Interfacial rheological behavior of ionic liquid-type imidazolium surfactant

• The dilational rheology of imidazolium surfactants [Cnmim] Br were studied.
• There is a shift of elasticity of [Cnmim]Br as increasing concentration.
• Interface elasticity of [Cnmim]Br increases to a maximum as concentration increases.
• The interface dilational modulus of [Cnmim]Br were affected by the steric effect.

The interface tension and interface dilational properties of a series of ionic liquid-type imidazolium surfactants solutions, [Cnmim]Br, at the decane–water interface are reported. The dilational rheology properties are measured by means of oscillating drop method. The influences of dilational frequency, bulk concentration and structure on dilational rheology properties have been investigated. The results reveal that the interface dilational elasticity of the imidazolium surfactants with longer hydrophobic chain is larger than that with shorter hydrophobic chain at lower concentration, but the opposite is observed at high [Cnmim]Br surfactants solution concentrations. At lower concentrations, with the length of hydrophobic chain increasing, the dilational modulus increase resulting from the greater hydrophobic interaction. However, at higher concentration, [Cnmim]Br surfactants with longer hydrophobic chain desorbs from the interface more easily, but more difficult to resist the interface deformation.

The interesting thing is that the interfacial dilational modulus of the three ionic liquid-type imidazolium surfactants ([Cnmim]Br) with long hydrophobic chain is larger than that with short hydrophobic chain at lower concentration, whereas the opposite is observed at higher surfactant concentration. At the lower concentration, the interaction of surfactant molecules is loose because of the few amount of surfactant molecules adsorbed at the interfacial. With the length of hydrophobic chain increasing, the dilational modulus increases resulting from the greater hydrophobic interaction. With increasing surfactant concentration, the rate of molecular diffusing between bulk and interface increases, also, surfactants which have longer hydrophobic chain have more compacting state at the interfacial. In other words, at higher concentration, surfactant with longer hydrophobic chain desorbs from the interface more easily, but more difficult to resist the interface deformation. Thus the longer the surfactant hydrophobic chain is, the easier the diffusion exchange becomes, resulting in more obvious decrease on the dilational modulus.Figure optionsDownload as PowerPoint slide