Aggregation and adsorption at the air-solution interface of the cetyltrimethyl ammonium tosylate with two poly(oxyethylene)-poly(oxypropylene)- poly(oxyethylene) block copolymers aqueous mixtures

Abstract

The aqueous mixed systems (EO76PO30EO76) (TBCP8400)-cetyltrimethyl ammonium tosylate (CTAT), and (EO97PO69EO97) (TBCP12600)-CTAT were studied to determine both the bulk aggregation and the adsorbed monolayer at the air/water interface. Results were interpreted with the pseudophase separation model plus the regular solution theory for aggregates and monolayers. The behavior is different for TBCP8400-CTAT and TBCP12600-CTAT mixtures, but it is strongly non-ideal in both cases. In bulk, TBCP8400-CTAT mixtures produce aggregates more close to CTAT micelles having TBCP8400 as a solubilizate than the inverse. At low CTAT content, the interaction is repulsive becoming attractive at high TBCP8400 content. The TBCP12600-CTAT aggregates strongly differ from the structure of both pure component micelles, and the interaction is always repulsive. In both cases, the interaction seems not to be cooperative but gradual. CTAT effect on copolymers aggregates seems to be more similar to that of a zwitterionic surfactant than to that of an ionic one. However, CTAT is not included in the aggregates as an ion pair, as revealed by the ionization degree results. It seems that cetyltrimethyl ammonium and tosylate ions have different effects on aggregates which in part are opposite. The adsorbed monolayers also show different behavior. In TBCP8400-CTAT system, the monolayer is mainly a CTAT one with inclusion of TBCP8400 as a monolayer-soluble impurity. However, the inclusion of the non-ionic surfactant alters the structure of the monolayer, which differs from that of the pure CTAT one. The area per adsorbed molecule (A0) is systematically higher than the ideal and computed ones. The system TBCP12600-CTAT shows a monolayer composition which is almost the same that the overall surfactant mixture composition, and the monolayer structure differs from both the pure-TBCP12600 and the pure-CTAT monolayers ones. The experimental A0 values are systematically lower than the ideal and the computed ones. Then, in both cases the A0 values for the pure components do not remain invariable in the mixed monolayer. The phenomenon is interpreted on the basis of the conformation of the copolymers adsorbed at the air/solution interface. © AOCS 2011.