On the interaction of dipalmitoyl phosphatidylcholine with normal long-chain alcohols in a mixed monolayer: a thermodynamic study

This study investigated the mixed monolayer behavior of dipalmitoyl phospha tidylcholine (DPPC) with normal long-chain alcohols at the air/water interf ace. Surface pressure-area isotherms of mixed DPPC/C18OH and DPPC/ C20OH mo nolayers at 37 degrees C were obtained and compared with previous results f or the mixed DPPC/C16OH system. The negative deviations from additivity of the areas and the variation of the collapse pressure with composition imply that DPPC and long-chain alcohols were miscible and formed non-ideal monol ayers at the interface. At lower surface pressures, it seems that the attra ctive intermolecular force was dominant in molecular packing in the mixed m onolayers. At higher surface pressures, the data suggest that the molecular packing in mixed DPPC/C16OH monolayers may be favored by the packing effic iency or geometric accommodation. Furthermore, negative values of excess fr ee energy of mixing were obtained and became significant as the hydrocarbon chain length of alcohols increased, which indicates there were attractive interactions between DPPC and long-chain alcohols. In each free energy of m ixing-composition curve, there was only one minimum and thus a phase separa tion did not exist for mixed DPPC/long-chain alcohol monolayers. (C) 2000 E lsevier Science B.V. All rights reserved.