Thermodynamic investigation of room temperature ionic liquid

Abstract

The molar heat capacities of the room temperature ionic liquid 1-butylpyridinium tetrafluoroborate (BPBF4) were measured by an adiabatic calorimeter in temperature range from 80 to 390 K. The dependence of the molar heat capacity on temperature is given as a function of the reduced temperature X by polynomial equations, C-p,(m) [J K-1 mol(-1)]= 181.43+ 51.297X -4.7816X(2)-1.9734X(3)+ 8.1048X(4)+11.108X(5) [X=(T-135)/55] for the solid phase (80-190 K), C-p,C-m [J K-1 mol(-1)]=349.96+25.106X+9.1320X(2)+19.368X(3)+2.23X(4)-8.8201X(5) [X=( T-225)/27] for the glass state (198-252 K), and C-p,C-m [J K-1 mol(-1)]= 402.40+21.982X-3.0304X(2)+3.6514X(3)+3.4585X(4) [X=(T-338)/52] for the liquid phase (286-390 K), respectively. According to the polynomial equations and thermodynamic relationship, the values of thermodynamic function of the BPBF4 relative to 298.15 K were calculated in temperature range from 80 to 390 K with an interval of 5 K. The glass transition of BPBF4 was observed at 194.09 K, the enthalpy and entropy of the glass transition were determined to be Delta H-g=2.157 kJ mol(-1) and Delta S-g= 11.12 J K-1 mol(-1), respectively. The result showed that the melting point of the BPBF4 is 279.79 K, the enthalpy and entropy of phase transition were calculated to be Delta H-m=8.453 kJ mol(-1) and Delta S-m=30.21 J K-1 mol(-1). Using oxygen-bomb combustion calorimeter, the molar enthalpy of combustion of BPBF4 was determined to be Delta H-c(m)0 =-5451 +/- 3 kJ mol(-1). The standard molar enthalpy of formation of BPBF4 was evaluated to be Delta H-f(m)0 =-1356.3 +/- 0.8 kJ mol(-1) at T=298.150 +/- 0.001 K.