Sorption von Fluiden in mesoporösen Silikamertialien: Modellierung des Sorptionsverhaltens und elastischer Verformungen

The mesoporous silica materials SBA-15 and MCM-41 are highly ordered and consist of long cylindrical pores having a pore radius of a few nanometers (2−10 nm = mesopores) arranged on a 2D-hexagonal lattice. These materials are used frequently in industrial applications such as filters, catalysts, templating materials and for gas storage. But they also provide a possibility of studying sorption processes and the accompanying material deformations on the nanoscale. In this work, scattering data from X-ray and neutron scattering experiments are modelled with a number of different approaches in order to study the sorption behavior of these materials. Additionally, a Molecular Dynamics simulation is coupled with a Finite Elements model into a simple multi-scale model for the sorption-induced elastic deformations. Small-angle X-ray scattering data from an adsorption experiment with dibromomethane in SBA-15 are fitted with a form factor model based on the radial density distribution in a single mesopore. It is shown that the fluid simultaneously adsorbs in the microporous corona and as a film on the pore surfaces where the maximum film thickness just before capillary condensation is about one monolayer of dibromomethane (0.5nm). Based on this model a 2D lattice model is introduced which reproduces the fluid adsorption using two heuristic filling mechanisms. Scattering curves produced by this model show a qualitative and often quantitative agreement with the experimental scattering data. By comparing the ratio of diffuse to Bragg scattering in simulation and experiment it can be shown that the mesopores in SBA-15 comprise 80% of the total porosity (φ = 0.68) while the rest (20%) is contibuted by additional micropores in the silica walls. Furthermore, a method for calculating the sorption isotherm from scattering data by using the integrated intensity in dependence on the fluid gas pressure is presented here. In addition, the self-assembly of amphiphilic molecules (surfactants) in SBA15 is examined. For this, neutron scattering data are fitted with a model function, where a form factor model of a hollow cylinder is used for the Bragg peaks in order to simulate an adsorbed film of surfactant micelles. The diffuse scattering is described by the Teubner-Strey-model for microemulsions. Combining these two scattering functions leads to excellent fitting results and shows that with increasing surfactant load the surfactant micelles form a perforated film on the pore surfaces, whose thickness is considerably lower than the diameter of free micelles. This is explained by the flattening of the micelles as an effect of the confined geometry. In order to examine the origin of the elastic deformations during fluid sorption a Molecular Dynamics study depicting a slit-pore with flexible particle walls and interactions via the Lennard-Jones potential is set up. This simulation shows that a compression of the pore can be measured when a certain fluid particle density is reached. From this compression the pressure that the fluid particle impose on the pore walls is calculated. Using realistic values for the interaction potential and the pore size this pressure for example for SBA-15 and pentane