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Electrospun nanofiber membranes incorporating PDMSaerogel superhydrophobic coating with enhanced flux and improved anti-wettability in membrane distillation Bhaskar Jyoti Deka, Eui-Jong Lee, Jiaxin Guo, Jehad Kharraz, and Alicia Kyoungjin An Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b07254 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 13, 2019

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Environmental Science & Technology

Electrospun nanofiber membranes incorporating PDMSaerogel superhydrophobic coating with enhanced flux and improved anti-wettability in membrane distillation

4 Bhaskar Jyoti Deka,*† Eui-Jong Lee,‡ Jiaxin Guo,† Jehad Kharraz,† and Alicia Kyoungjin An*†

5 6 7

†School

of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong,

8 9

China ‡Department

of Environmental Engineering, Daegu University, 201 Daegudae-ro, Jillyang, Gyeongsan-si,

10

Gyeongbuk 38453, Republic of Korea

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*Corresponding

12

Tel: +852-3442-9626, Fax: +852-3442-0688, E-mail: [email protected] (A.K. An)

authors: Dr. Alicia Kyoungjin An

13 14

ABSTRACT:

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Electrospun nanofiber membranes (ENMs) have garnered increasing interest due to their

16

controllable nanofiber structure and high void volume fraction properties in membrane

17

distillation (MD). However, MD technology still faces limitations mainly due to low permeate

18

flux and membrane wetting for feeds containing low surface tension compounds.

19

Perfluorinated superhydrophobic membranes could be an alternative, but it has negative

20

environmental impacts. Therefore, other low surface energy materials such as silica aerogel

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and polydimethylsiloxane (PDMS) have great relevancy in ENMs fabrication. Herein, we have

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reported the high flux and non-wettability of ENMs fabricated by electrospraying

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aerogel/polydimethylsiloxane (PDMS)/polyvinylidene fluoride (PVDF) over electrospinning

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polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) membrane (E-PH). Among

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various concentration of aerogel, the 30% aerogel (E-M3-A30) dual layer membrane achieved

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highest superhydrophobicity (~ 170˚ water contact angle), liquid entry pressure (LEP) of 129.5

1 ACS Paragon Plus Environment


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± 3.4 kPa, short water droplet bouncing performance (11.6 ms), low surface energy (4.18 ±

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0.27 mN m-1) and high surface roughness (Ra: 5.04 µm) with re-entrant structure. It also

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demonstrated non-wetting MD performance over a continuous 7 days operation of saline water

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(3.5% of NaCl), high anti-wetting with harsh saline water containing 0.5 mM sodium dodecyl

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sulfate (SDS, 28.9 mN m-1), synthetic algal organic matter (AOM).

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Keywords: Algal Organic Matter, Cassie-Baxter state, Electrospraying, Membrane

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Distillation, Silica Aerogel, Membrane Surface Energy.

34 35

INTRODUCTION

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Membrane distillation (MD) to treat complex industrial effluents is of increasing importance

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for the production of high quality water, as well as the potential recovery of valuable resources,

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which can increase the economic viability of the process.1 However, surfactant-like compounds

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can easily penetrate membrane pores, even during operational periods, leading to significant

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wetting issue.2 Moreover, even partial membrane wetting can reduce the temperature difference

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between the permeate and feed sides across the membrane, consequently decreasing the driving

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force and resulting in a sudden flux decline.3 Improvements in membrane characteristics, such

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as hydrophobicity, roughness, reduced surface energy, and re-entrant (convex texture)

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structures lead to an effective control of membrane wetting preventing fouling and partial/full

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wetting.4–6 The overhang mushroom like morphology, and spherical or cylindrical shapes re-

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entrant structure can provide barrier to transition from stable Cassie-Baxter state to Wenzel

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state.7,8 Therefore, the development of a sustainable membrane with anti-wetting, anti-fouling

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and high flux properties is essential for ensuring the practical industrial implementation of MD

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in the future.


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Electrospun nanofiber membranes (ENMs) fabricated by electrospinning technique can offer

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such properties. Electrospinning, through application of strong electric field to suitable

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polymers, allows for the fabrication of nanofibrous hydrophobic membranes with non-woven

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structures,9–11 which demonstrate high porosity, high roughness, tortuosity reduction, a

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decrease in thermal loss, and chemical stability.12 Furthermore, membrane surface coating via

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electrospraying of low surface energy polymers results in a dual layer electrospun membrane

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that can reduce the overall surface energy, further enhancing the hydrophobicity and surface

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roughness.13

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Therefore, significant attention has been given to the electrospinning/spraying of

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superhydrophobic membranes through the use of various polymers and micro/nano materials

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such as polydimethylsiloxane (PDMS), polyvinylidene fluoride (PVDF),13–15 polyurethane,

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and aerogel (cellulose/polyuria), to produce various synthetic porous membranes. Application

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of this inert, inexpensive, nontoxic, and non-flammable polymers for electrospraying can

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impart a highly intrinsic deformability and flexible hydrophobic characteristics to membrane

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surfaces.16–18 Specifically, PDMS possesses excellent film-forming properties, significant

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physical and chemical stability, and is highly water-resistant.19 Moreover, application of micro-

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porous silica aerogel can strengthen the anti-wetting and roughness properties of the

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membranes. Silica aerogels, which is comprised of 96% air by volume and a wispy matrix of

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silicon dioxide,20–25 possess exclusive features desirable for MD membranes, such as poor

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conductivity of heat (0.017-0.021 W m-1 K-1), low bulk density (0.003-0.05 g cm-3), high

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porosity (80-99.8%), high internal surface area (100-1600 m2 g-1) and good mechanical

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strength.26

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In our previous work on PDMS/PVDF membrane fabrication,13 a high contact angle (CA) of

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156.9° and a low hysteresis angle of 11.3˚ was achieved by simple PDMS spray-coating. The

