Materials Chemistry and Physics 72 (2001) 30–41 Silicon dioxide surface modified with cholesterol derivatives Boguslaw Buszewski a,∗ , Marta Jezierska a , Barbara Ostrowska-Gumkowska b a Faculty of Chemistry, Department of Environmental Chemistry and Ecoanalytics, Nicholas Copernicus University, 7 Gagarin Street, PL-87100 Toru´ n, Poland b Faculty of Chemistry, Department of Physical Chemistry, Nicholas Copernicus University, 7 Gagarin Street, PL-87100 Toru´ n, Poland Received 1 December 2000; received in revised form 18 January 2001; accepted 1 February 2001 Abstract The major aim of the presented work was focused on the preparation and characterization of surface architecture of home-made packing materials for liquid chromatography and related techniques. Four stationary phases were synthesized on the basis of the same batch of silica gel. Two of them were obtained using compounds possessing liquid crystalline properties, one consisted of alkylamide ligands and one was conventionally prepared hydrocarbonaceous material. Properties of the silica matrix as well as the preparation of phases were discussed. Various physicochemical techniques were applied for the characterization of the surface of obtained packings. The progress of the modification reactions was confirmed by elemental analysis, which allowed calculation of the surface coverage density with bonded ligands. Created undersurface structure was examined by means of the solid-state NMR and FT-IR spectroscopy. Thermal analysis revealed the presence of phase transitions. The chromatographic experiments were designed and performed to prove the evidence of the non-homogenous nature of the surface of the packing materials and to create a relative hydrophobicity scale. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Silica; Surface modification; Physicochemical characterization; Chromatography 1. Introduction At the background of the popularity of chromatographic techniques, mainly high performance liquid chromatogra- phy (HPLC) lies the possibility of quantitative and qualita- tive analysis of compounds of different properties — acidic, neutral or basic ones. This method permits the separation of enantiomers or precise determination of the composition of complex mixtures. By means of HPLC, compounds with even small differences in spatial structure of the molecules can be distinguished [1,2]. Solution of such wide scale of separation problems can be found in a great variety of commercial columns and packings. The most often applied adsorbents are based on silica gel modified with hydrocar- bon chains of various lengths (C 2 ,C 8 ,C 18 ,C 30 ), containing polar functional groups (–NH 2 , –CN, –DIOL) or fashion- able lately shielded phases [1–7]. Despite of their universal character and broad range of applications, complexity of physicochemical processes influencing chromatographic elution extorts to develop a new stationary phases with specific structural properties. ∗ Corresponding author. Tel./fax: +48-56-61-14-837. E-mail address: bbusz@chem.uni.torun.pl (B. Buszewski). One of the materials, which nowadays attract the atten- tion of scientists, is packing with chemically immobilized cholesterol [2,8–13]. It became popular because of interest- ing properties of cholesterol moiety. Cholesterol is an im- portant component of biological membranes. It constitutes the third group of polar membrane lipid sterols beside phos- pholipids and glycolipids. Cholesterol molecules decrease the permeability of the phospholipid bilayer and increase its stability [14]. All these properties make cholesterol stationary phases convenient for modeling of xenobiotics behavior in natural systems [15]. Potential liquid crystalline properties of such material can be utilized to specific sep- aration purposes [8,10–13]. In the literature, two paths of synthesis are described. Siouffi and co-workers [8] bonded cholesterol derivative to the commercial siliceous material covered with aminopropyl ligands (LiChrosphere NH 2 ). Surprisingly, few of the reported data, e.g. DSC measure- ments, appears to have mistakes. The second route via formation of hydride intermediate and reaction with olefin was reported by Pesek et al. [11–13]. In the former method, the cholesterol was placed on the undecenoate spacer, but the achieved coverage density was diminished. In the current work, the results of physicochemical characterization of home-made chemically modified silica supports have been reported. Series of stationary phases 0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0254-0584(01)00320-0