Pore size distribution is one of the important properties of porous materials.
The main determination methods are:
- Mercury intrusion method
- Gas adsorption-desorption isotherm method (physical adsorption)
- Calorimetry (divided into wettability calorimetry and thermal porosity method)
- Rejection method
- Gas bubble pressure method
- Liquid-liquid elimination
- Fluid flow method
- Permeability porosity
The visual information of the cross section and surface of the membrane can be obtained by using the microscopy technology, and the results of porosity and pore diameter can be obtained by further analyzing the image. The microscopic techniques used to characterize membrane pore size mainly include environmental scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), atomic force microscope (AFM) and scanning tunneling microscope (stem). Although the micro technology can directly observe the pore size type and size of porous materials, the micro electron microscope can only observe the pore size of the membrane in a very small range, which has great limitations. Moreover, the preparation of samples will affect the results, and the price of the instrument is generally expensive.
2. Mercury intrusion method
With the help of external force, the liquid metal mercury that does not infiltrate the material surface is pressed into the dry porous sample, and the volume of mercury in the sample is measured to change with the external pressure. The relationship between the pore volume and the pore diameter of the sample can be determined through calculation. Due to the large surface tension of mercury, the smaller the pore size, the higher the pressure required. For example, for 1.5nm pores, the pressure is as high as 450MPa, and the high pressure may damage the membrane structure. In addition, the pores measured by the mercury injection method include the U-shaped pores of the material, which have no effect on filtration and separation.
3. Gas adsorption-desorption isotherm method
This method usually uses an inert gas such as nitrogen as the adsorbate, the relative partial pressure of the adsorbate is changed at a constant temperature, and the adsorption capacity of the porous material in the adsorption process of the adsorbate and the desorption amount in the adsorbate desorption process are respectively determined, and the adsorption isotherm is obtained. Lines and desorption isotherms, pore size distributions calculated from the data using different models. The pore volume of the sample is calculated from the amount of gas adsorbate adsorbed at the boiling temperature. This method will be affected by the support when measuring the pore structure of the supported membrane. It is often used for the measurement of unsupported membranes, and is generally used to measure porous membranes with a pore size below 30 nm. However, the process of this method is more complicated, and the calculation model is different according to the pore diameter and isotherm.
The principle of immersion calorimetry is to measure the enthalpy change of the “dry” membrane material when immersed in different liquids, and the enthalpy change is related to the pore structure. For hydrophilic oxides, water is usually used as the immersion liquid, while for hydrophobic substances, organic substances such as benzene and n-hexane are used as the immersion liquid. The molecular size of the immersion solution was changed, and the immersion rate and enthalpy change of the immersion process were measured to determine the pore size of the membrane. This method is mainly used to determine the surface area and pore size of membranes with a pore size of less than 1 nm, such as carbon membranes. Thermoporosimetry uses the Gibbs-Thompson effect of the liquid-solid phase transition in the capillary to determine the pore size and distribution of the membrane. The principle is that the freezing point of the liquid in the pore size is lower than the normal state, and the deviation value is inversely proportional to the size of the pore size.
5. Rejection method
The retention rate method uses protein, polyethylene glycol, etc. as reference materials to determine the degree of retention of a certain molecular weight reference material by the membrane.
CF refers to the concentration of the reference substance in the raw liquid, and CP refers to the concentration of the reference substance in the permeate. Generally, the molecular weight with the rejection rate greater than 90% is taken as the rejection rate index of the membrane. Therefore, the higher the rejection rate, the narrower the rejection range, indicating that the better the separation performance of the membrane, the narrower the pore size distribution. However, the rejection rate of the membrane is not only related to the pore size and distribution of the membrane, but also related to the properties of the membrane material, the pore structure of the membrane and the structure, properties and operating conditions of the reference material. The determination process is also troublesome.
6. Gas bubble pressure method
The measurement of membrane pore size distribution by gas bubble pressure method is mainly based on the capillary tension of liquid in the pores and the flow mechanism of gas in the pores. The relationship between the flow rate of gas through the liquid impregnated membrane and the pressure difference is measured, and the pore size of the membrane is calculated by Laplace equation. This method has become the recommended method of ASTM.
The gas bubble pressure method is simple, convenient, accurate and reliable for measuring the pore size of tubular and sheet porous membranes. At the same time, this method is different from the gas adsorption desorption method, mercury injection method and calorimetry. It measures the pore size distribution of active pores, that is, the pores that can pass through the fluid, so it is more practical. The gas bubble pressure method is more convenient and practical for measuring the maximum pore size or defect size of membrane and the sealing performance of membrane components in characterizing industrial products. However, due to the influence of surface tension of wetting agent, the minimum pore size measured by gas bubble pressure method is generally about 0.5 μ M.
7. Liquid-liquid elimination
The measurement principle of this method is the same as that of the gas bubble pressure method, but two immiscible liquids are used as the penetrant and the wetting agent, that is, the liquid penetrant replaces the gas penetrant of the gas bubble pressure method. Because the interfacial tension between liquids is much lower than the surface tension between gas and liquid, the pressure required to measure the pore size of the same size is lower, and the measurable pore size is smaller. Pore size distribution of ultrafiltration membranes.
The capillary action in the membrane pores is determined by the Laplace equation or the Cantor equation; the relationship between the liquid permeation rate in the cylindrical pores and the pressure difference can be determined by the following Hagen-Poiseuille equation.
8. Fluid flow method
By measuring the permeation flux of the fluid (gas or liquid), the mean pore size (ie hydraulic radius) of the membrane is calculated from the mass transfer model. This method is simpler, but the resulting pore size reflects the overall fluidity of the membrane.
► Liquid osmosis.
► Gas permeation method. This method determines the average pore size of the membrane from the permeation mechanism of the gas by measuring the relationship between the permeation flux of the non-condensable gas and the pressure difference.
9. Permeability porosity
This method is one of the important methods for measuring the pore size of inorganic ultrafiltration membranes. It combines the advantages of adsorption-desorption method and gas permeation method, uses a gas-vapor mixture, controls the relative vapor pressure, and condenses the vapor components (carbon tetrachloride, methanol, ethanol and cyclohexane) in part of the pores, The gas permeation flux in the no-condensation pores was determined. According to the adsorption-desorption theory, the measurement adopts a desorption process, that is, starting from a relative vapor pressure of 1, so that all membrane pores are blocked by condensate, and no gas permeates the membrane at this time. In the process of gradually reducing the relative vapor pressure, the pores of the membrane are opened in order from large to small, and the permeation amount of another gas (nitrogen or oxygen) through the membrane is measured at the same time. The pore size distribution of the membrane can be determined by measuring the gas permeability of the membrane under a certain relative vapor pressure. This method can directly measure the active pores of the membrane, and the minimum measurement pore size can reach 1.5nm. However, this method requires the use of steam mixed gas, which requires higher control of the device.