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Self-Assembled Monolayers (SAM) are supramolecular organizates resembling, in some respects, the well-known Langmuir-Blodgett (LB) built-up films while displaying other distinct and rather Special features. Much of the interest in SAM stems from their potential in wide range of scientific and technological applications. The first application of SAM in chromatographic separation science was developed at the University of Delaware by Fatunmbi and Wirth. The bonding technique allows ordered monolayers of functional molecules to be chemically immobilized on solid substrates, such as, silica and alumina.

The technique of bonding was termed “horizontal polymerization” due to the fact that there is significant Si-O-Si bridging parallel to the silica substrate. This is achieved by reaction of trifunctional silanizing agent with the silica substrate under anhydrous condition, except for a monolayer of water on silica. This contrasts with conventional polymerization of trifunctional silanes, referred to as “vertical polymerization,” where water is deliberately added to polymerize the reagents before attachment to the surface. The key structural difference here is that horizontal polymerization provides much higher ligand density at the silica surface boundary.

The bonding chemistry utilized for chromatographic applications results in a monolayer coverage with typical thickness in the range of 6-22 Å, depending on the length of the attached ligand. For example, when a C18 ligand is attached, a film thickness of about 22 Å is obtained. Thickness in this range ensures bonding of the ligand both in and around the substrate material without blocking of the pores. Results from the relaxation time measurements, using solid state nuclear magnetic resonance spectroscopy (NMR), showed that the spacer molecules are evenly dispersed within the functional ligand and are not clumped together.