Mixing Temperatures of Bilayers Not Simply Related to Thickness Differences between Lo and Ld Phases.

Micron-scale coexisting Lo and Ld liquid phases can appear in lipid bilayers composed of a ternary mixture of a low-melting temperature lipid, a high-melting temperature lipid, and cholesterol. A priori, temperatures at which membranes demix, Tmix, are not simply related to differences in thicknesses, Δh, between Lo and Ld phases. Here, we use fluorescence microscopy to measure Tmix and we use atomic force microscopy at 22°C to measure Δh for a series of bilayers composed of different ratios of the three components. Our data illustrate cases in which a change in Tmix or Δh does not result in a change in the other parameter. The data provide a context in which to evaluate recent reports of a correlation between Tmix and Δh.