Over the last years, inverse gas chromatography (IGC) proved to be a versatile and sensitive analytical technique for physicochemical properties. However, the comparability of results obtained by different users and devices remains a topic for debate. This is the first time, an interlaboratory study using different types of IGC instruments is reported. Eight organizations with different IGC devices defined a common lab measurement protocol to analyse two standard materials, silica and lactose. All data was collected in a standard result form and has been treated identically with the objective to identify experimentally observed differences and not potentially different data treatments. The calculated values of the dispersive surface energy vary quite significantly (silica: 22 mJ/m2 - 34 mJ/m2, lactose 37 mJ/m2 - 51 mJ/m2) and so do the ISP values and retention volumes for both materials. This points towards significant and seemingly undiscovered differences in the operation of the instruments and the obtained underlying primary data, even under the premise of standard conditions. Variations are independent of the instrument type and uncertainties in flow rates or the injected quantities of probe molecules may be potential factors for the differences. This interlaboratory study demonstrates that the IGC is a very sensitive analytical tool, which detects minor changes, but it also shows that for a proper comparison, the measurement conditions have to be checked with great care. A publicly available standard protocol and material, for which this study can be seen as a starting point, is still needed to judge on the measurements and the resulting parameters more objectively.

Surface-modified porous silica is a well-established composite material. To improve its embedding and application behavior, adsorption studies of various probe molecules have been performed using the technique of inverse gas chromatography (IGC). For this purpose, IGC experiments were carried out in the infinite dilution mode on macro-porous micro glass spheres before and after surface modification with (3-mercaptopropyl)trimethoxysilane. To provide information about the polar interactions between probe molecules and the silica surface, in particular, eleven polar molecules have been injected. In summary, the free surface energy for pristine silica ( γ S t o t a l = 229 mJ/m2) and for (3-mercaptopropyl)trimethoxysilane-modified silica ( γ S t o t a l = 135 mJ/m2) indicates a reduced wettability after surface modification. This is due to the reduction of the polar component of the free surface energy ( γ S S P ) from 191 mJ/m2 to 105 mJ/m2. Simultaneously, with the reduction of surface silanol groups caused by surface modification of silica and, therefore, the decrease in polar interactions, a substantial loss of Lewis acidity was observed by various IGC approaches. Experiments with all silica materials have been conducted at temperatures in the range from 90°C to 120°C to determine the thermodynamic parameters, such as adsorption enthalpy ( Δ H a d s ) and adsorption entropy ( Δ S a d s ), using the Arrhenius regression procedure evaluating the IGC data. With the help of the enthalpy–entropy compensation, two types of adsorption complexes are assumed between polar probe molecules and the silica surface because of different isokinetic temperatures. Identical adsorption complexes with an isokinetic temperature of 370°C have been assigned to alkanes and weakly interacting polar probes such as benzene, toluene, dichloromethane, and chloroform. Polar probe molecules with typical functional groups such as OH, CO, and CN, having the ability to form hydrogen bonds to the silica surface, exhibit a lower isokinetic temperature of 60°C. Quantum chemical calculations of the probe molecules on a non-hydroxylated and hydroxylated silica cluster supported the formation of hydrogen bonds in the case of a strong polar adsorption complex with a bonding distance of 1.7 nm–1.9 nm to the silica surface.

Surface modified silica nanoparticles are well-established composite materials. To improve their embedding and application behavior, surface energy analysis has been performed before and after silanization with mercapto-propyltrimethoxysilane (MPTMS) using inverse gas chromatography technique (IGC). Experiments with two silica materials have been conducted at infinite dilution to determine the dispersive component of the surface energy (gamma(d)(s)) as well as the specific component (gamma(sp)(s)) using the van Oss theory and a least-squares procedure evaluating the IGC data of 8 polar probe molecules collectively (instead of evaluating only the data of a pair of monopolar probes as is often the case in IGC studies). After surface silylation, the total surface energy (gamma(t)(s)) of pyrogenic silica nanoparticles decreased from 225 mJ/m(2) to 149 mJ/m(2) referring to both a reduced wettability and an increased hydrophobicity of the MPTMS-modified sample. Moreover, the acidity/basicity parameters according to the van Oss and the Gutmann approach indicated that the acidity of the silica surface decreases by MPTMS grafting. In addition, IGC at finite concentration (using isopropanol as probe molecule) was applied to obtain the energetic heterogeneity of the silica surface. The results showed a bimodal energy distribution with maxima at about 18 and 25 kJ/mol and a reduction of the higher energetic sites from 65% to only 35% after MPTMS treatment. Overall, it can be seen that the dominance of specific interactions on silica surfaces is maintained even after extensive silanization. These findings are in agreement with the results of Si-29 CP MAS NMR measurements which confirm the assignment of higher energetic sites to silanol groups. For the interpretation of the IGC results, quantum chemical calculations proved the preferred interaction of isopropanol onto silica surfaces via hydrogen bridging.

