Understanding the interfacial properties of bmim-AOT reverse micelles for their application as nanoreactors

Abstract

In this work, we have investigated the absorption and emission behavior of the cationic hemicyanine trans-4-[4-(dimethylamino)-styryl]-N-methylpyridinium iodide (HC) as molecular probe in 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (bmim-AOT) reverse micelles (RMs) to gain more insight about the microenvironment created by these aggregates. This information is crucial to be able to use RMs as nanoreactors. Our results show that the HC photophysical behavior is strongly altered inside bmim-AOT RMs in comparison with the well-known sodium 1,4-bis-2-ethylhexylsulfosuccinate (Na-AOT) RMs. Thus, HC emits from a locally excited state upon excitation in bmim-AOT RMs, which indicates that the polar core of bmim-AOT RMs exhibits a microenvironment with low electron-donating capacity, result that strongly impact when them are used as nanoreactors. In contrast, HC undergoes an intramolecular charge-transfer process upon excitation in Na-AOT RMs, highlighting the great impact of the bmim+ counterion on the physicochemical properties of the inner polar cores of RMs. Next, we evaluated bmim-AOT RMs as reaction media for chitosan nanoparticles (Ch-NPs) synthesis. Our results showed that bmim-AOT RMs also can be used as nanoreactors, obtaining highly monodisperse Ch-NPs in aqueous suspension. Also, the crosslinking degree and consequently the size of the nanoparticles are quite different from those observed when Na-AOT RMs are used as nanoreactors. Given the low electron-donating capacity of the microenvironment created by bmim-AOT RMs, Ch-NPs are less cross-linked (and larger) than those prepared from Na-AOT RMs. Our findings demonstrated how valuable is the information provided by HC on the interfacial properties of the RMs, since the physicochemical properties showed to have a great impact when they are used as a nanoreactor.