Surface activity of poloxamines at the interfaces between air–water and hexane–water

Poloxamines are tetra functional block copolymers, which contain four polyethylene oxide (PEO)–polypropylene oxide (PPO) chains joined to a central ethylene diamine moiety by way of the nitrogen atoms. Surface and interfacial tensions at the air–water and hexane–water interfaces have been determined over a wide concentration range for several of these block copolymeric amphiphiles. Titration data indicates that these molecules are dibasic. Two pKa values—corresponding to proton loss from the central nitrogen atoms of the molecule—have been determined by fitting titrimetric data to a thermodynamic model of the titration process. The values obtained are fairly constant for different poloxamines, which suggests that varying the block size has little impact upon these pKa values. However, the values obtained are lower than those observed for ethylene diamine and indicate that the attachment of the PEO–PPO chains to the nitrogen atoms does reduce their basicity. The Gibbs adsorption isotherm at the air–water interface is similar to the adsorption pattern reported for the poloxamers-linear PEO–PPO–PEO block copolymers [Langmuir 10 (1994) 2604] and shows that the highly branched poloxamines have similar interfacial properties as the poloxamers. Data is also reported on adsorption of poloxamines at the hexane–water interface. In the main it is concluded that those block copolymers with high PPO to PEO ratios and which are, therefore, relatively hydrophobic are more effective in reducing surface and interfacial tensions. These effects are, however, attenuated by changes in aqueous phase pH, which alters the charged nature of the poloxamine molecule.