Physicochemical properties and esterolytic reactivity of oxime functionalized surfactants in pH-responsive mixed micellar system
Abstract
We present comprehensive study of the aggregation properties of zwitterionic micelles of oxime-functionalized surfactants and their mixed micellar systems with conventional cationic CTABr and nonionic Tween (R) 80 surfactants.Analysis of the micellar effects on the reaction rates toward activated esters is also completed. Specifically, aggregation properties of micelles of amphiphilic 1-alkyl-3-methyl-2-(oximinomethypimidazolium and 1-alkyl-3(1-oximinoethyl)pyridinum bromides (alkyl = CnH2n+1,where n = 12, 16) and their mixed micellar systems with CTABr and Tween (R) 80 have been investigated.The changes in micellar properties and reaction rates toward 4-nitrophenyl esters of diethyl phosphoric (NPDEP),diethyl phosphonic (NPDEPN),and toluenesulphonic (NPOTos) acids,with increase of pH ensuring deprotonation of specific oximate moiety,have been studied.We focused on the changes of micellar properties of mixed micelles depending on the mixture composition and the deprotonation degree of the functional oximate group.The critical micelle concentration (cmc) values,degrees of counterion binding (beta),and size of micellar aggregates were obtained using surface tension measurements,dynamic light scattering method, bromide selective electrode, and by solubilization of hydrophobic dye Orange OT.The surface excess concentration (T-max, mu mol/m(2)),minimum area per molecule (A(min)),interaction parameters (beta(m)),standard Gibbs free energy of adsorption (Delta G(ads)(o)) and micellization (Delta G(m)(o)) and excess free energy of micellization (Delta G(ex)) have been evaluated.Micellar effects of the systems studied on the acyl transfer reaction rates were shown to increase with higher fraction of deprotonated oxime and can be treated in the framework of a pseudophase partitioning model.These results provide new information on (i) control the reactivity of the organized molecular systems and (ii) elaboration the basis for designing pH responsive supramolecular assemblies.