Reassessment of the surface activity of ferrocenyldimethylammonium surfactants

The paper reports an investigation of the surface behavior of N,N-dimethylferrocenylmethylundecyl ammonium bromide, Br−Fc(CH2)N+(CH3)2(CH2)10CH3, where Fc is ferrocene, by employing a simple molecular thermodynamic model and by comparing the predictions of the model to experimental measurements. Whereas a past study (Colloids Surfaces A: Physicochem. Eng. Aspect 94 (1995) 243) has reported that chemical oxidation (using AgNO3) of Fc(CH2)N+(CH3)2(CH2)10CH3 to Fc+(CH2)N+(CH3)2(CH2)10CH3 (where Fc+ is the ferrocenium cation) leads to small (<4 mN m−1) changes in the surface tension of its aqueous solutions at 25 °C, the molecular thermodynamic model reported here leads to predictions of changes in surface tension at 25 °C that are larger than the past experimental study. In order to reconcile the predictions of our model and the past experiments, we performed new measurements of the surface tension of aqueous solutions of Fc(CH2)N+(CH3)2(CH2)10CH3 and Fc+(CH2)N+(CH3)2(CH2)10CH3, where the oxidized surfactant was prepared electrochemically and all measurements were performed at 40 °C (above the measured Krafft temperature of 37 °C). We find the new measurements to be in good agreement with the molecular thermodynamic model (∼15 mN m−1 changes in surface tension upon oxidation of Fc(CH2)N+(CH3)2(CH2)10CH3 to Fc+(CH2)N+(CH3)2(CH2)10CH3). The results of our model and experimental measurements also support our proposition that electrostatic interactions between the cationic groups of these surfactants dominate the surface behavior of these surfactants.