Partitioning of 4-nitrophenol in aerosol-OT reverse micelles

Abstract

Partitioning of 4-nitrophenol in a reverse micellar system consisting of aerosol-OT (AOT)–H₂O–isooctane was monitored spectrophotometrically. At pH 10.0, the ionized form 4-nitrophenolate in the water pool absorbs visible light with a maximum peak at 402 nm. However, that partitioned into the interface region is not ionized due to interactions with the negatively charged polar head of the surfactant. The partitioning depends on the water content of the system. In some intermediate [H₂O]/[AOT] molar ratio values, two absorption peaks were clearly observed, which can be utilized in the partition coefficient estimation. The partitioning also depends on the buffer used. While partitioning of 4-nitrophenol into the interface is observed in carbonate buffer, the partitioning disappeared in 2-amino-2-methylpropanol buffer presumably due to displacement of 4-nitrophenol from the interface region into the water pool. This displacement is not a salt effect but is due to the amino group of 2-amino-2-methylpropanol, because tert-butylamine, rather than isobutanol, induced the replacement. When the surfactant concentration was increased, while keeping the system water content constant, the absorption peak at 402 nm increased with a concomitant decrease in the A₃₁₀ peak, which demonstrated the affinity of the non-ionized 4-nitrophenol with the surfactant. Multiple apparent pK_a values of 4-nitrophenol were observed in the AOT reverse micellar system. We propose a model of the AOT reverse micelles with a gradient micro-polarity in the water pool that results in a continuous influence on the ionization of 4-nitrophenol in the water pool of the system.