Novel asymmetrical bis-surfactants with naphthalene and two amide groups: Synthesis, foamability and foam stability

A series of novel asymmetrical bis-surfactants (CnNCS, n = 10, 12, 14, 16), comprising a naphthalene group and a long chain hydrocarbon (as hydrophobic moieties), a quaternary ammonium group, one ether group and double amides (as hydrophilic moieties), and ethylidene as linkage groups, were designed, prepared, and characterized through proton and carbon nuclear magnetic resonance, Fourier-transform infrared, and electrospray ionization mass spectrometry analyses. The surface activity was studied by surface tension measurements. CnNCS exhibited low critical micelle concentrations (0.708-0.004 mmol L-1) and excellent surface tension reducing abilities. Moreover, the introduction of naphthyl and amido groups into C12NCS significantly enhanced the foamability and foam stability. This is attributed mainly to cooperative noncovalent interactions such as hydrophobic interactions of the long chain hydrocarbons, pi-pi stacking of the naphthyl groups, hydrogen-bonding of the amido groups of C12NCS itself and hydrogen bonding of C12NCS with water, and electrostatic interaction, which induced a decrease in the surface tension, leading to a compact and dense arrangement of the C12NCS molecules at the air-water interface to form a tight film. Both features enabled foam formation, slowed liquid drainage from the foam, and prevented the gas from diffusing through the monolayer, thus enhancing the foam stability. In addition, the long-acting emulsifying ability of C12NCS and good antibacterial activity of CnNCS were also demonstrated. Moreover, hydrogen-bonding of the amido groups and pi-pi stacking of the naphthyl groups of CnNCS were verified from the H-1 NMR and fluorescence spectra, respectively. This work provides a feasible strategy for the synthesis of highly efficient foaming, emulsifying, and bacteriostatic agents. (C) 2021 Elsevier B.V. All rights reserved.