Microemulsions as microreactors: a Monte Carlo simulation on the synthesis of particles

doi:10.1016/S0022-3093(98)00572-9

Journal of Non-Crystalline Solids

Volumes 235–237, 2 August 1998, Pages 688-691

https://doi.org/10.1016/S0022-3093(98)00572-9 Get rights and content

Abstract

The formation of particles in microemulsions is studied by Monte Carlo computer simulation. Two different types of particle size distributions can be obtained (unimodal and bimodal), depending on surfactant film flexibility and concentration. A bimodal particle size distribution is obtained only if two conditions are fulfilled: Growth by autocatalysis is the predominant process during nanoparticle formation and both growth processes (autocatalysis and ripening) take place at different times. The results allow us to explain the appearance of bimodality in recent experimental results.

Introduction

Nanometersized particles of metal and alloys exhibit properties which differ from the bulk [1]. The possibility of preparing ultrafine particles of homogeneous morphology and size could be of great interest in technological [2](magnetic recording) and theoretical [3](quantum size effect) fields. In the mid-seventies S. Friberg and later F. Gault proposed an original method using microemulsions to prepare monodisperse nanometersized particles. Microemulsions can be used as microreactors to carry out chemical reactions in confined geometries [4]. Although progress in this field has been extremely important, much has yet to be done in order to understand the properties of monodisperse particle systems, and also to obtain better control of the nanostructure of these materials.

A critical parameter in the preparation of the microemulsion is the flexibility of the film containing a microdroplet, which depends on the surfactant and oil used. It was shown experimentally [5]and by simulation [6]that the mean size and the monodispersity of particle size depend on film flexibility. But recent experimental results [7]show that film flexibility also affects the type of particle size distribution. This paper analyses the effect of film flexibility on the type of particle size distribution. Our Monte Carlo simulation study allows us to explain why a bimodal particle size distribution is found when the largest concentrations are used.

Section snippets

Simulation Procedure

The computer simulation of the formation of nanoparticles in microemulsions was carried out using the model previously reported 6, 8. Briefly, each simulation began with 1000 microemulsion droplets randomly located on a two-dimensional square lattice (volume fraction ϕ=10%). Droplets diffused on the lattice by performing random walks to nearest neighbour sites, subject to the exclusion principle (cyclic boundary conditions). Five hundred droplets carried c molecules of A and 500 c molecules of

Results

Fig. 1 shows the variation of the particle sizes as the concentration increases, using two different surfactants, i.e. two different values of film flexibility. Symbols in Fig. 1(A) show the variation of gold particle diameter as a function of precursor AuCl₃ concentration synthesized in DOBANOL 9.5%–hexane 90%–water 0.5% microemulsion (data obtained from Ref. [7]). In the DOBANOL–hexane–water microemulsion, particles of one size are obtained (unimodal particle size distribution). Size

Discussion

It is well-known that particle size increases monotonicly as precursor concentration increases and approaches a plateau at larger concentrations. This result can be observed in Fig. 1. The most common type of particle size distribution is unimodal, but the fact that the same reaction using different surfactants leads to different types of distributions is a recent result, which requires explanation. The simulation results allow us to conclude that bimodal or unimodal particle size distributions

Conclusions

It has been shown that a bimodal particle size distribution is obtained at high concentration and by using rigid films. All other cases give rise to unimodal distributions.

Monte Carlo simulations allow us to explain recent results concerning the influence of concentration and film flexibility on particle size distributions.

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A Monte Carlo model has been developed to describe the formation of bimetallic nanoparticles via the microemulsion route. The motivation stems from the need to understand the kinetics of nanoparticle formation in microemulsion droplets in order to determine the best experimental conditions to synthesize a nanoparticle with a given structure. We focus our study on the influence of the homogeneous and heterogeneous critical nucleus sizes of both metals on nanoparticle structure, as well as the role played by the surfactant film flexibility. The study reveals that the final structure is sensitive to changes in the critical nucleus numbers, because these parameters determine the rate of nucleation. An increase in the difference between nucleation rates of both metals gives rise to a better segregation of metals in the final nanoparticle. Likewise, as long as the formation of heterogeneous seeds is faster, the degree of alloying is greater. Finally, a fast material intermicellar exchange leads to a better mixture of metals, so the influence of the critical nucleus sizes on nanoparticle structure becomes less pronounced as the flexibility of surfactant film is increased.
Simulation of the kinetics of nanoparticle formation in microemulsions
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The simulation method and structural analysis are similar to our previous works on the influence of different parameters on the synthesis of nanoparticles in microemulsions [4,5]. This model was successfully applied to explain different experimental results [5–7]. The algorithm was run to simulate the kinetic course of nanoparticles formation in microemulsion media.
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Monte Carlo simulations were performed to study the influence of critical nucleus size on nanoparticle formation in microemulsions. It was found that critical nucleus size strongly affected nucleation and growth rates, as well as final nanoparticle sizes. An increase of critical nucleus leads to a slower nucleation process. In contrast, it gives rise to acceleration of the growth process. Final nanoparticle sizes increase as the critical nucleus value increases. It is predicted that this dependence will be less pronounced when a high reactant concentration is used. We have compared the simulation results with experimental data taken from different authors. Good agreement between the two kinds of results supports the conclusions of this paper.
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Microemulsions are very versatile reaction media which find nowadays many applications. Microemulsions have been used to control the particle size of many inorganic and organic materials because they induce drastic changes in the reagent concentrations and this can be particularly used for tuning the reaction rates. The influence of the chemical reaction rate on the final size of nanoparticles prepared in microemulsions is studied by computer simulation. We found that different chemical reaction rates lead to different final nanoparticle size. This result is explained mainly due to the ripening contribution to nanoparticle growth.

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Journal of Non-Crystalline Solids

Microemulsions as microreactors: a Monte Carlo simulation on the synthesis of particles

Abstract

Introduction

Section snippets

Simulation Procedure

Results

Discussion

Conclusions

Nanostruct. Mater.

J. Magn. Magn. Mater.

Chem. Rev.

Modelling of nano-alloying and structural evolution of bimetallic core-shell nanoparticles obtained via the microemulsion route

Simulation of the kinetics of nanoparticle formation in microemulsions

Reduced discrete population balance model for precipitation of barium sulfate nanoparticles in non-ionic microemulsions

Microemulsion-assisted precipitation of particles: Experimental and model-based process analysis

Critical nucleus size effects on nanoparticle formation in microemulsions: A comparison study between experimental and simulation results

Effects of the reaction rate on the size control of nanoparticles synthesized in microemulsions