where Wt is the total weight of the particles prepared perkilogram of micellar solution, W is the weight of one particle,and

Wt ¼ ½ions� �Mparticle

xð6Þ

where Mparticle is the molecular weight of the particle and x isthe number of metal atoms per particle. The weight of oneparticle is given by

W ¼ 43k

d2

� �3

Mv;particle ð7Þ

where d is the diameter of the particle measured by electronmicroscopy and Mv,particle is the volumetric mass of theparticle.

All the experimental and computed data are reported in Table1 for Ni2B particles.

Fig. 6. Variation in the probability of having k Ni(II) ions per aggregate for theCTAB 18%/hexanol 70%/H2O 12% microemulsion [Ni(II)] (molal): (a)1×10−2, (b) 5×10−2, (c) 7.5×10−2.


The diameter of the particles is systematically higher than thediameter of the inner water cores. For all the particlessynthesized, we calculated the proportionality factor F bysystematically varying the value of the minimum number ofions required to form a nucleus (i). If i=1 or i>2, the values offactor F vary considerably (not shown). However, if i=2, itsvalues are reasonably constant (see Table 1). The order ofmagnitude of the factor F is always 10−3. This means that at thevery beginning of the reduction, i.e., when the nuclei areformed, only one aggregate per thousand leads to the formationof metal boride particles.

There is another indication that the nucleation occurs at thevery beginning of the reduction. Indeed, the average radii ofthe water cores used for the calculation of the formationparameters of colloidal particles are measured for the systemcontaining only three fourths of the total amount of water,which is the composition of the solution before the addition ofthe reducing agent. If the final composition is used, however,no clear-cut correlation can be obtained between the numberof nanoparticles and either the Ni(II) concentration or thewater content.

The order of magnitude of the factor F is constant, but itsvalue decreases with increasing water content in the micro-emulsion (see Table 1). This phenomenon can be easilyunderstood because the rearrangement rate of the microemul-sion decreases with the water amount and hence the number ofaggregates reached by the reducing agent before rearrangementdecreases.

As the number of nuclei formed decreases at a constantconcentration of precursor ions, the particle size increases withthe water content in the system.

The diameter of the particles is plotted as a function ofmicellar droplet concentration in Fig. 5. The values of theparticle size are those obtained by interpolation of previousresults in the presence of 0.161 molal aqueous metal ion for theCTAB/hexanol/water systems. Particles prepared in the AOT/

Fig. 5. Size of nanoparticles prepared in various microemulsions as a function ofmicellar droplet concentration.

heptane/water system at a much lower droplet concentration areincluded for comparison. The Co2B particles were also obtainedin Triton X-100/decanol/water systems.

The size of the particles decreases linearly with the micellardroplet concentration. This is a strong indication that the finalsize obtained for the particles is governed by the presence ofreverse micellar aggregates. Indeed, if initial nucleation takesplace in the water cores, then nucleation should be related to themicellar droplet concentration of the system. Further, the greaterthe number of micellar droplets, the greater the number ofnucleation sites possible (the aqueous metal ion concentrationbeing obviously maintained constant). The results of Table 1also allow us to explain the minimum in the particle size as afunction of the concentration of ions (see Fig. 4).

For a constant microemulsion composition, at low ionconcentration, only a few water cores contain the minimumnumber of ions (two) required to form a nucleus; hence, only afew nuclei are formed at the very beginning of the reduction,and the metal boride particles are relatively large. When the ion

Fig. 7. Variation in (●) the number of nuclei formed per aggregate (Nn: numberof nuclei; NM: number of water cores) and (○) the probability of having two ormore ions per aggregate as a function of Ni(II) concentration in themicroemulsion CTAB 18%/hexanol 70%/water 12%.

Fig. 8. Model of the preparation of colloidal Ni2B particles from a water-in-oilmicroemulsion.

