Interaction-Limited Aggregation: Fine-Tuning the Size of pNIPAM Particles by Association with Hydrophobic Ions

Abstract

We have investigated the formation of stable clusters of poly(N-isopropylacrylamide) (pNIPAM) chains in water at temperatures above the lower critical solution temperature (LCST), induced by the presence of sodium tetraphenylborate, NaPh4B. The hydrophobic Ph4B– ions interact strongly with the pNIPAM chains, providing them with a net effective negative charge, which leads to the stabilization of pNIPAM clusters for temperatures above the LCST, with a mean cluster size that depends non-monotonically on salt concentration. Combining experiments with physical modeling at the mesoscopic level and atomistic molecular dynamic simulations, we show that this effect is caused by the interplay between the hydrophobic attraction between pNIPAM chains and the electrostatic repulsion induced by the associated Ph4B– ions. These results provide insight on the significance of weak associative anion–polymer interaction driven by hydrophobic interaction and how this anionic binding can prevent macroscopic phase separation. Harvesting the competition between attractive hydrophobic and repulsive electrostatic interaction opens avenues for the dynamic control of the formation of well-calibrated polymer microparticles.

Solvation of Nanoions in Aqueous Solutions

“…In recent years it has been discovered that highly charged ions of nanometer size like polyoxometalates, , tetraphenylborate, , or dodecaborate clusters share a number of unique properties despite their drastically different chemical nature: they associate with neutral polar surfaces, surfactants, , and small nonionic molecules, they show high-affinity binding to host molecules like cyclodextrins, and they induce a significant increase of the cloud point , of nonionic surfactants. These properties give rise to a rich spectrum of applications, ranging from biological applications as membrane carrier − or as inhibitors of HIV protease to formulation applications like foam stabilization or alternatives to traditional surfactants .…”