Phase behavior, dynamics, and structure of W/O microemulsions of the system aqueous solution/Synperonic 13_6.5/1-hexanol/isooctane were studied, with the goal of determining their effect on Mn-Zn ferrite nanoparticle formation, kinetics and characteristics. Microemulsion structure and dynamics were studied systematically by conductivity, dynamic light scattering (DLS), differential scanning calorimetry (DSC), and small-angle neutron scattering (SANS). The main effect of cosurfactant 1-hexanol was a decrease in microemulsion regions as compared to the systems without cosurfactant; nevertheless, overlap of microemulsion regions in the systems with precursor salts (PS) and precipitating agent (PA) was achieved at lower S/O ratios, compared to the system without cosurfactant. At 50 °C, PA microemulsions are nonpercolated, while PS microemulsions are percolated. SANS indicates small prolate ellipsoidal micelles with the absence of free water up to 18 wt % PS solution; DSC studies confirm the absence of free water in this composition range. Kinetic studies show an increase in the reaction rate with increasing concentration of the aqueous solution; but the most significant effect in reaction kinetics was noted when cosurfactant was used, regardless of microemulsion dynamics and structure. On the other hand, the main difference regarding the characteristics of the obtained nanoparticles was observed when bicontinuous microemulsions were used as reaction media which resulted in 8 nm nanoparticles, versus a constant size of ~4 nm obtained with all other microemulsions regardless of aqueous solution content, dynamics, and presence or absence of cosurfactant. The latter effect of constant size is attributed to the fact that the water present is dominantly bound to the EO units of the surfactant.
In this work, the formation of water-in-oil (w/o) microemulsions with high aqueous phase uptake in a nonionic surfactant system is investigated as potential media for the synthesis of Mn-Zn ferrite nanoparticles. A comprehensive study based on the phase behavior of systems containing precursor salts, on one hand, and precipitating agent, on the other hand, was carried out to identify key regions on (a) pseudoternary phase diagrams at constant temperature (50 °C), and (b) pseudobinary phase diagrams at constant surfactant (S):oil(O) weight ratio (S:O) as a function of temperature. The internal structure and dynamics of microemulsions were studied systematically by conductivity and self-diffusion coefficient determinations (FT PGSE (1)H NMR). It was found that nonpercolated w/o microemulsions could be obtained by appropriate tuning of composition variables and temperature, with aqueous phase concentrations as high as 36 wt % for precursor salts and 25 wt % for precipitating agent systems. Three compositions with three different dynamic behaviors (nonpercolated and percolated w/o, as well as bicontinuous microemulsions) were selected for the synthesis of Mn-Zn ferrites, resulting in nanoparticles with different characteristics. Spinel structure and superparamagnetic behavior were obtained. This study sets firm basis for a systematic study of Mn-Zn ferrite nanoparticle synthesis via different scenarios of microemulsion dynamics, which will contribute to a better understanding on the relationship of the characteristics of the obtained materials with the properties of the reaction media.
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