Table 2Specific surface area (SBET) of the Ni2B nanoparticles prepared in the 18.0%CTAB/70.0% hexanol/12.0% H2O microemulsion

H2Oa Before washing After washing

hyd(%)

d b

(nm)Spart

c

(m2)Bads

d

(%)Bads

e

(%)SBET

f

(m2/g)Spart′ g

(m2)Spart/Spart′

SBET(m2/g)

Spart′(m2)

Spart/Spart′

15 5.5 44 85 15 60.6 19.4 2.3 124.2 39.9 1.1

a % H2O hydrating the Ni2B nanoparticles; [Ni(II)]=5×10−2 molal.b Diameter of the Ni2B 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)]=5×10−2 molal.d Percent boron adsorbed on the particles before washing.e Percent boron adsorbed on the particles after washing.f SBET: specific surface area of nanoparticles determined by N2 adsorption.g Spart′ =WSBET whereW is the total weight of the particles synthesized in 100 g

of microemulsion.


concentration increases, the distribution of precursor ions in themicroemulsion is very different (Fig. 6), and the number ofnuclei obtained by reduction increases faster than the totalnumber of ions (Fig. 7). This results in a decrease in the particlesize. When more than 80% of the water cores contain two ormore ions, the number of nuclei formed remains quasi-constantwith increasing ion concentration. Hence the size of theparticles increases again.

Fig. 4 also shows the particle size as a function of watercontent in the microemulsion for different Ni(II) concentrations.An increase in the average diameter is observed with increasingproportion of water. The decrease in the number of micellaraggregates (NM) with water (Table 1) is accompanied by anincrease in their size. For the same Ni(II) concentration withrespect to water (i.e., for the same probability of collisionbetween the ions in the same water core), the total number ofnuclei formed in the early stage of the reduction decreases withincreasing water concentration, and more ions can participate inthe 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 shownif the size of the particles is plotted as a function of micellardroplet concentration (Fig. 5).

For most of the systems studied, a monotonous decrease inthe size with increasing NM is observed. These results reinforcethe hypothesis leading to the computation of the number ofnuclei and underline the importance of the water cores asreaction cages. The F value for nickel boride is equal to ca.2×10−3.

The essential parameters for the formation of monodispersecolloidal particles are thus quantified. We have shown thattwo metal atoms are required to form a stable nucleus andthat nucleation occurs only in the aggregates that are reachedby the reducing agent before rearrangement of the system canoccur (1 per 1000 aggregates). Fig. 8 illustrates quite well themechanism of reduction in a water-in-oil microemulsion.

After fast diffusion of the reducing agent, nucleation occursin the water droplets where the preceding conditions aresatisfied. The nucleus is stabilized by the adsorbed surfactantmolecules. The growth of the particles requires an exchangebetween different water cores. Finally, surfactant-protectedmonodisperse particles are formed that can be used directly orby being deposited on a support.

In the treatment just discussed, the nucleation step couldnot yet be clearly described. The model is based on thepresence of discrete water pools in the microemulsion,whereas conductivity measurements showed that percolationalready occurred in these systems, favoring the exchangebetween water pool contents [28]. More experiments areneeded to determine the formation of the first nuclei using fastkinetics measurements.

Nevertheless, the stabilization of the nuclei by the surfactantmolecules at the interface could play a definite role incontrolling their number formed at the very beginning of thereduction. The method of addition of the reducing agent in theaqueous solution is indeed very important, because if higheramounts of microemulsion systems are used, larger particles areobtained.

Table 3Analysis of Pt particle size as a function of the number of nuclei Nn

[K2PtCl4](molal vs. H2O)

nPtCl42− a, b d c(nm) Wt ×10

3

(g) dW×1019

(g) d, eNn×10

−16 f

0.001 0.54 1.5±0.3 0.98 0.38 2.60.01 5.4 2.5±0.3 9.8 1.75 5.60.05 27.0 5.0±0.3 49.0 14 3.50.1 54.0 9.0±1.0 98.0 81.9 1.20.3 162.0 13.0±1.5 294.0 247 1.2

a Number of PtCl42− ions per inner water core.

b NM (number of inner water cores)=5.56×1018 per kg solution; rM=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/cm3.f Number of nuclei per kg solution.

Fig. 9. Variation of the average Pt particle diameter as a function of K2PtCl4concentration with respect to water prepared according to Scheme I of Fig. 3.


