The Relationship between the Gibbs Surface Excess of a Surfactant at the Air/Water, Air/Oil and Oil/Water Interfaces

The thermodynamic relationship among the Gibbs surface excesses at the three interfaces (A/W, O/N and A/O) has been established by using the relation of the Neumann's triangle on the oil lense. Glycerol trioleate (TO) and N-dodecyl-β-alanine (amphoteric) were used as the oil and the surfactant respectively. The adsorbed amounts of the surfactant at the A/W and the A/O interfaces and that of the oil at the A/W interface have been measured by radiotracer method using tritium and ¹⁴C-labeled compounds respectively. Interfacial tensions at the three interfaces have been determined by a Wilhelmy tensiometer to evaluate the surface excesses of the surfactant and the oil (TO) at the O/W interface separately as those obtained independently at the A/W and the A/O interfaces. It has been shown for the first time that the adsorbed film at the O/W interface consists of the adsorbed monolayer of the surfactant and the oil, similarly those at the A/W and the A/O interfaces. It has been found that the surface excess of the surfactant is slightly greater than the calculated from the Guggenheim's isotherm.