ARTICLE pubs.acs.org/Langmuir
Aqueous-Mixed Ionic Liquid System: Phase Transitions and Synthesis of Gold Nanocrystals K. Srinivasa Rao, Tejwant Singh, and Arvind Kumar* Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR), G. B. Marg, Bhavnagar-364002, Gujarat, India
bS Supporting Information ABSTRACT: Micellevesiclemicelle (MVM) transitions are observed in the aqueous-mixed ionic liquid (1-butyl-3-methylimidazolium octyl sulfate and 3-methyl-1-octylimidazolium chloride) system. The surface activity of mixed ILs, phase behavior, and solution structures in the system have been thoroughly characterized using conductometry, tensiometry, fluorimetry, dynamic light scattering (DLS), viscometry, turbidity, atomic force microscopy (AFM), transmission electron microscopy (TEM), and 1H NMR techniques. Synergetic interactions between the two ILs in monolayers at the air/water interface and in micelles/vesicles have been determined using the regular solution approach, and the origins of spontaneous vesicle formation in this novel system are discussed. Using a photoreduction method, the formation of stable gold nanoparticles (GNPs) and microscale nanosheets of different shapes and sizes in the micellar and vesicle solutions has been reported. The studies show the potential of a mixed IL system in constructing stable micelles/supramolecular assemblies, such as bilayer vesicles, which are effective in the preparation of the desired nanomaterials.
1. INTRODUCTION Synthetic surfactant vesicles, as supramolecular aggregates, have been successfully used to understand the cell membranes and find potential applications in important areas such as in drug delivery and as microreactors for the production of advanced materials.15 Spontaneous vesicle formation can occur from single surfactants under varying conditions of pH, temperature, or salt addition or from combinations of binary surfactants such as cationic/anionic, ionic/nonionic, cationic/cationic, and anionic/ zwitterionic by adjusting ratios of the mixture components614 (because a large number of studies are available, only the representative or recent references are cited). Normally more stable vesicles are formed from mixtures of oppositely charged surfactants because of synergic interactions, and such systems display a wide variety of phase behavior and structure.9 Transitions from micelles to vesicles are of particular interest because such a phenomenon offers an easy way of encapsulating active agents by dissolving them in the micellar phase prior to vesicle formation. Amphiphilic ionic liquids (ILs, salts with a melting temperature of
