Film pressure (p) vs. surface area (a) isotherms were determine at 20?for single and mixed monolayers of Triton X-100 and Triton X-305 with polypropylene glycol (PPG), stearic acid (SA), or cetyl alcohol (CA) on the surface of 40% aqueous NaNO3 equimolar mixtures of TX 100/SA, TX 100/CA, TX 305/SA, and TX305/CA have p-a curves which show small pos. deviations from calculated average curves based on a simple additivity rule. As D. Shah and S. Shiao (1975) suggested, the expansion of mixed monolayers arising from unequal chain length probably is caused by thermal motion of the hydrocarbon chains. Equimolar mixtures of TX100/PPG and TX 305/PPG have p-a curves which show neg. deviations from calculated average curves. A previously proposed model in which the polyethylene glycol chain may be forced to enter the water phase under pressure can explain the results. A simple anal. of the shape of the compression curves of the surfac film indicates that in all these mixed monolayers, the 2 components are miscible on the surface of water.

Key words: INTERFACES, SURFACE ACTIVITY, POLYETHER, NON IONIC SURFACTANTS, MONOMOLECULAR FILMS, MONOLAYER