Surfing silica surfaces superciliously

The present mini-review summarizes the experience gathered by our group in developing different classes of novel quaternarized heterocyclic compounds able to modulate and reverse the electroendoosmotic flow (EOF) in a most peculiar manner. The first class comprises mono-salt compounds, with the determinant omega-iodoalkyl chains of different lengths (typically C4-C8), able to be adsorbed by silicas, at alkaline pH, and spontaneously alkylate ionised silanols, thus becoming covalently affixed to it. The second class is constituted by di-salt compounds, attached at the termini of an alkyl chain of variable lengths (here too, typically, C4-C8). This second class is unable to bind covalently silica surfaces, although, in thin-layer chromatography, it exhibits an extraordinary affinity for silica beads, contrary to the first one. On the basis of the strikingly different behaviour, structural rules are derived for the minimum requirements for general classes of amines to bind to silica walls and modify EOF. For compounds unable to bind covalently to the wall, the most important structural motif is two quaternary nitrogens spaced apart by a C4 chain: this seems to be the average distance (i.e., 0.8 nm) between two adjacent, ionized silanols for a snug fit. The other structural binding motif is the "hydrophobic decoration", i.e., the ratio of charged groups to alkyl residue in the various amines; amines with high levels of such alkane groups (i.e., with higher hydrophobicity), seem to bind more tenaciously to the wall, probably due to hydrophobic interaction not to the wall but among the amine derivatives themselves, when carpeting the silica.