Interfacial properties of mixtures of sorbitan sesquioleate with a block copolymer surfactant at the water/air and water/oil interfaces

Surface pressure—area (π-A) isotherms for the non-ionic surfactant sorbitan sesquioleate (Arlacel 83) have been measured at both the water/air and water/oil interfaces. The effect of high concentrations of ammonium nitrate on the monolayer was assessed. Equations of state are applied Gaines' equation is found to be particularly useful. Mixed monolayers with a non-ionic A-B-A block copolymer, where A is poly(12-hydroxystearic) acid B is polyethylene oxide, have been examined. The two surfactants show efficient packing at both the water/air and water/oil interfaces. It is shown that there is no significant effect of the electrolyte on the surfactant close-packed molecular areas up to the maximum electrolyte concentration studied (8.75 mol dm −3 ). There is evidence to suggest that at the water/oil interface the block copolymer improves the stability of the sorbitan sesquioleate film by reducing its tendency for dissolution into the oil phase.