Why does the pore size attributed to nanoparticles in our mixed matrix membranes (MMMs) appear larger when measured by positron annihilation lifetime spectroscopy (PALS) (0.81 nm) compared to the value obtained from Brunauer–Emmett–Teller (BET) analysis (0.63 nm)? What factors could contribute to this discrepancy between membrane-based measurements and BET results, considering that BET primarily reports the accessible pore size to adsorbate gases such as nitrogen? Furthermore, if PALS detects a pore size of 0.50 nm that is likely associated with the intersegmental gaps formed within the polymer matrix during membrane fabrication, what approximate accessible pore size should be expected in BET analysis for such a structure?
Siraj Ahmad
1. Different physical principles. PALS: This method focuses on the detection of positrons that annihilate with electrons in materials. Thus, PALS can provide information on pore size and structure based on the spatial distribution of positive ions in the membrane.BET: This method is based on gas adsorption and determines the surface area and volume of pores, which may be more sensitive to small pores and adsorption-related effects.
2. Statistical nature of the measurements.
BET results may include only those pores that participate in the adsorption process, while PALS can take into account a larger number of pores, including those that are impermeable to gas but still accessible to positrons. 3. 3/Calibration and accuracy. Measurement methods may differ in calibration and accuracy. PALS generally provides higher resolution in the case of small structures, while BET may be less sensitive to changes at the nanoscale. 4. Structural features. The structure of the mixed matrix membrane can affect the accessibility of the pores for different methods, which will also lead to differences in measurements. For example, due to aggregation or changes in the pore structure, pore sizes may appear larger with one technique compared to another.
Yuri Mirgorod, Thank you for the detailed explanation. Your answer highlights the fundamental differences between PALS and BET, including their measurement principles, sensitivity, and how structural features of mixed matrix membranes can influence pore accessibility. This helps me better understand why PALS shows larger pore sizes than BET, and how each technique offers unique insights into the membrane’s microstructure. Much appreciated.
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