Effect of surfactant concentration on the responsiveness of a thermoresponsive copolymer/surfactant mixture with potential application on Smart foams formulations

We studied a system formed by a mixture of a thermoresponsive negatively charged graft copolymer (Alg-g-PNIPAAm) with a brush-type structure, and an oppositely charged surfactant (DTAB), in bulk and at the air-solution interface. We performed experiments of surface tension, electrophoretic mobility, dynamic and static light scattering and atomic force microscopy in order to characterize the complexes formed as a function of DTAB concentration and temperature. We found that these polymer-surfactant complexes are able to respond by changing their sizes, both in bulk and at the air-solution interface, when T is increased above the coil-globule transition temperature (LSCT) of the copolymer. However, the thermoresponse was found to be dependent on surfactant concentration, cs: for cs < 2.8 mM, the size of the aggregates decreases as T increases but, for cs >= 2.8 mM, the opposite behavior takes place, i.e. the size increases with T. At the interface, the intensity of the effect produced on the surface tension by increasing T above LCST diminishes continuously as cs increases, reducing the ability of the interfacial complex to respond to temperature changes. We studied the stability of aqueous foams formulated with these mixtures as a function of T and cs. We found that the stability of the foam can be modulated by changing T, but we observed that this effect is dependent on the surfactant concentration range. We found a correlation between changes in the aggregates sizes, the surface tension behavior and the responsiveness of foam stability to changes of temperature.