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

Abstract

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, solvatochromic 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.