Separation and identification of oligomeric phenylethoxysiloxanols by liquid chromatography-electrospray ionization mass spectrometry

• A method for separation and identification of oligomeric phenylethoxysiloxanols is described.
• The method can be used to monitor the distribution of oligomers and the structure growth in the sol–gel process.
• The oligomer species evolve from linear to cyclic structures with increasing amount of water in the reaction mixture.
• The phenyl and ethoxy moieties in the oligomer molecule are responsible for solute retention in reversed-phase systems.

Liquid chromatography-electrospray ionization mass spectrometry has been applied to qualitative analysis of oligomeric phenylethoxysiloxanols, a class of organosilanols as active intermediates to polyhedral silsesquioxanes. The phenylethoxysiloxanol samples were prepared by controlled acid-catalyzed hydrolysis and condensation of phenyltriethoxysilane at various molar equivalents of water (r1) and characterized by standard spectroscopic techniques. Using a gradient binary water–methanol mobile phase, these reaction products were resolved on octadecylsiloxane silica stationary phase and subsequently identified by online electrospray ionization mass spectrometric detection. Results show that the reaction products are composed of a multitude of linear and monocyclic siloxanol oligomers with various numbers of silicon atoms and hydroxyl groups, depending upon the reaction conditions used. With the r1 value increasing from 0.5 to 2.0, the chain lengths of the oligomers increase slightly but the numbers of hydroxyl groups increase considerably, accompanying by structural evolution from chains to rings. Characterization of the retention behavior of these oligomers indicates that hydrophobic interactions of phenyl and ethoxy groups with the stationary phase are responsible for their retention in reversed-phase liquid chromatography.