exchange and exclusion. The original method employed by Tswett involved surface adsorption where
the relative polarities of solute and solid stationary phase determine the rate of movement of that solute
through a column or across a surface. If a liquid is coated onto the surface of an inert solid support, the
sorption process is one of partition, and movement of the solute is determined solely by its relative
solubility in the two phases or by its volatility if the mobile phase is a gas. Both adsorption and partition
may occur simultaneously, and the contribution of each is determined by the system parameters, i.e. the
nature of the mobile and stationary phases, solid support and solute. For example, a stationary phase of
aluminium oxide is highly polar and normally exhibits strong adsorptive properties. However, these
may be modified by the presence of adsorbed water which introduces a degree of partition into the
overall sorption process by acting as a liquid stationary phase. Conversely, paper (cellulose) is
relatively non-polar and retains a large amount of water which functions as a partition medium.
Nevertheless, residual polar groups in the structure of the paper can lead to adsorptive effects.
The third sorption phenomenon is that of ion-exchange. Here, the stationary phase is a permeable
polymeric solid containing fixed charged groups and mobile counter-ions which can exchange with the
ions of a solute as the mobile phase carries them through the structure.
The fourth type of mechanism is exclusion although perhaps 'inclusion' would be a better description.
Strictly, it is not a true sorption process as the separating solutes remain in the mobile phase throughout.
Separations occur because of variations in the extent to which the solute molecules can diffuse through
an inert but porous stationary phase. This is normally a gel structure which has a small pore size and
into which small molecules up to a certain critical size can diffuse. Molecules larger than the critical
size are excluded from the gel and move unhindered through the column or layer whilst smaller ones
are retarded to an extent dependent on molecular size.
In each chromatographic technique, one of the four mechanisms predominates, but it should be
emphasized that two or more may be involved simultaneously. Partition and adsorption frequently
occur together and in paper chromatography, for example, ion-exchange and exclusion certainly play
minor roles also.
Sorption Isotherms
Ideally the concentration profile of a solute in the direction of movement of the mobile phase should
remain Gaussian at all concentrations as it moves through the system. However, sorption characteristics
often change at high concentrations resulting in changes in the distribution ratio. If no such changes
occurred, a plot of the concentration of solute in the mobile phase as a function of that in the stationary
phase at constant temperature would be linear and the concentration profile symmetrical (Figure 4.11
(a)). Plots of this type, known as sorption isotherms, can show curvature towards either