On the search of rigorous thermo-kinetic model for wet phase inversion technique

Thermodynamics and kinetic analysis of membrane: Challenges and perspectives

The ultimate structure of the membrane is determined using two important effects: (i) thermodynamic effect and (ii) kinetic effect. Controlling the mechanism of kinetic and thermodynamic processes in phase separation is essential for enhancing membrane performance. However, the relationship between system parameters and the ultimate membrane morphology is still largely empirical. This review focuses on the fundamental ideas behind thermally induced phase separation (TIPS) and nonsolvent-induced phase separation (NIPS) methods, including both kinetic and thermodynamic elements. The thermodynamic approach to understanding phase separation and the effect of different interaction parameters on membrane morphology has been discussed in detail. Furthermore, this review explores the capabilities and limitations of different macroscopic transport models used for the last four decades to explore the phase inversion process. The application of molecular simulations and phase field to understand phase separation has also been briefly examined. Finally, it discusses the thermodynamic approach to understanding phase separation and the consequence of different interaction parameters on membrane morphology, as well as possible directions for artificial intelligence to fill the gaps in the literature. This review aims to provide comprehensive knowledge and motivation for future modeling work for membrane fabrication via new techniques such as nonsolvent-TIPS, complex-TIPS, non-solvent assisted TIPS, combined NIPS-TIPS method, and mixed solvent phase separation.

In-situ photoreduction strategy for synthesis of silver nanoparticle-loaded PVDF ultrafiltration membrane with high antibacterial performance and stability

Ultrafiltration (UF) technology using polyvinylidene fluoride (PVDF) membrane has been widely applied to water and wastewater treatment due to its low cost and simple operation process. However, PVDF-based UF membrane always encountered the issue of membrane biofouling that greatly impacted the filtration performance. In this study, we prepare a silver nanoparticle (AgNP)-loaded PVDF (Ag/PVDF) UF membrane by an in-situ photoreduction method to mitigate the membrane biofouling. Different from the previously reported method, AgNPs were synthesized in-situ by a UV photoreduction process, in which Ag⁺ ions were reduced to zero-valent Ag nanoparticles by the photo-induced reducing radicals. Antibacterial experiments showed that the inhibition efficiency of Ag/PVDF membrane to Escherichia coli reached up to ~ 99% after antibacterial treatment for 24 h. In comparison with the pristine PVDF membrane, Ag/PVDF membrane possessed a lower water contact angle (83.7° vs. 38.1°), and its pure water flux increased by 23.7%, and a high bovine serum albumin (BSA) rejection efficiency was maintained. In addition, the high stability of the Ag/PVDF composite membrane was confirmed by the extremely low releasing amount of Ag. This study provides a novel strategy for the preparation of metal nanoparticle-incorporated Ag/PVDF ultrafiltration composite membrane showing favorable antibacterial performance and stability.

Phase-Field Simulation of the Effect of Coagulation Bath Temperature on the Structure and Properties of Polyvinylidene Fluoride Microporous Membranes Prepared by a Nonsolvent-Induced Phase Separation

We used the phase-field model of the existing Nonsolvent Induced Phase Separation (NIPS) method to add the variable of temperature in simulating the changes in the process of membrane formation. The polyvinylidene fluoride (PVDF) membrane system was applied to examine the influence of coagulation bath temperature change on the skin-sublayer of the membrane structure, thereby elucidating the development process of membrane structure under different conditions and shedding light on the most suitable coagulation bath temperature ranges. It was found that as coagulation bath temperature increased, the number of interface pores in the outer skin layer decreased, but the size increased. As a result, it changed from the crack shape to round-hole shape, thus making the pore structure looser. In the sublayer, the mesh support structure was increased, which enhanced the mechanical strength of the membrane. Relevant experiments also verify the effectiveness of the model.

A molecular scale analysis of TEMPO-oxidation of native cellulose molecules

The native cellulose, through TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation, can be converted into individual fibers. It has been observed that oxidized fibers disperse completely and individually in water. It is believed that electrostatic repulsive forces might be responsible for such observations. In order to study the TEMPO-oxidation of cellulose molecules, we used Density Functional Theory (DFT) calculations and Flory-Huggins theory combined with molecular dynamics (MD). The surface electrostatic potential in native cellulose and TEMPO-oxidized cellulose were calculated using DFT calculations. We found that TEMPO-oxidized cellulose accommodates a threefold screw conformation where the negatively charged (–COO–) functional groups are pointed away from the surface in all spatial directions. This spatial orientation causes that TEMPO-oxidized cellulose molecules repulse each other due to strong negatively charged surface. At the same time, the spatial orientation increases the hydrophilicity in TEMPO-oxidized cellulose molecules. These observations explain the improved dispersion in water and separability of TEMPO-oxidized cellulose molecules. We obtained large and positive Flory–Huggins interaction parameters for TEMPO-oxidized cellulose molecules indicating their higher dispersion once in water.

Modeling the Effects of Interfacial Characteristics on Gas Permeation Behavior of Nanotube-Mixed Matrix Membranes

Theoretical approaches that accurately predict the gas permeation behavior of nanotube-containing mixed matrix membranes (nanotube-MMMs) are scarce. This is mainly due to ignoring the effects of nanotube/matrix interfacial characteristics in the existing theories. In this paper, based on the analogy of thermal conduction in polymer composites containing nanotubes, we develop a model to describe gas permeation through nanotube-MMMs. Two new parameters, "interfacial thickness" (a_int) and "interfacial permeation resistance" (R_int), are introduced to account for the role of nanotube/matrix interfacial interactions in the proposed model. The obtained values of a_int, independent of the nature of the permeate gas, increased by increasing both the nanotubes aspect ratio and polymer-nanotube interfacial strength. An excellent correlation between the values of a_int and polymer-nanotube interaction parameters, χ, helped to accurately reproduce the existing experimental data from the literature without the need to resort to any adjustable parameter. The data includes 10 sets of CO₂/CH₄ permeation, 12 sets of CO₂/N₂ permeation, 3 sets of CO₂/O₂ permeation, and 2 sets of CO₂/H₂ permeation through different nanotube-MMMs. Moreover, the average absolute relative errors between the experimental data and the predicted values of the proposed model are very small (less than 5%) in comparison with those of the existing models in the literature. To the best of our knowledge, this is the first study where such a systematic comparison between model predictions and such extensive experimental data is presented. Finally, the new way of assessing gas permeation data presented in the current work would be a simple alternative to complex approaches that are usually utilized to estimate interfacial thickness in polymer composites.