Characterization by inverse gas chromatography of the surface properties of silicas modified by poly(ethylene glycols) and their models (oligomers, diols)

Abstract

Silicas are modified by esterification of the surface silanol groups either with poly(ethylene glycol), its oligomers or diols of the same chain length. A whole series of samples carrying grafts differing in chemical nature, molecular weight and number are prepared. These samples are examined by inverse gas chromatography. The London component of the surface energy, the acid/base interaction parameter and the enthalpy of adsorption of alkanes and polar probes are measured. It is apparent for PEG-grafted silicas that the most important variable is the surface coverage, i.e., the number of monomer units of the grafted PEG per unit surface area, and not its molecular weight. The fixation of PEG changes drastically the surface properties of the silicas. According to the surface coverage, silica may acquire base-like properties. The strong PEG-silica interactions cause the polymer to adopt a flat configuration by spreading on the solid surface. Grafted diols behave entirely differently: the conformation of the longer grafts is best described by a sandwich structure in which the outer surface layer is formed by associated alcohol groups, whereas the middle part is formed by the highly organized aliphatic parts of the chains. IR spectroscopy supports these conclusions.