2.3.3. Characterization of the Ni2B nanoparticlesThe nature of the boride particles was determined by XPS

and EDX. Table 2 shows the specific surface area of the nickelboride particles. The composition of the nanoparticles wasdetermined by EDX measurements and it corresponds to Ni2Bstoichiometry.

The as-prepared boride nanoparticles adsorb large amountsof BO2

− and CTAB+ ions. The amount of adsorbed BO2− ions

was measured using 11B nuclear magnetic resonance (NMR). Itwas determined as the difference between the total amount ofNaBH4 added and the final concentration of boron in themicroemulsion after precipitation of the nanoparticles.

The amount of boron adsorbed on the Ni2B nanoparticles isabout 85% of the total boron present in the system as BO2

− ions.The borate ions are eliminated by two successive washings

with an aqueous solution of HCl and three successive washingswith distilled water. After washing, the remaining adsorbedboron is ca. 15%.

The precipitated particles adsorb a non-negligible amount ofwater from the microemulsion inner water core. This amountcan rise to about 15% of the initial water present in themicroemulsion (Table 2).

The specific surface area of the particles was determined onboth the as-prepared and the washed nanoparticles. In Table 2the SBET values are compared, the total surface of thenanoparticles obtained in 100 g of microemulsion anddetermined from the diameter of the particles (Spart′ ), and thetotal surface of the particles (Spart) obtained from BETmeasurements. It is seen that the unwashed particles presenta surface 2.3 times smaller than the theoretical surfaces (Spart/Spart′ ). After washing, the surface is almost clean for the Ni2Bparticles, because the Spart/Spart′ ratio is close to 1.

2.3.4. Sizes of platinum and gold particles

2.3.4.1. CTAB–hexanol–water microemulsion. Colloidal par-ticles of Pt and Au were prepared following Scheme I of Fig. 3.The monodisperse Pt particles prepared from H2PtCl6 dissolvedin the CTAB/hexanol/water microemulsion had an averagediameter of 4.0±0.5 nm, and their size was not dependent on theH2PtCl6 concentration (5×10−3–2×10−2 molal with respect to

water) [29]. The aqueous solution of hydrazine containing a 10-fold molar excess of hydrazine with respect to H2PtCl6 had aninitial pH of 10. The metal particle precursor is soluble in boththe dispersed inner water core and the continuous (or hexanol)phases. If it is assumed that the nucleation occurs in bothphases, the particle size is dependent only on its stabilization bythe adsorbed surfactant molecules [3,29,30].

It is interesting to note that particles of a similar size wereobtained, independently of water and H2PtCl6 concentrations,from the AOT/heptane/water microemulsion [31].

If K2PtCl4 is used instead as the particle precursor (for thesame hydrazine to K2PtCl4 ratio), a complex behavior isobserved as a function of pH. At low pH values (1<pH<4), noPt particles could be obtained. At 5<pH<8, dispersed Ptparticles were formed, but the reduction was not complete evenafter 24 h of reaction. For high pH values (pH>9), completereduction of the Pt salt occurred, but the particles thus obtainedwere aggregated.

It is thus clear that the surface charge does influence theaggregation of the metal particles. In addition, the adsorption ofthe surfactant molecules, also pH dependent, can greatlyinfluence the particle aggregation.

2.3.4.2. PEGDE–hexane–water microemulsion. Colloidal Ptparticles 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 PEGDE9.5%/hexane 90%/water 0.5%. Only K2PtCl4 was tested as aprecursor salt, however, because it is insoluble in the organicmedium. Table 3 and Fig. 9 show the variation in the size of thePt particles obtained following Scheme I as a function of initialK2PtCl4 concentration. The particle diameter increases monot-onously with increasing K2PtCl4 concentration and approachesa plateau at high concentration. This behavior seems to bedifferent from those previously observed for the Pt particlesusing H2PtCl6 [31,32] and for the Ni2B particles. In the firstcase, a constant particle size was obtained irrespective of theinitial H2PtCl6 concentration, and in the second case a minimumwas observed in the particle size (Ni2B) versus NiCl2concentration curve.