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fabricated superhydrophobic membrane outperformed the control PVDF membrane and 3 ACS Paragon Plus Environment


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showed steady desalination performance. However, partial wetting with seawater containing

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0.1 - 0.2 mM sodium dodecyl sulfate (SDS) was observed. Building on the aforementioned

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characteristics, in this study, we hypothesized that this issue could be prevented by employing

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aerogel in electrospun membrane fabrication, which can enhance surface roughness and

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superhydrophobicity.27 We will prove this hypothesis by fabricating a non-wettable membrane

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with

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aerogel/PDMS/PVDF over a supporting scaffold layer of electrospun polyvinylidene fluoride-

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co-hexafluoropropylene (E-PH). For the first time, we have reported the effects of different

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concentrations of aerogel incorporated in PDMS/PVDF to determine the optimum dual layer

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membrane with enhanced surface properties, water/ethanol/SDS CA, liquid entry pressure

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(LEP), pore size, porosity, surface roughness and membrane performance especially for MD

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feeds containing low surface tension solvents.

re-entrant

structures

(i.e.

convex

topography)

through

electrospraying

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EXPERIMENTAL SECTION

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Chemicals

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The dope solution for electrospinning was composed of dissolving polyvinylidene fluoride-co-

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hexafluoropropylene (PVDF-HFP, Mw = 455,000 g mol−1, Sigma-Aldrich), which is referred

92

as PH, in N,N-Dimetylformamide (DMF, >99 % pure, ACROS) and acetone (certified ACS

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grade, >99.5%, Sigma-Aldrich). The solution for electrospraying was prepared by melting

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PDMS/PVDF in a mix of DMF and tetrahydrofuran (THF, inhibitor free, 99.9%, Sigma-

95

Aldrich) solvents.13 THF, a high-solubility solvent for PDMS, was employed to assist in

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controlling PDMS swelling.28 Additionally, calculated fractions of hydrophobic silica aerogel

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(Enova Aerogel IC3100, particle size and density 2-40 µm, pore diameter ~20 nm, 120-150 kg

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m-3, 0.012 W m-1 K-1 of thermal conductivity at 25°C, surface area 600-800 m2 g-1) were


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introduced in the dope electrospray solution followed by 1 h sonication. Sodium dodecyl

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sulfate (SDS, ACS reagent, ≥ 99.0%,) was introduced to reduce the surface tension of the saline

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feed water. Diiodomethane (Sigma-Aldrich, 99%) was used as solvent to calculate surface

102

energy of membranes by measuring contact angle. The synthetic algal organic matter (AOM)

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solution was prepared by mixing sodium-alginate (SA) sodium salt (Sigma-Aldrich), humic-

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acid (HA) sodium salt (Sigma-Aldrich) and bovine serum albumin (BSA, Sigma-Aldrich).29

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As a comparative reference point, a commercial PVDF (C-PVDF, Millipore Company)

106

membrane with a nominal pore size of 0.45 μm was used.

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Dual Layer Membrane Fabrication by Electrospinning and Electrospraying

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The supporting nanofiber layer was fabricated by electrospinning the dope solution as shown

109

in Figure 1a. The detailed configuration of each dope solution is presented in Table 1. The

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electrospinning dope solution was prepared by mixing the PH polymer with DMF and acetone

111

in a ratio of 15:59.5:25.5, wt. % in the presence of a slight concentration of LiCl. Further, the

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solution was mixed continuously at 70 °C for 12 hours until the polymers dissolved completely.

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The polymeric dope solution was collected in a syringe of 10 mL volume with flat metal needle.

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The collector drum was then rotated at 150 rpm speed amassing charged (16 kV) E-PH

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nanofibers from the needle with a separation distance of 15 cm. The system was adjusted to

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eject the dope solution at 0.8 mL h-1 from the nozzle. The E-PH fibers were collected on an

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aluminium foil that was wrapped the drum.

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The schematic flow chart for the electrospraying process is presented in Figure 1b. In

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continuation of our previous research13, the optimised combination of PDMS/ PVDF i.e. 3%

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PDMS with 2% PVDF previously used was adopted in this study as well. The PMDS was

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dissolved in DMF and THF (1:1 wt. %) solvents and sonicated for 1 h followed by a 4 h mixing

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at 80°C. PVDF was then added to the solution and followed by stirring for 4 h at 70°C.

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Additionally, certain concentrations of aerogel (10%, 20%, 30%, 40%, 50% and 70%) based 5 ACS Paragon Plus Environment


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on weight were mixed into the solution followed by a 1 hour sonication as presented in Table

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1. Thereafter, the dope solution was placed for electrospraying over the E-PH membrane. The

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electrospraying was conducted at optimised 18 kV (Figure S1) with reduced nozzle to collector

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drum distance of 8 cm and at 1.5 mL h-1 flow rate. Finally, the fabricated dual layer membrane

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was carefully removed from the drum and oven dried for more than 1 day at 60 °C. All

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membranes were fabricated at room temperature and under less than 50% relative humidity.

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Figure 1. Illustration of dual layer membrane fabrication by combine electrospinning and

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electrospraying technique.

