The synthesis of quaternary N-alkyl tropinium cationic surfactants and study on their properties: effect of temperature, hydrophobic chain length and anions

Abstract

• A series of quaternary N-alkyl tropinium cationic surfactants (TILS) with different anion were synthesized and characterized. • By changing counter ions, the krafft point was significantly reduced (from 313.15 K to below 273.15 K) • By increasing the hydrophobicity of anions, the critical micelle concentration ( CMC ) value of TILS decreased about 4-5 times (being compared with bromide). • The effects of temperature, hydrophobic chain length, and anions on the micellar properties of TILS were investigated. A series of quaternary N-alkyl tropinium cationic surfactants (TILS) were synthesized and characterized. The effect of temperature, hydrophobic chain length and anions on their properties were studied. By changing the structure of anion, the krafft point was significantly reduced (from 313.15 K to below 273.15 K) and wide temperature window was obtained. The critical micelle concentration ( CMC ) value of TILS based on electrical conductivity method was found to be decreased about 4-5 times by increasing hydrophobicity of anions. At lower temperature and longer hydrophobic chain, TILS had lower CMC value and better surface activity. The thermodynamic properties of micellization Gibbs free energy Δ G mic , enthalpy Δ H mic and entropy T Δ S mic of micelles were obtained by Mass action model. The result indicates that the micellization process of the TILS was spontaneous and more stable with the increase of anionic hydrophobicity. In addition, with the increase of hydrophobic side chain length, the driving force of surfactant micellization may be enthalpy driven at higher temperature and entropy driven at lower temperature. The micro-environment of TILS solution with different concentrations was studied by UV spectrophotometry with curcumin as a probe and the change of micro-environment was used to determine the CMC of TILS. The CMC values were similar obtained by two methods.