Table 5Average diameter, d, of Au particles showing the bidispersion in PEGDE/hexane/water microemulsions

[AuCl3] (molal vs. H2O) d (nm) d (nm)

(a) PEGDE 9.5%/hexane 90%/water 0.5%0.001 – 2.9±0.40.01 – 3.0±0.30.05 8.2±1.2 3.6±0.40.1 11.0±1.9 3.8±0.50.3 13.9±2.5 4.4±0.5

(b) DOBANOL 9.5%/hexane 90%/water 0.5%0.001 3.3±0.50.01 7.1±1.10.05 9.7±1.20.1 11.5±1.40.3 13.2±1.6


For low initial K2PtCl4 concentration (up to 0.01 molal withrespect to water), Nn increases as a function of Pt concentration.This behavior was observed earlier for the case of Ni2Bparticles. However, for higher K2PtCl4 concentrations, the Nn

value decreases, leading to larger particles. The probablenucleus is a surfactant-stabilized Pt atom that is able to form thefinal Pt particle [33].

If the particles are prepared following Scheme II, where thetwo microemulsions containing the precursor K2PtCl4 and thereducing agent N2H4, are mixed together, smaller sizes areobtained. Indeed, the Pt particles prepared from the micro-emulsion with [K2PtCl4]=0.1 molal with respect to water havea 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 thevariation of the average diameter of the Pt particles as a functionof the concentration of K2PtCl4 prepared by Scheme I.

The larger size of the Pt particles obtained by the method ofScheme I can be explained in a first approximation by thediffusion of the aqueous solution through the organic phasebeing slower than the exchange between the water cores.Although in the PEGDE/hexane/water microemulsion noseparate spherical droplets are present, the water is probablythe dispersed phase in the microemulsion. The structure of themicroemulsion is better represented as a lamellar aggregatewhere the surfactant molecules are associated head to headalong a cylinder.

2.3.4.3. DOBANOL–hexane–water microemulsion. Particlesof 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 thePEGDE system [36].

Table 4 illustrates the influence of DOBANOL and PEGDEsurfactants. Within experimental errors, the same diameter isobtained for both systems.

The Au particles were obtained from the precursor AuCl3.In the PEGDE/hexane/water microemulsion at low precursorconcentrations (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 (about10 nm diameter) particles were formed at higher precursorconcentrations.

Table 4Average diameter, d, of Pt particles synthesized from PEGDE 9.5%/hexane90%/water 0.5% and DOBANOL 9.5%/hexane 90%/water 0.5%microemulsions

[K2PtCl4] (molal vs. H2O) d (nm)

PEGDE DOBANOL

0.001 1.9±0.2 2.3±0.30.01 2.2±0.3 2.7±0.40.05 2.6±0.3 2.7±0.40.1 2.8±0.3 2.8±0.40.3 3.8±0.4 3.9±0.4

In the DOBANOL/hexane/water microemulsion only onetype of particle is obtained, the size of which increases withincreasing precursor concentration (Table 5b).

The Pt–Au particles were prepared in both microemulsionsystems (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 ofthe precursor salts. The large particles are clearly formed byaggregation of the small particles. The nanoparticles are truemixed Pt–Au particles, as was shown by scanning transmissionelectron microscopy (STEM)/EDX measurements.

2.3.5. Characterization of the silver halide nanoparticles inmicroemulsions

The silver halide nanoparticles were prepared followingScheme II of Fig. 3, where the precursor salts AgNO3 andNaX were dissolved in AOT/heptane/water microemulsions ofsimilar compositions. In the numerous studies concerning thesynthesis of nanoparticles in microemulsion media, thelocation of water after the nanoparticle synthesis has neverbeen determined. Two models can be proposed (Fig. 11). In

ig. 10. Variation in gold particle diameter as a function of precursor AuCl3oncentration versus water synthesized in PEGDE 9.5%/hexane 90%/water.5% microemulsion. The presence of larger particles shows particleggregation.

