Micellar rate effects-assumptions and approximations

Abstract

Surfactants with ionic or polar head groups and extended apolar groups form micelles in water with hydrocarbon-like interiors and ionic or polar groups at the surface and bind ionic and nonionic solutes. They act as reaction regions, i.e., as pseudophases distinct from the bulk solvent, and can accelerate or inhibit reactions, depending on rate constants and reactant concentrations in the two regions. Theoretical treatments and experimental evidence of reactant partitioning between water and micelles allow estimation of rate constants in the micellar pseudophase for both uni-and bimolecular reactions, provided that they are slower than reactant transfers between water and micelles, but some treatments apply only to dilute electrolyte. Competition between reactive and inert ions inhibits bimolecular ionic reactions and is treated by ion-exchange equations and some local ionic concentrations can be estimated by dediazonization trapping or physical methods. Ionic micelles affect rates of unimolecular and bimolecular watercatalyzed reactions because the reaction region at the micelle-water interface is less polar than water. Zwitterionic micelles have no net charge but they interact with ions, although ion-binding is weaker than with ionic micelles and there are limitations in the use of the theoretical treatments applied to ionic micelles. Micelles can control product composition, but product isolation limits use of surfactants in some reactions

Physicochemical Properties and Supernucleophilicity of Oxime-Functionalized Surfactants: Hydrolytic Catalysts toward Dephosphorylation of Di- and Triphosphate Esters

“…Among kinetic approaches to treat experimental data on reaction occurring in micellar solution there are two most often used by researchers, namely pseudophase ion exchange (PIE) and pseudophase partititoning (PPM). The (PIE) model takes into account an exchange taking place in Stern layer between counterions and added ions including those being capable to interact chemically with a substrate bound at the micellar interface.…”

“…From this standpoint, amphiphilic systems composed of surfactant and nucleophilic moiety such as oximes, hydroxamates, peroxides o -iodosyl carboxylates, hydroxybenzotriazoles, and hypochlorite, etc., are of great significance. A promising method includes introduction of a nucleophilic moiety into the headgroup of surfactants to produce so-called “functionalized surfactants” with high solubilizing power. − Nucleophilic functionalized surfactants are among the most powerful reactants in cleavage of organophosphorous compounds. − The most useful are α-nucleophilic moieties which are to be deprotonated in their reactive anionic form, and pure functionalized micelles are zwitterionic. − Although it has long been known that cationic micellar mediums enhance the reaction rates, much interest has been devoted to the study of cosurfactants and mixed surfactant systems that are required to enhance solubility of sparingly soluble substrates and reduce the cost of mixed surfactant systems, and to find additional tools for tuning reactivity. − Micellar properties of functionalized micelles and p K a are the most sensitive to the micelle composition parameter that can be changed in the mixed systems . The higher the cationic surfactant content, the lower is the p K a, and, thus, “milder” the decontamination micellar system .…”

SN2 Mechanism of Cationic Micelles on the Hydrolysis of Bis-p-Methoxyphenyl Phosphate Ester

“…The reactions of phosphate bis-ester were strappingly catalysed at different concentrations of CTABr at which pseudo first order rate constants were obtained. Investigation of micelles catalysed hydrolysis (Bunton, 2011;Reale et al, 2010;Silva et al, 2009) of bis-p-MPPE with hydroxide ion has been carried out at temperature (40±0.2 °C) in presence or absence of detergent (10 -3 to 10 -4 mol/dm) at pH-8.0 to 10.0 using borate buffers. Effect of cationic (CTABr) detergent on the rate of hydrolysis of bis-p-MPPE in presence of hydroxide ion has been measured spectrophotometrically by the rate of appearance of inorganic phosphate (Kumar and singh, 2011) (Table 1) and rate increases sharply at CTABr concentration greater than the critical micelle concentration (CMC) for CTABr at pH-9.0 in 2.5×10 -3 mol/dm 3 borate buffer CMC = 80×10 -3 M determined by dye method (Jaks et al, 2010;Duynstee and Grunward, 1959).…”

Reactivity in organised assemblies

“…Mejuto and co-workers 5 reviewed the fading of carbocations in micellar and vesicular solutions as well as in microemulsions. Bunton 6 reviewed the area of micellar rate effects in aqueous solutions of ionic and zwitterionic surfactants, with a focus on micelle-models and approaches allowing the analysis of kinetic data.…”