The influence of mobile phase flow-rate in RP-LC on thermodynamic parameters studied for polar compounds

Thermodynamic parameters, such as standard enthalpy (DH0) and entropic contributions (DS0/R+volume phase ratio) of the partition between mobile and stationary phases, for several polar model compounds of pharmaceutical interest, were estimated in RP-LC for different flow-rates and using an octadecyl-silicagel stationary phase. The temperature range used to plot van’t Hoff curves was 25–50 C, which is the usual range for separation in RP-LC. Below 20 C, van’t Hoff plots become non-linear for all studied compounds, due to the changes of the balance between hydrophobic and solvophobic forces that influence the retention process. Other significant deviations were observed for one compound that is involved in inter-changeable tautomeric equilibria (as discussed for piroxicam). Regression parameters of the linear dependences obtained between retention (natural logarithm of retention factor, ln k’) and reciprocal absolute temperature (1=T) together with the
estimated phase ratio were used to calculate the thermodynamic parameters of the model compounds. A slight variation was observed when flow-rate was changed within the interval from 0.6 to 2.0 mL=min. The enthalpy value decreased with increasing flow-rate up to a minimum at 1.4mL=min; at higher flow-rates these values increased slightly or remained constant.