Theories of Equilibrium Inhomogeneous Fluids
I review two theoretical explanations for the existence of inhomogeneities in a fluid bilayer, such as the mammalian plasma membrane, which one might well expect to be homogeneous. The first is the existence of a phase separation. If biologically relevant temperatures are below the critical temperature of the separation, then these inhomogeneities are simply inclusions of one phase within the other. One has to understand, however, why macroscopic separation is not seen in the plasma membrane. If biologically relevant temperatures are above the critical temperature, then the inhomogeneities could be ascribed to critical fluctuations. There are difficulties with this interpretation which I note. The second possible interpretation is that the dynamic heterogeneities are evidence of a two-dimensional microemulsion. Several mechanisms which could give rise to it are discussed. Particular attention is paid to the coupling of membrane height fluctuations to composition differences. Such a mechanism naturally gives rise to a length scale which is of the correct order of magnitude for the domains postulated to exist in the plasma membrane.
KeywordsRafts Phase separation Critical phenomena Modulated phases Microemulsions
I have been working in this area for many years now and have been fortunate in my colleagues. First and foremost are the “amphiphilophiles” with whom I meet weekly: Sarah Keller, Lutz Maibaum, and their students, both current and former, like Sarah Veatch, Aurelia Honerkamp Smith, and Matt Blosser, and Post-Doctoral Fellows, Marcus Collins and Thomas Portet. I thank my own Post-Doctoral Fellows, Roie Shlomovitz and Ha Giang for many hours of stimulating conversation. I have enjoyed interactions on the theory of this subject with former colleagues, Marcus Mueller and Friederike Schmid, and am grateful to the experimentalists who have shared their knowledge with me: Erwin London, Gerry Feigenson, and John Katsaras. Finally I am indebted to the National Science Foundation for their constant support. This work was supported by the NSF on Grant No. DMR-1203282.
- 7.Veatch S, Keller S (2005) Miscibility phase diagrams of giant vesicles containing sphingomyelin. Phys Rev Lett 94:148101-1–148101-4Google Scholar
- 8.Elliott R, Szleifer I, Schick M (2006) Phase diagram of a ternary mixture of cholesterol and saturated and unsaturated lipids calculated from a microscopic model. Phys Rev Lett 96:098101-1–098101-4Google Scholar
- 10.Putzel G, Schick M (2011) Insights on raft behavior from minimal phenomenological models. J Phys Condens Matter 23:284101:1–284101:5Google Scholar
- 27.Gompper G, Schick M (1994) Self-assembling amphiphilic systems. Academic, San DiegoGoogle Scholar
- 28.Prigogine I, Defay R (1954) Chemical thermodynamics. Longmans Green, LondonGoogle Scholar
- 37.Hirose Y, Komura S, Andelman D (2012) Concentration fluctuations and phase transitions in coupled modulated bilayers. Phys Rev E 86:021916-1–021916-113Google Scholar
- 46.Schick M (2012) Membrane heterogeneity: manifestation of a curvature-induced microemulsion. Phys Rev E 85:031902-1–031902-4Google Scholar
- 49.Safran S (1994) Statistical thermodynamics of surfaces, interfaces, and membranes. Addison-Wesley, ReadingGoogle Scholar
- 50.Diamant H (2011) Model-free thermodynamics of fluid vesicles. Phys Rev E 84:061123-1–061123-7Google Scholar