Engineering Surface Energy and Nanostructure of Microporous

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Engineering Surface Energy and Nanostructure of Microporous Films for Expanded Membrane Distillation Applications Chanhee Boo, Jongho Lee, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02316 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 10, 2016

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Environmental Science & Technology

Engineering Surface Energy and Nanostructure of Microporous Films for Expanded Membrane Distillation Applications

Chanhee Boo, Jongho Lee, and Menachem Elimelech*

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286

* Corresponding author: email: [email protected]; Tel. +1 (203) 432-2789

ACS Paragon Plus Environment


ABSTRACT

1 2 3

We investigated the factors that determine surface omniphobicity of microporous membranes

4

and evaluated the potential application of these membranes in desalination of low surface tension

5

wastewaters by membrane distillation (MD). Specifically, the effects of surface morphology and

6

surface energy on membrane surface omniphobicity were systematically investigated by

7

evaluating wetting resistance to low surface tension liquids. Single and multi-level re-entrant

8

structures were achieved by using cylindrical glass fibers as a membrane substrate and grafting

9

silica nanoparticles (SiNPs) on the fibers. Surface energy of the membrane was tuned by

10

functionalizing the fiber substrate with fluoroalkylsilane (FAS) having two different lengths of

11

fluoroalkyl chains. Results show that surface omniphobicity of the modified fibrous membrane

12

increased with higher level of re-entrant structure and with lower surface energy. The secondary

13

re-entrant structure achieved by SiNP coating on the cylindrical fibers was found to play a

14

critical role in enhancing the surface omniphobicity.

15

chemically modified by the FAS with a longer fluoroalkyl chain (or lower surface energy)

16

exhibited excellent surface omniphobicity and showed wetting resistance to low surface tension

17

liquids such as ethanol (22.1 mN m-1). We further evaluated performance of the membranes in

18

desalination of saline feed solutions with varying surface tensions by membrane distillation

19

(MD). The engineered membranes exhibited stable MD performance with low surface tension

20

feed waters, demonstrating the potential application omniphobic membranes in desalinating

21

complex, high salinity industrial wastewaters.

22

Membranes coated with SiNPs and

TOC Art

23

1 ACS Paragon Plus Environment

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INTRODUCTION

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Membrane distillation (MD) is a thermal desalination process driven by a vapor pressure

26

gradient between a hot feed stream and a cold permeate (distillate) stream.1, 2 A hydrophobic,

27

microporous membrane serves as a medium for (water) vapor transport but a barrier to liquid

28

penetration, thereby enabling separation of volatile (i.e., water) and non-volatile species (i.e.,

29

salts).3,

30

membrane is maintained, which makes it possible to use low-grade heat as the energy source for

31

separation.5, 6 Productivity of MD is not significantly affected by the feed salinity because the

32

solution vapor pressure changes only marginally with salt concentration.7 MD can be an efficient

33

desalination process for highly saline wastewater, such as brines from shale gas produced water,

34

where the application of conventional pressure-driven membrane processes (i.e., reverse osmosis)

35

is limited due to the considerably high osmotic pressure of such wastewaters.8-10

36

4

Desalination by MD is possible as long as a vapor pressure gradient across the

Microporous

membranes

prepared 12

from

hydrophobic

polymers

polytetrafluoroethylene (PTFE),13,

14

such

as

37

polyvinylidenefluoride (PVDF),11,

38

(PP)15 have been widely used for MD applications. However, deployment of such conventional

39

hydrophobic membranes for desalination of challenging wastewaters that contain diverse low

40

surface tension contaminants is limited due to potential pore wetting of the MD membranes.8

41

Constituents of concern for pore wetting include oil, alcohol, and surfactants that are ubiquitous

42

in wastewater streams.16 These low surface tension substances can easily wet the hydrophobic

43

pores, thereby compromising water permeability and salt selectivity of the MD membrane.

and polypropylene

44

Omniphobic membranes that resist wetting to both water and low surface tension liquids,

45

such as oil, can extend MD applications to emerging industrial wastewaters where the use of

46

conventional hydrophobic membranes is limited.17-19 The shale gas and oil industry consumes

47

substantial amount of water for drilling and hydraulic fracturing of a shale gas well

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(approximately two to four million gallons).8 Brines from shale gas produced water are highly

49

saline with total dissolved solids (TDS) concentrations ranging from ~66,000 mg/L to ~261,000

50

mg/L.20 Further, these brines contain high levels of oil and grease, organic compounds, and

51

chemicals (e.g., surfactant) with low surface tension.20 Removal of volatile organics (e.g.,

52

alcohols) in the feed stream is another example in which omniphobic membranes can extend

53

applications of MD because low surface tension organic substances can facilitate wetting of 2 ACS Paragon Plus Environment


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hydrophobic MD membranes.21-23 For example, MD has recently been proposed as a low-

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temperature separation process to recover bioproducts, such as butanol, from fermenters by

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employing nonpolar organic solvents with low volatility as a permeate stream.24

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Despite the potential of omniphobic membranes in MD applications, studies on the

58

fabrication of such membranes are rather scarce. To better guide strategies for omniphobic MD

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membrane design, we conducted a comparative study to elucidate the factors that determine

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surface omniphobicity. The effects of surface morphology and chemistry on surface

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omniphobicity of microporous membranes were systematically investigated by modifying a glass

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fiber substrate with silica nanoparticles and fluoroalkylsilane. Based on the observed results, we

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elucidated the mechanisms governing the omniphobicity of the modified fibrous microporous

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membranes, focusing on the role of surface morphology and chemistry. We also demonstrated

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the potential application of our modified omniphobic membrane in desalination of a highly saline,

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low surface tension wastewater by membrane distillation.