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Table 1. Ingredients and typical membrane morphology for electrospinning/electrospraying

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dope solution. Typical membrane Ingredient and condition of each process morphology A. Electrospinning for the support layer (E-PH) Fractional ratios of PH polymer with DMF and acetone solvents = 15:59.6:25.5 (wt. %)


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B. Surface modification by electrospraying (E-M3) Fractions of polymers with solvents 3% PDMS in DMF:THF (1:1 wt. %) 2% PVDF in PDMS-DMF-THF solution C. Surface modification of E-M3 with aerogel (wt. % of PDMS & PVDF) 3% PDMS - 2% PVDF - 10% aerogel (E-M3-A10) 3% PDMS - 2% PVDF - 20% aerogel (E-M3-A20) 3% PDMS - 2% PVDF - 30% aerogel (E-M3-A30) 3% PDMS - 2% PVDF - 40% aerogel (E-M3-A40) 3% PDMS - 2% PVDF - 50% aerogel (E-M3-A50) 3% PDMS - 2% PVDF - 70% aerogel (E-M3-A70) 135 136

Membrane Characterization

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Surface Morphology, Roughness, and Fouling Layer Composition

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The surface morphology of the completely dried membranes was characterized by a Field

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Emission Scanning Electron Microscope (Hitachi SU5000 Variable Pressure FE-SEM). The

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presence of chemical bonds in the optimised lab made dual layer (E-M3-A30) membrane and

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C-PVDF were detected by a Fourier transform infrared spectroscopy (FTIR) analysis. The

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FTIR spectra were obtained in attenuated total reflectance (ATR) mode with 16 scans in the

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650 - 4000 cm-1 range with a resolution of 2 cm-1. The X-ray diffraction (XRD) was conducted

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on the fabricated membranes for conducting a detailed investigation regarding the presence of 7 ACS Paragon Plus Environment


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functional groups. The X-ray diffractometer (X'Pert3 Powder, PAN analytical, Netherlands)

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functioning at 40 kV/40 mA was used and the diffraction peaks were transcribed by Cu Kα

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radiation. Furthermore, precision surface roughness and 3D surface height measurements of

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the fabricated and commercial membranes were analysed by optical profiler (Wyko NT9300,

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Vecco, U.S.A.).

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Hydrophobicity, Wettability, Droplet Bouncing Effect and Surface Energy Calculation

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The CA for all membranes was computed with Kruss’s EASYDROP Contact Angle Measuring

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System (Germany). The mean results are reported for all parameters based upon repeating the

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measurements three times. The water droplet bouncing on the membrane surfaces was assessed

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by releasing a droplet from a fine stainless-steel needle coupled with a syringe pump (KD

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Scientific). Contact was monitored by a high-speed camera (Photron) with a 1/3600 s shutter

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speed (frame rate of 3600 fps) and the dynamics of the water droplets were recorded. Finally,

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all the recorded images were processed with ImageJ software.

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The surface energy of the membranes were calculated (Equation 1) using the Owens-

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Wendt method.30 Surface energy was analysed by measuring CA with water and diiodomethane

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solvent.

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0.5 𝜎𝐿.(𝑐𝑜𝑠𝜃 + 1) √(𝜎𝐿𝐷)

√(𝜎𝐿𝑃)

= √(𝜎𝑆𝑃) √(𝜎𝐿𝐷) + √(𝜎𝑆𝐷)

(Eq. 1)

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Wherein, σS is the overall surface energy of the solid membrane surface, σSD is the dispersive

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component of the solid membrane surface, σSP is the polar component of the solid membrane

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surface, and θ is the contact angle.

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Surface tension (σL, mN m-1), the dispersive component (σLD) and the polar component (σLP) of

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the aforementioned solvents at room temperature are presented in Table S1.


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Pore Size, Liquid Entry Pressure and Porosity

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The mean pore size and liquid entry pressure (LEP) of all membranes were measured by

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porometer (PoroLuxTM, Germany), and porosity for commercial and fabricated membranes was

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measured by the gravimetric method. Briefly, the dry and fully wetted (by ethanol) weights of

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each membrane sample (9.0 cm2 area) was recorded. The porosity (ε) of each membrane was

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then computed by considering volume of ethanol absorbed and densities of polymer/ethanol as

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per equation (Eq. 2). 𝑊1 ― 𝑊2

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𝜀=

𝐷𝑒

[

]+

𝑊1 ― 𝑊2

𝑊2

𝐷𝑒

𝐷𝑝

(Eq. 2)

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where, 𝑊1 and 𝑊2 represents wet and dry weights (g) of the membrane, respectively; the

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densities (g m-3) of ethanol (e) and polymer (p) are denoted with 𝐷𝑒 and 𝐷𝑝, respectively.

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Physical, Chemical and Mechanical Stability Testing

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In order to verify the physical stability of aerogel-PDMS coating layer, 400 g loading was

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applied to the E-M3-A30 membrane, and sandpaper (1500 mesh) was used as the abrading

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surface. The surface was dragged in one direction for 10 cm under 10 kPa pressure.31 The CA

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was measured after each 30 cycle. Chemical stability was investigated by immerging the

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membrane in acidic (pH 3) and basic (pH 11) solution for a duration of 2 h. The mechanical

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stability of membrane surface coating was evaluated by adopting ultrasonication. The

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submerged membrane was sonicated for 60 min by an ultrasonic cell crusher (Ultrasonic

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processor FS-250 N) at a frequency of 50 kHz and power of 250 W. Further the immerged

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membranes were collected after 2 h and placed in oven dry. Finally, the CA of membranes

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were evaluated before and after the immersion for both chemical and mechanical testing.



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Experimental Procedure for Direct Contact Membrane Distillation (DCMD)

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The applicability and performance of DCMD operation of electrospun and C-PVDF

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membranes was studied as shown in the schematic diagram described in Figure 2. A custom-

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made lab-scale MD module with an active membrane size of 5.5 cm x 1.8 cm was employed

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with other accessories, such as feed and permeate containers, pumps, and a temperature

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controller. To maintain feed and permeate temperatures, at 60 °C and 20 °C respectively, a hot

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plate with temperature controller and a chiller were used. The salt rejection during the operation

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of DCMD were calculated based on equation 3.

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𝛿𝑝 × 𝑉2

(Eq. 3)

𝑅 (%) = 𝛿𝑓 × 𝑉1

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Where, the conductivity value of permeate water is δp, the volume of permeate water is V2, the

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initial conductivity of feed solution is δf, the initial volume of feed solution is V1.

