  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.
