liquidexpanded

Liquid-expanded, abbreviated LE, is a fluid phase observed in lipid monolayers at the air–water interface. It describes a state in which lipid molecules are more expanded and less tightly packed than in the adjacent liquid-condensed (LC) phase. Some literature and data sets write the term without a space as liquidexpanded, but the conventional designation is LE.

In the LE phase, the area per molecule is larger, the monolayer is more compressible, and the hydrocarbon chains are less ordered, often adopting more gauche conformations. Compared with LC, LE features lower packing density and greater molecular mobility. The LE region typically occurs at lower surface pressures and can transition to LC with increasing compression, decreasing temperature, or changes in lipid composition.

Phase behavior in Langmuir monolayers often shows coexistence between LE and LC domains near phase boundaries. Cholesterol and other sterols can modify the balance between LE and LC, and may give rise to ancillary phases such as liquid-ordered (Lo) in certain lipid mixtures. The LE–LC balance is influenced by chain length, saturation, headgroup, and temperature, making LE a useful model for studying lipid condensation, packing, and the mechanical properties of membranes.

LE is commonly studied using Langmuir troughs and related techniques (surface pressure–area isotherms, fluorescence or Brewster angle microscopy) to understand how lipid composition and external conditions affect monolayer organization. The LE state provides insight into membrane fluidity, domain formation, and the physical basis of lipid–protein interactions in simplified model systems.