Singlet molecular oxygen-mediated photooxidation of nitrophenolic compounds in water-in-oil microemulsions. A kinetic study

Abstract

The aerobic singlet molecular oxygen-mediated photooxidation of 2-nitrophenol (2NP) and 4-methyl-2-nitrophenol (4M2NP) has been studied in water-in-oil (W/O) microemulsion media. The microemulsions used were sodium bis(2-ethylhexyl)sulfosuccinate (AOT)–heptane–water or benzylhexadecyldimethylammonium chloride (BHDC)–benzene–water. The kinetic profile of the reaction was investigated as a function of variables such as (a) surfactant structure and concentration and (b) amount of water dispersed in the microemulsion. Rate constants for overall quenching (k_t) and chemical reaction (k_r) for the interaction of 2NP and 4M2NP with singlet molecular oxygen in the microemulsion media were determined. The quantum efficiencies for the nitro-derivatives' photooxidation (φ_r) in both anionic and cationic W/O microemulsions are one to two orders of magnitude larger than in aqueous solutions. A mechanism based on polarity effects and probe location in the microemulsions is proposed in order to rationalize the results.