Phase Transitions in Phospholipid Monolayers: Theory Versus Experiments.

Area of Science:

• Statistical Mechanics
• Soft Matter Physics
• Surface Science

Background:

• The Doniach lattice gas (DLG) model describes ternary mixtures of water, ordered lipids, and disordered lipids at a 2D lattice.
• It maps to a spin-1 model, representing water, ordered, and disordered lipid states.
• Previous analyses used mean-field and pair approximations.

Purpose of the Study:

• To explicitly compare theoretical predictions of the DLG model at the pair approximation with experimental data.
• To analyze Langmuir monolayer compression experiments for DMPC and DPPC phospholipids.
• To assess the model's ability to capture observed phase transitions.

Main Methods:

• Utilized the Doniach lattice gas (DLG) model at the pair approximation.
• Performed isothermal monolayer compression experiments on DMPC and DPPC.
• Fitted model parameters to experimental data to generate phase diagrams.

Main Results:

• Model parameters derived from experimental data yielded phase diagrams qualitatively consistent with DMPC and DPPC monolayer phase transitions.
• The model predicted the absence of a low-density gas phase.
• Quantitative agreement between model predictions and experimental results was limited due to experimental reproducibility issues.

Conclusions:

• The DLG model at the pair approximation provides a qualitatively useful framework for understanding lipid monolayer behavior.
• Experimental factors like kinetic effects challenge precise quantitative validation.
• Further refinement may be needed for quantitative prediction of lipid monolayer phase behavior.