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

Jordi Faraudo; Arturo Moncho-Jordá; Delfi Bastos-González; Carlos Drummond

doi:10.1021/acs.macromol.3c00132

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

Macromolecules. 2023 Mar 17;56(6):2246-2257. doi: 10.1021/acs.macromol.3c00132. eCollection 2023 Mar 28.

Authors

Jordi Faraudo¹, Arturo Moncho-Jordá^{2

3}, Delfi Bastos-González², Carlos Drummond⁴

Affiliations

¹ Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, E-08193 Bellaterra, Spain.
² Biocolloid and Fluid Physics Group, Department of Applied Physics, University of Granada, Avenida Fuentenueva 2, E-18071 Granada, Spain.
³ Institute Carlos I for Theoretical and Computational Physics, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva S/N, 18071 Granada, Spain.
⁴ Centre de Recherche Paul Pascal Université Bordeaux, CNRS, CRPP, UMR 5031, F-33600 Pessac, France.

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.