Undecanoic and 10-Undecenoic Acid-Based Amidobetaines and Amidoamine Oxides

Three types of undecanoic and 10-undecenoic acid-based surfactants were synthesized in the present work: amphoteric amidobetaines, cationic amidobetaine chlorides and nonionic amidoamine oxides. Structural characterizations of synthesized compounds were based on nuclear magnetic resonance (NMR) (¹H and ¹³C) and mass spectrometry. Surface properties, such as critical micelle concentration (CMC), surface tension at cmc (γ _cmc), efficiency of surface adsorption (pC20), surface excess (Γ_max) and minimum area per molecule (A _min) at the air–water interface, were determined by surface tension methods. Fluorescence probing techniques were also employed for the measurement of CMC, as well as steady state anisotropy (r) at the micellar core. The CMC of the studied surfactants follow the order: amidobetaine > amidobetaine chloride > amidoamine oxide. The influence of the terminal double bond in the hydrophobic alkyl chain on CMC was also assessed, and a significant increase in CMC was found due to the introduction of the double bond in the cases of amidobetaine chlorides and amidoamine oxides. These two types of surfactants showed higher rigidity at the micellar core compared to their corresponding unsaturated counterparts. However, such influence of unsaturation on the hydrophobic moiety was not observed in the case of amidobetaines. In all three types of surfactants, the saturated surfactant exhibited a lower γ _cmc and A _min, but higher Γ_max, r and pC20 compared to its unsaturated counterpart.