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 °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 °C in contact mode showed significant contrasts in deformation elasticity, adhesion, and jump-to-surface. These physical property differences were kept below 40 °C, while they almost disappeared over 40 °C. In addition, the reversibility of the properties for the temperature change was also found.