Estudo das relações termodinâmicas em materiais nanoestruturados aplicados a adsorção e sistemas com interfaces curvas

Abstract

The synthesis of new hierarchically structured porous materials, in nanometric scale, has raised questions about the current characterization methods, by physical adsorption. Due to the complexity of a real interconnected pore system, the adsorption isotherm can be a complex combination of several pore segments and difficult to interpret. In heterogeneous systems, the properties can be modified and controlled by material processing, due to the change of the internal microstructure. Capillary effects can act as a driving force, or they can act as a barrier in material processing. In this work a critical analysis of the review of the characterization of hierarchical interconnected materials, by gas adsorption, was carried out, using the current model of independent pores. A silica micro-mesoporous material (SBA-15) was synthesized by the Zhao route (1998), analyzed by FTIR and characterized by gas adsorption (N2 .77 K). The FTIR spectra of the mesoporous silica samples showed a C-H stretching band (2800 to 3100 cm-1) and a C-H deformation band (1250 to 1500 cm-1) referring to the structure-directing agent P123, which was removed after calcination. The surface area calculation for SBA-15 silica showed correlation coefficients r > 0.9990 and C > 0, validating the calculation of the surface area of micropores present in the samples. Pore size distribution calculated by macroscopic methods based on the Kelvin equation (BJH, HK, SF) underestimate the size of mesopores and micropores compared with the microscopic method based on DFT (Density Functional Theory). The influence of the amount of wash water washing the precipitated solids on the pore size distribution was investigated. The use of 100 ml (3 samples) showed reproducible results with uniform distribution of mesopores, in a pore size range between 4.0 and 6.0 nm. The effects of curved interfaces in heterogeneous two-phase systems were performed by deriving thermodynamic equilibrium conditions and applying the equations for the selected systems: unary (L-V) and (L-S) and binary system (S-S). The application of thermodynamic equilibrium conditions for heterogeneous two-phase systems, with curved interfaces, showed a change in properties for particles with dimensions smaller than 1 µ. The excess Gibbs energy (associated with curved interfaces), calculated from thermodynamic equilibrium conditions, made it possible to predict the effect of curved interfaces on the properties of heterogeneous unary and binary systems with nanometric particles.