67 68

MATERIALS AND METHODS

69

Materials and Chemicals. (3-Aminopropyl)triethoxysilane (99% APTES), acetate buffer

70

solution (pH 4.65), silica nanoparticles (Ludox® SM, 30 wt%), hydrogen peroxide (ACS reagent,

71

30 wt%), sulfuric acid (ACS reagent, 95.0 − 98.0%), and sodium dodecyl sulfate (SDS) were

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purchased

73

(nonafluorohexyl)triethoxysilane (C12H19F9O3Si, hereafter denoted as 9−FAS), (heptadecafluoro-

74

tetrahydrodecyl)triethoxysilane (C16H19F17O3Si, hereafter denoted as 17−FAS) (Gelest,

75

Morrisville, PA), and hexane (ACS reagent, J.T. Baker, ≥98.5%) were used for the surface

76

fluorination of the glass fiber (GF) membrane.

from

Sigma-Aldrich

(Sigma-Aldrich,

St.

Louis,

MO).

77

Surface Modification of Glass Fiber Membrane. A glass fiber (GF) membrane with a

78

nominal pore size of 0.4 µm (determined based on particle retention) and an average thickness of

79

560 µm was used as a substrate (GB-140, Sterlitech, WA). The GF membrane is abundant in

80

hydroxyl functional groups that allow surface modification via well-established silane

81

chemistry.25 Further, the cylindrical morphology of the glass fiber (GF) substrate provides a

82

primary re-entrant structure.26, 27 A secondary re-entrant structure was obtained by coating the

83

GF substrate with silica nanoparticles (SiNPs) via a simple dip coating protocol. The GF 3 ACS Paragon Plus Environment

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membrane was treated with 1% v/v APTES in anhydrous ethanol for 1 h under gentle stirring to

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functionalize the surface with amine terminal groups, rendering the GF membrane positively

86

charged.28 The APTES-coated GF membrane was immersed in an aqueous SiNP solution for 1 h

87

under gentle mixing. During this step, the negatively charged SiNPs bind to the positively

88

charged GF membrane via electrostatic attraction. The aqueous SiNP solution was prepared by

89

dispersing 0.005 wt% SiNPs in acetate buffer with pH adjusted to 4 to ensure positive and

90

negative surface charges of the APTES-coated GF membrane and SiNPs, respectively.

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GF membranes with and without SiNPs were modified using 9−FAS or 17−FAS to lower the

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surface energy via liquid-phase silanization. The GF membranes were immersed in 1% v/v FAS

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solutions in hexane for 24 h, followed by thorough rinsing with hexane. Next, the FAS-treated

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GF membranes were subjected to heat treatment at 120 °C for 2 h. The FAS covalently binds to

95

the GF as well as SiNPs via hydrolysis and condensation, lowering the surface energy of the GF

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membrane.

97

Membrane Characterization. The elemental composition of the modified GF membranes

98

was analyzed by X-ray photoelectron spectroscopy (XPS, Kratos Analytical, Manchester, UK).

99

The sample was irradiated with a beam of monochromatic Al Kα source operating at 1.486 keV

100

and 140 W beam power. The base pressure of the sample analysis chamber was 2.0 × 10-9 Pa.

101

Spectra were collected in hybrid mode using electrostatic and magnetic lenses from a nominal

102

spot size of 300 µm × 700 µm. Surface morphology of the modified GF membranes was

103

investigated by scanning electron microscopy (SEM, Hitachi SU-70). Before imaging,

104

membrane samples were sputter-coated with a chromium layer (BTT-IV, Denton Vacuum, LLC,

105

Moorestown, NJ). Acceleration voltage of 5.0 kV was applied to image all membrane samples.

106

Fiber diameter distribution was determined using the ImageJ software (National Institutes of

107

Health, Bethesda, MD) by randomly measuring the diameter of 200 fibers from multiple SEM

108

images of the sample. The mean pore size and pore size distribution of the GF membranes before

109

and after SiNP coating were analyzed by a wet-dry capillary flow method using a custom-built

110

porometer.29

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Contact angles for the modified membranes with pure liquids with a wide range of surface

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tensions, including water (γ = 72.8 mN/m), mineral oil (γ ≈ 30 mN/m), and ethanol (γ = 22.1

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mN/m) were measured by a goniometer (OneAttension, Biolin scientific instrument) using the 4 ACS Paragon Plus Environment


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sessile drop method. A 2-µL liquid droplet was placed on the membrane sample and

115

photographed using a digital camera for 10 s. The left and right contact angles were analyzed

116

from the digital images by a post-processing software (OneAttension software). The

117

measurements were conducted on a minimum of two random locations with three different

118

membrane samples and the data were averaged.

119

Intrinsic Contact Angle and Surface Energy. The intrinsic contact angles of water,

120

methylene iodide, mineral oil, and ethanol on a surface modified with 9−FAS and 17−FAS were

121

measured using a silicon wafer (Mechanical grade 1196, University Wafer) with perfectly

122

smooth surface topology. The silicon wafer was first cleaned using a piranha solution (a mixture

123

of sulfuric acid and hydrogen peroxide at 3:1 volume ratio) for 2 h to maximize silanization

124

efficiency by removing organic contaminants.30 After thorough washing with DI water, followed

125

by drying on a hot plate (~120 °C) for 2 h, the silicon wafer was silanized with 9−FAS and

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17−FAS following the procedure outlined earlier. The liquid contact angles of the FAS modified

127

silicon wafer were measured by a goniometer as described in the previous section, and

128

determined to be the intrinsic contact angles.

