Analogy between micelles and polymers of ionic surfactants: A capillary isotachophoretic study of small ionic aggregates in water-organic solutions

Abstract

Capillary isotachophoresis (ITP) was applied to monitoring the synthesis of cationic polymers by γ-irradiation of various dodecyland hexadecylallyldimethylammonium and dodecylvinylimidazolium salts in aqueous solutions at concentrations considerably exceeding their respective critical micellar concentrations (CMC). The isotachopherograms obtained under both acidic and basic watermethanol electrolyte conditions showed distinctive steps for polymer and monomer surfactants, and also for amine (oxide) degradation products. This enables one to determine the amount of remaining monomer as a function of irradiation time, surfactant counter ion and initial monomer concentration. It was found, as far as the polymerization yield was concerned, that halide counter ions played a special (favourable or unfavourable) part in the polymerization reaction, and that bromide was the most suitable ion associated with allylic surfactants. An optimum irradiation time of ca. 25 h was determined for these allylic species, above which the ratio of degradation products was observed to increase markedly. Likewise, the initial monomer concentration should not exceed a second critical concentration, beyond which the polymerization yield would fall drastically. In addition, it was ascertained that when the reaction was allowed to take place in an isotropic medium ( i.e., at a concentration below the CMC), only low-mobility degradation products were obtained. ITP also appeared to be well adapted to the physico-chemical characterization of these ionic polymerized surfactants. This method enables polymers to be compared with dynamic micelies. The mobility of the polymers was found to be much greater than that of the starting monomer and close to that of a classical dynamic micelle. The analysis of fractions obtained from ultrafiltration assays revealed that their molecular weight was higher than 5000 g/mol. Hence it can be expected to be of the same order of magnitude than that of dynamic micelles. However, unlike the latter, the polymers remain stable in methanolic solutions and for monomer equivalent concentrations less than the CMC. Lastly, the degree of counter-ion binding, also determinable from isotachopherograms, is much higher for the polymers than for the ionic micelies. Nevertheless, these properties overall justify the term polymerized micelles currently applied to these polymers.