Fc0a

Table 6Average diameter, d, of Pt–Au particles as a function of Pt mole fraction xshowing the bidispersion in both systemsa

x d (nm) d (nm)

(a) PEGDE 9.5%/hexane 90%/water 0.5%0.16 9.5±1.8 3.4±0.40.33 11.3±2.0 3.2±0.40.50 10.7±1.9 3.7±0.50.66 13.5±2.4 2.9±0.30.80 9.2±1.3 3.4±0.4

(b) DOBANOL 9.5%/hexane 90%/water 0.5%0.16 11.2±2.2 2.6±0.40.33 12.5±2.5 2.9±0.40.50 14.6±3.2 2.6±0.40.66 12.7±2.5 2.7±0.50.80 12.1±2.6 2.8±0.4a[AuCl3]+ [K2PtCl4]=0.1 molal vs. H2O.


the first one the particles are surrounded by a layer of water,and in the second the surfactant molecules (the AOT) aredirectly adsorbed onto the particles and only a small amountof water molecules is present.

In order to discriminate between these two models, 2H NMRmeasurements on deuterated water in microemulsions havebeen carried out. Two NMR lines were observed in the 2H NMRspectra (Fig. 12) for the various microemulsions withoutparticles of silver bromide.

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 adsorbedonto the particle.

Fig. 12. NMR spectra of the deuterated water in the AOT/heptane/watermicroemulsion for R=3.1.

If the same spectrum is taken for a very low R value, such asR=0.5 (Fig. 13), three NMR lines are observed. These lines arenot due to the presence of impurities — in fact, their intensitydoes not decrease as the amount of water decreases — so theselines stem from different types of water molecules. This isillustrated by the measurements of their relaxation times T1. Infact, for R=1 the following three relaxation times T1 wereobtained at 273 K: 321 ms for the broader line, 804 ms for theline situated at −3.50 ppm, and 1087 ms for the line situated at−3.95 ppm. As the variation of the relaxation time withtemperature indicates that we are in a region where therelaxation time increases with the decrease of temperature,these two lines correspond to water molecules that are lessmobile and, therefore, more in contact with the surfactantmolecules.

Generally, three kinds of water may exist in a microemulsionmedium: “bulk” water in the center of the water core, “bound”water that interacts with the hydrophilic part of the surfactantmolecule, and “trapped” water that is trapped at the interface inthe form of monomers or dimers [34]. Bulk water molecules arenormally not present for R values below 6–10, where all thewater 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=[H2O]/[AOT] is 3.1, only two kindsof water molecules would be expected. Therefore, it is assumedthat the two NMR lines observed here correspond to boundwater and trapped water. In order to check this assumption, thesame experiment was carried out for higher R values. The

Fig. 13. NMR spectrum of deuterated water in the AOT/heptane/watermicroemulsion for R=0.5 at T=297 K.

Fig. 14. (a) Variation of the 2H chemical shift as a function of the R factor, (b)Variation of the line width as a function of the R factor.


chemical shift increases with the R value until it reachesapproximately that of the pure deuterated water (used asreference) while the line width at half-height decreases with R(Fig. 14).

Such variation has already been observed [35] and is theresult of a fast exchange (faster than 2×1010 s−1) between thebulk water and the bound water. At low R values, the observedchemical shift comes from the variation of the number ofhydrogen bonds in which the water molecules are involved. Infact, the water molecules adsorbed at the interface (or solvatingthe Na+ ions) form fewer hydrogen bonds, provoking a high-field chemical shift. The smaller number of hydrogen bonds haspreviously been shown by Wong et al. [36] using 1H NMRexperiments.

Furthermore, if the NMR spectra are recorded at lowertemperatures, the NMR line corresponding to the bound waterdecreases due to the freezing of this kind of water (thebandwidth becomes too large to be detectable) (Fig. 12). In fact,the freezing point of bound water seems to be about 243 Kinside the inverted micelles. This corresponds to the decrease ofthe freezing point of water with the size of the droplet; forexample, the freezing point of water in a droplet correspondingto R=4.5 in AOT/water/2,2,4-trimethylpentane is at around241 K [37]. On the other hand, the line corresponding to thetrapped water shows no freezing and its intensity remains quasi-constant.

