Influence of inorganic and organic counter-cations on the surface properties and self-assembly of cyclic lipopeptide surfactin

Abstract

The cyclic lipopeptide surfactin (SF), an anionic biosurfactant that gaining interest because of its unique characteristics and promising applications, forms various self-assembled structures in solution. Although counter-cations are known to affect the surface properties and self-assembly of anionic surfactants, the influence of counter-cations on the behaviour of SF requires further investigation. Here, we report the self-assembly properties of SF with different organic and inorganic counter-cations including potassium (SF-K), monoethanolamine (SF-MEA), diethanolamine (SF-DEA), and triethanolamine (SF-TEA). We studied the influence of these counter-cations on the surface properties of SF as compared to the SF sodium salt (SF-Na) using the Wilhelmy plate method. The critical aggregation concentrations (CACs) of SF-K, SF-DEA, SF-TEA, and SF-MEA were 2.8–4 times higher than that of SF-Na (2.7 × 10 −5 M). Furthermore, the CACs increased as the organic counter-cations became less hydrophobic (SF-TEA (8.7 × 10 −5 M) < SF-DEA (9.8 × 10 −5 M) < SF-MEA (1.1 × 10 −4 M)). In addition, circular dichroism measurements revealed that a negative band corresponding to β-sheets increased with increasing organic counter-cation hydrophobicity (SF-MEA < SF-DEA < SF-TEA). Thus, more hydrophobic organic counter-cations may enhance intramolecular and intermolecular hydrogen bonding to promote SF self-assembly in aqueous solution. Furthermore, mixtures of SF-Na and alkylammonium bromides with different alkyl chain lengths provided stable colloidal suspensions, which consist of vesicles with a diameter around 100 nm. These findings provide further understanding of the effects of counter-cations on SF self-assembly, which may contribute towards the innovation of cosmetic and pharmaceutical formulations.