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Water and vapour diffusion were circulated in opposite directions to maximize the driving force

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due to the difference in temperature. To conduct a long-term study of desalination performance,

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the optimized electrospun double layer membrane (E-M3-A30) was tested for 7 day under

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DMCD operational conditions. Once the initial flux dropped, the membrane was cleaned with

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DI water for 10 mins.

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Anti-wetting and anti-fouling study were carried out with low surface tension feed water

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comprised of saline solution containing SDS. The permeate side initially consisted of deionized

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(DI) water while the synthetic feed solution was composed of DI water, 3.5% NaCl and 0.1

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mM SDS. Additionally, the SDS concentration of the feed was gradually increased by 0.1mM

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per an hour. The surface tension of 0.5 mM SDS was calculated as 28.9 mN m-1 which is

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approximately equivalent to the results found in previous studies by Matijevic and Pethica’s

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and Wang et al.10,32 Further, the adsorption of dissolved natural organic matter (NOM) or algal


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organic matter (AOM) by the membrane was studied by mixing 5 mg L-1 of each HA, SA, and

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BSA compounds to the synthetic feed. The weight change in permeate water during the DCMD

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operational period was monitored by a weight balance connected to a computer. Fluxes were

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subsequently automatically computed at certain predefined periods with the aid of MS Excel

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software. Furthermore, a conductivity meter (HQ40d, Hach) was installed on the permeate side

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for continuous monitoring of conductivity.

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Figure 2. DCMD process flow diagram with accessories used in this experiment

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RESULTS AND DISCUSSION

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PDMS-Aerogel Coating of Dual Layer Membrane Fabrication

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The dope solutions were ejected from the nozzle into an electric field and subjected to

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electrostatic force, coulombic force, drag force, gravitational force, surface tension and

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viscoelastic force. Electrospinning and spraying forms dual layer of microspheres

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PDMS/PVDF/Aerogel on top of nanofibers PH layer as illustrated in Figure 1. The formation

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of nanofibers or microspheres depends on entanglement of the polymer, viscosity, and

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concentration. Low surface energy PDMS polymer alone is not capable of forming microsphere 11 ACS Paragon Plus Environment


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due to its low molecular weight, therefore PVDF was introduced to control the degree of

229

entanglement and for better conductivity.

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The charged nanofibers were collected on the rotating drum as a supporting bottom layer for

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subsequent charged microspheres. We utilized PVDF/PDMS (2% PVDF - 3% PDMS) for

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membrane surface modification of the supporting E-PH layer. This present an ideal case as the

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presence of a larger fluoride (F) ratio in PH is responsible for the increase in its hydrophobicity.

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Further improvement to the hydrophobicity and roughness membrane in this study was

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obtained by the introduction of hydrophobic silica-based aerogel as shown in Figure 3.

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Optimization of the aerogel concentration in PVDF/PDMS was necessary to achieve the best

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shape and structure of the membrane surface, as nano-size treads and beads were observed in

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the membrane surface with the introduction of aerogel. These unique surface microsphere

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properties are formed mainly because of the interfacial behaviour, caused by the rate of solvent

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evaporation and polymer viscosity. The PDMS is dissolved in THF only and moves towards

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the surface of the microspheres due to THF’s high solvent evaporation rate along with the

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modified, uniform and smaller aerogel particles after desorption of solvents.

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While Silica aerogel can absorb and desorb both compatible and miscible solvents THF and

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DMF, the PVDF polymer is dissolved only in DMF. However, a few fractions of PVDF may

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also be migrated to microsphere surfaces as DMF can mix with THF. These dominate semisolid

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PDMS/aerogel with a few fractions PVDF are incapable of diffusing back inside the

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microspheres after solvent evaporation. Evaporation of both solvents caused water

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condensation over the microspheres due to surrounding cold air and phase separation as DMF

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is a non-solvent for PDMS after it has completely dried. Therefore, the overall microsphere

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possessed rough textures with uniform and smaller aerogel particles as observed in FE-SEM

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images (Figure 4d).


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Figure 3. Illustration of dual layer membrane fabrication process

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Re-entrant Structures of PDMS-Aerogel

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Figure 4b-g of FE-SEM images show the silica aerogel assisted PDMS microspheres. Due to

256

the interaction between PDMS microspheres and aerogel particles, relatively smaller

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microspheres were observed in the case of 10%, 20% and 30% aerogel i.e. E-M3-A10, E-M3-

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A20 and E-M3-A30 membranes as seen Figure 4b, 4c and 4d, respectively. The dissection of

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polymeric microspheres with solvents during aerial travel between the needle and collector was

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the main cause of the formation of these smaller spheres with nano-thread networks.

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Rising aerogel agglomeration was observed with increase in aerogel concentration from 40%

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onwards i.e. E-M3-A40/50/70 as seen in Figure 4e, 4f, and 4g. This significant agglomeration

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of dominant aerogel over smashed microspheres of PDMS was the prime cause for the loss of



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essential hydrophobic property due to lose of the convex texture. This agglomeration of aerogel

265

particles occurred even after severe mixing and sonication. It should be noted that the 30%

266

aerogel i.e. E-M3-A30 (Figure 4d), has a relatively higher portion of smaller microspheres.

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Overall the membrane surface resembled a hillock-valley like topography with re-entrant

268

spherical structures (convex texture) leading to the enhancement of the membrane properties.

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Figure 4. FE-SEM images of the (a) C-PVDF membrane, electrospun (b) E-M3-A10, (c) E-

271

M3-A20 (d) E-M3-A30, (e) E-M3-A40, (f) E-M3-A50 and (g) E-M3-A70.

