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.
Author: RPW Scott
Book:The Thermodynamics of Chromatography
Section:Thermodynamics Basics
Thermodynamics Basics
The retention
of a solute in a chromatographic system is determined firstly, by the magnitude
of the distribution coefficient of the solute between the two phases and
secondly, by the amount of stationary phase available to the solute for
interaction. This is fully discussed in Plate Theory
and Extensions of this series. In addition, the
mechanism of distribution has been considered exclusively on the basis
of molecular interactions in The Mechanism of Chromatographic
Thermodynamics Basics
Author: RPW Scott
Book:Gas Chromatography
Section:YES Gas-Supplies Flow-Programmers
If the mobile phase is compressible the
relationship between retention volume, flow rate and inlet pressure is given
by,
(1)
Where (Vr)
is the true retention volume of the solute,
(Vr(0))
is the retention volume measured at the outlet.
and (g)
is the inlet/outlet pressure ratio
(for the
derivation of this equation see
The Thermodynamics of Chromatography)
Thus,
Now, from
The Thermodynamics of Chromatography where (e) is a constant
Thus, (2)
If (g) is large
compared with unity, Then
YES Gas-Supplies Flow-Programmers
Author: RPW Scott
Book:The Thermodynamics of Chromatography
Section:Thermodynamics Methylene-Methyl-Group
The Interactive Energy Difference Between that of a Methylene Group and that of a Methyl Group with an Alkane Stationary Phase
Before
discussing some examples of the use of thermodynamics in chromatography, it should
be pointed out that there is an immense amount of accurate and precise
retention data available in the literature. Unfortunately, much of this data
was published before 1980 and, thus, is not identified by many computer search
engines and needs to be found by older and conventional literature search
methods. So much retention data has been published that, in the thermodynamic
study of distribution systems, additional measurements are frequently
Thermodynamics Methylene-Methyl-Group
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Retention Thermodynamics
Prior to
discussing the parameters that determine the magnitude of (K) and (Vs)
and how they can be changed, it is useful to develop the thermodynamic approach
to the problem of solute retention in chromatographic separations.
The Thermodynamic Explanation of Retention
Classical
thermodynamics provides an expression that describes the change in free
energy of a solute when transferring from one phase to the other as a
function of the equilibrium constant (distribution coefficient). The expression
is as follows,
RT ln K = -DGo
where (R) is the gas constant,
(T) is the absolute temperature,
and
(DGo) is the Standard Free
Principles Retention Thermodynamics
Author: RPW Scott
Book:The Thermodynamics of Chromatography
Section:Thermodynamics Other-Methods Chiral-Separations
of either a particular stationary phase, or solvent mixture, for the separation
of closely eluting solutes must be carried out over a
range of temperatures.
 
Optimum Operating Conditions for Chiral Separations in Liquid Chromatography
Thermodynamic
reasoning need not be used exclusively to examine a chromatographic problem but
can also be employed together with other aspects of chromatography theory to
achieve a practical goal. The following example shows how thermodynamics can be
used with appropriate optimizing equations to identify the optimum conditions
for particular difficult types of separation.
In gas
chromatography (GC), chiral selectivity is controlled by choice of stationary
phase and operating temperature. From a practical point of view, chiral
selectivity is achieved by introducing spatially oriented groups into the
stationary phase molecules and, as a consequence, an additional entropic component to the standard energy of
Thermodynamics Other-Methods Chiral-Separations
Author: RPW Scott
Book:Plate Theory and Extensions
Section:Plate-Theory Mobile-Phase-Compressibility
The Compressibility of the Mobile Phase
By measuring
the retention volume of a solute, the distribution coefficient can be obtained.
The distribution coefficient, determined over a range of temperatures, is often
used to determine the thermodynamic properties of the system; this will be
discussed in
The Thermodynamics of Chromatography
. Thermodynamic studies are used largely as a diagnostic
tool to investigate the nature of the distribution. Thus, an accurate
measurement of (V'r) can be extremely important and those factors
that reduce the measurement accuracy need to be examined. The retention
volume of a solute (from equation (13)) is given by,
Vr = Vm + KVS
Plate-Theory Mobile-Phase-Compressibility