Thermal and spectroscopic investigations on three phosphonium based ionic liquids for industrial and biological applications

Abstract

Abstract We report a comprehensive result of investigations on three phosphonium based room temperature ionic liquids (ILs) of same cation, trihexyltetradecylphosphonium ([PC6C6C6C14]), with different anions viz. chloride [-Cl]− [Cl]−, dicyanamide [-N(CN)2]− and bis(trifluoromethylsulfonyl) imide [-NTf2]− for catching the effect of size, symmetry, dipole moment, as well as electro-negativity of anions on glassy dynamics and charge transport of the formed ILs. The effort was made for tuning the property for getting superior conductivity, improved thermal stability, better electrode-electrolyte interaction to enable it for diverse biological and technological applications. The ILs were characterized with differential scanning calorimetry (DSC), thermogravimetry (TGA), broadband dielectric spectroscopy (BDS) along with other routine measurements like FT-IR, FT-Raman spectroscopies and computational investigations with density functional theory (DFT). The applications were evaluated by means of electrochemical and biological activities. From DSC and TGA measurements, the samples showed good thermal stability up to 573 K to project them as excellent electrolyte and better heat transfer fluid for high temperature needs in various technological fronts. Dielectric response revealed that the conductivity relaxation manifested above glass transition temperature (Tg) to be well represented by Dyre's random barrier model and it followed a Vogel–Fulcher–Tamman (VFT) behaviour against temperature. The sub-Tg relaxation reflected a Cole-Cole response with an Arrhenius T-dependence. The dielectric data revealed a slight correlation with Tg while anticorrelates with the fragility and low temperature activation energy (Eβ). The DFT calculations gave the structure and other parameters in good agreement with the experimental data. The three ILs fulfilled most of the requirements needed for pharmaceuticals on evaluating pharmacokinetics, bioavailability and toxicity via in silico analysis. On exploring the preliminary biological studies, these ILs showed very good anti-bacterial, anti-oxidant and good anti-cancer activities highlighting their relevance and significance for pharmaceutical applications.