Based on the available surface state equation for phospholipid monolayers, a generalized equation was derived by introducing a third parameter—comparative expanding factor. It was supposed that the phospholipid monolayer surface characteristics have the close relation with not only the interaction between lipid intermolecules, but also the interaction between subsolution molecules and lipid ones. Therefore the third parameter—comparative expanding factor was defined as the ratio of the two interactions. Associating the reduced temperature T_r (based on the phase transition temperature T_c) and the reduced molecular area A_r (based on the molecular area of the liquid condensed film A_c) with the third parameter, the surface state equation was generalized to relate the π-A isotherm data of 1,2-dimyristoyl-sn-glycero-3-phosphocho- line, 1,2-dimyristoyl-sn-glycero-3-phosphatidic acid, and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine with small errors. The results show that the generalized state equation could describe the surface characteristics of phospholipid monolayer during the expansion-compression circulation, and the data of the third parameter might reflect the rates of different phospholipids expanding to monolayers.

Key words: phospholipid monolayer, state equation, π-A isotherm, generalization