Indian Journal of Chemistry Yol. 32A, March 1993, pp. 239-242 Effect of cationic micelles on pKa of acridine: A spectroscopic study R Hazarika, R K Dutta & S N Bhat* Department of Chemistry, North Eastern Hill University, Shillong 793003 Received 25 March 1992; revised 28 September 1992; accepted 6 November 1992 The interaction of acridine with cationic surfactants, viz., dodecyltrimethylammonium bromide, tetradecyl- trimethylammonium bromide and hexadecyltrimethyl- ammonium bromide have been investigated spectro- scopically. The apparent association constants at a constant pH have been determined. Using the concept of preferential association of the deprotonated form of the indicator with the micelles, the apparent pKa's of the indicator as a function of the surfactant concentra- tion have been predicted and the values are found to be in agreement with the determined values. The micellar effects on pKa have been quantitati- vely explained on the basis of surface potential of the micellel,2, dielectric property of micelle2, and pseudophase ion-exchange3,4. Drummond et al.5 found that in the case of charged aqueous micel- lar solutions, the changes in pKa can be attributed to the differences between the mean intrinsic sol- vent properties of the interfacial and bulk phases with an additional contribution from the electro- static micellar surface potential. The study of the equilibrium of preferential as- sociation of such indicator systems with surfactant micelles of same charge should provide valuable information regarding the pKa changes in such micellar media including its dependence on the concentration of the surfactant. Acridine, a simple A+Bo indicator (a representative example of a class of DNA intercalators)6 was chosen for such studies with three cationic surfactants, viz., dode- cyltrimethylammonium bromide (DTAB), tetrade- cyltrimethylammonium bromide (ITAB) and he- xadecyltrimethylammonium bromide (CTAB). Experimental Acridine (2,3,5,6-dibenzopyridine), a Sigma product, was recrystallized from ethanol-water mixture. Dodecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide were obtained from Sigma and tetradecyltrimethylam- monium bromide _(cetrimide) was from Sisco Res- earch Laboratories. The surfactants were recrys- tallized from acetone before use. The spectra were recorded by a Hitachi-330 Spectrophotometer using a matched pair of one cm path length cells fitted with a thermostated cell holder. The pH values were measured using a Systronics Digital pH Meter Model-335. All the experiments were carried out at 298 (± 0.1)K. Results and discussion The variation of spectral absorption of aqueous acridine at pH 4.5 with varying amounts of ITAB is shown in Fig. 1. With the increase in concen- tration of ITAB, there is a decrease in the ab- sorbance in the 355 nm band in addition to that of the visible region, (373 to 450 nm bands) and there is slight increase in absorbance in 360 to 373 nm region. There are two isosbestic points at 373 nm and 360 nm. The effect of added ITAB on the spectra of acridine at a constant pH is si- milar to that of increasing pH in the absence of the surfactant. Acridine solution which was origin- ally yellow becomes colourless on increasing the' 0-61- u-1 2 3 4 5 0'4' - .. - 6 •• v c tl -e 0 .",- .<), <{ ~50 400 450 Wavelength (nm) Fig. I-Spectra of acridine (lAX 10-4 mol dm-3) in aqueous TIAB solutions at pH 4.53; concentrations of TIABx 10-2 mol dm - J: 1 = 0.00, 2 = 0.50, 3 = 1.25, 4 = 2.50, 5 = 3.50, " 6 = 6.00, 7 = 10.00 and 8 = 20.00.