Assembly of Amphiphilic Hyperbranched Polymeric Ionic Liquids in Aqueous Media at Different pH and Ionic Strength

Abstract

We demonstrated the assembly of amphiphilic hyperbranched protic ionic liquids (HBP-ILs) based on aliphatic hyperbranched polyester (HBP) in aqueous media in a wide range of pH and ionic conditions. The series of new branched polyionic liquids with different terminal groups, HBP-ILs, was synthesized by neutralization of carboxylic and sulfonic terminal acid groups of hypebranched core with <i>N</i>-methyl­imidazole (Im) and 1,2,4-1<i>H</i>-triazole (Tr). HBP-IL compounds with triazole and imidazole counterions form 12–16 nm core–corona micelles at pH 11.6. We found that the introduction of long hydrophobic terminal groups such as <i>n</i>-octadecylurethane tails to initial hydrophobic HBP core has larger effect on the size of micellar assemblies than the introduction of ionic terminals groups. Furthermore, tuning the hydrophilic/hydrophobic balance of HBP-ILs can be achieved by changing the degree of ionization of terminal groups and counterions by reducing pH from 11.6 to 5.2 or ionic strength to 0.1 M. These changes caused the formation of much larger micellar aggregates with the size of 150–200 nm due to reduced ionization of carboxylic groups. At the same time, for sulfonate-containing HBP-ILs the micelle size increased modestly (to 25–40 nm) because of the higher degree of ionization of sulfonate terminal groups. The diverse aggregation behavior of these branched polymeric ionic liquids enables control over their micellar morphologies in solution and bulk states