Mixed micelles of sodium perfluorooctanoate and imidazolium based ionic liquids in aqueous solution: A SANS and Tensiometric study

Abstract

Micellization and microstructure of mixed micelles from mixtures of an anionic fluorocarbon surfactant sodium perfluorooctanoate (NaPFO) and cationic alkylmethyl-imidazolium based non-amphiphilic and amphiphilic ionic liquids (ILs) with different alkyl chain length (C4, C6, C8, C10 and C12) and anions viz. chloride (Cl−), tetrafluoroborate (BF4−), hexaflurophosphate (PF6−), octylsulphate (C8SO4−) and trifluoromethane-sulphonate (CF3SO3−) were examined extensively by surface tension and small-angle neutron scattering (SANS) measurements. Low critical micelle concentration (CMC) and large negative value of interaction parameter (β) showed synergism in all the mixed systems and it depends on the molecular characteristics of ILs. Thermodynamic parameters for mixed monolayers and mixed micelles were evaluated and discussed. Micellar growth, shape changes and micelle to vesicle transitions obtained from these oppositely charged mixed systems depend on the surface activity of ILs (alkyl chain and counter ions). Influence of ILs which differ in counter ions with similar chain length and similar counter ions with different chain lengths on aggregation patterns of NaPFO micelles have been examined using SANS. The data for size, shape and aggregation number for aggregates formed by these mixed systems evaluated from the fitted SANS data are reported and discussed in terms of their distinct interaction with NaPFO micelles. The present study is a significant addition to existing knowledge on ionic liquid induced morphological changes of sodium perfluorooctanoate micelles.