Adsorption, micellization and antimicrobial activity of formyl-containing cationic surfactant in diluted aqueous solutions

Abstract

Surfactants based on dynamic covalent bonds have attracted significant interest over the past few decades due to the coexistence of covalent and non-covalent bonds. However, studies on the fundamental properties of their precursors are rare. Herein, we synthesized a series of formyl-containing cationic surfactants (N-alkyl-N,N-dimethyl-N-(2-(4-formyl-phenoxy)ethyl) ammonium bromides) bearing different hydrophobic chains, which can form dynamic imine bonds. The adsorption, micellization and antimicrobial activity were systematically investigated using surface tension, fluorescence, electrical conductivity, and ITC techniques. The results showed that the insertion of the 2-(4-formyl-phenoxy)ethyl group (equivalent to ~7 methylene groups) endows the surfactant with an asymmetrical double-chain structure, enhancing the hydrophobic interaction. As a result, the critical micelle concentration (cmc) and contribution per methylene group to the standard Gibbs free energy of micellization (∆Gmo) were significantly decreased, compared to conventional single surfactant. Upon increasing the hydrophobic chain length, the cmc, the negative logarithm of the surfactant concentration in the bulk phase required to produce a 20 mN·m−1 reduction in the surface tension of the solvent (pC20) and maximum surface excess amount of the adsorbed surfactant (Γmax) all decreased accompanied by an increase in the minimum molecular occupation area (Amin), while the ∆Gmo and standard Gibbs free energy of adsorption (∆Gadso) become more negative and thus more favorable for the formation of micelles and the adsorption film. Moreover, micellization changed from being entropy-driven to enthalpy-driven and is more sensitive toward NaCl than urea. In addition, the current formyl-containing cationic surfactants exhibit excellent antimicrobial activity against both Gram-positive and Gram-negative bacteria and fungi, especially the surfactant with a hydrophobic chain containing ~20 atoms. Such a study will be favorable for the design of new dynamic covalent surfactants and an understanding of the change in their aggregation.