e d i to ri a l
Enabling Monoliths T
his Editorial is the second in a new Analytical Frontier series that discusses “enabling” science and technology. In this case, the topic is columns for monolithic separations. What were key elements in their innovation and development? Self-supporting monolithic columns have had a substantial impact in chromatographic and electrophoretic sciences, and their study has spawned a large literature. A central attraction is their high axial permeability, coupled with a large internal pore surface area and the flexibility of modifying them chemically with stationary phases or ion exchange properties. Their high permeability means low back pressure, compared with packed columns, and fast analytical separations without sacrificing efficiency. Because so many researchers use combinatorial libraries in chemical discovery these days, the latter characteristic is a big deal. Monolithic columns prepared by in-place polymerization of organic monomers were introduced by Hjerten in 1989 (under the descriptor “continuous bed”) and Svec in 1990. The use of sol–gel chemistry to prepare porous silica monoliths was introduced into chromatography by Tanaka in 1996. Many refinements have been made, and continue to be made, in these porous column materials; for example, monolithic columns are available in various lengths, from microcolumns to longer capillary versions. The theme of high mobile-phase flow continues to drive research on new versions. The enabling sciences in the developments to date have been polymer and ceramics chemistry, which emerged as modern disciplines in the middle of the 20th century. Poly(methyl methacrylates) were first prepared in 1931; methacrylate gels were used by Hjerten in his early work. Indeed, both Hjerten and Svec made contributions to polymer chemistry that predate their chromatographic inventions. Sol–gel chemistry has been known since the synthesis of alkoxysilanes—at least, I presume so, because those chemicals undergo condensation polymerization so readily. Alkoxysilane-based surface modifications produced bonded-phase chromatographic stationary phases in the 1960s and the first chemically modified electrodes in 1975. Sol–gel polymerization became an active component of ceramic science in the 1970s. Thus, monoliths sit atop a deep and incompletely tapped well of chemistry that feeds other areas.
© 2005 AMERICAN CHEMICAL SOCIETY
A major subtheme of monolithic columns is the chemistry of pore formation. A rod of polymer or glass with few or poorly connected pores can be readily made, but it is chromatographically useless. A central goal, for both organic and silica polymers, is the formation of the column body from chemically or photochemically initiated polymerization that, with post-polymerization processing, leaves a network of interconnecting meso- and macropores distributed uniformly across and down the structure. In the case of organic monomers, solvents can be chosen to induce the forming polymer to collapse into connected beads or tendrils. Another, more general approach involves adding a nonreactive solvent component—aptly called a porogen—to the polymer reaction, to form tiny solvent bubbles within the polymer phase. These are washed out in post-polymerization processing to leave behind an interconnected pore structure. Sol–gels are formed by condensation of alkoxysilanes to form an Si–O–Si network (gelation phase) that gradually collapses and extrudes (aging phase) much of the internal aqueous volume. This hydrogel structure can be rendered macroporous by subsequent internal gas generation from suitable reagents or by inclusion of suitable porogens in the sol–gel reaction feed. Heating to higher temperatures—short of those forming molten glass—dries the monolith body of water and porogen, leaving a porous structure. When it comes to pore-formation processes in both organic and silica polymerizations, my impression is that more is understood about the behavior of the final product than about the molecular details. Porogens are actively used in syntheses of numerous materials, including membranes, scaffolding for tissue engineering, breathable clothing, dielectric films, cushioning, and the manufacture of sorbants. Chromatographic science is not the only one profiting from the chemistry of pore formation. The interconnectedness of the chemical sciences is an enormous, continuing, and underappreciated strength.
A U G U S T 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
277 A
