Probing the interfacial transition of acetonitrile/AOT/n-heptane microemulsion through in situ silver colloid synthesis

Abstract

Very recently, it was suggested that non-aqueous acetonitrile/AOT/n-heptane microemulsions may undergo a interfacial transition from reverse micelle (RM) to bi-continuous microemulsion (BMC) above an intermediate ws (= [acetonitrile]/[AOT]). Herein, we develop a unique strategy for probing interfacial transition of the microemulsion by using microemulsion as a template for synthesis of hydrophobic silver colloids. Precursors (silver nitrate) and reducing agents (ascorbic acid) are insoluble in n-heptane phase and thus, Ag colloids formation occur solely inside the acetonitrile fraction and therefore, the structural morphology of the Ag colloids provides valuable information regarding the structure of the template where Ag colloids formation occur. At low ws (≤ 1), synthesized silver colloids exhibits remarkably strong surface plasmon resonance (SPR) band but the strength of SPR remarkably drops at ws = 2 and almost vanishes at ws = 3. TEM measurements revealed the presence of isolated spherical monodispersed silver nanoparticles (AgNPs) of ∼6 nm diameter at lower ws and larger distribution of nanoparticle sizes at ws = 2. Most interestingly, interconnected Ag colloids packed channels were found at ws = 3 which directly shows footprint of the BMC nature of the microemulsion template. Thus, the characteristics of Ag colloids (i.e. size distribution and optical quality) provide valuable information about interfacial structure of the microemulsion and suggested that microemulsion switches from RM to BMC nature at an intermediate ws.