The world faces a growing freshwater scarcity crisis driven by population growth, industrialization, climate change, and water contamination. Desalination, particularly reverse osmosis (RO) membrane desalination, presents a sustainable solution, but current industrial-scale membranes suffer from fouling. Fouling reduces flux, necessitates periodic chemical cleaning (with associated environmental and energy costs), and shortens lifespan. This paper addresses this critical limitation by introducing a new class of smart, self-cleaning membranes. The challenge lies in designing antifouling membranes that maintain high productivity and rejection ability. While progress has been made with advanced membranes exhibiting improved permeability, selectivity, and fouling resistance, fundamentally new approaches are required. Smart gating membranes, inspired by stimuli-responsive cell channels, offer one potential solution. These membranes dynamically adjust pore size or surface properties in response to stimuli like temperature, light, pH, or electric fields. Hybrid membrane approaches, combining stimuli-responsive materials with traditional membranes, have shown promise, particularly those using hydrogels. This research explores the use of thermosalient (TS) crystals – a class of dynamic crystalline materials that expand or move upon thermal stimulation – as a novel approach to create smart, self-cleaning membranes. TS crystals offer a unique advantage: their rapid and reversible conversion of heat into mechanical work at a millisecond timescale, a property potentially beneficial for efficient, on-demand fouling removal. Stabilizing TS crystals within a soft matrix like hydrogels is crucial for maintaining their integrity and cyclic operation. The study aims to demonstrate the feasibility and effectiveness of this approach for improving the performance and longevity of desalination membranes.

Existing literature highlights the limitations of conventional desalination membranes due to fouling. Numerous studies have explored advanced membrane materials and surface modifications to mitigate fouling, including the use of nanoparticles, zwitterionic polymers, and hydrogels. The concept of smart gating membranes, capable of dynamic self-regulation, has also been investigated, utilizing various stimuli-responsive materials. Hydrogels, known for their responsiveness to external stimuli and excellent permeation properties, have been widely studied in this context. This study, however, proposes a novel approach utilizing thermosalient crystals, a relatively new class of dynamic materials, not yet extensively explored for membrane applications. The review of existing literature on TS crystals highlights their potential for actuation and sensing applications, but their use in self-cleaning membranes is a new research area.

The research employed a hybrid membrane approach, combining porous polyvinylidene fluoride (PVDF) membranes with a thin polyvinyl alcohol (PVA) hydrogel layer containing dispersed thermosalient (TS) crystals of 1,2,4,5-tetrabromobenzene (TBB). TBB, chosen for its readily accessible phase transition slightly above room temperature, serves as an energy-efficient dynamic material. Membrane preparation involved mixing aqueous PVA and glutaraldehyde (GA) solutions, adding varying amounts of TBB crystals, treating with hydrochloric acid, and casting the mixture onto the PVDF support. Polymerization occurred at room temperature. The resulting P-P-T membranes (PVDF-PVA-TBB) were characterized for their morphology, mechanical properties, and transport characteristics. Characterization methods included optical microscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), contact angle measurements, zeta potential analysis, and ATR-FTIR spectroscopy. Mass transport properties were evaluated using osmotic distillation (OD) and direct contact membrane distillation (DCMD) setups at varying temperatures, both with pure water and solutions containing model foulants (bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA)). Gas permeation measurements (using CO2, H2, and N2) were conducted to assess the membrane's pore size distribution and transport mechanisms. Finally, variable-temperature single-crystal X-ray diffraction was used to study the thermal expansion of TBB crystals, and electrochemical techniques examined dielectric properties of the membranes. Osmotic distillation tests were conducted using a lab-scale OD plant with a flat membrane module. Transmembrane flux (J) and NaCl rejection were calculated. Fouling tests involved multiple OD cycles with foulant solutions, assessing flux decline and the effect of temperature cycling on membrane performance. DCMD tests utilized a similar setup but with a hypersaline feed solution and a temperature gradient. Transmembrane flux, salt rejection, and specific thermal energy consumption (STEC) were evaluated over multiple cycles. The antifouling behavior was analyzed through transmembrane flux and mass transfer coefficient (Bm) measurements. Data analysis included Arrhenius plots to analyze temperature dependence and exponential fitting to model flux over time.

