MIXED MONOLAYERS OF HEPTADECANOIC ACID WITH CHLOROHEXADECANE AND BROMOHEXADECANE - EFFECTS OF TEMPERATURE AND OF METAL-IONS IN THE SUBPHASE

Mixed monomolecular films of heptadecanoic acid (HpA)-chlorohexadecane (ClHx) and heptadecanoic acid-bromohexadecane (BrHx) at the air/water interface were studied on ultrapure water and on aqueous substrates c ontaining cadmium sulfate and barium chloride at 15 and 25 degrees C. The surface pressure-area (pi-A) isotherms were determined by the Wilh elmy plate method. ClHx and BrHx by themselves do not form insoluble m onolayers at the air/water interface. The shape of the mixed isotherms varies with the composition, the temperature, and the presence of met al ions in the subphase. For many compositions, the alkyl halide is pa rtially squeezed out of the monolayer before the final collapse. It wa s observed that this initial collapse of the mixed monolayer decreases with the mole fraction of the alkyl halide and with temperature and v aries with the halogen atom in the guest molecule: BrHx is preferred o n the pure water subphase while, in the presence of Cd2+, ClHx is pref erred. The presence of Cd2+ in the subphase promotes a condensing effe ct of the mixed monolayers, decreases the initial collapse surface pre ssure, and induces a second collapse in the two mixtures studied for a mole fraction of alkyl halide x(2) > 0.3. The condensing effect of Cd 2+ is more pronounced in the system with chloride than in the presence of bromine, On the basis of monolayer collapse surface pressures and mean molecular areas, the two-dimensional miscibility was investigated . The range of miscibility decreases with the surface pressure and wit h the temperature.