Synthesis, characterization, biodegradation, and evaluation of the surface-active properties of non-ionic gemini surfactants derived from lauryl diethanolamide

Abstract

The surface activities and application properties of aqueous solution surfactants are greatly influenced by their structure, especially the spacer group that connects the polar head groups. Herein, four new non-ionic Gemini surfactants with different spacers were designed and synthesized, and their surfactant properties and biodegradability were studied. The synthesis of these compounds involves a two-step procedure. The first step is the formation of an amide from lauric acid and diethanolamine. The second step is the reaction of lauryl diethanolamide with four different spacers, the latter being flexible-hydrophilic, and rigid-hydrophobic in structure, respectively. Their structures were characterized using ¹H NMR, ¹³C NMR, FT-IR, and ESI-MS. The critical micelle concentration (CMC), the surface tension at CMC (γ_CMC), the efficiency of these compounds to reduce the surface tension by 20 mN/m (C₂₀ and pC₂₀), the effectiveness (π_CMC), the maximum surface excess (Γ_max), and the minimum surface area (A_min) were measured at 20, 40, and 50°C. The molecular architecture of the spacers in these compounds strongly influences the thermodynamic parameters, such as the standard change for Gibbs free energy of adsorption (ΔG°_ads) and the standard change for Gibbs free energy of micellization (ΔG°_mic). The ability of these surfactants to reduce surface tension is particularly good, but their distinguishing characteristic is their high relative propensity to form micellar aggregates. This aggregation ability improves as the hydrophilicity and flexibility of the spacer increase. Finally, in less than 30 days, all non-ionic Gemini surfactants were determined to be 99% biodegradable in river water.