Colloids and Surfaces A: Physicochemical and Engineering Aspects
Inverse gas chromatography for determining the dispersive surface energy of porous silica
Introduction
Porous silicates, such as silica gel, porous glasses and ordered mesoporous materials, are widely used in many applications, in particular where surfaces are involved. These include the fields of separation, catalysis and sensors. An improvement of the performance of porous silica is possible by tailoring the surface properties of the solids. Therefore, organic compounds can be used to achieve a wide range of surface modifications, e.g., hydrophobic silica surfaces can be established by reaction with 1,1,1,3,3,3-hexamethyldisilazane (HMDS) so that the resistance to water adsorption increases significantly [1]. A current application of this modification procedure is the design of new carrier systems for well controlled drug release, e.g., the optimisation of ibuprofen release from MCM-41 [2]. Furthermore, the modification with 3-amino-propyltriethoxysilane (3-APTS) enables many applications in biotechnology, e.g., the immobilisation of enzymes [3]. The development of new pesticides sensors on the basis of (modified) porous glass–enzyme composites is a recent example [4].
The thermal treatment of porous silicates changes their surface chemistry. Siloxane groups are formed through condensation of silanol groups. The concentration of remaining hydroxyl groups can be controlled by thermal treatment conditions (time, temperature) [5]. In the case of porous glasses, surface heterogeneities caused by different boron species lead to a further modification of the surface chemistry [6].
The measurement of the surface modification effect in porous materials might seem to be difficult, but can be accomplished by surface sensitive techniques like inverse gas chromatography (IGC). IGC is a dynamic sorption technique that allows to investigate relations between surface modification and surface properties by detecting thermodynamic and kinetic parameters correlated to the solids surface. Moreover, it can be used to determine surface areas, pore size distributions and adsorption isotherms [7]. IGC is helpful for understanding the performance of materials in applications and for a better comprehension of the surface chemistry. For IGC measurements the solid under investigation is used as stationary phase in the IGC column while gaseous probe molecules with known properties are used as mobile phase. The solid's surface properties have an impact on the retention time of the probe molecules, hence enabling the calculation of physicochemical parameters. Such parameters are for example the surface energy of solids, adsorption isotherms, diffusion coefficients and differential enthalpy and entropy of adsorption [7].
IGC is widely used to study the surface energy of porous materials [8]. The surface energy consists of a dispersive term , caused by non-polar interactions based on London forces, and a specific term , caused by polar interactions. Due to the high sensitivity of the dispersive component of surface energy for changes in the surface chemistry, is an appropriate parameter for the characterization of (modified) porous materials. Recently, Thielmann [9] used IGC technique for the characterization of MCM-41, activated carbons and porous alumina via determination of the dispersive surface energy .
The aim of this study was the investigation of correlations between dispersive surface energy, surface chemistry, surface modification and microstructure of porous silicates. Porous glass beads and silica gels acted as model systems for the study of these correlations. The surface properties of these porous silicates were modified by (i) thermal treatment, (ii) reaction with HMDS and (iii) reaction with 3-APTS to investigate the dependency of dispersive surface energy on changes of the surface chemistry. Furthermore, different ordered mesoporous silica materials (MCM-41, SBA-15 and SBA-16) were characterized by inverse gas chromatography, because their surface chemistry is still under discussion [10].
Section snippets
Materials
Mesoporous glass beads were synthesized by the following procedure. Initial glass beads (70 wt.% SiO2, 23 wt.% B2O3 and 7 wt.% Na2O) were phase separated at 550 °C for 48 h, followed by a combined acid and alkaline leaching treatment. The resulting porous glass beads were washed with deionised water and dried at 80 °C for 24 h (sample 550/48).
Commercial silica gels (ACROS Organics) with 4, 8 and 15 nm mean pore diameter were used without further purification (samples SG4, SG8 and SG15).
The surface of
Theoretical analysis and calculations
The principle of inverse gas chromatography is to conduct gaseous probe molecules with known properties over the surface of the solid of interest. The retention time of the probe molecules is then influenced by the interaction with the surface of the solid. Physicochemical properties can be calculated from the retention times, e.g. surface energy of the solids, adsorption isotherms, differential enthalpy and entropy of adsorption and diffusion coefficients [7], [9].
The surface energy can be
Results and discussion
The IGC method described in the previous section was utilized to determine the dispersive surface energy of various porous silicates. The focus of this investigation was to study the effect of several surface modifications and the influence of microstructure on the surface energy.
Conclusions
IGC has been shown to be a useful technique to characterize various porous materials. In this study, the dependency of the dispersive part of the surface energy on the surface chemistry-concentration of hydroxyl groups, type of porous silicates (silica gel, porous glass, ordered mesoporous materials), surface modifications-could be demonstrated. A correlation between the surface modification with HMDS and the dispersive surface energy was observed. In the case of a modification with amino
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