Effects of inclusions and the presence of an adsorbing surface on lateral and flip-flop phase transitions in fluid membranes

Abstract

In this paper, we first investigate the effects of inclusions on the critical behavior of a fluid bilayer biomembrane made of two incompatible amphiphiles A and B. Upon variation of certain appropriate parameters (temperature, lateral pressure, etc.), these lipid molecules can undergo a lateral or transverse phase separation into macro-phase domains. Such transitions are of ordered liquid-disordered liquid type, and the free energy allowing the description of the physical system is a two-order parameters field theory. We show that the presence of a random distribution of inclusions (proteins, enzymes, etc.) may generate a high order coupling interaction term Ωφ2ψ2, which tends to favor flip-flop transition. We evaluate in particular the effect, close to the consolute point, of inclusions on the behavior of the composition fluctuations, φ and ψ, as well as that of the partial and the overall compressibilities in terms of temperature T and chemical potential difference Δμ. Moreover, we show that the presence of inclusions (such as translocases) leaves the values of the critical exponents unchanged, but that the amplitudes are shifted to their high values. We have also established clear links with some experimental work, which concerns a vesicle membrane, proving in particular the existence of a critical temperature Tc in the medium coupling regime. In the second part of this work, we investigate how the critical behavior of the system is affected near an adsorbing wall. We assume that one or both amphiphiles are strongly adsorbed on the surface. We assist therefore to a quenched composition on the wall. The segregation is assumed to occur at low temperature. The influence, on the critical properties of the fluid membrane, of the quenched surface fluctuations is discussed, in all cases, and the composition profile determined.