129

Surface energy originates from two components: dispersion ( γ svd ) and dipole-hydrogen (polar)

130

bonding forces ( γ svp ) (i.e., γ sv = γ svd + γ svp ). These two surface energy components were estimated

131

by the Owens-Wendt method31 using the experimentally determined intrinsic contact angles of

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water ( θWater with γ Water = 72.8 mN/m) and methylene iodide ( θ M − I with γ M − I = 50.8 mN/m):

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d p γWater γ svd + γWater γ svp =

134

γ Md −I γ svd + γ Mp −I γ svp =

(1+ cos (θWater))γWater 2

(1 + cos (θM −I ))γ M −I

(1)

(2)

2

135

d The surface energy derived from dispersive and polar components of water, γ Water = 21.8 mN/m

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p and γ Water = 51.0 mN/m, and those of methylene iodide, γ Md − I = 49.5 mN/m and γ Mp − I =1.3 mN/m,

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were taken from the literature.31

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Membrane Distillation Performance Tests. MD performance of the modified GF

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membranes was evaluated by a laboratory-scale direct contact membrane distillation (DCMD) 5 ACS Paragon Plus Environment

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unit using feed solutions with different surface tensions. The edges of the membrane coupon

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were covered by a water-proof tape firmly attached by silicon glue to reduce the effect of uneven

142

hydrodynamics at the inlet and outlet of the cross-flow membrane cell. The membrane area

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exposed to the feed and permeate streams was 10 cm2 (5 cm × 2 cm), which was smaller than the

144

projected area of the flow channel (7.7 cm × 2.6 cm). NaCl solution (1 M) at 60 °C and

145

deionized (DI) water at 20 °C were used for the initial feed and permeate solutions, respectively.

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A slightly higher cross flow rate for the feed stream, 0.4 L/min or a cross flow velocity of 8.5

147

cm/s, than for the permeate stream, 0.3 L/min or a cross flow velocity of 6.4 cm/s, was used to

148

facilitate the detection of membrane pore wetting. In such flow conditions, the feed solution

149

penetrates through the MD membrane to the permeate side when pores are wetted, resulting in an

150

increase of water flux and a decrease in salt rejection.

151

To evaluate wetting resistance of the modified GF membranes, the surface tension of the feed

152

solution was progressively lowered by adding sodium dodecyl sulfate (SDS) every 2 h during the

153

DCMD experiment. The SDS concentrations in the feed solution (1 M NaCl at 60 °C) after

154

sequential SDS addition were 0.1, 0.2, and 0.3 mM, and the corresponding feed surface tensions

155

were ~42, ~33, and ~31 mN/m, respectively.32 The feed solution (initial volume of 1 L) was

156

replenished after 50 mL volume loss to maintain variation of SDS concentration within 5%. The

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water vapor flux across the membrane, Jw, was determined by measuring the increase in

158

permeate weight. Electric conductivity of the permeate solution was monitored to determine the

159

NaCl concentration in the permeate solution, CP, from which the salt (NaCl) rejection, RNaCl, was

160

determined using

161

 V C /J A t R NaCl = 1 − P P w m  100 CF  

162

where Vp, is the permeate volume, CF is the initial NaCl concentration in the feed (1.0 M), Am is

163

the membrane area, and t is time.

(3)

164 165 166

RESULTS AND DISCUSSION 6 ACS Paragon Plus Environment


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Surface Properties of Modified Membrane. We modified the surface morphology and

168

surface chemistry of the GF substrate using silica nanoparticles (SiNPs) and FAS having two

169

different fluoroalkyl chain lengths (Figure 1). SEM images depicting surface morphologies of

170

the modified membrane substrates are shown in Figure 2. Membrane surface morphologies

171

before and after SiNP coating were markedly different, while surface fluorination by FAS did

172

not affect surface morphology. It is evident from the SEM images that SiNPs evenly coat the

173

glass fiber, thereby creating multi-level surface roughness. Because both spherical SiNPs33 and

174

cylindrical glass fibers26 are geometries that provide a re-entrant structure, which is critical for

175

achieving surface omniphobicity, our SiNPs decorated membranes (Figures 2B and 2D) feature

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multi-level re-entrant structure.

177

FIGURE 1

178

FIGURE 2

179

The nominal pore size and pore size distribution of the membrane substrates did not change

180

much after SiNP coating (Figure S1), indicating that the small SiNPs (diameter ~30 nm) were

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selectively bound to the GF (diameter ~242 nm) without significant aggregation and pore

182

blocking (Figure S2). The irreversibility of SiNP binding on the fibers was assessed by

183

subjecting the SiNP grafted membranes to bath sonication. The SiNPs remained intact on the

184

fibers after 10-minute bath sonication, demonstrating robust binding of SiNPs to fibrous

185

substrate via electrostatic attraction (Figure S3).

186

Results from XPS analysis for the FAS-modified membranes with attached SiNPs and

187

without SiNPs are compared in Figure 3. Silicon (Si, blue) and oxygen (O, gray) are two major

188

elements for all modified membranes, which is consistent with the chemistry of the glass fibers.