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Re-entrant structures (i.e. convex topography with texture angle β < 90˚) as shown in Figure 5

273

play a vital role in maintaining superhydrophobic characteristics in membrane surface

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morphology.33,34 Liquids releases their free energy on textured rough surface either in a Cassie-

275

Baxter (CB) state or a Wenzel state of complete wetting. The re-entrant rough membrane

276

surface morphology is responsible for significant air trapping which restricts the transformation


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of CB state to Wenzel state during the course of MD application.5,35 Membrane hydrophobicity

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can be sustained until the liquid entry into the rough textured surface is equal to the contact

279

angle (θ). However, if θ < β, the downward capillary force leads to imbibition of liquid over

280

the textured surface, proceeding to the Wenzel state of wetting. Re-entrant (convex texture)

281

structures were observed in the case of 10%, 20%, and 30% aerogel concentration (i.e. E-M3-

282

A10, E-M3-A20, and E-M3-A30 membranes) as seen in respective FE-SEM images. Silica

283

aerogel, which is soluble in both DMF and THF solvents, can migrate to the surface of the

284

microspheres along with PDMS as seen in Figure 5e which causes the rough surface texture.

285

The rough and re-entrant surface morphology of the aerogel assisted PDMS microspheres can

286

trap a large amount of air inside its interspaces, minimizing the direct contact area of water

287

droplets between the feed and membrane surface. Therefore, in terms of surface morphology

288

the 30% aerogel membrane (E-M3-A30) appeared to be the most optimized hybrid mixture of

289

polymers and aerogel.



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Figure 5. Illustration of (a) concave texture (β > 90⁰), (b) convex texture (β < 90⁰) and re-

292

entrant surface assembly for superhydrophobicity, (c) Cassie-Baxter (convex texture) state of

293

E-M3-A30 membrane, (d) Wenzel wetting state and (e) aerogel assisted rough PDMS

294

microsphere of E-M3-A30 membrane.

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Liquid Entry Pressure, Pore Size, Porosity and Thickness

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Table 2 contains the liquid entry pressure (LEP), pore size (dp), porosity (ε), and thickness (δ)

297

of the commercial and fabricated membranes. The commercial PVDF membrane demonstrated

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a minimum porosity of 68%, mean pore size of 0.46 μm, LEP of 110 kPa and thickness of

299

100.5 µm. In comparison, all the electrospun dual layer membranes possessed significantly

300

high porosities of 77-85%, with the highest porosity, 85.8%, being recorded for the E-M3-A30

301

with a LEP of 129.5 kPa, 0.47µm pore size, and 91.1 µm thickness.


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Membranes with high porosity can lead to superior performance in terms of permeate flux due

303

to large thermal resistance facilitated by the presence of air in membrane voids. Larger pore

304

sizes found in the E-PH membrane when compared to C-PVDF are responsible for higher

305

porosity. We found that an increasing concentration of aerogel up to 30% demonstrated higher

306

porosity but decreased after this level. The LEP of membranes is generally dependent on the

307

geometry of pores (evaluated by the tortuosity factor), maximum pore size, liquid surface

308

tension, as well as liquid CA. Rosa-Fox et al36 reported that homogenous aerogel and PDMS

309

hybrid material have higher a rupture modulus and lower a Young’s modulus when compared

310

to pure aerogel, leading to elastomer behavior in fabricated membranes and further influencing

311

the LEP.

312

The LEP of the C-PVDF is higher than the E-PH membrane because of the relatively smaller

313

pore size. However, the surface coated dual layer electrospun membranes posed superior LEP

314

compared to the E-PH supporting layer because of the smaller and controlled pore size,

315

resulting in superhydrophobic membranes. Due to the presence of small bumps on

316

microspheres surfaces which composed of PDMS with highly porous aerogels, efficient

317

prevention of LEP drop was also achieved, especially in case of the E-M3-A30 membrane.

318 319

Table 2. Characteristic of Membranes Parameters

Type of membrane

LEP (kPa)

Pore size, dp

Thickness, δ Porosity, ε (%)

(µm)

(µm)

C-PVDF

110.0 ± 1.5

0.46 ± 0.05

68.2 ± 0.25

100.5 ± 1.3

E-PH

95.3 ± 1.1

0.56 ± 0.02

89.4 ± 0.13

83.1 ± 2.2

E-M3

111.0 ± 2.1

0.42 ± 0.03

77.2 ± 0.18

87.3 ± 1.8



Page 18 of 36

E-M3-A10

112.0 ± 1.2

0.43 ± 0.02

77.3 ± 0.20

88.2 ± 2.2

E-M3-A20

114.0 ± 1.7

0.46 ± 0.02

78.7 ± 0.16

89.5 ± 1.7

E-M3-A30

129.5 ± 3.4

0.47 ± 0.05

85.8 ± 0.28

91.1 ± 1.2

E-M3-A40

129.4 ± 2.9

0.48 ± 0.02

84.7 ± 0.22

91.0 ± 2.0

E-M3-A50

128.3 ± 1.8

0.48 ± 0.03

84.2 ± 0.19

91.9 ± 2.7

E-M3-A70

126.8 ± 2.6

0.49 ± 0.05

84.3 ± 0.22

91.6 ± 1.9

320 321

Hydrophobicity, Surface Roughness and Surface Energy Determination

322

The surface coated double layer electrospun membranes achieved higher CA when tested with

323

water, 0.5 mM SDS, and 0.5 mM ethanol when compared to the E-PH and C-PVDF membranes

324

(Figure 6) which makes them highly wetting resistant and hydrophobic in nature. The E-PH

325

membrane demonstrated a high-water CA (138.4˚) and a cylindrical fiber morphology when

326

compared to the C-PVDF membrane (118.6˚) resulting in higher resilience to the intrusion of

327

low surface tension liquids. As shown in Figure 5, further improvement in the contact angle

328

for respective solvent media was achieved in the electrospun membranes after surface

329

modification by PVDF/PDMS/aerogel. The water CAs of E-M3-A10~70 membranes were

330

greater than 150° (referred to as superhydrophobic14,37) compared to E-PH (138.4°) and C-

331

PVDF (118.6°) membranes. The highest water CA of 170° was achieved by the E-M3-A30

332

electrospun membrane, which was considered as being the most optimised electrospun

333

membrane with superhydrophobic properties and was consequently considered further for

334

DCMD experiments.

