Temperature effect on nanometer-scale physical properties of mixed phospholipid monolayers.

Mixed dipalmitoylphosphatidylcholine (DPPC) and dioleoylphosphatidylcholine (DOPC) monolayers have been deposited on mica using Langmuir-Blodgett technique, as a model system for biomembranes. Nanometer-scale surface physical properties were quantitatively characterized with the gradual temperature change using the atomic force microscope. At 25 degrees C, tapping mode imaging revealed the clear phase-separation in the form of microscopic DPPC domain embedded in a DOPC matrix and the obvious step height between the higher DPPC phase and the lower DOPC phase. Surface force measurement made at 25 degrees C in contact mode showed significant contrasts in deformation elasticity, adhesion, and jump-to-surface. These physical property differences were kept below 40 degrees C, while they almost disappeared over 40 degrees C. In addition, the reversibility of the properties for the temperature change was also found.