Interaction between two homologues of cationic surface active ionic liquids and the PEO-PPO-PEO triblock copolymers in aqueous solutions

Abstract

The interaction of nonionic triblock copolymers of poly(ethyleneoxide) (PEO) and poly(propyleneoxide) (PPO) (PEOnPPOmPEOn) with a series of cationic surface-active ionic liquids in aqueous solutions have been investigated. The cationic surface-active ionic liquids include 1-alkyl-3-methylimidazolium bromide (CnmimBr, n = 8, 10, 12, 14, 16) and N-alkyl-N-methylpyrrolidinium bromide (CnMPB, n = 12, 14, 16). For different polymer-surfactant systems, the critical aggregation surfactant concentration (cac), the surfactant concentration to form free micelles (C m), and the saturation concentration of surfactant on the polymer chains (C 2) were determined using isothermal titration microcalorimetry (ITC) and conductivity measurements. The structure of the formed aggregates depended strongly on the hydrophobicity of the surfactant and the ratio of polymer/surfactant concentration. For C8mimBr, there were not any micelle-like surfactant–polymer clusters detected in the solution, and only micelles appeared. For other surfactants, the polymer–surfactant aggregates were formed in the solution, which was verified by the appearance of a broad endothermic peak in the ITC thermograms. The intensity of polymer–surfactant interaction increased with the hydrophobicity of the surfactants and the polymers but was not affected by the surfactant headgroups.