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 containing cadmium sulfate and barium chloride at 15 and 25 °C. The surface pressure-area (π-A) isotherms were determined by the Wilhelmy plate method. ClHx and BrHx by themselves do not form insoluble monolayers at the air/water interface. The shape of the mixed isotherms varies with the composition, the temperature, and the presence of metal ions in the subphase. For many compositions, the alkyl halide is partially squeezed out of the monolayer before the final collapse. It was observed that this initial collapse of the mixed monolayer decreases with the mole fraction of the alkyl halide and with temperature and varies with the halogen atom in the guest molecule: BrHx is preferred on the pure water subphase while, in the presence of Cd²⁺, ClHx is preferred. The presence of Cd²⁺ in the subphase promotes a condensing effect of the mixed monolayers, decreases the initial collapse surface pressure, and induces a second collapse in the two mixtures studied for a mole fraction of alkyl halide x₂ > 0.3. The condensing effect of Cd²⁺ 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 with the temperature.