Contrasts in the surface activity of polyoxypropylene and polyoxybutylene-based block copolymer surfactants

Previous studies have shown that polyoxybutylene surfactant hydrophobes are less polar than polyoxypropylene hydrophobes. Polyoxyalkylene hydrophobes can be modified by ethoxylation to give terminal polyoxyethylene block hydrophilic groups. Polyoxybutylene/polyoxyethylene copolymer surfactants exhibit differentiated interfacial tensions, and wetting and foaming properties when compared to structurally equivalent polyoxypropylene/polyoxyethylene copolymers. There have been no reported comparisons, however, of polyoxybutylene/polyoxyethylene copolymers and polyoxypropylene/polyoxyethylene analogues in terms of fundamental parameters, such as critical micelle concentration, area per molecule at the interface, efficiency, and effectiveness. In one phase of this work, four polyoxybutylene/polyoxyethylene block copolymer surfactants were compared to structurally analogous polyoxypropylene/polyoxyethylene materials. Findings showed that polyoxybutylene/polyoxyethylene copolymers exhibited enhanced cotton wetting and lower surface and interfacial tensions compared to polyoxypropylene/polyoxyethylene analogues. Polyoxybutylene-based surfactants were typically one order of magnitude better in their ability to reduce surface tension at interface saturation. Polyoxybutylene/polyoxyethylene copolymers pack more efficiently at the interface, as evidenced by a smaller area per molecule at the interface. Critical micelle concentration values were also lower for polyoxybutylene/polyoxyethylene copolymers. A second phase of experiments focused on the surface activity of polyoxypropylene/polyoxyethylene triblock copolymers with higher molecular weight hydrophobes. Enhanced surface activity was found when compared to lower-molecular weight polyoxypropylene/polyoxyethylene copolymers.