A facile one-step preparation method of recyclable superhydrophobic polypropylene membrane for oil–water separation

Abstract

A facile one-step dip-coating method is reported here to prepare recyclable superhydrophobic polypropylene membrane. Membranes with nanoscale surface roughness and contact angle (water) greater than 150° were created by dipping polypropylene fabric film in a solution of silica nanoparticles networked with alkylsiloxanes and they exhibited excellent oil flux for oil–water separation in a gravity-induced simple separation system. Taking into consideration its separation selectivity, productivity flux, reusability and endurance, the membranes show promise in oily wastewater treatment and oil spill cleanup.

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Introduction

Nature has its own materials which show excellent water repellent (hydrophobic) and self-cleaning properties. Inspired by nature, synthetic superhydrophobic materials have been explored for application in oil–water separation, which is a global challenge because of tougher regulations concerning oily industrial wastewater discharges, and the need for water recycling and reuse, as well as frequent crude oil leakages.^1,2

There are a variety of reported superhydrophobic materials^3–9 that can selectively absorb oil contaminants from a water–oil interface. These superhydrophobic materials have been developed using various methods such as the metal-catalyzed homocoupling polymerization of 1,3,5-triethynylbenzene,³ surface coating of manganese oxide nanowire with chemical vapor deposition of polydimethylsiloxane molecules,⁴ carbonaceous nanofibers based on a template-directed hydrothermal carbonization process,⁵ nanoporous polydivinylbenzene by a solvothermal route,⁶ electrospun cellulose acetate nanofibers and poly(m-phenylene isophthalamide) nano fibers with fluorinated polybenzoxazine-incorporated SiO₂ nanoparticle functional layer by in situ polymerization,^7,8 polymethylsilsesquioxane from sol–gel process.⁹

In order to get high flux for practical application in oil–water separation, metallic mesh and textile sheets coated with these superhydrophobic materials have also been developed, such as carbon nanotubes grown on stainless steel mesh by thermal chemical vapor deposition with a diffusion barrier of Al₂O₃ film,¹⁰ 1-hexadecanethiol-grafted copper mesh through solution-immersion in nitric acid solution and sequential modification with 1-hexadecanethiol,¹¹ teflon-coated stainless steel mesh by spray-and-dry method,¹² waste-potato-residue/candle-soot/silica/attapulgite-coated meshes by a one-step spray-coating method,^13–15 polyacrylamide hydrogel-coated stainless steel mesh from a photoinitiated polymerization process with acrylamide, N,N′-methylene bisacrylamide, 2,2′-diethoxyacetophenone and polyacrylamide,¹⁶ block [poly(2-vinylpyridine)-b-polydimethylsiloxane] copolymer-grafted non-woven textiles and polyurethane sponges by a block copolymer-grafting strategy,¹⁷ surface coating of polyester textile via chemical vapor deposition of trichloromethylsilane¹⁸ and electrospinning, dip-coating and spin-coating of heptadecafluorodecyl polyhedral oligomeric silsequioxane + cross-linked poly(ethylene glycol)diacrylate, or poly(methylmethacrylate) onto stainless steel wire mesh.^19,20 However, all these above fabrication procedures are either costly or complicated. Furthermore, the above coated metal mesh membranes have a limitation in emulsified oil–water separation as the pore size in these membranes is generally large due to typical mesh sizes in the 50 to 500 μm range. An additional limitation for their usage in oil spill cleanups is related to their recyclability, owing to the particle nature of the coated membrane material having intrinsic difficulty in retaining its physical structure after each separation cycle. Recently, suspended silica nanoparticles (∼20 nm) along with trichloro(octadecyl)silane and polyfluorowax (diameter < 15 μm) in toluene were used to modify polyester fabric, copper mesh and sponges by a simple dip-coating process.²¹ In another reported work,^22,23 tetraethoxysilane in the presence of n-hexadecyltriethoxysilane was first hydrolysed in basic ammonia-saturated ethanol by a modified Stöber method to prepare ∼350 nm silica nanoparticles with attached hexadecylethyl groups which were then coated over spongy polyester fabric of 1 or 5 mm thickness by dip-coating. Silica modified with alkylsiloxanes was also used for fabricating translucent superamphiphobic coatings and superhydrophobic/superoleophilic filtering materials for oil–water separation.^24,25

Herein, the facile and inexpensive one-step fabrication of recyclable superhydrophobic polypropylene membranes of 0.11 mm thickness is reported, with direct relevance to the present situation as they could be of used for practical oil–water separation. Hydrophobic silica nanoparticles (7 nm) tethered with alkylsiloxanes of different alkyl chain types were used to coat polypropylene film by a dip-coating process for membrane preparation.

