Poly(ethylene oxide) alkyl ether C18E20 was in combination with each of three n-fatty alcohols (1-octanol, n-dodecanol and n-octadecanol) respectively as a model of non-ionic surfactant and polar organic substance composites, which was spreaded onto air-water interface and formed a mixed monolayer on a Langmuir trough. The stability of each mixed monolayer was analyzed by use of π-A isotherms at various mole fractions of the C18E20-fatty alcohol pair. It was found that the stability of the C18E20 monolayer could be enhanced by combining with fatty alcohols, based two points. The one is the expanding degree of the monolayer decreases as the surface pressure of the mixed monolayer is under the inflexion of the π-A curve, and the other is that the plateau length of the curve decreases after the surface pressure is above the inflexion of the π-A curve. The condensability of the mixed monolayer increases with the mole ratio of fatty alcohol, or with the hydrophobic chain length of the alcohol more close to that of C18E20. For almost all of the π-A isotherms of the mixed monolayers, the inflexion and the plateau always appeared near the surface pressure 30 mN/m, which showed that a phase separating occurs at a higher surface pressure. It was also observed that the mixed monolayer is in a favorable condition to the compatibility of the multiple surfactants, as the mole ratio of C18E20 and octadecanoic alcohol is equal to 3 ∶1.
