Thermodynamic study of the liquid chromatography retention process for some pesticides on C8 and C18 silica-based stationary phases

Thermodynamic standard enthalpy (∆H0) and entropy (∆S0) of the liquid chromatographic retention process for several pesticide-type compounds by reversed-phase (RP-LC) mechanism were estimated using an octyl- (C8) and octadecyl-silica (C18) based stationary phase. The temperature range used to plot van’t Hoff dependences was 20–45°C, which is the usual range for separation in RP-LC. Regression parameters of the linear dependence obtained between retention factor (ln k’) and absolute temperature (1/T) together with the phase ratio were used to calculate the above thermodynamic parameters of the chosen compounds. Chromatographic conditions were following: acetonitrile/water as mobile phase composition in isocratic elution; and 1 mL/min as flow-rate. Chromatographic columns were Zorbax Eclipse XDB-C8 or C18 with characteristics: 4.6 mm inner diameter; 150 mm length, and 5µm particle size. The values of standard enthalpy and entropy were situated within the intervals (-12; -3 kJ/mol) and (-1; -26 J/mol K), respectively, which are in good agreement with data reported for other compounds in the literature.