Synthesis and Properties of Surface Chemically Pure Alkylamidoamine‐N‐oxides at the Air/Water Interface

Abstract

AbstractA homologous series of surface active 2‐(alkanoylamino)ethyldimethylamine‐N‐oxides, EDA‐p(O), and 3‐(alkanoylamino)propyldimethylamine‐N‐oxides, PDA‐p(O), were synthesized. Their aqueous stock solutions were processed by the automatically operating apparatus to remove surface‐active contamination and provide chemical purity at the air/water interface. In case of 3‐(tetradecanoylamino)propyldimethylamine‐N‐oxide, PDA‐14(O), the difference between equilibrium surface tension values of the purified surfactant solutions and the corresponding values of the solutions prepared from the “as received” compounds amounts to 15 mN m−1. Moreover, in presence of the surface‐active contaminants time needed to reach equilibrium surface tension values is over 2 h. For the solution of the “surface‐chemically pure” grade the change of the surface tension within adsorption time is negligible and the equilibrium is reached instantaneously. Physicochemical properties of obtained surface‐chemically pure aqueous solutions of N‐oxides of alkylamidoamines and adsorption parameters (standard free energy of adsorption, ΔG°ads, surface excess by saturation surface concentration, Γ∞, minimum surface area demand per molecule adsorbed, Amin) were evaluated from the equilibrium surface tension versus concentration isotherms at the air/water interface using Gibbs's equation. The introduction of the CH2 moiety into the intermediate part of molecule causes a slight decrease of the hydrophobic character of surfactant. Also the minimum surface area demand, Amin, is slightly greater for PDA series than for the corresponding EDA derivatives. Surface potential measurements were performed in addition to surface tension studies. Electric surface potential versus concentration isotherms was determined. Surface potential increases with increasing surfactant's bulk concentration for all investigated compounds. At highest concentrations, where interface is almost saturated, changes of surface potential become almost negligible.