The volume of stationary phase is extremely important for the estimation of thermodynamic information such as entropy and Gibbs free energy in HPLC. However, the definition of this volume strongly depends on the conditions chosen. In the present mini review the partitioning model of small molecules at low concentrations and methanol as modifier is assumed. All the other conditions that could influence the volume of the stationary phase like the thickness of the phase, the area of the phase, the order of C18-chains, the density of the chains, the temperature etc. had to be considered and are therefore reviewed in the present work. The results of the theoretical considerations and a new method of surface estimation were used to calculate the stationary phase volume.
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