Monolayer properties of mixed chlorophyll a-digalactosyldiacylglycerol at the nitrogen–water interface

Abstract

Surface pressure, surface potential, and ellipsometric–area isotherms have been measured to investigate the properties of chlorophyll a – digalactosyldiacylglycerol monolayers at the nitrogen – water interface. The surface pressure – area isotherms of the mixed film show a pronounced negative deviation with respect to ideality, whereas the surface potential isotherms follow the additivity law for mixed films. As the mole fraction of Chl a increases from 0.1 to 0.5, each of the corresponding ellipsometric isotherm is shifted towards more negative values in the liquid expanded region. As the mole fraction is increased above 0.5, the ellipsometric δΔ values become more positive, with this trend continuing throughout the 0.5 to 0.9 mole fraction range. The combined effects of anisotropy in both the refractive indices and the absorption coefficient can explain the turnover observed in the ellipsometric δΔ isotherms as the mole fraction of Chl a becomes larger than 0.5.Hydrophilic interactions have been ruled out on the basis of the agreement with the additivity law, which is observed for surface potentials of the mixed monolayers. The observed deviation exhibited in the π–A isotherms is interpreted in terms of the intermolecular cavity effect. Key words: chlorophyll a, digalactosyldiacylglycerol, mixed monolayer, ellipsometry.