335

The higher water CA and better droplet shape over the E-M3-A30 electrospun membrane

336

surface can be attributed to the low surface energy of the E-M3-A30 membrane when compared

337

to the surface tension of water, causing a strong repulsive force with regards to the liquid

338

droplets and enhancing the wetting resistance property. Additionally, the poor interaction 18 ACS Paragon Plus Environment

Page 19 of 36


339

among the liquid and membrane caused a decreased in the interfacial tension between the E-

340

M3-A30 membrane and water. The same trend of higher CAs was also observed in the 0.5 mM

341

SDS and 0.5 mM ethanol solutions, however the superhydrophobic property was achieved only

342

by the E-M3-A30/40/50 membranes when tested with the 0.5 mM SDS solution. When tested

343

with 0.5 mM ethanol, the superhydrophobicity was lost for all electrospun membranes as well

344

as the C-PVDF, but the highest CA of 146.2° was still maintained by the E-M3-A30 membrane.

345 346

Figure 6. CA with water, 0.5 mM SDS and 0.5 mM ethanol for the commercial PVDF,

347

supporting electrospinning E-PH, and surface modified electrospraying membranes E-M3, E-

348

M3-A10, E-M3-A20, E-M3-A30, E-M3-A40, E-M3-A50, and E-M3-A70.

349

Due to higher hydrophobicity (higher water CA) of electrospraying aerogel/PDMS/PVDF

350

membrane, it can trap air beneath the interfaces of surface re-entrant microspheres which can

351

be explained by Cassie equation.43

352

cos 𝜃1 = 𝑓1(𝑐𝑜𝑠𝜃2) ― 𝑓2 = 𝑓1(𝑐𝑜𝑠𝜃2 + 1) ― 1

353

where θ1 and θ2 are the water CA on rough and smooth surfaces, respectively; f1 and f2 are the

354

factors for fraction area of solid surface and air in contact with the water droplet. Considering

(𝑓1 + 𝑓2 = 1)


(Eq. 4)


355

E-PH (θ1 = 140.64˚) as the base smooth surface, and E-M3 (θ2 = 151.55˚) and E-M3-A30 (θ2 =

356

169.8˚) as the rough modified membrane surfaces, f2 factor can then be calculated as 0.478 and

357

0.913 for E-M3 and E-M3-A30 membranes, respectively. Compared to E-M3, the increase in

358

f2 factor for E-M3-A30 clearly demonstrates its larger aspect ratios of microspheres containing

359

relatively higher air fractions. Therefore, relatively higher anti-wetting and water repellency

360

properties were recorded for the E-M3-A30 membrane.

361 362

Figure S2 and Table S2 illustrate the FTIR spectra of the silica aerogel, C-PVDF and

363

electrospun membranes, and their respective chemical bands, respectively. Absorption bands

364

observed at 763, 1072, 1402 cm-1, 881, 1072, 1176 cm-1, and 1230 cm-1 are attributed to the α,

365

β and γ phase of PVDF polymorphs, respectively, whereas β + γ phase of the PVDF

366

polymorphs was observed at 842 cm-1. Similar observations were also recorded by other

367

researchers.38 Due to the higher mechanical elongation that occurred under the extreme

368

electrical field during electrospinning,39–41 the β-phase was relatively stronger than the α-phase,

369

as the unoriented β-phase was achieved via crystallization of the PVDF. After a successful

370

coating with aerogel, the peak of the α-phase was almost non-existent, with a presence of a

371

strong β-phase that is attributed to the typical characteristics of nanofiber mats and the presence

372

of higher negatively charged fluorine atoms.5 The peaks at 795, 1280 and 1455 cm-1

373

corresponds to the Si-CH3 bands, while the peak at 1084 cm-1 corresponds to Si-O-Si bands.

374

The C-C bond peak for the C-PVDF as well as the electrospun E-M3-A30 membrane was

375

observed at 877-880 cm-1. The peaks at 881 cm-1, 1176, and 1430 cm-1 correspond to stretching

376

of CF and CF2 bands.

377

Figure S3 shows XRD pattern of aerogel, PVDF and electrospun membranes, while Table S3

378

presents analysis of each peak. The conspicuous β phase (20.4°) of PVDF is attributed to the


Page 20 of 36

Page 21 of 36


379

crystallization during electrospinning. The new peaks appeared at 22° correspond to the

380

amorphous silica structure of aerogel, while the peak at 36.5° represents the Si-C (PDMS)

381

spectra. On the contrary, increasing aerogel concentration weakened the diffraction peak at

382

around 18° which is indicative of the α and γ phase of PVDF. This implies that aerogel/PDMS

383

was effectively coated onto the E-PH membrane.

384

The significant superhydrophobicity of the E-M3-A30 membrane can be mainly attributed to

385

their surface roughness (Ra) as they recorded the highest (5.04 µm) when compared to the C-

386

PVDF (0.55 µm), E-PH (2.23 µm) and E-M3 (2.88 µm) membranes as computed by optical

387

profiler images as shown in Figure 7a-d, respectively. Increases in surface roughness are

388

mainly attributed due to the presence of branched O-Si-(CH3)3 groups of silica aerogel.42

389 390

Figure 7 Roughness images (Optical profiler) of (a) C-PVDF, (b) E-PH, (c) E-M3 and (d) E-

391

M3-A30 membranes.



