The Formation of Me(AOT)_n Micelles as Nanoreactors, Crystallizers, and Charging Agents: Cation-Exchange Solvent Extraction versus Direct Injection Solubilization

ABSTRACT

The composition and hydrodynamic diameter of reverse micelles Me(AOT)_n (n = 1–3) were determined during injection solubilization of nitrate salts of Li⁺, Na⁺, K⁺, Cs⁺, NH₄⁺, Ag⁺, Ni²⁺, Co²⁺, Zn²⁺, Cu²⁺, Cd²⁺, Ca²⁺, Al³⁺, Fe³⁺, and La³⁺ cations and upon solvent extraction from their concentrated (1‒2 mol/L) solutions. It was found that solvent extraction by NaAOT reverse micelles can increase the concentration of cations in microemulsion by a factor of 10‒20 as compared to injection solubilization. Therewith, the fraction of super equivalent extraction of cations as nitrate salts into the aqueous cores of reverse micelles can reach 150%. According to the study, the higher is the hydration energy of cation, the greater is the number of successive extraction steps (up to 4) required for a virtually complete (more than 99%) replacement of Na⁺ in AOT micelles. The hydrodynamic diameter of Мe(AOT)_n micelles was in the range from 3.2 to 13.9 nm, and the content of aqueous pseudophase, from 1.2 to 10.4 vol.%. It was demonstrated that the obtained extracts could be used for enhancing the productivity of micellar synthesis of nanoparticles (Ag and CdS) and isothermal micellar crystallization of salts (LiNO₃, KNO₃, CsNO₃), and also as the charging agents (Cu(AOT)₂, Ni(AOT)₂, and Fe(АОТ)₃) of SiO₂ nanoparticles in the n-hexadecane-chloroform mixture.

KEYWORDS:

• Extraction
• reverse micelles
• solubilization
• AOT
• cations
• hydration
• synthesis
• crystallization
• nanoparticles
• dynamic light scattering

Acknowledgments

Authors would like to thank Dr E. A. Maximovskii (SEM), Dr V. I. Zaikovskii (HRTEM) and I. I. Alferova (recording of IR spectra). The research (Nikolaev Institute of Inorganic Chemistry SB RAS) was supported by the Ministry of Science and Education of the Russian Federation.

Notes

¹ Solubilization capacity of micellar solution (V_s/V_o) is the ratio of the volume of solubilized aqueous phase (V_s) to the total volume of organic phase (V_o).