Indian Journal of Chemistry Vol. 40A, February 2001. pp. 155-158 Acid dissociation constants of N-alkyl pia-hydroxy styryl pyridinium dyes in surfactant solutions Bijay K Mishra·, Pradipta K Behera & Rajalaxmi Rath Centre of Studies in Surface Science and Technology, Department of Chemistry, Sambalpur University, Jyoti Vihar 768019, India Received 4 September 2000; revised 9 November 2000 Some N-alkyl pia-hydroxy styryl pyridinium halides, the alkyl moieties being methyl, hexyl and decyl, have been synthesised and the acid dissociation constants of these compounds in aqueous and surfactant solutions have been determined using spectrophotometric method. The alkyl chain does not have any substantial effect on the pKa values in aqueous and cationic surfactant (cetyl trimethylammonium bromide: CTAB) system. However, with longer alkyl chain the pKa values increase in anionic surfactant (sodium dodecyl sulphate: SDS). These results have been attributed to the variation in solubilization sites of the surfactants. Acid-base equilibrium in micellar media differs significantly from that in the aqueous media'-3. The shifts in the pK. values of indicators have been paid much attention due to their role to investigate the environment characteristics 4 - 6 • It has been suggested that the shift in the pK. may be due to (i) electrostatic potential at the micellar surface, (ii) ion exchange between the micelle surface and bulk aqueous phase, and (iii) specific molecular interaction such as ion- pair formation between oppositely charged indicator species and head groups of monomeric surfactants 7 - 9 . Dutta et al. W have proposed a model for the indicator- surfactant systems in which either the acid or the base form of the indicator predominantly binds to the micelle. In this model, the shifts in pKa of an indicator in micellar solutions are calculated from the knowledge of the association constant of the indicator with the micelles at any desired concentration of the surfactant. Moulik et at." have investigated the acid- base equilibrium for dye, neutral red, in AOT micelles and have proposed the localization site of the dye in the micellar system. Similarly, Paul and Ismail 12 , proposing the residence site of neutral red to be the interfacial region of SDS microemulsion, have studied the acid-base equilibria of neutral red in benzene-in- oil microemulsion and they reported the pK. to be 6.88 and 7.05 in microemulsions with n-propanol and n-butanol as the cosurfactant respectively. In continuation of our earlier work in dye- surfactant interactions '3-17, herein we report an investigation of acid dissociation constants of some N-alkyl olp-hydroxy styryl pyridinium iodide/ bromide (I) in cationic and anionic surfactant systems. Materials and Methods The compounds N-methyl 4-(p-hydroxy styryl) pyridinium iodide (P-OHC,), N-methyl 4-(o-hydroxy styryl) pyridinium iodide (o-OHC,), N-hexyl 4-(p- hydroxy styryl) pyridinium iodide (P-OHC 6 ), N-hexyl 4-(o-hydroxy styryl) pyridinium bromide (o-OHC 6 ), N-decyl 4-(p-hydroxy styryl) pyridinium bromide (p- OHC w ) and N-decyl 4-(o-hydroxy styryl) pyridinium bromide (o-OHC w ) were prepared by condensing appropriate y-picolinium salts with plo-hydroxy benzaldehyde'5.'8. The uncorrected melting point values recorded in acid bath are reported in Table I. The IR spectral data obtained from spectra recorded in a Perkin-Elmer RXI-Ff IR spectrophotometer are in accordance with the structures of the compounds. The experimental concentrations of the hydroxy compounds were maintained at 2xlO- 5 M in 5% methanol (E-Merck, AR) solution. CTAB and SDS (E-Merck) were purified before use'9. The pH was maintained at 7-12 with buffers prepared from citric acid, boric acid, phosphoric acid, sodium hydroxide and hydrochloric acid in different compositions 2o • An Elico LI-120 digital pH meter was used for pH measurements. The electronic spectra were recorded on a Shimadzu UV -vis-200 spectrophotometer maintaining temperature at 300±0.5 K by using INSREF Ultracryostat. The pKa values have been determined by using the absorbance values of the