Gas adsorption method for specific surface and pore size analyzer - Database & Sql Blog Articles

Gas adsorption is an excellent method for obtaining a comprehensive characterization of porous materials. It can reflect information on specific surface and pore size distribution. However, this requires a detailed understanding of the adsorption process, including the adsorption and phase changes of the porous material to the fluid and its effect on the adsorption isotherm, which is the basis for surface analysis and pore analysis.
The pore width, pore shape and effective adsorption energy are related to the pore filling process. If it is a so-called microporous (according to IUPAC classification, pore width <2 nm) pore filling is a continuous process; if it is a mesoporous (mesopore, pore width between 2nm-50nm), the pore filling is gas in the pore The internal condensation process, which is expressed as a first gas-liquid phase transfer.
We have previously introduced the capillary condensation theory (BJH) for mesoporous analysis and the microporous analysis models (HK and SF), which are macroscopic thermodynamic analysis methods, which cannot unify micropores and mesopores by the same method. The so-called classic macroscopic thermodynamic concept is based on the assumption of a certain pore filling mechanism. Methods based on the Kelvin equation (such as the BJH method) are related to capillary condensation in the pores, so they can be applied to mesoporous distribution analysis, but not to the description of micropore filling, even for narrower mesopores. Incorrect. Other classical theories, such as the Dupinin-Landkovic (DR) method, and the radius-method processing methods (such as the HK and SF methods) are only devoted to describing micropore filling and cannot be applied to mesoporous analysis. If the material contains both micropores and mesopores, we must at least have two different methods to obtain the pore size distribution from the adsorption/desorption isotherm. In addition, the accuracy of macroscopic thermodynamic methods is limited because it assumes that the fluid in the pore is a free fluid with similar thermophysical properties. Recent theoretical and experimental work has shown that the thermodynamic properties of confined fluids are quite different from those of free fluids, resulting in at least critical points, displacements of freezing and triple points. Thus, a more advanced aperture analysis method, density function theory, etc., has been proposed. Non-localized density function theory (NLDFT) and computer simulation methods (such as molecular dynamics and Monte Carlo simulation) have evolved into effective methods for describing the adsorption and phase behavior of heterogeneous fluids that are limited by porous materials. These methods can accurately describe the structure of simple restricted fluids, such as the distribution of oscillation densities near solid surfaces, or fluid structures that are limited by certain simple geometries such as slit holes, cylinders, and spheres.
Density function theory (DFT) and molecular simulation methods (MC, Monte Carlo simulation method) are molecular dynamics methods compared to those of macroscopic research methods. They not only provide a microscopic model of adsorption but also more realistically reflect the thermodynamic properties of the fluid in the pore. Those theories based on statistical mechanisms reflect the macroscopic nature of molecular behavior. Therefore, in order to make a more objective description of the adsorption phenomenon and a more comprehensive and accurate analysis of the pore size, it is necessary to establish a bridge between the molecular level and the macroscopic inquiry. The DFT and MC simulation methods of the non-uniform fluid are exactly the same. at this point. These methods consider and calculate the equilibrium density distribution of the fluid adsorbed on the surface and the fluid in the pore. From this, the adsorption/desorption isotherms, heat of adsorption, neutron scattering mode and transfer characteristics of the model system can be derived. Density distribution is obtained by MC simulation and DFT theory, and the interaction between fluid-fluid and fluid-solid interactions is calculated. The parameters of the fluid-fluid interaction are determined by regenerating their macroscopic overall properties (such as the nature of nitrogen and argon at low temperatures). The parameters of the solid-fluid interaction are obtained by fitting the adsorption isotherms of standard nitrogen and argon on a smooth surface.
The DFT method does not produce a strong fluid density distribution vibration at the solid-fluid interface, which leads to inaccurate description of the adsorption/desorption isotherms, especially for narrow pores. Conversely, non-localized DFT (NLDFT) and Monte Carlo computer simulation techniques more accurately provide fluid structures in narrow pores. Figure 1 shows the density distribution of the oscillations of such features. The density profile indicates that the gaseous and liquid phases of the fluid coexist in a wedge-shaped mesopores (fracture holes). The density of the coexisting gas (spherical) and the liquid (square) is a function of the distance between the walls of the pores, and the adsorption layer close to the pore walls is reflected as a multilayer adsorption, and the density decreases as the distance from the pore walls increases. The density profile of Figure 1 clearly indicates that pore agglomeration is inherent in the core region of the pore, which results in the formation of a seemingly unconstrained core fluid in a larger mesopores (here, a pore width of 20 molecules), as if It is the same in the core area of ​​the hole; and this is the trend of the density distribution that is essentially non-fluctuating.
The DFT method calculates the equilibrium density profile from all locations in the well, which is obtained by minimizing the free energy function. The pore system that is in equilibrium with the mobile phase (i.e., the state in which the adsorption experiments are carried out) has a large potential energy or free energy that constitutes the conditions of attraction or repulsion of fluid-fluid interactions between fluid-fluid walls. The difficulty with this approach is to establish a correct description of the fluid-fluid interaction. Because of this, different DFT research methods have been used in the past decade. The so-called localized DFT (LDFT) and non-localized DFT methods.