Lipid membranes that exhibit fluid–fluid phase coexistence are widely studied as models for the physical state of lipids in raft-forming biomembranes. Yet, whereas model membranes are typically symmetric, biomembranes maintain different lipid compositions in their two leaflets. In the plasma membrane, only the extracellular but not the cytoplasmic leaf contains a raft-competent lipid mixture. This raises the question if a raft-like state in the extracellular layer is able to induce a similar state also on the cytoplasmic side. Recent studies have thus begun to investigate the problem of trans-monolayer domain coupling in model membranes. Experimental work on asymmetric membranes has revealed that a tendency to phase-separate in one leaf can induce phase separation on the other leaf. Conversely, the lack of such a tendency can prevent phase separation in the apposed leaf. The strength of the coupling between domains can be described by a composition-dependent surface tension or, for small compositional changes, by a single coupling constant. Theoretical work has demonstrated how the coupling constant affects the phase behavior of a binary membrane. We summarize previous experimental findings and modeling efforts. In addition, three possible physical mechanisms of trans-monolayer domain coupling are discussed: electrostatic coupling, cholesterol flip-flop, and dynamic chain interdigitation. We argue that dynamic chain interdigitation likely provides the main contribution to the coupling constant. The charges of acidic lipids tend to reduce the coupling constant – but the reduction is insignificant even at low salt content.

In one of the most commonly used phenomenological descriptions of the electrical double layer, a charged solid surface and a diffuse region of mobile ions are separated from each other by a thin charge-depleted Stern layer. The Stern layer acts as a capacitor that improves the classical Gouy–Chapman model by increasing the magnitude of the surface potential and limiting the maximal counterion concentration. We show that very similar Stern-like properties of the diffuse double layer emerge naturally from adding a nonelectrostatic hydration repulsion to the electrostatic Coulomb potential. The interplay of electrostatic attraction and hydration repulsion of the counterions and the surface leads to the formation of a diffuse counterion layer that remains well separated from the surface. In addition, hydration repulsions between the ions limit and control the maximal ion concentration and widen the width of the diffuse double layer. Our mean-field model, which we express in terms of electrostatic and hydratio...

We suggest a minimal model for the coupling of the lateral phase behavior in an asymmetric lipid membrane across its two monolayers. Our model employs one single order parameter for each monolayer leaflet, namely its composition. Regular solution theory on the mean-field level is used to describe the free energy in each individual leaflet. Coupling between monolayers entails an energy penalty for any local compositional differences across the membrane. We calculate and analyze the phase behavior of this model. It predicts a range of possible scenarios. A monolayer with a propensity for phase separation is able to induce phase separation in the apposed monolayer. Conversely, a monolayer without this propensity is able to prevent phase separation in the apposed monolayer. If there is phase separation in the membrane, it may lead to either complete or partial registration of the monolayer domains across the membrane. The latter case which corresponds to a three-phase coexistence is only found below a critical coupling strength. We calculate that critical coupling strength. Above the critical coupling strength, the membrane adopts a uniform compositional difference between its two monolayers everywhere in the membrane, implying phase coexistence between only two phases and thus perfect spatial registration of all domains on the apposed membrane leafs. We use the lattice Boltzmann simulation method to also study the morphologies that form during phase separation within the three-phase coexistence region. Generally, domains in one monolayer diffuse but remain fully enclosed within domains in the other monolayer.

We model the influence of rigid inclusions on the curvature elasticity of a lipid membrane. Our focus is on conelike transmembrane inclusions that are able to induce long-range deformations in the host bilayer membrane. The elastic properties of the membrane are described in terms of curvature and tilt elasticity. The latter adds an additional degree of freedom that allows the membrane to accommodate an inclusion not only through a curvature deformation but also via changes in lipid tilt. Using a (mean-field level) cell model for homogeneously distributed inclusions in a small membrane segment of prescribed (mesoscopic-scale) spherical shape, we calculate the optimal microscopic-scale deviation of the membrane shape around the intercalated inclusions and the corresponding free energy, analytically. We show that the lipid tilt degree of freedom can lead to local softening of the inclusion-containing lipid bilayer segment. The predicted softening requires a sufficiently small value of the tilt modulus; its origin lies in the reduction of the excess membrane-inclusion interaction energy. We compare our results to the case of suppressed microscopic shape relaxation. Here, too, local softening of the membrane is possible.

Primitive cell models help to understand the role that compartmentalization plays in origin of life scenarios.