Bending Elasticity of Charged Surfactant Layers:  The Effect of Mixing

• Published in: Langmuir 2006-08, Vol.22 (16), p.6796-6813
• Main Author: Bergström, L. Magnus
• Format: Article
• Language: English
• Subjects: aqueous mixtures; bilayer; cationic surfactants; Chemistry; Elasticity; Exact sciences and technology; General and physical chemistry; Hydrophobic and Hydrophilic Interactions; membranes; Membranes, Artificial; Models, Chemical; monolayers; NATURAL SCIENCES; NATURVETENSKAP; phase-behavior; spontaneous curvature; Surface-Active Agents - chemistry; thermodynamics; vesicle formation
• ISSN: 0743-7463; 1520-5827; 1520-5827
• DOI: 10.1021/la060520t

Expressions have been derived from which the spontaneous curvature (H 0), bending rigidity (k c), and saddle-splay constant (k̄ c) of mixed monolayers and bilayers may be calculated from molecular and solution properties as well as experimentally available quantities such as the macroscopic hydrophobic−hydrophilic interfacial tension. Three different cases of binary surfactant mixtures have been treated in detail:  (i) mixtures of an ionic and a nonionic surfactant, (ii) mixtures of two oppositely charged surfactants, and (iii) mixtures of two ionic surfactants with identical headgroups but different tail volumes. It is demonstrated that k c H 0, k c, and k̄ c for mixtures of surfactants with flexible tails may be subdivided into one contribution that is due to bending properties of an infinitely thin surface as calculated from the Poisson−Boltzmann mean field theory and one contribution appearing as a result of the surfactant film having a finite thickness with the surface of charge located somewhat outside the hydrophobic−hydrophilic interface. As a matter of fact, the picture becomes completely different as finite layer thickness effects are taken into account, and as a result, the spontaneous curvature is extensively lowered whereas the bending rigidity is raised. Furthermore, an additional contribution to k c is present for surfactant mixtures but is absent for k c H 0 and k̄ c. This contribution appears as a consequence of the minimization of the free energy with respect to the composition of a surfactant layer that is open in the thermodynamic sense and must always be negative (i.e., k c is generally found to be brought down by the process of mixing two or more surfactants). The magnitude of the reduction of k c increases with increasing asymmetry between two surfactants with respect to headgroup charge number and tail volume. As a consequence, the bending rigidity assumes the lowest values for layers formed in mixtures of two oppositely charged surfactants, and k c is further reduced in anionic/cationic surfactant mixtures where the surfactant in excess has the smaller tail volume. Likewise, the reduction of k c is enhanced in mixtures of an ionic and a nonionic surfactant where the ionic surfactant has the smaller tail. The effective bilayer bending constant (k bi) is also found to be reduced by mixing, and as a result, k bi is seen to go through a minimum at some intermediate composition. The reduction of k bi is expected to be most pronounced in