Experimental

Materials and method

A commercial non-woven polypropylene (PP) fabric film (Viledon novatexx 2432 ND) of 110 μm thickness was purchased from Freudenberg Vliesstoffe KG Weinheim. Trichloro(1H,1H,2H,2H-perflourooctyl)silane (FS), trichlorododecylsilane (DS), n-octadecyltrichlorosilane (OS) and fumed silica (Sigma-Aldrich product number S5130, surface area 370–420 m² g⁻¹, size 7 nm) were purchased from Sigma-Aldrich. All the solvents used in this work were purchased from Merck India. All the chemicals were of analytical grade and were used as received without any further purification.

Preparation of superhydrophobic polypropylene (PP) membrane

The preparation procedure of the membrane is shown in Scheme 1. The required amount of the fumed silica powder, 2% (w/v), was dispersed in toluene solvent and ultrasonicated for 40 min at room temperature to form a colloidal suspension, after which trichloro(alkyl)silane was added dropwise. The weight ratio of silica to trichloro(alkyl)silane was 1 : 1. The solution was then refluxed at 60 °C for 3 h for chemical reaction between surface hydroxyl groups of silica and chloride of trichloro(alkyl)silane. The resultant colloidal solution was allowed to cool at room temperature for subsequent use as dip-coating solution. Superhydrophobic PP membrane was prepared by a simple dip-coating technique in which the PP fabric was dipped in the above coating solution for 10 minutes for optimal sorption uptake. The PP fabric was then removed from solution and dried at 60 °C in an oven for 30 min.

Scheme 1 PP membrane preparation scheme comprising steps of (i) colloidal silica solution preparation, (ii) reaction of silica with trichloro(alkyl)silane to form alkylsiloxane–silica and (iii) dip-coating of PP fabric with the alkylsiloxane–silica; (A) and (B) represents structure of the membrane at different length scale; (C) is the optical image of the membrane.

Characterization of superhydrophobic PP membrane

The membrane morphology of all the samples was examined by TEM (JEOL, JEM 2100), SEM (FE-SEM, JSM-7100F), and AFM (NT-MDT). The AFM experiment was carried out to check surface roughness of the sample. The average roughness and root mean square (RMS) roughness were calculated by using Nova_P9_Ntegra_2.1.0.800 software. For AFM analysis 1 × 1 cm² was cut and dried in a vacuum chamber at a temperature of 60 °C for 1 hour. The sample was analyzed immediately. The hydrophobicity of sample contact angle (CA) measurement was carried out on a DSA100 Kruss GmbH instrument. For the CA measurement 3 × 3 cm² was cut and stuck on a glass plate. The amount of water dispersed per droplet on the sample surface was 4 μl. The measurement was carried out 6 times using different areas of the sample to get an average value of CA. The formation of –Si–O–Si– linkages was confirmed by NMR spectroscopy on a Bruker AVANCE-II 500 MHz instrument. Dynamic light scattering measurements were performed on NaBiTec Spectro Size 300 light scattering apparatus (NaBiTec, Germany) with a He–Ne laser (633 nm, 4 mW).

Sorption experiment and reusability

A sorption experiment was carried out to check the capacity to absorb different organic solvents such as hexadecane, carbon tetrachloride (CCl₄), dichloromethane (DCM), silicon oil, crude oil and toluene. In the experiment, a membrane sample of size 2 × 2 cm² was cut, its initial dry weight (W₀) was measured, and it was then dipped in the desired solvent for 1 h, after which the final wet weight (W) was measured. The sample was then washed with acetone and dried at 60 °C in an oven for 1 h before testing its recyclability. The recycling test was carried out for 10 cycles to evaluate the reusability of the sample. The absorption capacity (S) of the sample was calculated by the following equation:

Oil–water separation experiment

Oil–water separation was carried out using superhydrophobic silica PP membranes. Separation was carried out by two methods: (i) direct oil–water separation; (ii) emulsion separation using span-80 surfactant. For direct separation, a mixture of 10 ml of oil and 10 ml of water was placed in a separating funnel, after which the water was coloured with methyl blue indicator. The separation process is shown in the video (Movie S1†). As representative examples, five different organic solvents, DCM, chloroform, toluene, petrol and hexadecane, were selected. The flux was calculated considering the effective area of membrane to be 8.48 cm². Similarly, the emulsion separation of the oil–water mixture was carried out using a prepared 9 : 1 (v/v) mixture of oil and water emulsified by using span-80 surfactant (0.4 g l⁻¹) (Movie S2†). Here, the effective area of membranes is 12.25 cm².