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The ratios of elemental fractions of silicon to oxygen before and after SiNP coating on the glass

190

fiber remain almost identical (Si/O ≈ 0.5) for 9−FAS and 17−FAS modified membranes because

191

the chemical composition of both glass fiber and SiNPs is based on the silicon dioxide (SiO2).34

192

A higher elemental fraction of fluorine (F, purple) for 17−FAS modified substrate than 9−FAS

193

modified substrate is expected because the longer fluoroalkyl chain of 17−FAS provides a higher

194

fluorine density on the GF than 9−FAS. It is noteworthy that SiNP coating on the membrane

195

substrate does not significantly affect the surface fluorination efficiency, thereby resulting in

196

similar fluorine to silicon (F/Si) elemental fraction ratios for both 9−FAS and 17−FAS modified 7 ACS Paragon Plus Environment

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membranes. The results may suggest that surface fluorination by FAS leads to comparable

198

surface energies for the modified fibrous substrates with and without SiNPs.

199

FIGURE 3

200

Wetting Behavior of Modified GF Membranes. The contact angles for the surface

201

engineered membranes were measured to assess their wetting resistances to pure liquids having a

202

wide range of surface tensions, including water (γ = 72.8 mN/m), mineral oil (γ ≈ 30 mN/m), and

203

ethanol (γ = 22.1 mN/m). As shown in Figure 4, all modified membranes had water contact

204

angles greater than 90°, indicating that the initially highly hydrophilic GF substrate (i.e., no

205

measurable water contact angle) was successfully modified to be hydrophobic after surface

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fluorination with low surface energy materials (i.e., 9−FAS and 17−FAS). The membranes

207

modified with 17−FAS show higher water contact angles compared to those modified with

208

9−FAS because the longer fluoroalkyl chain of 17−FAS imparts significantly lower surface

209

energy than 9−FAS. Surface energies of 17−FAS and 9−FAS modified membrane substrates

210

were estimated by the Owens and Wendt method based on the measured intrinsic liquid (water

211

and methylene iodide) contact angles on a silicon wafer (Table S1).31 The results confirm a

212

significantly lower surface energy for 17−FAS (13.1 mN/m) compared to that of 9−FAS (23.9

213

mN/m). Higher water contact angles were observed on the substrates coated with SiNPs for both

214

9−FAS and 17−FAS modified membranes compared to those with no SiNPs. SiNP coating on

215

the fibers increases surface roughness by creating air-gaps underneath the liquid.35 Because air

216

itself is strongly liquid repulsive, the membrane substrate features higher hydrophobicity after

217

SiNP coating when the surface is modified with the same FAS.

218

FIGURE 4

219

Although the 9−FAS modified membrane substrates exhibited a relatively high

220

hydrophobicity (water contact angle ~127°), they were oleophilic and readily wetted by mineral

221

oil and ethanol (i.e., no measurable contact angles, Figure 4). SiNP coating on the fiber did not

222

increase the wetting resistance of the 9−FAS modified membrane to the low surface tension

223

liquids. Upon contact with the membrane surface, mineral oil penetrated the porous membrane

224

and formed a very small contact angle ~25°, while ethanol readily wetted the 9−FAS modified

225

membrane. In contrast, the 17−FAS modified membrane was not wetted by mineral oil and 8 ACS Paragon Plus Environment


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showed a high mineral oil contact angle (~125°) even without SiNP coating. This observation

227

indicates that surfaces consisting of cylindrical fibers can resist wetting against oil and be

228

oleophobic after surface modification with ultralow surface energy material (i.e., γsv = 13.1

229

mN/m for 17−FAS). Further, this result is consistent with previous studies showing that

230

electrospun fibrous mats comprising an array of cylindrical fibers exhibited omniphobicity after

231

surface fluorination to lower the surface energy.36, 37 Notably, the 17−FAS modified membrane

232

failed to resist wetting against ethanol as indicated by the observed instant wicking. However,

233

coating the membrane substrate with SiNPs and surface fluorinating by 17−FAS (indicated as

234

17−FAS GF with SiNPs in Figure 4) imparted wetting resistance against ethanol. The contact

235

angle of ethanol on the 17−FAS modified membrane with attached SiNPs was ~100°. The

236

observed wetting behaviors of pure liquids with different surface tensions suggest that both

237

surface morphology and surface chemistry influence the wetting resistances of the GF

238

membranes.

239

Morphology and Chemistry Required for Surface Omniphobicity. As shown in the

240

previous subsection, substrate surfaces with different morphology and chemistry resulted in

241

membranes with different wetting resistance to low surface tension liquids. Achieving anti-

242

wetting property against water, which has relatively high surface tension (γ = 72.8 mN/m), is

243

relatively simple compared to creating an omniphobic surface that repels both water and low

244

surface tension liquid (e.g., oil). In addition to the low surface energy and rough surface texture

245

required for high surface wetting resistance to water, to achieve omniphobicity, surfaces must

246

also have features with a re-entrant structure.

