Electrification at water–hydrophobe interfaces Item Type Article Authors Nauruzbayeva, Jamilya; Sun, Zhonghao; Gallo Junior, Adair; Ibrahim, Mahmoud; Santamarina, Carlos; Mishra, Himanshu Citation Nauruzbayeva, J., Sun, Z., Gallo, A., Ibrahim, M., Santamarina, J. C., & Mishra, H. (2020). Electrification at water–hydrophobe interfaces. Nature Communications, 11(1). doi:10.1038/ s41467-020-19054-8 Eprint version Publisher's Version/PDF DOI 10.1038/s41467-020-19054-8 Publisher Springer Nature Journal Nature Communications Rights This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. Download date 05/10/2021 16:06:59 Item License https://creativecommons.org/licenses/by/4.0 Link to Item http://hdl.handle.net/10754/665657 ARTICLE https://doi.org/10.1038/s41467-020-19054-8 OPEN Electrification at water–hydrophobe interfaces Jamilya Nauruzbayeva1,3, Zhonghao Sun 2,3, Adair Gallo Jr.1,3, Mahmoud Ibrahim1, J. Carlos Santamarina 2 & ✉ Himanshu Mishra 1 The mechanisms leading to the electrification of water when it comes in contact with hydrophobic surfaces remains a research frontier in chemical science. A clear understanding of these mechanisms could, for instance, aid the rational design of triboelectric generators and micro- and nano-fluidic devices. Here, we investigate the origins of the excess positive 1234567890():,; charges incurred on water droplets that are dispensed from capillaries made of poly- propylene, perfluorodecyltrichlorosilane-coated glass, and polytetrafluoroethylene. Results demonstrate that the magnitude and sign of electrical charges vary depending on: the hydrophobicity/hydrophilicity of the capillary; the presence/absence of a water reservoir inside the capillary; the chemical and physical properties of aqueous solutions such as pH, ionic strength, dielectric constant and dissolved CO2 content; and environmental conditions such as relative humidity. Based on these results, we deduce that common hydrophobic materials possess surface-bound negative charge. Thus, when these surfaces are submerged in water, hydrated cations form an electrical double layer. Furthermore, we demonstrate that the primary role of hydrophobicity is to facilitate water-substrate separation without leaving a significant amount of liquid behind. These results advance the fundamental understanding of water-hydrophobe interfaces and should translate into superior materials and technologies for energy transduction, electrowetting, and separation processes, among others. 1 King Abdullah University of Science and Technology (KAUST), Water Desalination and Reuse Center (WDRC), Division of Biological and Environmental Sciences and Engineering, Thuwal 23955 - 6900, Saudi Arabia. 2 King Abdullah University of Science and Technology, Ali I. Al-Naimi Petroleum Engineering Research Center (ANPERC), Division of Physical Science and Engineering, Thuwal 23955 - 6900, Saudi Arabia. 3These authors contributed equally: Jamilya ✉ Nauruzbayeva, Zhonghao Sun, Adair Gallo Jr. email: [email protected] NATURE COMMUNICATIONS | (2020) 11:5285 | https://doi.org/10.1038/s41467-020-19054-8 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19054-8 ater becomes electrified when it comes in contact with Methods). Glass capillaries allowed us to precisely control the Whydrophobic media. Electrification underlies various solid-liquid interfacial tension. For instance, freshly cleaned glass curious phenomena, such as the electrostatic manip- capillaries were superhydrophilic, characterized by ultralow 1–3 θ ulation of droplets placed on hydrophobic surfaces and Kelvin apparent contact angles, r 5°. To render them hydrophobic, we generators4–6. The electrification of water against hydrophobic covalently grafted perfluorodecyltrichlorosilane (FDTS) onto surfaces (hereafter referred to as water-hydrophobe interfaces) them through a molecular vapor deposition technique (Methods). plays an important role in various applied and natural contexts, We characterized wetting by measuring advancing (θA) and 7–9 10–12 θ such as pipetting , triboelectric power generation , hydro- receding ( R) contact angles using sessile deionized water droplets gen generation13,14, mitigating dust deposition on solar panels15, of volume 2 μL dispensed/retracted at 0.2 μLs−1, and found fi fl 16 θ = θ = preventing re hazards in granular ows , and precipitation and them to be A 105° ± 1° and R 72° ± 1° respectively thundercloud