Nanoparticle formation, microemulsion

Fig. 15 Formation of PbS-coated CdS nanocomposite in microemulsion a mixing of a microemulsion containing Cd(N03)2 aqueous solution with a microemulsion containing (NH4)2S aqueous solution b formation of CdS nanoparticle in microemulsion c,d Pb + ions in a third microemulsion replace the Cd + in the Cd-S band and diffuse through the PbS layer to form the PbS shell [142]...

Note, C., Ruffin, J., Tiersch, B. and Koetz, J. (2007) The influence of polyampholytes on the phase behaviour of microemulsion used as template for the nanoparticle formation. /. Disp. Sci. Tech, 28, 155-164. 

The formation of nanoparticles from microemulsions need not essentially follow the template shape. Pileni [32] (as quoted by Ganguli and Ganguli) has shown that with water/isooctane/Cu( AOT)2 shapes like sphere to cylinder to mixed spherulites and cylinders to other polydisperse shapes were possible with increasing to. According to Pileni [33], the presence of salt anions can control the shape while chloride ions favour formation of nanorods, nitrate ions hinder it. The surfactant content also can have a say on the shape of nanoparticles. The infrequently observed morphologies of nanoparticles, viz. wires, trigons, hexagons, cubes etc. have so far no specific and reliable reasons for formation in micro emulsion templates. 

Nanoparticle Formation in Rapid Expansion of Water-in-Carbon Dioxide Microemulsion into Liquid... 

Xin et al. [10] evaluated the synthesis of catalyst nanoparticles using microemulsions like cyclohexane. In this work no pure metal were observed, only alloys of the catalyst exhibited as seen from X-ray diffraction (XRD). The composition and particle was controlled using microemulsions [11,12]. The alloy formation of platinum, which has a FCC structure, included shifting of the diffraction peak to a higher angle (20), indicating a decrease in lattice parameter. 

Instead of continuing the previous series of review papers [3,4], I place emphasis in this contribution on the fundamental aspects of monodisperse nanoparticle formation. We shall see how the inner water cores of the microemulsion systems work as microreactors. Moreover, the nucleation process is approached, and a minimum number of atoms forming the nuclei is proposed. The role of the surfactant and the cosurfactant are analyzed in the light of the formation of the first nuclei. Finally, the role of the adsorbed molecules in the monodisperse nature of the particles is examined. The different parts are illustrated taking into account the available literature. 

Numerous investigations in nanoparticle formation through W/O microemulsions have used extensive ranges of relative water content (indicated by the ratio w = [water]/[surfactant]) not only to obtain optimized conditions of synthesis, but also to examine the effect of relative water content on the size of the aqueous droplets and the particles synthesized therefrom. Such control parameters are not linked with merely the water content perse. It has been shown (as discussed below) that the structural arrangements and chemical bondings of the water molecules in the droplets of W/O microemulsion can vary with water content. 

Mn-doped CdS nanoparticle formation (and fixation in xerogels) has been reported by Counio et ai [355]. As in several other cases, the two-microemulsion method was used for sulfide formation. The basic system was AOT/heptane and the two microemulsions contained aqueous solutions of (i) Cd(N03)2 + Mn(N03)2 and (ii) Na2S the concentrations (mol/1) were [Cd ] = 0.20, [Mn T = 0.12, [S ] = 0.38. Microemulsion (i) was added to (ii) of equal volume for synthesis and pyridine added to cap the particles and avoid agglomeration. The capped particles were washed with petroleum ether to remove AOT, dispersed in pyridine and the dispersion mixed with a sol obtained from CH3Si(OC2H5)3 for fabrication of bulk disks or thin films with entrapped CdS Mn (1-2 nm). 

For obtaining CdS/ZnS nanoparticles (CdS coated by ZnS), the CdS core was first prepared by mixing two microemulsions containing Cd-acetate and Na2S aqueous solutions in a 1 2 volume ratio (to provide excess S "). After formation of CdS nanoparticles, a microemulsion containing an aqueous solution of zinc acetate was added to the CdS-containing solution in a volume ratio of 1 3. Particles of ZnS coated by CdS were prepared by a similar method. Specific size measurements of the different products could not be performed, but analysis of optical spectra showed them to be a few nanometers in size. 

The computer simulation of the formation of nanoparticle in microemulsions was carried out using the model previously reported [7-9]. Briefly, each simulation began with a random... 

Taken into account these former results, the nanoparticle formation was realized at point A and B marked in the phase diagram, which represent the w/o and the bicontinuous microemulsion, respectively. The microemulsions were modified by using the poly(ethylene glycol) samples PEG I and PEG II at polymer concentrations of 10, and 20wt.%. 

Nanoparticle Formation in Microemulsions Mechanism and Monte Carlo Simulations... 

Tojo, C., Blanco, M.C., and L6pez-Quintela, M.A. 1998. The influence of reactant excess and film fiexibility on the mechanism of nanoparticle formation in microemulsions A Monte Carlo simulation. Langmuir, 14, 6835-6839. 

Kumar, A.R., Hota, G., Mehra, A., and Khilar, K. 2004. Modeling of nanoparticles formation by mixing of two reactive microemulsions. AIChE Journal, 50, 1556-1567. 

Jain, R. and Mehra, A. 2004. Monte Carlo models for nanoparticle formation in two microemulsion systems. Langmuir, 20, 6507—6513. 

Tojo, C., Barroso, F., and de Dios, M. 2006. Critical nucleus size effects on nanoparticle formation in microemulsions A comparison study between experimental and simulation results. Journal of Colloid and Interface Science, 296, 591-598. 

This review summarizes our findings on the effect of some operating and microemulsion variables on nanoparticle uptake in reactive and nonreactive microemulsion systems. Mixing was found to increase the rate of nanoparticle formation and shorten the time needed to reach the nanoparticle uptake for reactive and nonreactive systems. Temperature increased nanoparticle uptake for exothermic nanoparticle formation reactions in reactive nficroemulsions, given stable reverse-micellar structure is maintained. The overall effect of temperature, nevertheless, was dependent on the final particle size. Nanoparticle uptake increased linearly with the surfactant concentration, for reactive and nonreactive surfactant systems, most probably due to the increase in the population of reverse micelles, nanoreactors. The effect of surfactant counterion and water to surfactant mole ratio, R, was dependent on their effect on the stability of the reverse micellar structure... 

Husein, M., Rodil, E., and Vera, J.H. 2003. Formation of silver chloride nanoparticles in microemulsions by direct precipitation with the surfactant counterion. Langmuir, 19, 8467-8474. 

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