AFM surface morphology and friction force studies of microscale domain structures of binary phospholipids.

We have studied friction forces on binary mixtures of phospholipid monolayer films. The phospholipid monolayer films have been prepared via the Langmuir-Blodgett (LB) technique on mica. The two-component phospholipids are distearoylphosphatidylcholine (DSPC) and dilauroylphosphatidylcholine (DLPC). At 25 degrees C the LB monolayer films give the gel-state DSPC domains surrounded by the liquid crystalline DLPC matrix. The friction forces measured on the DSPC domain region are significantly greater than those on the DLPC matrix region at this temperature. An increased temperature results in a decreased friction of the DSPC domain region, and above the gel-to-liquid crystalline phase transition temperature of DSPC, the difference in the friction forces measured on the two phospholipids becomes negligible. This means that the phase state is a key factor in determining friction forces on the phospholipid monolayer films.