Interactions of Ionic Surfactants with Aqueous Solutions of Tetraalkylammonium Cation-Based Ionic Liquids: Tensiometric and Viscometric Measurements

The present work reports surface tension and viscosity studies of the cationic surfactant, DTAB (dodecyltrimethylammonium bromide) and the anionic surfactant, SDS (sodium dodecylsulphate) in 0.01 mol‧kg−1 aqueous solutions of the ionic liquids (ILs) tetraalkylammonium bromide (R4NBr), tetraalkylammonium nitrate (R4NNO3) and tetraalkylammonium acetate (R4NOAc) where alkyl (R) is propyl (Pr), butyl (Bu) or pentyl (Pen). Experimentally determined values of surface tension have been further analysed in terms of surface-active (interfacial) parameters including the surface excess at the air–water interface ( $$\Gamma_{\max }$$ ), minimum area per surfactant molecule ( $$A_{\min }$$ ), surface pressure at the CMC ( $$\pi_{{{\text{cmc}}}}$$ ), efficiency of surfactant in reducing surface tension ( $$pC_{20}$$ ) and adsorption at air/water interface relation to micellization ( $${{CMC} \mathord{\left/ {\vphantom {{CMC} {C_{20} }}} \right. \kern-\nulldelimiterspace} {C_{20} }}$$ ). The thermodynamic parameters of micellization and adsorption viz. change in standard Gibbs free energy of adsorption ( $$\Delta G_{{{\text{ad}}}}^{{\text{o}}}$$ ), change in standard free energy of micellization ( $$\Delta G_{{\text{m}}}^{{\text{o}}}$$ ) and change in standard free energy of transfer ( $$\Delta G_{{{\text{tr}}}}^{{\text{o}}}$$ ) for both DTAB and SDS have also been calculated. The viscometric data have been utilized to compute relative viscosity ( $$\eta_{r}$$ ) and viscous relaxation time ( $$\tau$$ ). All these parameters afford insight into structural rearrangement of amphiphilic molecules at the interface and the relative involvement of hydrophobic and electrostatic interactions between surfactant and IL molecules.