Electrospinner: Molecular confinement induced Janus structure fiber: giving the surface high potential to achieve fast collection of fog water!

Background

Collecting fog water composed of differently charged droplets is one of the potential strategies to deal with the freshwater crisis. The electrostatic attraction between the charged surface and the droplets can effectively improve the capture efficiency, thereby achieving efficient collection of fog water. However, the strategy of enhancing electrostatic attraction by introducing charges faces the challenge of persistence.

Recently, the team of Professor Cai Zaisheng of Donghua University published a research result entitled "A Molecular Confine-Induced Charged Fiber for Fog Harvesting" in Advanced Fiber Materials. This work uses a wet spinning process to successfully prepare Janus-PAN fibers with persistent high surface potential through molecular intrinsic polarity regulation and wettability gradient design. The harp collector made of this fiber can achieve a water collection efficiency of 1775 mg/(cm2·h), which is 2.6 times that of traditional low surface potential, non-wetting gradient fiber collectors. This study provides new ideas for the structural design and controllable preparation of a new generation of fog collection fiber materials.

The main point of this paper

The spontaneous charging of water droplets, which generates attraction and coalescence, is an important reason for the formation of fog. This charging phenomenon is mainly caused by three factors in Figure 1a. (1) Embedded charge: gravity and airflow promote collisions, causing charged particles to merge into water droplets; (2) Ionized charge: water molecules dissociate during evaporation and condensation, producing protons and hydroxide ions; (3) Polarized charge: the polarity of water molecules causes internal charge imbalance.

Generally, the larger the molecular polarity, the higher its surface potential, which is more conducive to the adsorption of water molecules. Among polymers, polyacrylonitrile (PAN) repeating units have a large dipole moment (3.6 D) and strong molecular polarity, making them ideal materials for preparing high surface potential fibers. Due to the large electronegativity of the cyanide group and the surface polarization caused during the preparation process, the surface of PAN fibers presents a high negative potential, which produces a strong electrostatic interaction with water molecules, which helps to improve the capture efficiency of fog (Figure 1b-e).

PAN fibers are prepared by wet spinning. As the alkalinity of the coagulation bath increases, the PAN molecules are partially hydrolyzed, resulting in the conversion of the original cyanide group into a carboxyl group. In addition, PAN fibers prepared in a neutral coagulation bath by treating hydroxylamine hydrochloride can obtain fibers with positive surface potential, which is helpful to study the effects of positive and negative potentials on fog water collection. In-situ molecular confinement modification is used to increase the absolute value of the fiber surface potential or change its polarity, which can ensure that the fiber potential is not affected by environmental humidity fluctuations.

The changes in PAN molecules during the spinning process were further verified by testing methods such as XRD, FTIR, and XPS. As can be seen from Figure 3g, as the pH of the coagulation bath changes from 3 to 13, the surface potential of PAN fibers increases from -12 mV to -43 mV. Due to the large electronegativity of oxygen atoms, the polarity of carboxyl groups is greater than that of cyano groups, which increases the surface potential of PAN fibers. In the reaction of hydroxylamine hydrochloride with nitrile groups, the conversion rate (CR) of nitrile groups in PAN changes with reaction time as shown in Figure 3h. After 5 h of reaction, CR reaches about 78%, and the surface potential of PAN fibers reaches +41 mV.

Efficient collection of fog water mainly depends on the effective capture of droplets and the rapid transmission of directional water. When the substrate surface presents a high potential, it will generate a strong electrostatic attraction to the droplets with the opposite potential, thus promoting the capture of the droplets (Figure 4a-c). In the test of collecting fog water, the higher the surface potential, the faster the water droplets gather on the fiber surface (Figure 4d); when the fiber surface potential is -43 mV, the water collection efficiency (WCR) reaches a peak of 774 mg/(cm2·h) (Figure 4e). The WCR of the vertically placed fiber is about 1.5 times that of the horizontally placed fiber (Figure 4f-g). For the positively charged PAN fiber, as the modification time increases, the surface potential gradually increases, and the WCR increases from 683 mg/(cm2·h) to 751 mg/(cm·h)

The fiber with a wettability gradient on the surface (Janus-PAN) can effectively and timely transport the captured water to the collector to re-expose the capture site. The WCR of the harp collector made of Janus-PAN fibers reached 1775 mg/(cm2·h), which is 2.4, 1.5 and 1.7 times that of PAN, HB-PAN and Janus-PAN grid collectors, respectively. In addition, the stability of the Janus-PAN harp collector is excellent.

summary

In summary, this work has developed a molecular confinement induced potential control technology that can stably maintain the surface potential of the material without being affected by humidity changes. The Janus-PAN fibers prepared using this technology can simultaneously achieve efficient capture of fog water and directional and rapid water transport. However, excessively high surface potential may hinder the shedding of water droplets, resulting in a decrease in collection efficiency. In addition, the wide applicability of the Janus-PAN harp collector in crop irrigation has been verified. This novel fog water collection strategy provides new inspiration for fluid management at asymmetric wettability interfaces.