Effect of Ion Specificity on the Interfacial Behavior of Amphiphilic Hyperbranched Copolymer H-P(OEGMA-co-LMA)

The interfacial aggregation behavior of two amphiphilic hyperbranched copolymers of poly[oligo(ethylene glycol) methacrylate-co-lauryl methacrylate] (H-P(OEGMA-co-LMA)) under neutral subphase conditions with different salts (NaCl, Na₂SO₄, NaNO₃, and NaSCN) was investigated by using the Langmuir film balance technique and atomic force microscopy. At the air/water interface, H-P(OEGMA-co-LMA) tends to form the dense network-like monolayer structures of core-shell micelles. At the low and middle salt concentrations, the limiting areas of the isotherms under various salt conditions are usually close to that without salt. At the high salt concentration, OEG side groups are more swollen/stretched at the interface due to the increased subphase density for various anions and the additional salting-in effect for the weakly hydrated anions. In hysteresis experiments, the OEG side groups being pressed into the water upon monolayer compression are less likely to return to the interface during the expansion process, and thus the hysteresis degrees with the addition of salts are larger than those without salt. For the two copolymers, the effects of the four anions on the stretching degrees of their OEG side groups are different due to their different densities in the micellar shells, which is not consistent with the Hofmeister series.