The aggregate formation and surface adsorption of aqueous binary mixture of short-chain phospholipids, dihexanoyl-phosphatidylcholine (DC6PC), and dioctanoyl-phosphatidylcholine (DC8PC), were examined by surface tension and dilational viscoelasticity measurements. The concentration versus composition diagram of aggregate formation was constructed by analyzing the surface tension data. The diagram demonstrated a synergetic effect that even when the mole fraction of DC8PC in the aqueous solution was 0.02, worm-like micelle was formed and furthermore the addition of DC6PC to DC8PC solution lowered the DC8PC concentration at which worm-like micelle formation starts, although DC6PC molecules do not form worm-like micelles solely. The phase diagram of adsorption was also constructed to elucidate the miscibility of two components in the adsorbed film. The calculated excess Gibbs energy of adsorption was negative, again suggesting a synergistic attractive intermolecular interaction between two species in the adsorbed film. A staggered arrangement was proposed as a favorable intermolecular arrangement. The dilational viscoelasticity was also measured by the electrocapillary wave method, and both storage and loss modulus showed a maximum at around the surface composition of the minimum of the excess Gibbs energy of adsorption.
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