Intramicellar nucleation

Figure 6. Formation of silica nuclei by hydrolysis ofTEOS in reverse micellar systems a, partial hydrolysis ofTEOS and association to reverse micelles and b, inter- and intramicellar nucleation.

Inaeasing precursor salt concentration increases the ion occupancy number per reverse micelle, which promotes intramicellar nucleation and growth and leads to higher uptake. On the other hand, high-ionic strength inside water pools of reverse micelles impacts the stability of the (w/o) microemulsions [63,64]. Moreover, the stabihty of the (w/o) microemulsions depends on the surfactant counterion, which results from ion exchange between the solubilized metal salt and sodium. Since the focus of this work was on nanoparticle uptake by (w/o) microemulsious, the concentrations of the metal precursors were limited to values which maintained stable reverse-micellar structures. 

The formation of the metal oxide/hydroxide nanoparticles commenced upon the addition of the precipitating agent, the NaOH solution, to reverse micelles containing the dissolved salt. The addition of the aqueous solution disrupts the equilibrium momentarily as the reverse micelles re-group to accommodate the added solution [27,28]. This dismption is manifested by the appearance of immediate cloudiness. Reaction R3.1 proceeds in the bulk water pools, upon re-grouping of the reverse micelles, and metal hydroxide nanoparticles form by means of intramicellar [27,28,56,57] and intermicellar [57,58] nucleation and growth. 

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