Acid–base behaviour of 4-nitrophenol and 4-nitrophenyl-2-sulphonate in water-in-oil microemulsions stabilized by aerosol-OT

Abstract

The weak acid 4-nitrophenol (NP) experiences a large (ca. 2 unit) increase in pK_a when solubilized in buffered water-in-oil (w/o) microemulsions stabilized by the surfactant sodium bis-2-ethylhexylsulphosuccinate (AOT), as compared with the aqueous solution value (pK⁰ _a= 7.0 at 25 °C). In addition both the phenol (NPH) and its conjugate base (NP^–) undergo a hypsochromatic shift of the UV–VIS spectrum, relative to that in aqueous solution, at low water contents. In contrast, the pK_a of 4-nitrophenyl-2-sulphonate, a structural analogue of NP bearing a net negative charge, was found to be similar to that in aqueous solution. Spectral hypsochromic shifts were once again observed for both the acid (NPSH^–) and conjugate base (NPS^2–). The observed change in pK_a of NP is shown to be consistent with the extensive partitioning of the phenol (NPH) out of the aqueous domain. The spectrophotometric behaviour of the charged nitrophenolic species is shown to be consistent with their confinement to the aqueous domain, the hypsochromic shifts at low water contents being attributed to changes in the structure of the microdispersed water. The spectrophotometric behaviour of NPH is consistent with a primarily interfacial location for this species. The spectra of the NPH/NP^– couple were analysed, in microemulsions at various water contents, to yield a series of isosbestic wavelengths convenient for the measurement of total (acid + base) NP. These are of value in studies of the kinetics of the hydrolysis of esters of NP in microemulsions.