Reversed adaptive system of PNIPAM-decorated nanoparticles induced by the ionic strength of the subphase

We investigate the effects of ionic strength on the self-assembly of poly(N-isopropyl acrylamide)-nanoparticles (PSiFe) at the air-water interface. These responsive nanoparticles exhibit temperature-dependent behavior, forming more compact films at high temperatures (above the lower critical solution temperature, LCST). Upon adding potassium chloride (KCl) to the subphase, we observe an unexpected increase in the distance between nanoparticles, attributed to the creation of ionic clouds around them. This shifts the isotherms significantly compared to those formed at pure water, as larger areas of the interface are required to accommodate the same number of particles. Moreover, our findings reveal that increasing ionic strength alters the PNIPAM conformation, inhibiting nanoparticle aggregation even at higher temperatures. Using Langmuir-Blodgett techniques, we confirm that ionic strength modulates particle spacing and assembly dynamics. These results provide new insights into the control of nanoparticle interactions at interfaces, with potential applications in the design of tunable, stimuli-responsive materials.