Partitioning of p-nitroaniline between pseudo-phases within a water-in-oil microemulsion. Effects of temperature and microwave irradiation

We have investigated the distribution of p-nitroaniline (pNA) between the oil, interfacial and water pseudo-phases of water-in-oil microemulsions stabilised by the anionic surfactant sodium bis-2-ethylhexyl sulphosuccinate (AOT). The UV/VIS spectra of pNA located in the different pseudo-phases of the microemulsion show large shifts in the wavelength of maximum absorption. Changes in the UV/VIS spectra as a function of the microemulsion composition are accurately fitted using a pseudo-phase model which yields the two partition coefficients describing the pNA distribution. The temperature dependence of the oil–interface partition coefficient shows that the standard enthalpy of transfer of pNA from the oil to the interfacial pseudo-phase is − 29.7 kJ mol−1, similar in magnitude to the enthalpy change for transfer of an alcohol from an alkane solvent to the oil/water interface. Transfer of pNA from the interface to the water pseudo-phase of the microemulsion shows no significant enthalpy change, suggesting that hydration of the interfacial pNA is virtually complete. Microwave irradiation of the microemulsion samples using a frequency of 2.45 GHz and total incident power of 25 W caused no significant changes in the pNA distribution.