Interaction of Cetyl-trimethylammonium Bromide with Swollen and Collapsed Poly(N-isopropylacrylamide) Nanogel Particles

The interaction of cetyl-trimethylammonium bromide (CTAB) with swollen and collapsed poly(N-isopropylacrylamide) (pNIPAM) monodisperse nanogel particles was investigated by electrophoretic mobility, dynamic light scattering, and potentiometric surfactant activity measurements. The surfactant binds to the nanogel particles as monomers in the whole CTAB concentration range and binds in the form of surfactant aggregates as well above a critical concentration (cac) in both the swollen and collapsed state of the pNIPAM. The swollen particle system is a thermodynamically stable solution. The collapsed nanogel particle system is an electrically stabilized colloid dispersion, which coagulates when the particles are near the electrically neutral state. An analytically undetectably small amount of surfactant binding (5 x 10(-7) mol/g of pNIPAM) leads to a dramatic effect on the stability of the pNIPAM nanogel system. The electrokinetic potential versus surfactant concentration functions unexpectedly strongly depend upon the temperature around the lower critical solution temperature (LCST) of the polymer, which was interpreted by the change of the polymer segment density in the surface layer of the collapsing nanogel particles.