The Thermodynamics of Chromatography - The Analysis of the Standard Energy of Distribution > Distribution of Standard Energy Between Different Chemical Groups > Page 11
Most van't Hoff curves have positive slopes and the negative intercepts. A negative intercept indicates that the standard entropy change results from the production of a less random and more orderly system during the distribution process.
The Analysis of the Standard Energy of Distribution
The standard energy of distribution (DG) can be divided into different parts each representing different energy sources that contribute to the equilibrium process. There are two major modes of standard energy distribution; portions of standard energy can be allotted to specific types of molecular interaction that can occur between the solute molecules and the two phases (e.g. energies involved in dispersive interactions, polar interactions and ionic interactions, or subdivisions of these interactive processes such as 'so called' complexation, hydrogen bonding etc.); alternatively, the molecule can be divided into different parts or chemical groups (e.g., methyl groups, methylene groups, phenyl groups etc.) and the interactions of each group allotted a portion of the standard energy. Due to the fact that it is extremely difficult to separate the different interactive processes that take place during distribution, the latter distribution of standard energy (i.e. between different chemical groups or atoms) has provided the more useful information.
Distribution of Standard Energy Between Different Chemical Groups
The distribution of standard energy between different chemical groups was a concept first suggested by Martin (2) almost at the inception of GC, and has been frequently used, over the years, to relate molecular structure to solute retention for many distribution systems. To start, it is necessary to choose a molecular group that would be fairly ubiquitous and that could be used as the first building block to develop the correlation. The carbon atom, or the methylene group (CH2) are obvious choices for simple aliphatic substances, as they are a common atom or group and (as will be seen later) can only interact dispersively with any stationary phase. This choice is substantiated by the relationship between the ln(K) and the number of carbon atoms in the solute for a homologous series of n-alkanes as shown in figure 4.