Partitioning of 4-nitrophenol in aerosol-OT reverse micelles

Partitioning of 4-nitrophenol in a reverse micellar system consisting of ae rosol-OT (AOT)-H2O-isooctane was monitored spectrophotometrically. At pH 10 .0, the ionized form 4-nitrophenolate in the water pool absorbs visible lig ht with a maximum peak at 402 nm. However, that partitioned into the interf ace region is not ionized due to interactions with the negatively charged p olar head of the surfactant. The partitioning depends on the water content of the system. In some intermediate [H2O]/[AOT] molar ratio values, two abs orption 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 carbo nate buffer, the partitioning disappeared in 2-amino-2-methylpropanol buffe r 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, rathe r than isobutanol, induced the replacement. When the surfactant concentrati on was increased, while keeping the system water content constant, the abso rption peak at 402 nm increased with a concomitant decrease in the A(310) p eak, which demonstrated the affinity of the non-ionized 4-nitrophenol with the surfactant. Multiple apparent pK(a) values of 4-nitrophenol were observ ed in the AOT reverse micellar system. We propose a model of the AOT revers e 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 poo l of the system.