Intermicellar exchange

Some investigations have emphasized the importance of micellar size as a control parameter of nanoparticle size [224]. It has been suggested that other factors also influence the nanoparticle size, such as the concentration of the reagents, hydration of the surfactant head group, intermicellar interactions, and the intermicellar exchange rate [198,225-228],... 

For instance, nanoparticles of silver chloride have been synthesized by mixing two mi-croemnlsions, one containing silver ions and the other containing chloride ions. It was shown that the average particle size, the polydispersity and the number of particles formed depend on the intermicellar exchange rate and/or the rigidity of the surfactant shell [228],... 

Bagwe RP, Khilar KC (2000) Effects of intermicellar exchange rate on the formahon of silver nanoparticles in reverse microemulsions of AOT. Langmuir 16 905-910... 

The intermicellar exchange process, governed by the attractive interactions between droplets, can be modified by changing the bulk solvent used to form reverse micellar solution (26). This is due to the discrete nature of solvent molecules and is attributed to the appearance of depletion forces between two micelles (the solvent is driven off between the two droplets) (26). When the droplets are in contact forming... 

ForCaCO, particles in W/O microemulsions of hexaethylene glycol dodecyl ether, however, the values of Nm/Np lie between 0.044 and 0.28, indicating that the particles form by the destruction of micelle-solubilizing aqueous Ca(OH)2 rather than by the intermicellar exchange process (5). In the case of the formation of CaCO-) particles in the Ca salt of Aerosol OT-cyclohexane system, as seen in Table 7.2.2, the particles... 

HjO]/[AOT] ratios influenced the equilibrium sizes of CdS particles generated rates of CdS growth, determined in a stopped-flow spectrophotometer, were consistent with the rate-determining intermicellar exchange of solubilizates... 

Nucleation is a rate phenomenon therefore, even though the condition oc > c is necessary, it is nonetheless insufficient to bring about nucleus formation. An additional requirement must be satisfied. That is, in order for cluster formation to proceed to completion, the monomers that represent potential candidates for a given critical nucleus must be retained for a sufficiently long period of time within a volume of molecular dimensions. If intermicellar exchange proceeds extremely rapidly, then the candidate monomers will be redistributed before nucleation can occur. Accordingly, it may be deduced that an increase in the communication rate will decrease the probability of nucleus formation. The expected trend of number of nuclei versus the rate of intermicellar exchange (A ex) is illustrated schematically in Fig. 5d. 

Stage II. This is the stage when intermicellar exchange takes place by fusion-fission. For well-defined dispersed droplets the exchange rate coefficient is approximately of the order of 10 -10" lower than the diffusion controlled reaction rate. 

The diffusion of the reducing agent (for particle formation) through the oil phase and toward the reverse micelle is faster than the intermicellar exchange process. 

The reaction is instantaneous however, it is also to be considered that the intermicellar exchange is slow and instrumental in particle growth. 

The intermicellar exchange takes place through binary fusion-fission (Section 3.7) with defined kinetics fusion of the micelles is rate-limiting, and fission of the dimer is instantaneous in relation to the other time... 

R. P. Bagwe and K. C. Khilar, Effects of the intermicellar exchange rate and cations on the size of... 

The effect of supercritical solvent continuous phase properties on particle growth behavior was investigated by Roberts et al. throngh comparison of Cu and Ag particles growth rates in snpercritical alkan and in normal liquid solvents at the same conditions. Favorable properties of SCFs solvent, such as lower density and solvent power, decreased solvent interaction with the surfactant tails and led to smaller nanocrystals and faster particle growth rate, due to the increased kinetics of the intermicellar exchange mechanism [42]. 

Abstract Monte Carlo simulations were carried out to study the different stmctures showed by bimetallic nanoparticles synthesized in microemulsions. It is observed that the difference in reduction rates of both metals is not the only parameter to determine the metals segregation, playing the interdroplet channel size a relevant role. The reduction rates difference determines nanoparticle structure only in two extreme cases when both reactions take place at the same rate a nanoalloy structure is always obtained if both reactions have very different rates, the nanoparticle shows a core-shell structure. But in the large interval between both extreme cases, the nanoparticle structure is strongly dependent on the intermicellar exchange, which is mainly determined by the surfactant film flexibility, and on reactants concentration. This result is very promising for the preparation of bimetallic nanoparticles with a given structure. 

The main conclusion is that nanoparticle structure is determined by the chemical reaction rates ratio, as in homogeneous media, only if both reactions take place at the same rate and if the reaction rates are very different. The first case leads to a nanoalloy and the second one to a core-shell structure. But in the common case of a nanoparticle composed by two metals with a moderate difference in reduction potentials, the dynamics of the intermicellar exchange causes that the synthesis variables, such as surfactant film flexibility or initial reactants concentrations, become more relevant, modifying the nanoparticle structure An increase of flexibility gives rise to alloys, and an increase of concentration implies a higher degree of mixture in the inner layers and an enrichment in the slower product in the outer layers. 

The use of the pseudophase model is convenient if the distribution of the reactants among micelles is not taken into account, i.e. for the first order reactions and also for the second order reactions if the intermicellar exchange by the reactant molecules occurs faster than the reaction in question. When the reaction is faster than or has a comparable rate to the intermicellar exchange by the reactants molecules, one should take into account the statistics of the intermicellar distribution of molecules. 

The most general model which takes into account the direct intermicellar exchange of the quencher molecules (the rate constant of this process is denoted fc ) was put forward by De Schryver et al. [44]. This model gives the following expression for the fluorescence decay after a pulse excitation ... 

Celade and De Schryver applied the same model to the fluorescence quenching in SDS micelles by neutral molecules [45,46] and in inverted micelles by halogene anions [47]. The intermicellar exchange of the counterions (iodine ions) was studied for cetyltrimethylammonium chloride (CTAC) micelles [48] and the value kg=9A X10 dm mole" s was obtained. 

The influence of the surfactant concentration upon the quenching process may be caused by the change of the micelles shape, rather than intermicellar exchange of quencher molecules, as it was supposed for pyrene fluorescence quenching by copper ions in SDS micelles [49]. 

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