The Effects of Alkyl Chain Combinations on the Structural and Mechanical Properties of Biomimetic Ion Pair Amphiphile Bilayers.

Ion pair amphiphile (IPA), a lipid-like complex composed of a pair of cationic and anionic surfactants, has great potentials in various pharmaceutical applications. In this work, we utilized molecular dynamics (MD) simulation to systematically examine the structural and mechanical properties of the biomimetic bilayers consist of alkyltrimethyl-ammonium-alkylsulfate (Cm TMA⁺-Cn S- ) IPAs with various alkyl chain combinations. Our simulations show an intrinsic one-atom offset for the Cm TMA⁺ and Cn S- alignment, leading to the asymmetric index definition of ΔC = m - (n + 1). Larger |ΔC| gives rise to higher conformational fluctuations of the alkyl chains with the reduced packing order and mechanical strength. In contrast, increasing the IPA chain length enhances the van der Waals interactions within the bilayer and thus improves the bilayer packing order and mechanical properties. Further elongating the Cm TMA⁺-Cn S- alkyl chains to m and n ≥ 12 causes the liquid disorder to gel phase transition of the bilayer at 298 K, with the threshold membrane properties of 0.45 nm² molecular area, deuterium order parameter value of 0.31, and effective bending rigidity of 20 kB T, etc. The combined results provide molecular insights into the design of biomimetic IPA bilayers with wide structural and mechanical characteristics for various applications.