The relative solution and interfacial hydrophobicity of ethylene oxide—propylene oxide—ethylene oxide block copolymers

By varying the ratio of the ethylene oxide (EO) to propylene oxide (PO) units in a series of poly(ethylene oxide—propylene oxide—ethylene oxide) (EPE) non-ionic copolymers we have been able to quantify the relative hydrophobic energy contribution of the EO and PO units in bulk solution and interfacial processes. These polymers are known to form aggregates in solution which consist of a hydrophobic PO core surrounded by a hydrophilic EO shell. In order to determine the hydrophobic character of the respective EPE polymers the partitioning of pyrene, a strongly hydrophobic probe, between the aqueous environment and the hydrophobic polymer environment was monitored as a function of polymer concentration. Aggregate hydrophobicity was found to be dependent on both the EO and PO chain lengths such that the hydrophobic contribution per PO unit is ten times the hydrophilic contribution per EO unit. The ability of EPE to stabilise an aqueous colloidal dispersion of hydrophobic particles (carbon black) was measured as a function of EPE concentration. The critical stabilisation concentration, which represents the onset of colloidal stability, was round to be inversely related to the hydrophobicity of the polymer aggregates in the bulk solution. The information gathered in this study allows the optimum EO:PO:EO ratio for stabilisation of a hydrophobic colloidal dispersion to be predicted.