Effect of chemical modification on surface free energy components of Aerosil silica powders determined with capillary rise technique

The knowledge about interfacial energy interactions and surface free energy is necessary for understanding and modeling many surface and interface processes. The aim of this paper is to study the effect of the chemical modification on the surface free energy components of Aerosil OX50. This fumed silica is chemically modified using the hydrophobic organosilane hexadecyltrichlorosilane (HTS). The modification process is studied by Fourier transform infrared spectroscopy (FTIR) and total carbon mass measurements performed on solid silica samples. The combination of these two techniques allows the determination of the adsorption isotherm of HTS onto silica showing the formation of a plateau for an initial concentration of 3 mM. The quantity adsorbed of HTS at maximum coverage is 2.14 μmol/m2. However, the chemical modification does not elaborate a compact layer. The HTS density is approximately 1.2 molecules/nm2 which is close to the density of hydroxyl groups on the surface. A recent method, developed by Chibowski and Perea-Carpio, based on imbibition of probe liquids into a thin porous layer (capillary rise), is used for the determination of the surface free energy components. The surface free energy γs decreases continuously from 54.2 mJ/m2 for bare silica to 46.7 mJ/m2 for silica treated at an initial concentration of 4 mM following a linear diminution of γs with the HTS adsorbed amount. The apolar Lifshitz-van der Waals component γsLW is reduced from 41.1 mJ/m2 to 35.8 mJ/m2. The electron-donor parameter γs− drops from 53.9 mJ/m2 to 27.5 mJ/m2 while the electron-acceptor parameter γs+ remains roughly constant (1 mJ/m2). Trends in the surface free energy components are notably linked to observations in infrared spectra and give additional information about the structure of the HTS layer.