Miscibility of octyldimethylphosphine oxide and decyldimethylphosphine oxide in the adsorbed film and micelle

The surface tension of the aqueous solution of octyldimethylphosphine oxide (OPO) and decyldimethylphosphine oxide (DePO) mixture was measured as a function of the total molality and the composition of the mixture at 298.15 K under atmospheric pressure. The experimental data were analyzed thermodynamically to evaluate the total surface density Γ^H, the composition in the adsorbed film X", and the composition in the micelle χ^M₂. The relations among the compositions of bulk solution, adsorbed film, and micelle were expressed in the form of phase diagram similar to the three-dimensional one; they were compared with the phase diagrams of ideal mixed film and micelle obtained theoretically. It was concluded that the OPO and DePO molecules mix ideally in the adsorbed film while the difference in their hydrophobic chain length causes a weaker interaction in the micelle because of a spherical shape of the micelle. This conclusion was supported by the fact that the 2-(octylsulfinyl)ethanol and 2-(decylsulfinyl)ethanol mixture behaves quite analogously.