247

To better explain the importance of a re-entrant structure to achieve surface omniphobicity,

248

we present a conceptual model illustrating the expected liquid-air interfaces on a 17−FAS

249

modified membrane (solid) with water, mineral oil, and ethanol in Figures 5A−5C. To maintain a

250

metastable Cassie-Baxter state for the liquid-air interface without surface (solid) wetting, the

251

direction of the capillary force (black arrow in Figure 5) should be upwards (i.e., dewetting

252

direction) and an air-gap between the liquid meniscus and the bottom line of the solid (i.e.,

253

surface of glass fiber located at one step below) must be guaranteed (i.e., h > 0 in Figures 5A and

254

5B).36 The intrinsic contact angle (i.e., the equilibrium contact angle of the liquid on a planar

255

smooth surface) on the textured surface can be expressed by an angle between the direction of 9 ACS Paragon Plus Environment

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the capillary force and the surface curvature (θ1 , θ2 , and θ3 in Figure 5).26 If the liquid has an

257

intrinsic contact angle greater than 90° (i.e., water, θ1 = 109° for 17−FAS modified silicon wafer,

258

Table 1), the liquid-air interface can be located at the upper half of the cylindrical fiber where a

259

convex curvature exists (Figure 5A). On the other hand, for liquids with an intrinsic contact

260

angle < 90° (i.e., mineral oil, θ2 = 70°, Table 1), a metastable Cassie-Baxter state of a liquid-air

261

interface is formed at the bottom half of the fiber, owing to its re-entrant, convex curvature

262

(Figures 5B). This condition allows for the existence of an air-gap between the liquid and the

263

solid surface (h > 0 in Figure 5B), while maintaining the direction of the capillary force upward

264

for a liquid with an intrinsic contact angle < 90°.

265

Liquids with significantly low surface tension, such as ethanol (γ = 22.1 mN/m), exhibit

266

small intrinsic contact angles even for ultralow energy surfaces. The intrinsic contact angle of

267

ethanol, measured on a silicon wafer surface fluorinated with 17−FAS, was 53° (Table 1; θ3 = 53°

268

in Figure 5C). Such a small intrinsic contact angle does not allow an air-gap between the liquid

269

meniscus and the next level surface structure, thereby leading to surface wetting even though the

270

surface features a re-entrant structure as described in Figure 5C. A secondary re-entrant structure

271

achieved by SiNP coating on the GF can enhance the surface omniphobicity, thereby increasing

272

the wetting resistances to liquids with low surface tension. Similar to cylindrical fibers, the

273

spherical SiNPs possess a re-entrant structure, thus offering an additional barrier to surface

274

wetting as illustrated in Figure 5D. This additional barrier created by the SiNPs allows a low

275

surface tension liquid (e.g., ethanol) to maintain a metastable Cassie-Baxter state by sustaining

276

an air-gap between the liquid and the solid surface.

277

FIGURE 5

278 279

MD Desalination Performance with Low Surface Tension Saline Waters. To

280

compare the desalination performance of our engineered membrane substrates with different

281

surface wettabilities, we performed direct contact membrane distillation (DCMD) experiments

282

using feed solutions of varying surface tensions. Sodium dodecyl sulfate (SDS), a representative

283

surfactant which is ubiquitous in wastewaters, was selected as a surface active agent.38, 39 We

284

note that when SDS is present in wastewaters of high salinity, such as shale gas produced



285

wastewater, it significantly reduces the surface tension of the medium, because the electrolyte

286

promotes migration of ionic surfactant molecules to the liquid-air interface.32, 40

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We conducted DCMD experiments for 2 h using 1 M NaCl feed solution at 60 °C (without

288

SDS) and DI water permeate at 20 °C to validate the MD performance of the modified

289

membranes. SDS was then introduced into the feed solution every 2 h during the DCMD

290

experiment to progressively increase the feed water SDS concentration and reduce the solution

291

surface tension. Specifically, the SDS concentrations in the feed after sequential additions were

292

0.1, 0.2, and 0.3 mM and the corresponding estimated feed surface tensions were ~42, ~33, and

293

~31 mN/m, respectively; these surface tension values were extrapolated from the data developed

294

by Matijević and Pethica.32 As shown in Figures 6A−B, the 9−FAS modified membrane

295

substrates showed a stable water flux and salt rejection before SDS addition regardless of SiNP

296

coating, demonstrating that the modified substrates properly function as a hydrophobic MD

297

membrane. However, after introduction of 0.1 mN SDS to the feed, the water flux started to

298

increase and salt rejection substantially decreased with the 9−FAS modified membranes with and

299

without SiNPs. Our DCMD results with the 9−FAS modified membrane suggest that an MD

300

membrane having a relatively high surface energy (i.e., 23.9 mN/m, surface energy of 9−FAS

301

provided in Table S1) is prone to wetting by low surface tension feed waters (~42 mN/m for 0.1

302

mM SDS in 1 M NaCl), even though it features a re-entrant structure.

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The 17−FAS modified membrane substrate exhibited a stable MD performance without

304

changes in water flux and salt rejection for feed SDS concentrations up to 0.2 mM, as presented

305

in Figure 6C. Although the 17−FAS modified GF membrane started to wet when SDS

306

concentration in the feed was 0.3 mM, as indicated by the drastically increased water flux and

307

reduced salt rejection, it demonstrated wetting resistance for feed solutions with a relatively low

308

surface tension (~33 mN/m for 0.2 mM SDS in 1 M NaCl at 60 °C).

