Interaction of ethylene oxide-propylene oxide copolymers with ionic surfactants studied by calorimetry: random versus block copolymers.

The present study used calorimetric techniques to follow the interaction of random and block ethylene oxide (EO)-propylene oxide (PO) copolymers with ionic surfactants. Features such as the intensity of the interaction (evaluated through their critical aggregation concentrations) and the profile of the isothermal titration calorimetry (ITC) curves were comparatively analyzed for random and block copolymers with similar composition (number of EO and PO units). Random copolymers displayed an interaction similar to that observed with other hydrophilic homopolymers with the additional characteristic that the intensity of the interaction increased with the increase in the copolymer hydrophobicity (as determined by its PO content), revealing that these copolymers display an intermediate behavior between PEO and PPO. For nonaggregated block copolymers (unimers) with large enough EO blocks (molar mass above 2000 g mol-1), ITC curves revealed that the anionic surfactant sodium dodecylsulfate (SDS) interacts with the PO and EO blocks almost independently, being more favorable with the PO block, which controls the critical aggregation concentration (cac) value. Effects of temperature and of the nature of the ionic surfactants on their interaction with these copolymers were found to agree with the previously reported trends.