Room-temperature ionic liquids (RTILs) which are salts in liquid phase have been received various applications as green solvents involves electrochemistry, extraction processes, catalysis and reaction media for organic synthesis regarding theirs hydrophobic and hydrophilic nature. During our theoretical and experimental studies on bulk and surface of ILs and also surface of organic liquids we found that both cations and anions greatly affects the hydrophilicity and hydrophobicity. For each model liquid system, [C4mim]PF6, [C4mim]BF4, we have performed gas phase ab initio calculation and DFT study by analysis the trajectory of a 120 ps long Car-Parrinello MD run of a system contains 5 ionic liquid ion pairs and 15 water molecule under bulk conditions. We compared the results of partial charges for each system which was obtained using standard methods e.g. CHELPG and ESP. Then, we have used these partial charges for bulk and liquid/vapor interface studies using the DL_POLY program at ambient pressure. We have found that partial atomic charges obtained from CPMD simulation under bulk condition for [C4mim]BF4, [Cation]1.04[Anion]-1.04, are near to the standard charge but the results for [C4mim]PF6 are scaled, [Cation]0.6[Anion]-0.6. Using these atomic charges for a classical molecular dynamics simulation leads to phase separation from water in the case of hydrophobic ionic liquid, [C4mim]PF6, and a homogeneous system for hydrophilic ionic liquid, [C4mim]BF4.

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