charging17,18. However, the causes and mechan- (Methods). The polypropylene surfaces of the pipette tips fi θ = θ = isms underlying this electri cation process are still intensely exhibited A 113° ± 2° and R 62° ± 2°. Hereafter, deionized debated11,14,19–37. water is referred to as water. Supplementary Fig. 1 presents AFM A variety of mechanisms have been put forth to explain elec- scans of the FDTS-coated glass and polypropylene surfaces. trification of water in contact with solid/liquid/gaseous hydro- Two complementary experimental techniques were deployed to phobes, including the specific adsorption of hydroxide ions38–48 investigate the electrification of water. The first technique used and hydronium ions14,49–55, the dipolar organization of inter- pendant droplets of controlled volume (10–20 μL) and surface facial water19,22,56, the partial charge transfer between the O and area, formed at the tip of hydrophobic and hydrophilic capillaries. H atoms of interfacial water57,58 or between interfacial water and We recorded the formed droplets’ behavior inside a parallel plate oil molecules59, the adsorption of bicarbonate ions due to the capacitor, which comprised of two 100 × 100 mm2 aluminum 60 61–66 fi dissolution of ambient CO2 , contamination , reactive che- plates that ensured a uniform electric eld in the central region of mical groups8,9,67–69, electrons trapped on the surface of insula- <4 × 4 × 4 mm3 occupied by the droplets (Fig. 1a, b, Supplemen- tors70–72 and mechanoradicals73,74. With the exception of tary Section 2, and Supplementary Fig. 2). Two scenarios were surface-bound electrons, these mechanisms assume that com- tested: (i) the capillary was fully filled with water before mon hydrophobic surfaces such as polypropylene and per- producing the droplet at the end of the capillary inside the fluorocarbons are electrically neutral in air. In this work, we capacitor (Fig. 1a); and (ii) the capillary was filled with air when designed elemental laboratory experiments to answer the fol- the droplet was formed, akin to standard pipetting (Fig. 1b). lowing interrelated questions: The second technique deployed to quantify the electrification of water used an ultrasensitive electrometer (with a detection i. Why do water-hydrophobe interfaces become electrically limit of 10 fC) equipped with a Faraday cup made of aluminum charged? sheet to shield external interferences76 (Fig. 1c, Supplementary ii. How do the properties of aqueous solutions, solid surfaces fi Fig. 3, Methods). Electrical charges on pendant droplets were and the environment impact the electri cation of water at measured by dispensing them into the Faraday cup. Together, water-hydrophobe interfaces? fi these two techniques enabled us to investigate the effects of iii. Could other liquids besides water become electri ed when surface wettability (hydrophilicity and hydrophobicity) and liquid brought into contact with hydrophobic surfaces? What is fi properties (ionic strengths, pH, and dielectric constants) on the the role of hydrophobicity in the context of electri cation at electrification of water. water-hydrophobe interfaces? Based on this experimental investigation, we deduce that the Pendant droplets under uniform electric fields. We used high- surfaces of common hydrophobes, such as polypropylene, FDTS, speed imaging to quantify the excess charges (q) carried by the and PTFE, are negatively charged. Thus, when these surfaces droplets through their deflections under uniform electric fields. come into contact with a liquid containing solvated ions, such as The balance of the electrostatic (FE) and gravitational forces (FG) water, cations form an electrical double layer at the interface in acting on the pendant droplets gave rise to tilting angles, α, accordance to the electrical double layer theory75. (Fig. 2a-b) as a function of the applied voltage (V) as: α ¼ = ð Þ Results tan FE FG 1 Experimental setup. We quantitatively investigated the elec- fi tri cation of deionized water droplets dispensed from poly- ¼ ð Þ propylene pipette tips and borosilicate glass capillaries (see FG mg 2 abWater-filled Air-filled c Coulombmeter circuit C Water Air Capillary Capillary Capillary + A V0 Water Water droplet droplet Capacitor Capacitor + + Faraday cup Electrometer Fig. 1 Schematics of the experimental set-ups. a Droplets formed at the tip of a water-filled capillary (hydrophobic or