As the precursor of sphingolipids, sphingosine is one of important constituents of cell membranes. In this work, the thermodynamic behavior of mixed systems of D-sphingosine with 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) has been investigated by analyzing the data of surface pressure-mean molecular area isotherms. The deposited Langmuir-Blodgett (LB) monolayers of different ratios were investigated with atomic force microscopy (AFM) under certain surface pressure. The results show that: (1) in D-sphingosine/DPPC system, the excess molecular area and excess Gibbs energy took on negative deviations from the ideal behavior at D-sphingo- sine ratios of XD-Sph＝0.2, 0.4, 0.6 and positive values at XD-Sph＝0.8; and (2) as for D-sphingosine/DPPE systems, the excess molecular area and excess Gibbs energy displayed negative values at the surface pressure of π＜25 mN•m－1, and positive at π≥25 mN•m－1. The interaction of mixtures was determined by molecular area and Gibbs energy. The negative deviations from ideality suggest attractive interaction between molecules, with aggregating phenomenon. Furthermore, positive deviation from linearity indicates repelling interaction between the two compounds, inducing phase separation. Moreover, the mixed monolayer is more stable if the value of excess Gibbs energy is lower. The compression and AFM experiments support the above findings effectively as well as interpretation.

Key words: monolayer, excess Gibbs energy, elastic modulus, atomic force microscopy