This article combines a systematic literature review on the fabrication of macroporous α-Al2O3 with increased specific surface area with recent results from our group. Publications claiming the fabrication of α-Al2O3 with high specific surface areas (HSSA) are comprehensively assessed and critically reviewed. An account of all major routes towards HSSA α-Al2O3 is given, including hydrothermal methods, pore protection approaches, dopants, anodically oxidized alumina membranes, and sol-gel syntheses. Furthermore, limitations of these routes are disclosed, as thermodynamic calculations suggest that γ-Al2O3 may be the more stable alumina modification for ABET > 175 m2/g. In fact, the highest specific surface area unobjectionably reported to date for α-Al2O3 amounts to 16–24 m2/g and was attained via a sol-gel process. In a second part, we report on some of our own results, including a novel sol-gel synthesis, designated as mutual cross-hydrolysis. Besides, the Mn-assisted α-transition appears to be a promising approach for some alumina materials, whereas pore protection by carbon filling kinetically inhibits the formation of α-Al2O3 seeds. These experimental results are substantiated by attempts to theoretically calculate and predict the specific surface areas of both porous materials and nanopowders.

Surface functionalization of porous materials with sulfonic acid (SO 3 H) groups is of particular interest in applications involving ion exchange, acidic catalysis and proton conduction. Macro-mesoporous silica monoliths are ideal support structures for these applications, as they combine advection-dominated mass transport in the macropores with short diffusion lengths and a large surface area (available for functionalization) in their mesoporous skeleton. Here, we report on SO 3 H functionalized sol–gel silica monoliths with bimodal pore systems exhibiting macro- and mesoporosity, prepared according to a simple, efficient in situ synthesis protocol. Based on the co-condensation approach, thiol groups were introduced homogeneously into the pore structure, followed by their oxidation to SO 3 H groups and the simultaneous removal of the template. The macropore size, specific surface area, and coverage with SO 3 H groups are easily adjusted in this synthesis route. Importantly, the hybrid monoliths have a substantially narrower mesopore size distribution (relative standard deviation ~25%) than conventional sol–gel materials (>40%) and can be engineered crack-free in a robust column design (suitable for high-pressure flow-through operation) with mean mesopore size down to ~7 nm. They are characterized by IR spectroscopy, thermogravimetry, and elemental analysis as well as 13 C and 29 Si solid state NMR to corroborate the simple, efficient combination of sol–gel-based material synthesis, surface functionalization, and template removal (i.e., polymer extraction). Complementary, inverse gas chromatography is presented as a new approach to characterize the incorporated SO 3 H groups via surface energy analysis and particularly resolve changes in the Lewis acid–base characteristics engendered by that functionalization.

Surface modification of porous glass beads by ethanol-based 3-mercaptopropyltrimethoxysilane (MPTMS) grafting solutions is directly evidenced by nitrogen adsorption, elemental analysis, thermogravimetry, infrared and Si-29 CP MAS NMR spectroscopy. Furthermore, the energetic characterization of the surface is essential to understand comprehensively the physico-chemical interactions between the pristine and MPTMS-modified surface and its gas/liquid-phase environment. In this study, inverse gas chromatography (IGC) is used to characterize the surface properties of porous glass (PG). By means of IGC at infinite dilution (IGC-ID), the dispersive component of the surface energy (gamma(d)(s)), the enthalpy and entropy of adsorption of C-6-C(10 )hydrocarbon probes were determined at temperatures between 30 and 120 degrees C. The specific component of the surface energy (gamma(sp)(s)) at the temperature of 120 degrees C has been obtained via the Van Oss theory and a least-squares procedure evaluating the IGC data of 8 polar probe molecules collectively. After surface silylation, the total surface energy (gamma(t)(s)) decreased from 402 to 255 ml/m(2) indicating both a reduced wettability and an increased hydrophobicity of the MPTMS-modified PG. Moreover, the acidity/basicity parameters according to the Van Oss and the Gutmann approach indicated that the acidity of the PG surface decreases by MPTMS grafting. Using n-octane and isopropanol probes, IGC at finite concentration (IGC-FC) was applied to obtain their adsorption isotherms and subsequently the BET specific surface areas. In addition, the surface heterogeneity of the studied PGs was also computed. The energy distribution functions of adsorption sites were monomodal (peak maximum at about 22 kJ/mol) for the n-octane probe, while isopropanol revealed a bimodal distribution function (maxima at about 18 and 25 kJ/mol) on both pristine and MPTMS-modified PG. Furthermore, the proportion of high energy sites (apparently assigned to Si-OH groups) has been reduced by surface modification from 65% to only 35% despite a high surface coverage of (1) over tilde0 MPTMS species/nm(2). These findings are in agreement with the results of Si-29 CP MAS NMR measurements and are supported by DFT calculations on the adsorption of isopropanol and n-octane on the surface of a silica cluster model. (C) 2019 Elsevier B.V. All rights reserved.