Topics - Thermodynamics

Thermodynamics The application of thermodynamics to a chromatographic separations explains how the distribution coefficient (which itself determines the magnitude of retention) is controlled by the standard energy of distribution and the absolute temperature. In fact, from thermodynamics, it can be shown that the distribution coefficient is equal to the negative exponent of the ratio of the standard energy of distribution to the product of the absolute temperature and the gas constant. Thus, the effect of temperature on chromatographic retention can be predicted. However, thermodynamics can explain further that the standard energy of distribution is equal to the standard enthalpy of distribution minus the product of the standard entropy of distribution and the absolute temperature. It follows that by obtaining a curve of log retention volume per ml of stationary phase against the reciprocal of the absolute temperature, a straight line will be produced and the slope of the curve will give a value for standard enthalpy of distribution and the intercept will give a value for standard entropy of distribution. It has been argued that the value for standard enthalpy of distribution will give a measure of the energies involved in the molecular interactions whereas the standard entropy of distribution will give a value for any spatial restriction that might occur during retention by exclusion or chiral selectivity.