Effect of organic and inorganic ions on the lower critical solution transition and aggregation of PNIPAM

Abstract

We have studied the effect of different ions belonging to the extended Hofmeister series on the thermosensitive polymer poly(N-isopropylacrylamide) (PNIPAM), by combining Differential Scanning Calorimetry (DSC) and Dynamic Light Scattering (DLS). The variations in the lower critical solution temperature (T_LCS) and enthalpy change during PNIPAM phase separation evidence the importance of considering both hydration and hydrophobicity to explain the interaction of ions with interfaces. The results obtained in the presence of inorganic ions can be explained by the tendency of water molecules to preferentially hydrate the PNIPAM chains or the ions, depending on the kosmotropic (highly hydrated) or chaotropic (poorly hydrated) character of the ions. On the contrary, tetraphenyl organic ions (Ph₄B⁻ and Ph₄As⁺) interact with the hydrophobic moieties of PNIPAM chains, inducing a significant reduction of the T_LCS. DLS results show that the aggregation state of PNIPAM above the T_LCS is also strongly influenced by the presence of ions. While macroscopic phase separation (formation of a polymer-rich phase insoluble in water) was apparent in the presence of inorganic ions, we observed the formation of submicron PNIPAM aggregates at temperatures above the T_LCS in the presence of the hydrophobic ions. Kinetically arrested monodisperse PNIPAM nanoparticles were formed in the presence of the Ph₄B⁻ anion, while a rather polydisperse distribution of particle sizes was observed in the presence of Ph₄As⁺. These results show that ionic specificity influences both the static (thermodynamic) and dynamic (kinetically controlled aggregation) states of PNIPAM in an aqueous environment.