Dispersion-Interaction
Dispersive interactions are one of the three basic interactive mechanisms that allow molecules to exert forces on one another (i.e., dispersive, polar and ionic). Dispersive forces (or London.s dispersive forces as they were once known) are not due to permanent dipole or induced permanent dipole interactions, or due to interactions between permanent charges on the molecules as in ionic interactions, but are due to transient, random charges, spontaneously generated continuously all over the molecule. Glasstone gave a good definition of dispersion forces,
“-although the physical significance probably can not be clearly defined, it may be imagined that an instantaneous picture of a molecule would show various arrangements of nuclei and electrons having dipole moments. These rapidly varying dipoles when averaged over a large number of configurations would give a resultant of zero. However, at any instant they would offer electrical interactions with another molecule resulting in interactive forces.”
Dispersive forces are the only type of molecular force that can exist to the exclusion of all others (e.g., interaction between one hydrocarbon and another).Polar interactions are always accompanied by dispersive interactions and ionic interactions usualy accompanied by polar interactions and always accompanied by dispersive interactions.
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Distribution-Coefficient Molecular Dispersion
Molecular Forces
All intermolecular forces are electrical in nature.
The three different types are termed dispersion forces,
polar forces and ionic forces. All interactions
between molecules are composites of these three forces.
Dispersion Forces
Dispersion forces were first described by London (3),
and for this reason were originally called 'London's dispersion
forces'. London's name has now been dropped and they are now simply
referred to as 'dispersion' forces. They arise from charge
fluctuations throughout a molecule resulting from electron/nuclei
vibrations.
Glasstone (4) suggested that dispersion
forces could be best described as follows,
"although the
Principles Distribution-Coefficient Molecular Dispersion
Author: RPW Scott
Book:The Mechanism of Chromatographic Retention
Section:Retention Chromatographic-Interactions Dispersive
offer electrical
interactions with another molecule, resulting in interactive forces".
London (3) was
the first to describe dispersion forces, which were consequently termed
'London's dispersion forces'. Unfortunately, over the years, London's name has
been dropped and the simpler term 'dispersion' forces is now used.
Dispersion
forces are ubiquitous and must arise in all molecular interactions. They can,
themselves, occur in isolation, but are always
present even when other types of interaction dominate. An example of
interactions that are exclusively dispersive are those between hydrocarbons.
The lower molecular weight hydrocarbons (hexane, heptane, octane etc.) are
liquids and not gasses due entirely to the dispersion forces that act between the
hydrocarbon molecules. Dispersive interactions are sometimes referred to as 'hydrophobic' or 'lyophobic'
interactions, particularly in the fields of biotechnology and biochemistry.
These terms appear to have arisen because
Retention Chromatographic-Interactions Dispersive
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Available-Stationary-Phase Phase-Loading
. A large sample is often necessary
in trace analysis to provide sufficient material for detection. Under such
circumstances the column may be overloaded giving a very broad asymmetric peak
which may obscure the trace materials of interest. This asymmetric dispersion
is due to solute-solute interaction in the mobile and stationary phases causing
a nonlinear adsorption isotherm. The subject of adsorption isotherms will not
be discussed here and it is sufficient to say that the asymmetric dispersion
can be reduced by increasing the stationary phase in the column.. A larger
amount of stationary phase, will, even with a larger charge, reduce the sample
concentration in the stationary phase and thus the deleterious high sample
concentrations are never reached
Principles Available-Stationary-Phase Phase-Loading
Author: RPW Scott
Book:The Mechanism of Chromatographic Retention
Section:Retention Chromatographic-Interactions
constraining their dispersion so that they are eluted discretely. To move the
peaks apart each solute must be retained to a different extent, which (as shown
by the plate theory, book 6) means that their distribution coefficients must
differ or they must interact with different volumes of stationary phase.
Retention is therefore controlled by regulating the magnitude of the distribution
coefficient or modifying the quantity of stationary phase available to each
solute. The former is employed in interaction chromatography and the
latter in exclusion chromatography. In practice, it is rare that either
procedure is exclusive in any given separation as both retentive processes are
usually present to some extent, particularly in liquid chromatography (LC).
Retention in gas chromatography (GC), using coated open tubes, is, perhaps, the
exception, as, in this distribution system, exclusion processes (aside from
chiral phases) are normally absent and, consequently, retention control is
purely
Retention Chromatographic-Interactions
Author: RPW Scott
Book:Preparative Chromatography
Section:Preparative Mass-Overload
with the benzene. In fact, as a result
of solute–solute interaction the benzene is, in effect, partially eluting itself.
The effect of
the mass overload of benzene on the other solutes is also clearly demonstrated.
The presence of the high concentration of benzene in the mobile phase increases
the elution rate of both the naphthalene and the anthracene. Its also seen,
however, that the effect of the high concentration of the benzene on the closer
eluting peak naphthalene is to produce band dispersion, whereas the anthracene
band does not suffer significant dispersion and the retention of both the front
and the rear of the anthracene peak appear to be linearly reduced with sample
mass. The chromatograms shown in figure
7 also show that the anthracene peak maintains its symmetry throughout all
sample sizes. The impact of the high concentration of benzene on the elution of
the other solutes only occurs while the solutes are still in contact with one
another at the beginning of the
Preparative Mass-Overload
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Introduction
of GC there has been a continuous interaction between
improved detector performance and improved column performance. Initially, separations
monitored by detectors with improved sensitivity permitted a precise column
theory to be developed and experimentally substantiated. This allowed new columns to be designed with reduced
dispersion and higher efficiencies. The improved efficiencies, however,
produced small volume peaks, small, that is, compared with the volume of the
detector sensor and the dispersion that took place in the conduits of the
detector system.. As a consequence, the ultimate efficiency obtainable from the
column was determined by the geometry of the fluid conduits of the detector and
not its sensitivity. This provoked detector redesign, with smaller sensor
volumes, different geometry and shorter connecting tubes between the column and
sensor. In turn, these modifications allowed much smaller particles to be used
in the column resulting in even lower column dispersion
HPLC-Detectors Introduction