The invention of rigid monoliths has led to an entirely new class of separation media that have been described as the fourth generation of stationary phases. Polymer monoliths offer some distinct advantages for certain types of separations. The synthesis of the polymeric monolith itself is the most important step in the production of a suitable porous polymer monolithic stationary phase. However, novel polymerization methods are needed to improve the structural homogeneity.

High internal phase emulsions (HIPEs) are concentrated mixtures of liquid droplets dispersed in another liquid, defined by a minimum droplet volume fraction of 74%.  HIPEs are commonly stabilised by surfactants. The polymerization of the continuous phase of HIPEs leads to porous polymeric monoliths with open-cellular structure. PolyHIPEs have the advantage of high permeability, easy preparation, uniform porous structure, facile control of pore size, etc. Studies have demonstrated their great potential as separation media. Until now, however, no practical applications of PolyHIPEs have been reported because they still have a disadvantage of low mechanical strength. A different approach to stabilizing PolyHIPE was developed by using an old Pickering concept.  In contrast to conventional polyHIPEs, have larger but closed-cell pores.

Using mixed or stacked emulsions stabilized with functional nanoparticles and polymeric surfactant having different surface chemistry enabled spatial “patterning” of functionality in the resulting monolithic structure. Their interconnected structure allows the convective flow of the mobile phase through the separation medium, producing low pressure drops at high flow rates as well as fast mass transfer of the solutes to the stationary phase. Transport by convection also ensures good column efficiency at high flow rates, producing rapid separations.