Acid-Base Behaviour of Methyl Red in Microemulsions

pK values of the first protonation of methyl red (MR) in benzene-water microemulsions (MEs) stabilized by sodium dodecyl sulfate (SDS) + butanol, SDS + propanol, and cetylpyridinium chloride (CPC) + propanol have been determined at 25°C as a function of R (molar ratio of water to surfactant) by studying the electronic spectra of MR in these MEs at various pH values. In two of the MEs, viz., benzene/SDS/butanol/water and benzene/SDS/propanol/water of R = 66, pK values of second protonation of MR have also been measured. The partitioning of the doubly protonated (H₂MR+), singly protonated (HMR), and deprotonated (MR-) species of MR in the different regions of the ME has been ascertained on the basis of the spectral data. The plot of pK of first protonation versus R exhibitis a minimum at low R (R ≈ 10) and a maximum at higher R (R ≈ 40) indicating different types of dependence of pK on R in the w/o, bicontinuous and o/w regions of the ME. pK is shown to be controlled by the effective dielectric constant at the site of protonation, interfacial electrical potential and the partition coefficients of the acidic and basic forms of MR with respect to the different domains of ME. A method for estimating the limiting pH at which first protonation of MR is complete has been suggested.