Topics - Enthalpy
The standard enthalpy of distribution is part of the standard energy of distribution, the negative exponent of which is equal to the distribution coefficient. The total standard energy is made up of the standard enthalpy together with the standard entropy which, together with the temperature, exclusively determine the magnitude of the distribution coefficient and, as a consequence, chromatographic retention. The standard enthalpy and standard entropy contributions to standard energy arise from two quite different physical processes in the distribution system. The standard enthalpy is a measure of the relative magnitude of the molecular forces that hold the solute in the two phases. It is determined by the nature and the strength of the molecular forces between the solute molecules and the molecules of the two phases and is, thus, controlled by the choice of the phase system. In contrast the standard entropy is a measure of the relative freedom or randomness of the solute molecules when changing from one phase to the other. For example, in exclusion chromatography the smaller molecules enter the pores of the stationary phase and, thus, are less random and have less freedom of movement than the larger molecules that are excluded from the stationary phase and are free to move about in the mobile phase. No distribution system can be exclusively controlled by either the standard enthalpy, or the standard entropy, but if the standard enthalpy dominates then the distribution is said to be energy driven. Conversely, if the standard entropy dominates, the distribution is said to be entropically driven.