In order to distinguish between the two models of AgBrstabilization (see earlier), the NMR experiments mentionedhave also been carried out in presence of silver bromidenanoparticles. As the only difference between the two

experiments is the presence of silver bromide particles, allobserved differences must be due to the particles. In thepresence of these particles, the quantity of trapped water islarger, as shown by a comparison of spectra in the presence andin the absence of nanoparticles (Fig. 15). The total intensity isalso higher in presence of silver bromide particles, alsostemming from the greater importance of the trapped water. Infact, this water freezes at a lower temperature. Furthermore, notall the water cores of the microemulsion are occupied by aparticle, only 1 water core out of 1.3×104 is occupied by aparticle. Hence, if the microemulsion structure stayed the same,with the same number of water molecules in each water core, no

Fig. 15. NMR spectra of the deuterated water in the AOT/heptane/watermicroemulsion (full line) and in presence of AgBr particles (dotted line) at263 K.


influence on the NMR spectra could be observed upon additionof AgBr.

The higher amount of trapped water is in favor of model (b),where the particles are in closer contact with the interfaciallayer. However, the NMR line of the adsorbed water couldoverlap that of the trapped water. In order to check thishypothesis, the number of water molecules per AOT wascalculated. The spectra in Fig. 15 have been decomposed in twobands corresponding, respectively, to the bound water and to thetrapped water. The difference in intensities of the two NMRlines corresponding to the trapped water in the spectra withoutand with AgBr particles gives the amount of water trapped oradsorbed on the particles. The number of AOT molecules perparticle has been calculated using a spherical surface of 4.6 nmdiameter and a surface area of 0.41 nm2 for the polar part of theAOT molecule [38]. It has been computed that if the whole lineintensity corresponded to the trapped water there would be 2000water molecules per AOT molecule. As the trapped water isconsidered to be in the form of a monomer or a dimer, this valueis too high to correspond only to water molecules trapped in theinterface. Hence, it has to be assumed that the additional watermolecules so computed are adsorbed on the AgBr particles andthe NMR lines of the trapped water and the adsorbed wateroverlap.

If it is assumed that all these additional water molecules areadsorbed on the particles, the number of water monolayers canbe calculated by using the van der Waals radius of a watermolecule. Approximately 1000 monolayers of water can be

Fig. 16. Structures of the

formed around the nanoparticles. These two arguments, theobservation of an NMR line corresponding to the adsorbedwater molecules and the estimation of the number of watermonolayers, are in favor of model (a). Hence, this model will beadopted.

In order to quantify by another method the amount of wateradsorbed on the nanoparticles, a microemulsion in which theparticles had sedimented was also examined. This microemul-sion was obtained by adsorption of pseudoisocyanine on theparticles. This dye causes a rapid sedimentation of the particles[39], and a 2H NMR spectrum was taken after sedimentation ofall the particles. From this spectrum, it was established that 68%of the water was adsorbed on the particles. The number of watermonolayers formed around the particles was calculated and avalue of about 4600 monolayers of water was obtained. Thisvalue is too large and physically impossible; in fact, the radiusof the corresponding water core should be 2.6 μm. These watercores should scatter the light, and as the colloidal suspension islimpid, the number of water molecules bound to the silverhalide particles must be overestimated in this approach. Such alarge amount of water in the precipitate can be explained only ifthe sedimented particles form a sort of gel where a large amountof water is required. This gelation was previously shown in thecase of Ni2B nanoparticles prepared from CTAB/n-hexanol/water [40] microemulsion. This high amount of adsorbed watermolecules is also in favor of model (a).

3. Synthesis of organic particles

3.1. General considerations

Different organic nanoparticles have been synthesized incertain microemulsions. The active compounds are cholesterol,rhodiarome, and rhovanil (Fig. 16). The microemulsions usedare AOT/heptane/water, Triton/decanol/water, and CTABr/hexanol/water.

The general preparation of these organic nanoparticlesconsists of the direct precipitation of the active compound inthe aqueous cores of the microemulsion. After their preparation,nanoparticles are revealed with iodine vapor and observed witha transmission electron microscope (Philips EM301) [41].