Page 22 of 36

392

In addition to membrane hydrophobicity, roughness, and re-entrant structures with convex

393

texture, low surface energy is also a pivotal parameter for achieving superhydrophobicity.43

394

The surface energy of C-PVDF, E-PH, E-M3, and E-M3-A30 membranes were computed in

395

Table 3. The E-M3-A30 membrane exhibits the lowest surface energy (4.18 mN m-1) which

396

can be attributed to the hydrophobic silica aerogel.42,44

397

Although we could lower the surface energy along with enhancing superhydrophobic

398

properties, we could not achieve Omniphobicity as silica aerogel has oil absorbing

399

characteristics as reported by Mahadik et al. and Malfait et al.44,45 We expect that further

400

introduction of nano particles could allow for maintaining the Cassie-Baxter state with organic

401

solvents (superhydrophobicity with omniphobic property), which requires further exploration.

402

Table 3. Estimation of Surface Energy of Membranes by Owens−Wendt method

Type of membrane E-M3-A30

Contact angle (˚)

𝟎.𝟓 𝛔𝐋.(𝐜𝐨𝐬𝛉 + 𝟏) √(𝛔𝐋𝐃)

√(𝛔𝐋𝐏) √(𝛔𝐋𝐃)

167.91 ± 4.00

0.17

1.53

Diiodomethane 120.36 ± 0.59

1.76

0.00

0.94

1.53

Liquid

Water

Water

151.55 ± 8.76

E-M3

Surface energy (mN m-1) 4.18 ± 0.27

16.92 ± 0.88 Diiodomethane

87.84 ± 1.16

3.70

0.00

Water

140.64 ± 0.77

1.77

1.53

Diiodomethane

71.25 ± 1.17

4.71

0.00

Water

115.46 ± 1.66

4.44

1.53

E-PH

25.87 ± 0.94

C-PVDF

29.02 ± 0.83 Diiodomethane

59.84 ± 1.15

5.35


0.00

Page 23 of 36


403

Water Bouncing, Chemical and Mechanical Stability Performance of E-M3-A30

404

Membrane

405

The unique bouncing phenomenon of water droplets on the E-M3-A30 membrane surface was

406

further confirmed by bouncing an accelerated droplet at 0.38 m s-1. As presented in Figure 8,

407

the water droplet was allowed to fall by gravity on the membrane surface. Upon complete

408

collision, the water droplet immediately rebounded and departed from the membrane surface

409

within 11.6 ms. As a superhydrophobic property of the E-M3-A30 membrane, the adherence

410

of the water droplet and the membrane surface was insignificant during this short impact

411

duration. This can be attributed to insufficient adhesive force, i.e. the dissimilarity among

412

‘Lotus effect’ and ‘Petal effect’.14,46,47 Additionally, the short bouncing duration of 11.6 ms

413

demonstrated the existence of an ample amount of air pockets which hindered the water droplet

414

falling under pressure from penetrating into the membrane surface. As demonstrated earlier in

415

the FE-SEM images (Figure 4), the hillock-valley surface morphology of the E-M3-A30 super

416

hydrophobic membrane with trapped air can act as an obstacle and consequently offer anti-

417

wetting and anti-fouling characteristics. E-M3-A30 membrane was found to be stable based on

418

observation of CA after physical (abrasion test) (Figure S5), chemical, and mechanical stability

419

tests (Figure S6), confirming the stability of the coating layer.

420

421 422

Figure 8. Water droplet dynamics on E-M3-A30 membrane, photographs representing the

423

whole rebounding phenomenon.



424

Wettability and Long Term DCMD Performance

425

In DCMD, the molecular diffusion mechanism by which water vapour moves across the

426

membrane pores is known as water flux.48 When compared to the other lab-made electrospun

427

membranes in terms of hydrophobicity and anti-wetting potential (highest LEP), as discussed

428

in previous sections, the dual layer E-M3-A30 membrane was ascertained as the best

429

performing membrane and was therefore considered for use in the DCMD experiments. During

430

MD operation for longer periods of time, the performance of membranes is mainly challenged

431

by wetting and fouling issues. As a result, the membranes should possess an anti-wetting

432

property for optimal seawater desalination in MD with liquids having poorer surface tension

433

compared to water (72.8 mN m-1 at 20 °C).32

434

To asses membrane performance, we introduced an amphipathic surfactant (SDS) compound

435

found in seawater. The presence of dissolved NOM/AOM in feed solutions can be adsorbed by

436

the membrane surface,29 triggering reduction in surface tension of the feed water. Therefore,

437

synthetic AOM (5 mg L-1 of each model compound of HA, SA and BSA) along with SDS

438

(0.1mM increment per hour) and NaCl (35 g L-1) was introduced further to study the

439

performance of electrospun membrane under extreme harsh conditions. The C-PVDF, E-PH,

440

E-M3 and E-M3-A30 membranes were used for DCMD performance study. The initial fluxes

441

were observed as 22.9±1.2, 36.2±1.9, 32.7±1.1 and 33.1±1.7 L m−2 h−1 respectively with saline

442

SDS solution illustrated in Figure 9. Francis et al. compared PVDF hollow fiber and

443

nanofibrous membrane and reported that electrospun nanofiber membranes (ENMs) showed a

444

higher flux of 36 LMH than that of Hollow fiber (31.6 LMH) at 80º.49 Similarly, our

445

electrospun PVDF-HEP (reported as E-PH) membrane exhibited higher flux of 36.2±1.9

446

compared to the commercial PVDF membrane (22.9±1.2) despite operating at lower

447

temperature of 60º. Hammani et al. fabricated electrospun nanofiber membranes incorporating

448

periodic mesoporous organosilica (PMO) nanoparticles (NPs) and reported a proportional


Page 24 of 36

Page 25 of 36


449

increase of hydrophobicity at higher nanoparticles content. For instance, upon increasing the

