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There is an increasing demand to solve the environmental dissemination problems of fluorosurfactants and improve their responsive behavior, surface properties, chemical stability and foaming performance. In this work, three pH-responsive fluorosurfactants, named F9-es-mor, F9-N3-mor and F9-N2-mor, were synthesized by facile one-step esterification or amidation. The F9-N2-mor efficiently reduced the water surface tension to 14.56 mN·m−1 with a low critical micelle concentration (CMC) of 0.20 mM. More importantly, both F9-N2-mor and F9-N3-mor obviously displayed reproducible pH-response foaming behaviors at an optimal pH of 6.0 and 8.0, respectively, probably owing to their rigid molecular structure, strong intermolecular H-bond and high membrane elasticity. Besides, the morpholine fluorosurfactants defoam at pH below 4.0, indicating that these fluorosurfactants could be recycled and their residue could be easily removed. Overall, the novel fluorosurfactants F9-N2-mor might provide new inspiration to recycle fluorosurfactants as well as construct pH-responsive foams.

Two series of novel gemini surfactants with morpholinium headgroup and benzene-based rigid spacers (abbreviated as (Mor)m-P-m and (Mor)m-BP-m, where m represents the carbon numbers of hydrophobic chains) were synthesized and characterized by 1H NMR, ESI-MS, and FT-IR spectra. The effect of lengths of rigid spacer and hydrocarbon chain on their solution properties and antibacterial activities were systematically investigated. Due to the existence of morpholinium, they have a superior surface activity to the classic gemini surfactants. (Mor)m-BP-m possess lower CMC and γcmc than (Mor)m-P-m, which is assumed to be related to the conformation change of spacer. The possible arrangement models for the adsorptions of (Mor)m-P-m and (Mor)m-BP-m at the air-water interface were proposed by comparing their surface parameters with those of other similar surfactants. The pre-micellar associations occur in aqueous solutions of (Mor)16-P-16, (Mor)14-BP-14 and (Mor)16-BP-16. The thermodynamic parameters indicate the micellar processes are spontaneous, and (Mor)m-BP-m have stronger aggregation tendency than (Mor)m-P-m. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) show they spontaneously self-assemble into vesicle, and the aggregate size of (Mor)m-BP-m is larger than (Mor)m-P-m. These surfactants showed excellent antibacterial properties against Bacillus subtilis and Escherichia coli. Compared with (Mor)m-P-m, (Mor)m-BP-m exhibit a higher activity.

Two nonionic Gemini fluorocarbon surfactants containing ecofriendly ester-bonded, named as MN-2C9F19 and EN-2C9F19 were designed and synthesized by one-step esterification. Their surface properties, wettability, thermal properties, and foaming characterization were systematically studied. Two surfactants exhibited high surface activity in aqueous solution, with surface tension (γcmc) of 14.20 mN/m for MN-2C9F19 and 13.71 mN/m for EN-2C9F19. Moreover, MN-2C9F19 and EN-2C9F19 showed high decomposition temperature (Td) of up to 165.4 °C and 176.7 °C respectively. Although the structural difference between methyl and ethyl group is negligible, their wettability and foaming properties are quite different. EN-2C9F19 could wet the low surface energy PTEF plate completely at low concentration (0.6 mM) but MN-2C9F19 needed higher concentration (1.2 mM). Also, EN-2C9F19 exhibited much better foaming property with the foam integrated value F up to 4,370,500 mL·s while MN-2C9F19 could reach up to 2,732,580 mL·s. This maybe ascribe to lower surface tension and higher viscosity of EN-2C9F19 solution. Furthermore, two surfactants showed stronger foaming properties at high temperatures than the conventional foam surfactant SDBS even at room temperature. This work provided an easy synthesis route to develop high-performance foaming agents at high temperatures.