We studied the adsorption and interactions of poly(NIPAM) particles (Volumetric Phase Transition at 32oC/90oF) at Air-Water and Oil-Water interfaces. Experiments with a Langmuir Trough in combination with characterization of the molar mass, allowed measuring the hitherto unknown equation of state. From it, we found that adsorbed particles are strongly stretched, giving rise to discernable surface pressures even at low coverage densities. Combining these results with measurements of how the surface pressure of a Pendant Bubble evolves in a freshly created pNIPAM solution, allowed to analyze the adsorption kinetics. Diffusive transport and the creation of a (growing) adsorption barrier as the adsorption proceeded, were corroborated. Remarkably, the final area per particle was practically the same for all bulk pNIPAM concentrations; suggesting a drastic change in the adsorption barrier and/or net adsorption energy as the coverage density reaches this value. Additional experiments at the Oil/Water interface using Pendant Drops afforded well-defined variations of both temperature and droplet area. Cycling between 24oC (75oF) and 36oC (97oF) indicated a full reversibility of the surface pressure, while large stepwise reductions in the droplet area resulted in surface pressure responses without a relaxation time. These findings suggest that the pNIPAM particles adsorb irreversibly. Remarkably, raising the temperature from 24oC to 36oC gave a significant increase in surface pressure, in spite of the considerable shrinkage of the particle in the bulk liquid. This trend will be tentatively explained.

Interactions between soft microgel particles at fluid interfaces

249th National Meeting (2015)

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