SPONTANEOUS CURVATURE AND THERMODYNAMIC STABILITY OF MIXED AMPHIPHILIC LAYERS

The elastic behavior of mixed bilayers composed of two amphiphilic com ponents with different chain length (and identical head groups) is stu died using two molecular level models. In both, the bilayer free energ y is expressed as a sum of chain, head group and interfacial contribut ions as well as a mixing entropy term. The head group and interfacial terms are modeled using simple phenomenological but general expression s. The models differ in their treatment of the chain conformational fr ee energy. In one it is calculated using a detailed mean-field molecul ar theory. The other is based on a simple ''compression'' model. Both models lead to similar conclusions. Expressing the bilayer free energy as a sum of its two monolayer contributions, a thermodynamic stabilit y analysis is performed to examine the possibility of spontaneous vesi cle formation. To this end, we expand the bilayer free energy as a pow er series (up to second order) in terms of the monolayer curvatures, t heir amphiphilic compositions and the average cross sectional areas pe r molecule; all variables are coupled, with the molecular composition and areas treated as degrees of freedom which are allowed to relax dur ing bending. Using reasonable molecular interaction parameters we find that a second order transition from a planar to a curved (vesicle) ge ometry in a randomly mixed bilayer is unlikely. Most of our analysis i s devoted to calculating the spontaneous curvature and the bending rig idity of the bilayer as a function of its amphiphile chain composition . We find that adding short chain amphiphiles to a layer of long chain molecules reduces considerably its bending rigidity, as already known from calculations involving only the chain contributions. However, we find that inclusion of head group and interfacial interactions modera tes the effect of the added short chains. We also find that the bendin g rigidity Of pure monolayers is approximately linear in chain length, as compared to the nearly cubic dependence implied by the chain free energy alone (at constant head group area). Our main result involves t he calculation of the spontaneous curvature as a function of compositi on. We find, for different chain mixtures, that upon adding short chai ns to long chain monolayers, the spontaneous curvature first increases nearly Linearly with composition and then (beyond mole fraction of ab out 0.5) begins to saturate towards the spontaneous curvature of a pur e short chain layer. Qualitative arguments are provided to explain thi s behavior. (C) 1995 American Institute of physics.