Ionic liquids as modulators of physicochemical properties and nanostructures of sodium dodecyl sulfate in aqueous solutions and potential application in pesticide microemulsions

Abstract

Emulsifiers for pesticide microemulsions contain a part of volatile organic compounds (VOCs), and ionic liquids (ILs) are becoming potential substitutes. In order to apply ILs in pesticide microemulsions, the study of interaction between ILs and surfactants is necessary. Therefore, the surface properties and aggregation of anionic surfactant sodium dodecyl sulfate (SDS) in ILs, including N-hexyl-N-methylmorpholinyl bromide ([C₆mm][Br]), N-hexyl-N-methylpiperidyl bromide ([C₆mp][Br]), N-hexyl-N-methylpyrrolidyl bromide ([C₆mpyr][Br]), N-octyl-N-methylpyrrolidyl bromide ([C₈mpyr][Br]), N-dodecyl-N-methylpyrrolidyl bromide ([C₁₂mpyr][Br]) and N-hexadecyl-N-methylpyrrolidyl bromide ([C₁₆mpyr][Br]), were investigated in terms of surface tension, conductivity, dynamic light scattering (DLS), viscosity, fluorescence, pseudo-ternary phase diagram and ¹H NMR measurements. Generally in agreement, the methods afforded the evaluation of various micellar parameters such as critical micelle concentration (CMC), degree of counterion ionization (α), the maximum surface excess concentration (Γ_max), the minimum area per surfactant headgroup (A_min) as well as some thermodynamic parameters, including standard free energy of micellization (ΔG⁰_m), standard enthalpy of micellization (ΔH⁰_m) and standard entropy of micellization (ΔS⁰_m). The results indicated a hydrophobic effect as the primary force of a spontaneous, exothermic, entropy driven micellization process. ¹H NMR technique was applied to reveal the solubilization site and interaction of ILs in aqueous SDS micellar solutions. Fluorescence, DLS and viscosity measurements revealed considerable micellar morphologies and various phase behaviors. Furthermore, the 10% difenoconazole microemulsion was successfully prepared and showed good stability and spreadability in mixtures which indicated ILs' potential application in pesticide formulation.