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 mu mol/m(2). However, the chemical modification does not elaborate a compact layer. The HTS density is approximately 1.2 molecules/nm(2) 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 decreases continuously from 54.2 mJ/m(2) for bare silica to 46.7 mJ/m(2) for silica treated at an initial concentration of 4 mM following a linear diminution of gamma(s) with the HTS adsorbed amount. The apolar Lifshitz-van der Waals component gamma(LW)(s) is reduced from 41.1 mJ/m(2) to 35.8 mJ/m(2). The electron-donor parameter gamma(-)(s) drops from 53.9 mJ/m(2) to 27.5 mJ/m(2) while the electron-acceptor parameter gamma(+)(s) remains roughly constant (1 mJ/m(2)). 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. (C) 2013 Elsevier B.V. All rights reserved.