Droplet core-shell

Fig. 17.10 Schafer et al.14 have observed single mode lasing from core shell CdSe/ZnS nano crystal quantum dots in a glycerine water mixture. The fluorescence spectrum (black line) showed clear peaks of WGM and single mode lasing (grey line) was observed for sufficiently small droplets ( 10 pm) and high pump laser intensities (53 mJ crrT2 in 10 ns pulses at 532 nm). Insert shows the droplet trapped between the electrodes. Reprinted from Ref. 14 with permission. 2008 American Chemical Society...

Among the synthetic methods, the reverse micelle technique shall only briefly be mentioned. It is based on the use of water droplets in an organic phase of a surfactant. bi Metal salts, dissolved in the water droplets, are reduced inside the nanometer-sized water volume, to result in nanoparticles trapped in the micelles. Alloy-like particles as well as bimetallic core-shell particles thus become available. Size distributions up to over 20% have to be put up with this technique. Another disadvantage of the reverse micelle procedure is the lack of crystallinity due to the low synthesis temperature (<100°C) requiring subsequent aimealing at 200-300 Fe, Co, and Ni... 

A number of workers have suggested that emulsion polymerization may not occur homogeneously throughout a polymer particle but either at the particle surface [75] or within an outer monomer-rich shell surrounding an inner polymer-rich core [76]. The latter has been referred to as the shell or core-shell model. The latter model has been proposed to explobserved constant rate behavior up to about 60 percent conversion, which according to Eq. (6.232) requires [M] to be constant, and the considerable experimental evidence which indicates that emulsified monomer droplets (serving as monomer reservoirs) disappear at 25 to 30 percent conversion and the monomer concentration drops thereafter. 

Nanoprecipitation can also be a very efficient method for the encapsulation of an aqueous core with a polymeric shell. Aqueous droplets containing an active component, e.g., the antiseptic chlorohexidine digluconate [193,194], can be obtained by miniemusification. The continuous phase of the miniemulsion consists of a mixture of a solvent (e.g., dichloromethane, DCM) and a non-solvent (e.g., cyclohexane) for the polymer (e.g., PMMA, PCL, or polymethylacrylate PMA). After miniemulsification, the solvent is carefully evaporated in a controlled manner and the polymer precipitates onto the aqueous droplet (Fig. 23), resulting in a core-shell structure of the system. 

FIGURE 9.1 Schanadc representation of the characteristics of capsules and the different types of encapsulation, (a) Dispersed, (b) core-shell, (c) droplets, (d) degree of cross-linking, (e) size, (f) swell or shrink, and (g) surface charge. (Reproduced from Shewan, H.M. and Stokes, J.R., J. Food Eng., 119(4), 781,2013. With permission.)... 

These droplets subsequently undergo solidification to obtain a matrix encapsulation microsphere, or they could undergo further encapsulation processes to generate uniform core-shell with single or multicore structures. 

The formation of microcapsules is greatly affected by the surfactant, which influences not only the mean diameter but also the stability of the dispersion. The surfactants used in the system have two roles, the first one to reduce the interfacial tension between oil and aqueous phases allowing formation of smaller microcapsules and the other one to prevent coalescence by its adsorption on the oil-water interface and therefore by forming a layer around the oil droplets. The synthesis of a core/ shell particle or other possible morphologies is mainly governed by the kinetic factors and thermodynamic factors. 

Fig. 20 Examples of composite core-shell colloids without (a) and with (b) PMMA outer shell. The particle core consists of polymerized MPTMS-droplets stabilized by magnetite (a). Adapted from [147] with permission of Wiley-VCH...

Initiators for the controlled living radical polymerization could also be introduced to silica particles. Nitroxide-mediated polymerization (NMP) conducted with styrene in miniemulsion led to the generation of core-shell particles, with styrene grafted to the central silica particle [131]. PBA could be polymerized from 20 nm silica beads by attaching an ATRP agent to the silica surface and subsequent miniemulsion polymerization [132]. Confining the polymerization to miniemulsion droplets could avoid gel formation, which was observed in the bulk reaction. Due to the limited monomer diffusion, only 25-35% of conversion could be obtained in bulk. 

The influence of the solvation shell on the viscosity data was also reported in a study on the system sodium alkylbenzenesulfonate (Na DBS)-hexanol-water-xylene. Here it was observed that the degree of solvation increased with increasing NaDBS/water ratio. This was attributed to the fact that with increasing ratio of surfactant layer thickness to droplet core the relative amount of bound solvent molecules increases, thereby increasing the hydro-dynamic volume of the droplets [58]. This shows that viscosity measurements are very sensitive to the state of solvation of colloidal particles and can be used to extract information about this property. 

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