450

percent of PMO NPs doping from 5 to 10 wt %, the water contact angle increased from 123°

451

to 142°.50 Moreover, the prepared membranes showed enhanced water vapor production as

452

high as 31 LMH with a 140% increase compared to the commercial PTFE flat sheet membrane

453

at a 65 °C feed inlet temperature. Likewise, in our present study, the incorporation of silica

454

aerogel (E-M3-A30 membrane) achieved higher CA (~170°) and higher flux (33.1±1.7

455

LMH) mainly attributed to its porosity and superhydrophobicity. In presence of highly

456

concentrated synthetic AOM with same saline SDS feed condition, the initial flux reduced

457

slightly to 20.9±1.5, 35.0±1.1, 32.3±1.7 and 32.3±0.9 L m−2 h−1 respectively. The higher flux

458

of the dual layer E-M3-A30 membrane can be mainly attributed to its porosity and

459

superhydrophobicity.

460

During DCMD operation, a gradual increase in conductivity on the permeate side was observed

461

for the C-PVDF membrane suggesting a fractional pore wetting triggered by the strong

462

adhesive force after 30 minutes of operation for both feeds. After 1 h of DCMD operation with

463

C-PVDF for a feed solution containing 0.1 mM SDS and 3.5 % NaCl, the salt rejection was

464

decreased to 95%. Presence of synthetic AOM, triggered salt rejection by 95% along with sharp

465

flux drop within 50 minutes of operation (Figure 9a). In case of E-PH membrane, similar trend

466

was observed with 0.1-0.2 mM SDS solution (Figure 9b). Stable salt rejection performance

467

was achieved by dual layer E-M3 membrane upto 0.3 mM SDS saline feed, however the

468

rejection efficiency started to decrease with further increased in SDS concentration to 0.4 mM

469

with sharp flux drop. Relatively less salt rejection efficiency was observed for E-M3 membrane

470

for feed containing AOM with saline SDS (Figure 9c). In contrast, both salt rejection and flux

471

of the lab made E-M3-A30 membrane were found to be stable when assessed through 5 h

472

DCMD operation with extremely low surface tension feed solution (0.5 mM SDS with 3.5%

473

NaCl).



474

In presence of saline AOM with SDS up to 0.3 mM, the E-M3-A30 membrane (Figure 9d)

475

presented highest salt rejection (>99%) efficacy and stable flux as compared to E-M3, E-PH

476

and C-PVDF membrane. However further increase in SDS concentration to 0.4 mM, the

477

performance of salt rejection (95%) decreased with reduction in flux. The anti-wetting, stable

478

salt rejection and flux of E-M3-A30 membrane can be attributed to their superhydrophobic

479

property and high LEP. Over and above, silica aerogel has extremely low thermal conductivity

480

of 0.009-0.012 W m-1 K-1 and very high porosity (>80%),51 which decreases the heat loss via

481

membrane conduction and stabilized flux for promising DCMD application. The presence of

482

nano/micro scale hillock and valleys like structures enhanced surface roughness as well which

483

increases the liquid-water vapour contact volume during DCMD operation, further increasing

484

the flux.

485 486

Figure 9. Water flux and salt rejection performance of (a) C-PVDF, (b) E-PH, (c) E-M3 and

487

(d) E-M3-A30 membranes. The feed solution consists of low surface tension saline water (0.126 ACS Paragon Plus Environment

Page 26 of 36

Page 27 of 36


488

0.5 mM SDS with 3.5% NaCl) and synthetic algal organic matter. In the case of both (a) and

489

(b), the experiment was stopped much earlier than planned due to poor salt rejection.

490 491

The prospective applications of optimized dual layer E-M3-A30 membrane was further studied

492

under longer DCMD operation as shown in Figure S6. The feed solution consisted of 35 g L-1

493

NaCl at 60˚C and the distillate permeate at 20˚C for 7-day duration of the DCMD experiments

494

to record long term performance. The electrospun membrane showed a higher and stable salt

495

rejection of 99.9%, along with stable flux of 34.6±1.9 L m−2 h−1. High porosity, satisfactory

496

mean pore sizes, wetting resistance, and the superhydrophobic nature of the E-M3-A30

497

membrane were found to be the key causes for the excessive salt rejection and stable flux. Our

498

methodology of fabricating electrospun E-M3-A30 membranes proves the stability and much

499

simplicity, which can be scaled up for fabrication in large industrial production unlike other

500

complicated process, resulting in lower overall manufacturing costs and product consistency.

501 502

Acknowledgement

503 504 505 506

This work was supported by the Research Grant Council of Hong Kong (Project No. 21201316 and No. 11207717), City University of Hong Kong by Applied Research Grant (No. 9667155), Innovation and Technology Commission through Innovation and Technology Fund (NO. ITS/262/17FX, and No. ITS/206/18FX).

507 508

Supporting Information Available

509

Figure S1. Contact angles under different applied voltages for the E-M3-A30

510

membrane

511

Figure S2. FTIR spectra of silica aerogel, C-PVDF, and electrospun membranes

512

Figure S3. XRD patterns of silica aerogel, C-PVDF, and electrospun membranes

513

Figure S4. Physical stability of E-M3-A30 membrane coating by abrasion test 27 ACS Paragon Plus Environment


514

Figure S5. Chemical and mechanical stability of E-M3-A30 membrane

515

Figure S6. DCMD performance of the C-PVDF and the fabricated E-M3-A30

516

membrane for continuous 7 days

517

Table S1. Surface energy of solvents used in this study

518

Table S2. FTIR spectral bands assignment for aerogel, C-PVDF and electrospun

519

membranes

520

Table S3. XRD peaks analyse for aerogel, C-PVDF, and electrospun membranes.

521

Conflict of interest

522

The authors declare no competing financial interests.

523 524


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