Driving Force for Spontaneous Perforation of Bilayers Formed by Ionic Amphiphiles in Aqueous Salt

Abstract

Spontaneous perforation of amphiphilic membranes is important in both living matter and technology because of an impact on functions of biological membranes and shape transitions of self-assembling structures. Nevertheless, no definite molecular mechanism has been established so far even for simple ionic surfactant systems. We show that spontaneous perforation of a bilayer formed by an ionic amphiphile is driven by electrostatics. Creation of large pores with a concave-convex geometry of the rim is promoted by lower electrostatic free energy than that for a flat nonperforated bilayer. The opposite effect comes from the elasticity of the hydrocarbon tails of the amphiphile that prefer flat geometry of a nonperforated bilayer. The balance between electrostatics and tail deformation controls the appearance of pores; this balance is modulated by added salt that screens the electrostatic interactions. We illustrate the proposed mechanism with the aid of classical aggregation model that has been extended by including an analytical description of the electrostatic contribution for the toroidal rim of a pore. Numerical solution of the linearized Poisson-Boltzmann equation confirms the role of electrostatic forces in formation of pores. For the ionic surfactants of CTAB family, we predict shape transitions including bilayer perforations and formation of branched micellar networks induced by changing salinity or temperature and demonstrate the effect of surfactant's molecular parameters on these transitions.

Correlation of Emulsion Chemistry, Film Morphology, and Device Performance in Polyfluorene LEDs Deposited by RIR-MAPLE

“…However, in both cases, the surfactant did not help the emulsion to form a well-defined cylindrical micelle structure even after simulating the system for an additional 50 ns (200 ns total). This type of intermediate structure is not uncommon for systems with surfactants. − Wang et al also observed such network cylinder structures for higher concentrations of polymer using dissipative particle dynamics (DPD) simulation . Almgren exclusively studied perforated bilayers and concluded that such intermediate structures appear while increasing the concentration of surfactants as there is a transition to the micelle structure .…”

Modeling Micellar Growth and Branching in Mixtures of Zwitterionic with Ionic Surfactants

“…7,8,16,17 A junction of three cylindrical aggregates is represented by a curvilinear triangular piece of bilayer rimmed by three toroidal elements (Figure 1). 15 For the junction, the standard free energy of aggregation is estimated as a linear combination 15,18 g g g (jun)…”

“…A junction of three cylindrical aggregates is represented by a curvilinear triangular piece of bilayer rimmed by three toroidal elements (Figure ). For the junction, the standard free energy of aggregation is estimated as a linear combination , where g (tor) and g (pl) are the free energies of the junction’s toroidal and planar elements, respectively; the factors χ tor and χ pl take into account relative contribution of these elements to the total free energy of the junction: Here n tor and n pl are the total aggregation numbers, n A + n B , in the toroidal and planar parts of the junction, respectively.…”