Soft microgel particles at fluid interfaces

Abstract

We investigate the use of soft microgel particles based on a thermosensitive poly- mer, poly-N-isopropylacrylamide (PNIPAM) to stabilize a fluid interface. We also study the effect of temperature on the adsorption kinetics and the interfacial behaviour of these particles on air-water and oil-water interfaces. To begin with we study the adsorption dynamics of microgel particles at an air- water interface. We experimentally establish the relation between the surface pressure and surface concentration (an EOS) using compression isotherms of spread monolay- ers of microgel particles. The isotherms detect measurable surface pressures at inter- particle distances that are much greater than the particle diameter. This suggests that the particles are strongly deformed. We establish separate EOS for various temper- atures. These experimental EOS have been used to study the adsorption kinetics of microgel particles. A simple model that takes into account diffusion limited adsorp- tion at short times and a barrier limited adsorption at long times has been developed. The solution to such a model is fitted to the experimental data using diffusion coef- ficient and adsorption rate constant as fit parameters. The diffusion coefficient thus extracted agreed well with ones measured using DLS. Further we look into microgel particles adsorbed on an oil-water interface. Our ob- servations confirm the fact that the particles are irreversibly adsorbed on the oil-water interface. The electrophoretic measurements suggest a 2.5 fold increase in the effec- tive surface potential over the range of temperatures studied. The particles interact via long range dipole-dipole interactions. In addition to this the particles also experience a short range attraction owing to the hydrophobic interactions between the loose poly- mer segments along the periphery of the particles. The net effect is that the interfacial microstructure goes from a dense, ordered structure at lower temperatures to a more open network of particles at higher temperatures. These insights will prove crucial if PNIPAM based microgel particles are to be used as Pickering stabilisers for foams and emulsions. The stimuli responsive nature of these particles provides us with a handle to control the particle interactions, their interfacial microstructure and interfacial rheology and consequently on the stability of foams and emulsions prepared using such particles as stabilisers.