The mechanism of the formation of nanoparticles has beenproposed previously [3,29,30]. This consists of several stages.The solution of the active compound in an appropriate solventpenetrates inside the aqueous cores by crossing the interfacialfilm. The solvent certainly plays a role in the transport of theactive compound inside the aqueous cores. The active

active compounds.

Fig. 19. Variation of the cholesterol nanoparticle size as a function of theconcentration.

Fig. 17. Variation of the cholesterol nanoparticle size as a function of R atdifferent concentrations in the AOT/heptane/water microemulsions.


compound precipitates in the aqueous cores because of itsinsolubility in water, and the nuclei are thus formed. The so-formed nuclei can grow because of the exchange of the activecompound between the aqueous cores. Finally, the nanoparti-cles are stabilized by the surfactants.

3.2. Nanoparticles of cholesterol prepared in differentmicroemulsions

Fig. 17 represents the evolution of nanoparticle size as afunction of R at different concentrations of the cholesterol solu-tion in chloroform in the AOT/heptane/water microemulsion.

It should be noted that the total amount of cholesterol addedincreases with increasing R, as the volume of chloroformsolution is equal to that of the water in the microemulsion. Themean particle size is 3–6 nm and a minimum is observed for a

Fig. 18. Variation of the cholesterol nanoparticle size as a function of R at a fixedconcentration (50 g/L).

certain R value. A hypothesis is the participation of water asa reaction medium in the precipitation reaction. In this case,for low values of R, the amount of water is not enough toenable the formation of an optimal number of nuclei. As theconcentration of water increases, the number of nucleiincreases and the size of the particles decreases. For a largeamount of water, the number of nuclei is already optimal, andthe size of the particles remains constant. In this case,nanoparticles of a certain size are thermodynamicallystabilized, hence this size remains constant. In this hypothesis,the LaMer diagram is followed for small R values in thesynthesis of the cholesterol particles. Another hypothesis toexplain the presence of minimum stems from the directparticipation of chloroform in the stabilization of thecholesterol nanoparticles. Indeed, the amount of chloroformincreases with R value and the relative amount of chloroformin the solvation sphere could depend on the size of theparticles. In order to check the veracity of this hypothesis,another series of experiments were carried out: the sameamount of chloroform solution of cholesterol (0.3 mL) wasadded to the various microemulsions with different R values(Fig. 18).

Fig. 20. Variation of the rhovanil nanoparticle size as a function of R.

Fig. 22. Variation of the rhodiarome (R) nanoparticle size as a function of R (or[H2O]/[surfactant]) before and after the recovery of the nanoparticles (A:acetone used as carrier).


In this case, the particle size is constant as a function of R.The size of the particles is hence controlled by the thermody-namic stabilization of the particles. But in this experiment, theamount of cholesterol also stays constant. Hence, the statementthat the variation of the chloroform concentration is responsiblefor the minimum observed in Fig. 17 cannot be accepted as aproof.

Fig. 19 shows the variation of nanoparticle size as a functionof the concentration of cholesterol in the same microemulsionsystem. Contrary to the previous graph (Fig. 17), no minimumappears. The size of the particles is thus controlled bythermodynamic stabilization with surfactant molecules.

Nanoparticles of cholesterol have also been synthesized intwo other microemulsion systems: Triton/decanol/water andCTABr/hexanol/water. Similar experiments have been carriedout. In these two cases, the nanoparticle size was independent ofboth the factor R and the concentration of the cholesterolsolution. The particles are thus thermodynamically stabilized bythe surfactants at certain favored sizes.

The nanoparticles were stable for months, no precipitateappeared, and the final solutions were still limpid.

3.3. Nanoparticles of rhodiarome (or Rhovanil) prepared in theAOT/heptane/water microemulsion

3.3.1. Influence of the factor R and the concentration of theactive principal on the nanoparticle size

An example is presented of the formation of nanoparticles ofrhovanil. A solution of rhovanil in acetone (50 g/L) was used.Fig. 20 presents the variation of the mean diameter as a functionof R.

