The relationship between the surface activity of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers and molecular composition

A systematic investigation of surface tension, as a function of copolymer concentration, has been undertaken for a range of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers in aqueous solution. The copolymers examined form three distinct families, containing 20, 70, and 80% ethylene oxide. The focus of the study was the change in surface tension observed below the critical micelle concentration of the triblock copolymers. Thus, in general, the plots of surface tension as a function of copolymer concentration showed a low concentration break. In some cases a high concentration break—corresponding to the onset of micellisation—was also observed. Estimates of molecular area have been made for molecules adsorbed at the air–water interface either side of the low concentration break. Below the break the molecular area does not appears to show any straightforward relationship with molecular composition. However, multiple linear regression analysis of the experimentally determined molecular area data above the break indicates that there does appear to be a weak relationship (adjusted R2 = 0.64) between molecular area and molecular composition (based upon the manufacturers’ data sheets). Estimates were also made of pC20 values (the negative logarithm of the copolymer concentration that reduces the surface tension of pure water by 20 mN m−1). Multiple linear regression analysis of the pC20 data indicates that pC20 values are related to molecular composition (adjusted R2 = 0.78). In addition, multiple linear regression analysis shows that surface tension values at a variety of aqueous copolymer concentrations are strongly correlated with molecular composition.