309

The membrane substrate coated with SiNPs and fluorinated using 17−FAS (referred to as

310

17−FAS GF with SiNPs) exhibited the highest wetting resistance to low surface tension feed

311

solutions. We attribute the results to the multi-level re-entrant structure and extremely low

312

surface energy of the membrane. The initial water flux and complete salt rejection with the

313

membrane coated with SiNPs and 17−FAS were maintained even after the addition of 0.3 mM

314

SDS to the feed, which corresponds to a solution surface tension of ~31 mN/m.32 The results 11 ACS Paragon Plus Environment

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demonstrate that a secondary re-entrant structure achieved by the spherical SiNPs on the

316

cylindrical GF plays a critical role in enhancing surface omniphobicity, thereby increasing

317

wetting resistance to low surface tension feed solutions in membrane distillation. FIGURE 6

318 319

In summary, our systematic approach to elucidate the factors affecting surface omniphobicity

320

has demonstrated that both ultralow surface energy and a re-entrant structure are critical for

321

membrane wetting resistance to low surface tension liquids. Previous research efforts on the

322

development of high performance MD membrane have focused on superhydrophobic surfaces

323

for enhanced wetting resistance to water. However, as we demonstrated in this study, surface

324

superhydrophobicity does not guarantee membrane wetting resistance to low surface tension

325

liquids. Therefore, engineering omniphobic membranes can be a key for successful applications

326

of MD to treat challenging industrial wastewaters, such as shale gas produced water that contains

327

diverse low surface tension contaminants (e.g., oils, organic solvents, and surfactants).

328 329

ASSOCIATED CONTENT

330

Surface

331

tetrahydrodecyl)triethoxysilane (17−FAS) (Table S1); nominal pore size and pore size

332

distribution of glass fiber (GF) substrate before and after silica nanoparticle (SiNP) coating

333

(Figure S1); diameter of glass fibers and SiNPs (Figure S2); SEM images of SiNPs attached to

334

GF membranes after exposure to bath sonication (Figure S3). This material is available free of

335

charge via the Internet at http://pubs.acs.org.

energies

of

(nonafluorohexyl)triethoxysilane

(9−FAS)

and

(heptadecafluoro-

336 337

ACKNOWLEGMENT

338

We acknowledge the support received from the National Science Foundation through the

339

Engineering Research Center for Nanotechnology-Enabled Water Treatment (ERC-1449500).

340

Facilities used were supported by the Yale Institute of Nanoscale and Quantum Engineering

341

(YINQE) and the Chemical and Biophysical Instrument Center (CBIC) at Yale. We also



Page 14 of 24

342

acknowledge Dr. Christopher M. Stafford at the National Institute of Standards and Technology

343

for his assistance with XPS characterization.

344


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345

REFERENCES

346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387

1. Khayet, M.; Matsuura, T., Chapter 1 - Introduction to Membrane Distillation. In Membrane Distillation, Elsevier: Amsterdam, 2011; pp 1-16. 2. Wang, P.; Chung, T. S., Recent advances in membrane distillation processes: Membrane development, configuration design and application exploring. J Membrane Sci 2015, 474, 39-56. 3. Alkhudhiri, A.; Darwish, N.; Hilal, N., Membrane distillation: A comprehensive review. Desalination 2012, 287, 2-18. 4. Lawson, K. W.; Lloyd, D. R., Membrane distillation. J Membrane Sci 1997, 124, (1), 125. 5. Al-Obaidani, S.; Curcio, E.; Macedonio, F.; Di Profio, G.; Ai-Hinai, H.; Drioli, E., Potential of membrane distillation in seawater desalination: Thermal efficiency, sensitivity study and cost estimation. J Membrane Sci 2008, 323, (1), 85-98. 6. Gingerich, D. B.; Mauter, M. S., Quantity, Quality, and Availability of Waste Heat from United States Thermal Power Generation. Environ Sci Technol 2015, 49, (14), 8297-8306. 7. Lin, S. H.; Yip, N. Y.; Elimelech, M., Direct contact membrane distillation with heat recovery: Thermodynamic insights from module scale modeling. J Membrane Sci 2014, 453, 498-515. 8. Shaffer, D. L.; Chavez, L. H. A.; Ben-Sasson, M.; Castrillon, S. R. V.; Yip, N. Y.; Elimelech, M., Desalination and Reuse of High-Salinity Shale Gas Produced Water: Drivers, Technologies, and Future Directions. Environ Sci Technol 2013, 47, (17), 9569-9583. 9. Li, J. F.; Guan, Y. S.; Cheng, F. Q.; Liu, Y., Treatment of high salinity brines by direct contact membrane distillation: Effect of membrane characteristics and salinity. Chemosphere 2015, 140, 143-149. 10. Leitch, M. E.; Li, C. K.; Ikkala, O.; Mauter, M. S.; Lowry, G. V., Bacterial Nanocellulose Aerogel Membranes: Novel High-Porosity Materials for Membrane Distillation. Environmental Science & Technology Letters 2016, 3, (3), 85-91. 11. Nejati, S.; Boo, C.; Osuji, C. O.; Elimelech, M., Engineering flat sheet microporous PVDF films for membrane distillation. J Membrane Sci 2015, 492, 355-363. 12. Woo, Y. C.; Kim, Y.; Shim, W.-G.; Tijing, L. D.; Yao, M.; Nghiem, L. D.; Choi, J.-S.; Kim, S.-H.; Shon, H. K., Graphene/PVDF flat-sheet membrane for the treatment of RO brine from coal seam gas produced water by air gap membrane distillation. J Membrane Sci 2016, 513, 74-84. 13. Shirazi, M. M. A.; Kargari, A.; Tabatabaei, M., Evaluation of commercial PTFE membranes in desalination by direct contact membrane distillation. Chem Eng Process 2014, 76, 16-25. 14. Duong, H. C.; Gray, S.; Duke, M.; Cath, T. Y.; Nghiem, L. D., Scaling control during membrane distillation of coal seam gas reverse osmosis brine. J Membrane Sci 2015, 493, 673682. 15. Gryta, M., Long-term performance of membrane distillation process. J Membrane Sci 2005, 265, (1-2), 153-159. 16. Munirasu, S.; Haija, M. A.; Banat, F., Use of membrane technology for oil field and refinery produced water treatment—A review. Process Safety and Environmental Protection 2016, 100, 183-202.