The nanoparticle size is relatively constant as a function ofR and is between 4.5 and 6.2 nm for the four concentrationsstudied. It is the same for rhodiarome, where the nanoparticlesize is independent of the factor R. The second parameterstudied is the concentration of the active principal in thesolvent. Fig. 21 shows a constant size between 4.5 and7.0 nm.

Fig. 21. Variation of the nanoparticle size as a function of the concentration ofrhovanil in acetone.

In the two cases, a hypothesis can be made: the nanoparticlesize is essentially determined by thermodynamic stabilizationby the surfactant molecules at a certain size as it is dependentneither on R nor on the concentration.

3.3.2. Recovery of the nanoparticles stabilized by surfactants ina microemulsion and their transfer into an aqueous

mediumSome potential pharmaceutical applications can be consid-

ered if less toxic solvents are used. Thus, the residual solvents(heptane, for example) are evaporated under vacuum and thenanoparticles stabilized by surfactants are recovered. Theseparticles are suspended in distilled water under ultrasoundenergy in order to obtain a limpid and stable dispersion. Fig. 22shows the variation of the nanoparticle size as a function of R.

The size lies between 5.7 and 6.3 nm and does not changeafter the recovery. The nanoparticles are thus thermodynami-cally stabilized by the surfactants. The change of the mediumdoes not influence the nanoparticle size.

Biocompatible microemulsions have also been employed inorder to allow their use in drug delivery [42].

4. Summary and conclusions

This chapter has placed emphasis on the mechanism offormation of the particles. Two models have been proposed: theLaMer diagram and the thermodynamic stabilization of theparticles. These two models are relatively simplistic. The LaMerdiagram is based on the separation between the nucleation andthe growth of the particles. It is consistent with the mechanismproposed by Lopez-Quintela and Rivas [43] for Fe nanoparti-cles obtained in AOT microemulsions using a stopped-flowtechnique and measuring the time-resolved small-angle X-rayscattering (SAXS) with synchrotron radiation. Nucleationimplies an increase in the number of scattering centers (numberof particles) for a given observation window, and, therefore, itgives an increase in the scattered intensity. On the contrary, thegrowth of particles is associated with a decrease of the scattered


intensity because the observation window corresponds to thediffraction of smaller particles, which are disappearing duringthe growth process. The presence of this maximum (althoughnot well defined) has also been spectrophotometricallydetected by Towey et al. [44] for the formation of CdSnanoparticles in AOT microemulsion. This is an illustration ofthe LaMer diagram, as according to this diagram thenucleation occurs only in the beginning of the reaction.Theoretical calculation has been carried out by Tojo et al. [45]involving the study of the influence of the concentration andthe film flexibility and of the kinetic exchange constantbetween the droplets using the difference between thenucleation and the growth of the particles. The thermodynamicstabilization is less documented in the literature, but anexample shows the formation of secondary monodispersespherical particles by coagulation of the primary particles [46].

Whether the reaction follows the LaMer diagram or thethermodynamic stabilization of the particles depends on themicroemulsion diagram used and on the nature of the particlessynthesized. As an example, the synthesis of AgBr particlesfollows the LaMer diagram in the AOT/heptane/water micro-emulsion system, but it follows the thermodynamic stabilizationof the particles for the AOT/p-xylene/water system. Thedifference between the two systems can arise from theadsorption of the p-xylene molecule on the particles of AgBr.In fact, the adsorption of p-xylene on the AgBr particles hasbeen shown in the study of the adsorption of pseudoisocyanineon these particles [47]. In the CTAB/hexanol/water microemul-sion, the formation of Ni2B particles follows the LaMerdiagram, but the formation of Pt follows the thermodynamicstabilization of the particles. The difference could stem from thedifferent adsorption of the surfactant on the particles. Mixedparticles have also been synthesized. The particles are nothomogeneous in composition, and the size of these particlesdoes not vary linearly with their composition.

In the case of the organic nanoparticles, all the particles seemto follow the thermodynamic stabilization (with the exceptionof the cholesterol synthesized in the AOT/heptane/microemul-sion). This can be due to a specific interaction of the surfactantwith the particles.

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