Bonded Phases - The Characteristics of Reverse Phases
The Characteristics of Reverse Phases
Reverse phases resolve substances on quite a different interactive process to silica gel that separates substances largely based on their polarity. The selectivity of a stationary phase (or a mobile phase for that matter) depends on the nature of the interactive forces that can occur between the solute molecules and the molecules of the respective phase.
Reverse phases have hydrocarbon chains bonded to the silica surface and consequently, the interactive forces between them and the solute (or solvent) will be dispersive in nature. That is to say, any molecular interactions with the reverse phase result from London's Dispersion Forces acting between the molecules. For some reason, the term hydrophobic forces has been employed as an alternative to dispersion forces to describe molecular interactions in reverse phase chromatography; a most unsuitable term, as 'water fearing' (a literal translation of hydrophobic) when applied to a molecular force has no rational meaning. It possibly originated from a rather confused association with the immiscibility of water and hydrocarbons where it was quite wrongly assumed by many that water molecules and hydrocarbon molecules repel one another. Despite, superficially, this could appear to be the case, molecules cannot repel one another unless they enter (and stay inside) each other's
Van der Waals radii
Water and hydrocarbons are immiscible because the polar forces between the water molecules and the dispersive forces between the hydrocarbon molecules are much greater in magnitude then the dispersive forces between hydrocarbon molecules and water molecules. As a consequence, water and hydrocarbon molecules interact far more strongly with themselves than they do with each other and, as a result, the two liquids become immiscible. From another standpoint, it is patently obvious that water and hydrocarbon molecules cannot repel one another, because water has a slight but finite solubility in hydrocarbons and hydrocarbons have a slight but finite solubility in water. Nonetheless, hydrophobic is a term that persists depicting the nature of dispersive interactions, irrespective of its illogical origin and the existence of a rational alternative. Throughout this book, however, the true physical chemical term dispersive will be employed to describe all non polar, non-ionic molecular interactions that are solely due to London's dispersion forces.