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17. Lin, S. H.; Nejati, S.; Boo, C.; Hu, Y. X.; Osuji, C. O.; Ehmelech, M., Omniphobic Membrane for Robust Membrane Distillation. Environmental Science & Technology Letters 2014, 1, (11), 443-447. 18. Lee, J.; Boo, C.; Ryu, W. H.; Taylor, A. D.; Elimelech, M., Development of Omniphobic Desalination Membranes Using a Charged Electrospun Nanofiber Scaffold. Acs Applied Materials & Interfaces 2016, 8, (17), 11154-11161. 19. Wang, Z. X.; Elimelech, M.; Lin, S. H., Environmental Applications of Interfacial Materials with Special Wettability. Environ Sci Technol 2016, 50, (5), 2132-2150. 20. Gregory, K. B.; Vidic, R. D.; Dzombak, D. A., Water Management Challenges Associated with the Production of Shale Gas by Hydraulic Fracturing. Elements 2011, 7, (3), 181-186. 21. Hoffmann, E.; Pfenning, D. M.; Philippsen, E.; Schwahn, P.; Sieber, M.; Wehn, R.; Woermann, D., Evaporation of Alcohol-Water Mixtures through Hydrophobic Porous Membranes. J Membrane Sci 1987, 34, (2), 199-206. 22. Izquierdo-Gil, M. A.; Jonsson, G., Factors affecting flux and ethanol separation performance in vacuum membrane distillation (VMD). J Membrane Sci 2003, 214, (1), 113-130. 23. Shaulsky, E.; Boo, C.; Lin, S. H.; Elimelech, M., Membrane-Based Osmotic Heat Engine with Organic Solvent for Enhanced Power Generation from Low-Grade Heat. Environ Sci Technol 2015, 49, (9), 5820-5827. 24. Liu, D. E.; Cerretani, C.; Tellez, R.; Scheer, A. P.; Sciamanna, S.; Bryan, P. F.; Radke, C. J.; Prausnitz, J. M., Analysis of countercurrent membrane vapor extraction of a dilute aqueous biosolute. Aiche J 2015, 61, (9), 2795-2809. 25. Cras, J. J.; Rowe-Taitt, C. A.; Nivens, D. A.; Ligler, F. S., Comparison of chemical cleaning methods of glass in preparation for silanization. Biosens Bioelectron 1999, 14, (8-9), 683-688. 26. Tuteja, A.; Choi, W.; Ma, M. L.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E., Designing superoleophobic surfaces. Science 2007, 318, (5856), 1618-1622. 27. Choi, W.; Tuteja, A.; Chhatre, S.; Mabry, J. M.; Cohen, R. E.; McKinley, G. H., Fabrics with Tunable Oleophobicity. Adv Mater 2009, 21, (21), 2190-2195. 28. Pasternack, R. M.; Amy, S. R.; Chabal, Y. J., Attachment of 3(Aminopropyl)triethoxysilane on Silicon Oxide Surfaces: Dependence on Solution Temperature. Langmuir 2008, 24, (22), 12963-12971. 29. Khayet, M.; Matsuura, T., Preparation and characterization of polyvinylidene fluoride membranes for membrane distillation. Ind Eng Chem Res 2001, 40, (24), 5710-5718. 30. Brzoska, J. B.; Benazouz, I.; Rondelez, F., Silanization of Solid Substrates - a Step toward Reproducibility. Langmuir 1994, 10, (11), 4367-4373. 31. Owens, D. K.; Wendt, R. C., Estimation of the surface free energy of polymers. Journal of Applied Polymer Science 1969, 13, (8), 1741-1747. 32. Matijevic, E.; Pethica, B. A., The Properties of Ionized Monolayers .1. Sodium Dodecyl Sulphate at the Air-Water Interface. Transactions of the Faraday Society 1958, 54, (9), 13821389. 33. Kota, A. K.; Mabry, J. M.; Tuteja, A., Superoleophobic surfaces: design criteria and recent studies. Surf Innov 2013, 1, (2), 71-83. 34. Atkins, P. W.; Shriver, D. F., Inorganic chemistry. 4th ed.; W.H. Freeman: New York, 2006; p xxi, 822 p. 15 ACS Paragon Plus Environment

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35. Liao, Y.; Wang, R.; Fane, A. G., Fabrication of Bioinspired Composite Nanofiber Membranes with Robust Superhydrophobicity for Direct Contact Membrane Distillation. Environ Sci Technol 2014, 48, (11), 6335-6341. 36. Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E., Robust omniphobic surfaces. P Natl Acad Sci USA 2008, 105, (47), 18200-18205. 37. Davis, A.; Mele, E.; Heredia-Guerrero, J. A.; Bayer, I. S.; Athanassiou, A., Omniphobic nanocomposite fiber mats with peel-away self similarity. J Mater Chem A 2015, 3, (47), 2382123828. 38. Hester, R. E.; Harrison, R. M., Fracking. Royal Society of Chemistry: Cambridge, UK, 2015; p xvii, 228 pages. 39. Final Report on the Safety Assessment of Sodium Lauryl Sulfate and Ammonium Lauryl Sulfate. J Am Coll Toxicol 1983, 2, (7), 127-181. 40. Rosen, M. J., Surfactants and interfacial phenomena. 3rd ed.; Wiley-Interscience: Hoboken, N.J., 2004; p xiii, 444 p.

