New insight into organosilane grafting onto titania and silica nanoparticles using volumetric techniques based on molecular probing
Résumé
This paper addresses the surface modification of oxide nanoparticles using organosilane molecules. To this aim,the surface of TiO2nanoparticles is modified with n-(6-aminohexyl)aminopropyl-trimethoxysilane (AHAPS) whilehexadecyltrichlorosilane (HTS) is applied as surface modifier of Aerosil OX50 silica particles. A detailed study is conducted to obtain new insights into the grafting
process using volumetric techniques based on molecular probing. More precisely, the evolution of the surface energy heterogeneity during the grafting of the organosilane moleculeshave been investigated by quasi-equilibrium low-pressure approaches using argon and nitrogen as probe molecules. DIS modeling of the distribution functions of condensation give accessto follow the evolution of the different energy sites when grafting organic molecules. Very few studies have directly addressed the contribution of physisorbed aminosilane molecules during the organosilane grafting process on titania. The main objectiveof this work is to show experimentally the importance of the physisorption during the grafting process of an aminosilane coupling agent n-(6-aminohexyl)
aminopropyltrimethoxysilane (AHAPS) on TiO2nanoparticles. The distinction between chemisorbed and physisorbed aminosilane molecules on TiO2is thoroughly analyzed. The surface of bare and modified TiO2particles has been characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) to gain a better understanding of the adsorption mechanism of AHAPS on TiO2. Quantitative information on surface energy of TiO2, in terms of adsorption energy sites and heterogeneity, has been investigated by quasi–equilibrium low–pressure adsorption technique using nitrogen and argonas probe molecules. The FTIR and XPS data are combined to estimate and discuss the chemisorbed and physisorbed contribution. The results demonstrate that both physisorption and chemisorption occurs but they display a different behavior.The physisorbed amounts are much higher than the chemisorbed amounts. This shows that the main part of the adsorbed layer is composed of physisorbed molecules. The physisorbed uptake depends highly on the AHAPS concentration while the chemisorbed amount remains constant.Quasi-equilibrium Ar derivative adsorption isotherms reveal that the AHAPS molecules are mostly located on the {101} and {001} faces of titania and that the 2 faces display the same reactivity toward AHAPS sorption.
Nitrogen adsorption experiments show that the sorption takes place on the 3 polar surface sites of high energy. The molecules are chemisorbed onto the site displaying the highest energy while they are physisorbed on the 2 lower energy sites. Although many works were conducted with the aim of preparing hydrophobic silica surfaces, the thorough analysis of the change of the silica surface properties which occurs during the chemical modification was never reported in the literature. The surface heterogeneity of a hydrophilic silica OX 50 is studiedbefore and after hydrophobic chemical modification. This fumed silica is chemically modified using the hydrophobic organosilane hexadecyltrichlorosilane (HTS). The modifications of surface heterogeneity due to HTS adsorption was analyzed using low-pressure adsorption techniques. For bare silica, the shape of the derivative experimental curve obtained with argon can be described by five local adsorption domains or local isotherms. This assesses that the silica surface is energetically heterogeneous due to the heterogeneity in the distribution of the adsorption sites. Conversely, for the totally covered silica (QHTS= 2.1 μmol/m2), only one local derivative isotherm is needed to represent the experimental data. The experimental curves corresponding to the partial HTS surface coverages (QHTS= 0.35-2.0 μmol/m2) can be represented by a linear combination of the different local models corresponding to bare silica and full HTS-covered silica, without any change of the local energy parameters. This suggests that the HTS grafting can be view as being patchwise with the formation of HTS aggregates on the surface leaving free patches of silica surface. Nitrogen is a Lewis base, and can be used to attest the polar−apolar nature of the solid surfaces. The shift of all the peaks at a high and medium energy from argon to nitrogen highlights the presence of polar surface sites on the surface of the silica since the HTS molecules are covalently linked to the surface polar hydroxyl groups through the Ti-O-Si bond. The information obtained from the low-pressure gas adsorption are used to reproduce the HTS grafted amount along its adsorption isotherm from cyclohexane. The data are compared to experimental data obtained using carbon mass measurements. The data derived from volumetry analysis compare well with those of TOC measurements. Consequently, the grafting content can be accurately reproduced through the DIS modelling.