Specific anion modulated adsorption of a Pluronic copolymer at the air-water interface

Amphiphilic block copolymers are ubiquitous in natural and engineered systems, where interfacial and self-assembly behaviour is highly sensitive to environmental factors such as temperature as well as electrolyte type and concentration. Pluronic^®F127 is one such triblock copolymer, and is composed of a hydrophobic poly(propylene oxide) moiety encased by two more hydrophilic poly(ethylene oxide) moieties. F127 is ubiquitous in biomedical and pharmaceutical applications. Herein, the interfacial behaviour of F127 in aqueous electrolyte solutions was investigated across a range of temperatures and concentrations using pendant drop tensiometry and neutron reflectometry (NR). Across all cases examined, F127 readily adsorbed at the air-water interface. In the absence of salt, temperature-induced modulations in the adsorbed amount were observed, however, only minimal changes in interfacial conformation were present; PPO blocks remained anchored at the interface and PEO blocks extended into the aqueous subphase. Surface tension measurements showed an interfacial accumulation in the presence of salting-in anions (e.g.) and a concentration-dependent shift from accumulation to depletion for salting-out anions (e.g.). However, only through complementary NR measurements can the specific species responsible for interfacial accumulation and depletion be unambiguously identified. In brief, NR revealed that at low temperatures, classically salting-out ions increased the adsorbed layer thickness, whereas salting-in ions decreased it. This indicates that at low temperatures, salting-out ions promoted the accumulation of F127 at the interface, while salting-in ions led to its depletion. At higher temperatures, all electrolytes manifested a decrease in the adsorbed layer thickness which translated to a depletion of F127 from the air-water interface. These results indicate that there is a complex balance between polymer solubility and ion hydration, whereby ion-induced effects are primarily driven by a delicate competition between ion and polymer for interfacial occupancy.