Study of the solvolytic cleavage of octadecyl ligands from the surface of RP-18-type stationary phases in TLC. Part 3. TLC and Raman spectroscopic quantification of the extent of cleavage for buffer pH from 1 to 10

An effort has been made to investigate the resistance of selected RP-18 TLC stationary phases to cleavage of the chemical bonds coupling the octadecyl ligands to the silica matrix. An attempt was made to cleave these bonds by repeated development of the layer with methanol-water-buffer solution (buf fer pH from 1 to 10) and to monitor the progress of the cleavage by two ind ependent experimental approaches. The first approach was indirect measurement of the density of coverage of t he silica matrix by octadecyl ligands, after one, two, four, and eight pass ages of the aforementioned methanol-water-buffer solutions through the laye r, by monitoring the retention of an homologous series of higher fatty acid s with an even number of carbon atoms per molecule, as test solutes, with m ixed methanol-water mobile phases. The second procedure used consisted in application of laser Raman spectrosc opy with a high working-power neodymium laser. RP-18 TLC stationary phase s amples were developed repeatedly with methanol-water-buffer solutions, thei r Raman spectra were measured, and quantification was performed by use of a very intense, broad, and highly structured spectral band, in the range 228 5 to 1130 cm(-1), quite typical of aromatic compounds. The origin of this b and is rather obscure, although it seems fairly probable that the temperatu re elevation induced by the laser radiation results in at least partial deh ydrogenation of the octadecyl ligands, then aromatization. It was demonstrated that cleavage of octadecyl ligands from the silica matr ix is not a negligible, or insignificant process, but that the extent of th e cleavage was quite substantial (usually ca 20%) at all pH other than neut ral (i.e. pH 7), A conclusion was also drawn about molecular-level aspects of the quantified phenomenon and, more specifically, about the number of th e chemical bonds anchoring octadecyl ligands to the silica surface.