Trioctylphosphonium room temperature ionic liquids with perfluorinated groups - Physical properties and surface behavior in comparison with the nonfluorinated analogues

A series of novel ionic liquids (ILs) based on trioctylphosphonium-cations with attached perfluorinated chains and the anions bis(trifluoromethanesulfonyl) imide (NTf2-), dicyanamide (N(CN)(2)(-)) and tricyanomethanide (C(CN)(3)(-)) was synthesized. Their important physicochemical properties such as thermal behavior, solvato-chromic polarities and viscosities were measured and compared to the alkyl analogues in order to investigate the influence of fluorination. The characteristic wetting behavior of these ILs on a hydrophobic modified surface was analyzed using the high-precision drop shape analysis (HPDSA) approach as this method is suitable to analyze highly non-symmetrical droplets. For the fluorinated ILs a more pronounced effect on the thermal transitions with increasing side chain length was found. The introduction of fluorinated alkyl groups leads to anion dependent decomposition temperatures, higher overall viscosities and a non-Newtonian flow shear thinning at higher shear rates. Furthermore, the fluorinated ILs showed a modified wetting behavior on the investigated surface, which was found to result in lower contact angle values and higher pinning compared to the non-fluorinated ILs.