Spread monolayers of poly(styrene)-poly(ethylene oxide) diblock copolymers (PSm-PEOn, m ) 38, n ) 90, 148, 250, and 445) have been studied at the air-water interface by measuring the surface pressurearea (π-A) isotherms at several temperatures. The π-A isotherms exhibit several regions which can be ascribed to different conformations of the polymer chains: a pancake structure at low surface pressures and high areas when the isolated chains are adsorbed by both the PS globule and the PEO segments at the interface; an intermediate structure, quasi-brush, when the PEO segments are solubilized in the subphase; and finally a brush developed at low surface areas when the PEO chains are obliged to stretch away from the interface to avoid overlapping. At surface pressures near 10 mN/m there is a transition between a high-density pancake and the quasi-brush regime. The compression and the subsequent expansion curves superpose at the transition and quasi-brush regions but not at the brush and pancake stages. This points to a high cohesion in the brush structure after compression and to some irreversible entanglement and hydration of the PEO chains when immersed in the subphase. These two local hystereses depend differently on the PEO chain length and temperature. The hysteresis observed at high surface pressures (brush conformation) decreases with the PEO length and temperature, whereas the low surface pressure hysteresis (pancake) increases with PEO chain length, decreases with temperature in the range 283-298 K, and increases in the range 298-315 K. A negative mean transition entropy change was obtained from the temperature dependence of the quasi-SSAL-quasi-brush transition. The results indicate that the extensive properties of the present diblock copolymers at the interface, such as the pancake limiting area and the mean transition entropy, when expressed by PEOmer, are independent of the PEO length.

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 x 2 > 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.