The study found that the incorporation of TBB crystals significantly enhanced the performance of the membranes. An optimal TBB loading of 1.0 mg cm⁻² maximized transmembrane flux in osmotic distillation (OD). At this loading, a flux increase exceeding 43% was observed at 48 °C (above the TBB phase transition temperature) compared to reference membranes without TBB. The Arrhenius plots showed deviations from typical behavior near the TBB phase transition temperature, indicating a different transport mechanism activated by the crystals' dynamic response to heating. Higher TBB loadings led to decreased flux, attributed to crystal aggregation and reduced membrane elasticity. The mechanical analysis demonstrated that TBB crystal inclusion significantly stiffened the PVA hydrogel layer, influencing swelling behavior and optimal loading range. The incorporation of TBB also improved the hydrophilicity of the membrane, decreasing the water contact angle. Fouling experiments demonstrated the self-cleaning capabilities of the TBB-loaded membranes. In OD tests with model foulants, the TBB-loaded membrane (P-P-T 1.0) exhibited substantially improved performance and resistance to fouling compared to the undoped PVDF-PVA membrane. After multiple cycles, the flux in the TBB-loaded membrane remained significantly higher (over 160%). SEM imaging revealed increased porosity in both doped and undoped membranes after thermal cycling, with larger pores or holes observed in the doped membranes possibly resulting from TBB crystal movement and partial disintegration. This porosity contributes to the enhanced flux in the presence of the crystals, and partially explains the cleaning effect. The gas permeability measurements showed that the presence of TBB crystals had a small effect on the transport of gases, primarily due to the dominant Knudsen flow mechanism which masks any selectivity effect in the gas transport. In DCMD tests with hypersaline (228 g L⁻¹ TDS) solutions and foulants, the P-P-T 1.0 membrane maintained a flux above 1 L h⁻¹ m⁻² over multiple cycles, with only a 7% decrease, unlike the undoped membrane, which experienced a 40% flux decline. The mass transfer coefficient (Bm) calculations further confirmed the improved antifouling properties of the TBB-loaded membranes. Variable-temperature single-crystal X-ray diffraction revealed anisotropic thermal expansion in TBB crystals, potentially contributing to the enhanced transport properties. Dielectric measurements showed a 25% higher dielectric constant for doped membranes compared to pristine membranes.

The findings demonstrate that the integration of thermosalient crystals into hybrid membranes provides a highly effective strategy for enhancing desalination membrane performance and addressing the persistent fouling problem. The observed increase in transmembrane flux, significantly exceeding that of conventional membranes, is attributable to the unique dynamic response of the TBB crystals to temperature changes. The self-cleaning capability arises from the mechanical instability and subsequent crystal movement during the phase transition, effectively dislodging and removing accumulated foulants. The optimal TBB loading represents a balance between enhanced transport properties due to hydrophilicity and the potential for crystal aggregation that would impede flux. The improved antifouling behavior is a crucial advantage, reducing the need for energy-intensive chemical cleaning. The results also suggest potential synergies between this approach and multiple-effect membrane distillation to further optimize energy efficiency. These findings have significant implications for advancing desalination technology and other separation processes. The ability to create self-cleaning, high-performance membranes could drastically reduce operating costs and environmental impact.

This research presents a novel approach to creating smart, self-cleaning desalination membranes by incorporating thermosalient crystals. The results demonstrate a significant enhancement in water flux and a remarkable resistance to fouling. The optimal TBB loading maximizes performance, balancing hydrophilicity and crystal aggregation. Future work could explore other TS crystals and membrane materials, optimize membrane fabrication processes, and investigate long-term operational stability under diverse conditions. The potential for this technology extends to various separation processes beyond desalination, offering a path towards more sustainable and efficient water purification.

The study focused on a specific TS crystal (TBB) and membrane composition. The long-term stability and durability of the membranes under continuous operation require further investigation. The study's fouling experiments used model foulants; more extensive testing with real-world foulants is needed to fully assess the membranes' antifouling capabilities under diverse conditions. While the observed increase in porosity likely contributes to cleaning, the exact mechanism warrants additional research. Finally, the study did not consider the cost-effectiveness of incorporating these TS crystals into large-scale manufacturing of the membrane, which is a crucial factor for practical implementation.