449



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Table 1. Intrinsic contact angles of water, mineral oil, and ethanol on silicon wafer surfaces treated by 9−FAS and 17−FAS. Contact Angle, θ (°) Silane

(nonafluorohexyl) triethoxysilane

Formula Water

Mineral Oil

Ethanol

C12H19F9O3Si

97 ± 2

51 ± 2

29 ± 1

C16H19F17O3Si

109 ± 2

70 ± 2

53 ± 1

(9−FAS) (heptadecafluorotetrahydrodecyl) triethoxysilane (17−FAS)

452 453 454

Standard deviations are based on two contact angle measurements from three different silicon wafer samples.

455 456 457


Page 19 of 24


458 459 460 461 462 463

Figure 1. Schematics of the surface morphology and chemistry of the modified glass fiber (GF) membranes. 9−FAS and 17−FAS indicate (nonafluorohexyl)triethxoysilane (C12H19F9O3Si) and (heptadecafluoro-tetrahydrodecyl)triethoxysilane (C16H19F17O3Si), respectively. SiNPs refers to silica nanoparticles.

464 465 466 467



Page 20 of 24

468 469 470 471

Figure 2. SEM micrographs of (A) GF membrane treated by 9−FAS, (B) GF membrane with attached SiNPs treated by 9−FAS, (C) GF membrane treated by 17−FAS, and (D) GF membrane with attached SiNPs treated by 17−FAS.


60 (A) 9-FAS GF 40 20 0

Si

40 20 0

O

Si

Area Fraction (%)

Area Fraction (%)

40 20

Si

F

F

O

Element

60 (C) 17-FAS GF

0

475 476 477 478 479 480 481

F

60 (B) 9-FAS GF with SiNPs

Element

472

473 474

Area Fraction (%)


Area Fraction (%)

Page 21 of 24

60 (D) 17-FAS GF with SiNPs 40 20 0

O

Si

Element

F

O

Element

Figure 3. XPS analysis of the surface of the FAS modified GF membranes. Fractions of silica (Si, blue), fluorine (F, purple), and oxygen (O, gray) relative to the sum of elements present at the surface of the (A) GF membrane treated by 9−FAS, (B) GF membrane with attached SiNPs treated by 9−FAS, (C) GF membrane treated by 17−FAS, and (D) GF membrane with attached SiNPs treated by 17−FAS. The elemental fraction was calculated using CasaXPS software package, using a Shirley-type background from the XPS survey scan.

482 483 484



150

486 487 488 489 490 491

9-FAS GF

9-FAS GF with SiNPs

Wicking

50

Wicking

100

0

485

Water (72.8 mN/m) Mineral Oil (~30 mN/m) Ethanol (22.1 mN/m)

Wicking Wicking

Contact Angle (°)

200

Page 22 of 24

17-FAS GF 17-FAS GF with SiNPs

Membrane Type Figure 4. Contact angles of modified GF membranes with water, mineral oil, and ethanol. Contact angles with a 2 µL liquid droplet were monitored for 10 seconds. “Wicking” indicates that no stable contact angle was measurable because the membrane was readily wetted by the liquid droplet. Surface tensions of the liquids are indicated in legends. Error bars represent standard deviations of two contact angles from three different membrane samples.

492


Page 23 of 24


493 494 495 496 497 498 499 500 501 502 503

Figure 5. Schematic describing the expected liquid-air interfaces on the glass fibers (solid with cylindrical morphology) modified by 17−FAS with (A) water, (B) mineral oil, and (C) ethanol. θ1, θ2, and θ3 indicate the intrinsic contact angles of water, mineral oil, and ethanol, respectively, on surfaces modified by 17−FAS. The red dotted line in Fig. 5(A) represents the center of the cylindrical fibers which also applies to Figs. 5(B) and 5(C). Figs. 5(D-a) and (D-b) represent the putative liquid-air interface on a surface of GF with SiNPs (red spheres) modified by 17−FAS with ethanol and describe how the secondary re-entrant structure achieved by SiNPs contribute to increasing surface omniphobicity.

504 505 506



SDS Concentration in the Feed (mM) Water Flux Salt Rejection

40

105 100

30

95

20

90

10

85

(A) 9-FAS GF 0

0

120

240

360

80 480

SDS Concentration in the Feed (mM) 0.1

Water Flux (L m-2 h-1)


0.3

40

100

30

95

20

90

10

85

(B) 9-FAS GF with SiNPs 0

0

120

105 100

30

95

20

90 85

10

(C) 17-FAS GF 0

0

120

240

360

80 480

360

80 480




Water Flux Salt Rejection

40

240

50

0.2

0.3


40

105 100

30

95

20

90 85

10

(D) 17-FAS GF with SiNPs 0

0

120

Time (min)

240

360

80 480

Salt (NaCl) Rejection (%)

0.3


50

0.2

105

Time (min)


0.3


Time (min)

507

508 509 510 511 512 513

50

0.2


50

0.2


0.1

Page 24 of 24

Time (min)

Figure 6. Water flux and salt rejection of modified MD membranes measured in DCMD using 1 M NaCl at 60 °C with varying SDS concentrations as a feed and DI water at 20 °C as a permeate. The SDS concentrations in the feed after sequential additions every 2 h were 0.1, 0.2, and 0.3 mM and the corresponding expected surface tensions of the feed solution were ~42, ~33, and ~31 mN/m, respectively.

514 515 516 517


Engineering Surface Energy and Nanostructure of Microporous

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