Surface-modified silica gels as solid stationary phases in gas chromatography

Abstract

Modifications of the precipitated silica gel were done by different treatments including calcination, hydrothermal treatment, silylation and treatment with alkali salt (NaCl). In addition, the hydrothermally treated silica gel was subjected to further treatments, namely silylation or modification by alkali metal. Density of surface hydroxyls ( N OH/100 Å 2) was determined thermogravimetrically and confirmed by infrared spectroscopy. Surface textural characteristics were investigated from nitrogen adsorption-desorption isotherms at −196 °C. It was found that the hydrothermal treatment has a pronounced influence in removing hydroxyls from the silica surface. Different pore systems were found to exist depending on the adopted treatments. The surface-modified silica gels were tested as GC solid stationary phases in terms of the separation efficiency for various groups of non-polar and polar solutes. Also, thermodynamic parameters (Δ H, Δ G and Δ S) were determined using n-hexane as a probe in order to show the sdsorbate-adsorbent interaction. It was observed that the non-polar solutes could be separated independent of the reactivity and porosity of the silica surfaces. The efficiency of the surface-modified silica gels to separate the aromatic hydrocarbons seemed to be strongly influenced by the density of the surface hydroxyls. It was suggested that the separation of the polar solutes ( n-alcohols) is enhanced by the inertness of the silica surface and a fraction of wider pores located mostly at 80 Å.