Mixed Monolayer Studies of Chlorophyll a and Plastoquinone 9 at the Nitrogen-Water Interface

The surface pressure-molecular area isotherm of the chlorophyll a-plastoquinone 9 binary mixture has been studied at various molar compositions at the nitrogen-water interface. The surface pressure-molecular area isotherms of the mixtures are interpreted using the additivity rule and surface phase rule in two dimensions The mixed system conforms to ideality regardless of the molar fraction. The mixed monolayers at molar fractions of chlorophyll a between 0.3 and 0.8 show two collapse pressures, one of which (the lower) varies with the film composition with the other collapse nearly constant at approximately 27 mN m^-1. Two collapses are also evident in the surface potential-molecular area isotherms. This apparent dependence of the collapse on the film composition is interpreted as a partial miscibility between the components, while complete miscibility is suggested at 0.9 mole fraction of chlorophyll a, where a single collapse pressure is observed.