Kinetic and rheological measurements of the effects of inert 2-, 3- and 4-bromobenzoate ions on the cationic micellar-mediated rate of piperidinolysis of ionized phenyl salicylate

The effects of the concentration of inert organic salts, [MX], (MX = 2-, 3- and 4-BrBzNa with BrBzNa = BrC6H4CO2Na) on the rate of piperidinolysis of ionized phenyl salicylate (PS−) have been rationalized in terms of pseudophase micellar (PM) coupled with an empirical equation. The appearance of induction concentration in the plots of kobs versus [MX] (where kobs is pseudo-first-order rate constants for the reaction of piperidine (Pip) with PS−) is attributed to the occurrence of two or more than two independent ion exchange processes between different counterions at the cationic micellar surface. The derived kinetic equation, in terms of PM model coupled with an empirical equation, gives empirical parameters FX/S and KX/S whose magnitudes lead to the calculation of usual ion exchange constant KXBr (=KX/KBr with KX and KBr representing cationic micellar binding constants of counterions X− and Br−, respectively). The value of FX/S measures the fraction of S− (=PS−) ions transferred from the cationic micellar pseudophase to the aqueous phase by the optimum value of [MX] due to ion exchange X−/S−. Similarly, the value of KX/S measures the ability of X− ions to expel S− ions from cationic micellar pseudophase to aqueous phase through ion exchange X−/S−. This rather new technique gives the respective values of KXBr as 8.8 ± 0.3, 71 ± 6 and 62 ± 5 for X− = 2-, 3- and 4-BrBz−. Rheological measurements reveal the shear thinning behavior of all the surfactant solutions at 15 mM CTABr (cetyltrimethylammonium bromide) indicating indirectly the presence of rodlike micelles. The plots of shear viscosity (η) at a constant shear rate (γ), i.e. ηγ, versus [MX] at 15 mM CTABr exhibit maxima for MX = 3-BrBzNa and 4-BrBzNa while for MX = 2-BrBzNa, the viscosity maximum appears to be missing. Such viscosity maxima are generally formed in surfactant solutions containing long stiff and flexible rodlike micelles with entangled and branched/multiconnected networks. Thus, 15 mM CTABr solutions at different [MX] contain long stiff and flexible rodlike micelles for MX = 3- and 4-BrBzNa and short rodlike micelles for MX = 2-BrBzNa.

Graphical abstractKinetic observation of the effects of [3-BrBzNa] on the rate of piperidinolysis of ionized phenyl salicylate in the presence of CTABr micelles.Figure optionsDownload full-size imageDownload high-quality image (63 K)Download as PowerPoint slideResearch highlights► Determination of ion exchange constants, KXBr, for counterions of cationic micelles. ► New kinetic model for studying the effects of inert salts, MX, on reaction rates in CTABr micelles. ► Relationship between magnitudes of KXBr and formation of wormlike micelles. ► Kinetic demonstration of the occurrence of independent ion exchange processes on micelle. ► Maxima of the plots of shear viscosity vs. [MX] at 15 mM CTABr are indicative of wormlike micelles.