Result and discussion

Formation of superhydrophobic polypropylene membrane

As shown in Scheme 1, silica was modified with different types of organosilane to make it superhydrophobic. The superhydrophobic silica was coated onto PP fabric by a simple dip-coating preparation method. After the coating process, the porous PP fabric was found to be packed with alkylsiloxane-networked-silica nanoparticles leading to durable superhydrophobic PP membranes (OS–PP, DS–PP and FS–PP) which are recyclable. Solid state ²⁹Si NMR spectra of the samples were used to confirm chemical bond formation by reaction of trichloro(alkyl)silane with surface hydroxyl groups (silanols) of the silica nanoparticle.

All the spectra showed a prominent peak at −110 ppm along with a shoulder peak at −100 ppm, which was assigned to the connecting SiO₄ tetrahedra, represented by Q₄ and Q₃, where the Q₄ and Q₃ signals correspond to silicon atoms bonded to four and three oxygen atoms,²⁶ respectively, and two other well-resolved peaks at −57 ppm and −68 ppm were assigned to the general alkylsiloxane structures (HO) (R)Si(OSi)₂ [isolated silanol, T₂] and (R)Si(OSi)₃ [siloxane, T₃], where R = alkyl.²⁷ The ²⁹Si NMR spectra of the samples are provided in Fig. 1. The presence of T₂ and T₃ peaks in the spectra indicates that the network structures exhibit two and three cross-links with silanols of the silica nanoparticles.

Fig. 1 NMR spectra of the fumed silica and modified silica with alkylsiloxane.

SEM and TEM characterization

The nanoparticle morphology of the membrane layers was observed by top-surface SEM images of the samples and the PP substrate (Fig. 2). Fig. 2A shows the surface morphology of the PP fabric while Fig. 2B–D show the surface morphology of coated PP fabric. Very fine deposition of inter-connected silica particles on the porous PP support is shown in the SEM images. Further examination of the membrane morphology using TEM images indicated that each nanoparticle was well inter-connected to form large-scale structures. The structures of OS–silica, DS–silica and FS–silica are shown in TEM images at two different magnifications at micrometer-length and nanometer-length scales (Fig. 3A–C).

Fig. 2 SEM images of the initial PP fabric and the final PP membranes. (A) PP fabric, (B) OS–PP, (C) DS–PP, (D) FS–PP.

Fig. 3 TEM images of the OS–PP, DS–PP and FS–PP at different magnifications.

AFM and CA measurements

The average surface roughness of the membranes was determined by AFM analysis (Fig. 4A–C). The roughness of the samples was found to be around 32, 47 and 53 nm, respectively for OS–silica, DS–silica and FS–silica membranes. The AFM surface images of the membrane samples were obtained at three different locations on each membrane surface to determine the average surface roughness values. The details of the AFM analysis are given in Table 1. All the samples exhibited average nanoscale roughness in the 25–41 nm range in increasing order for OS–PP < DS–PP < FS–PP.

Fig. 4 AFM surface images and contact angle (water) of the OS–PP, DS–PP and FS–PP.

Table 1 AFM surface roughness analysis of the membranes

Sample	Peak to peak, Sy (nm)	Mean value, (nm)	Roughness average, Sa (nm)	Mean square, Sq (nm)
OS–PP	221.09	112.56	25.21	31.62
DS–PP	330.42	164.77	36.67	46.83
FS–PP	430.73	231.33	41.28	52.59

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This nanoscale surface roughness of the membranes implied that the silica nanoparticles are arranged in an orderly manner on the PP substrates, playing a major role in the superhydrophobicity of the PP surface. The relationship between nanoscale structure of alkylsiloxane-networked silica and superhydrophobicity is described below on the basis of the lotus leaf effect. It has long been known that lotus leaves show excellent water-repellent properties (superhydrophobicity) due to surface roughness caused by branch-like nanostructures on top of the micropapillae and to the low surface energy of the epicuticular wax.²⁸ Similarly, the surfaces of the as-prepared membranes are composed of nanoscale silica structures networked with long alkyl chains accounting for the low surface energy, and showed an average contact angle (water) > 150°; these values remained stable even after 1 hour of water dispersal, which indicated the superhydrophobicity remains stable for a long time and atmospheric moisture does not affect the wettability of the PP membrane (Fig. 4D–F). Among all the samples, FS–PP showed a slightly higher CA value than the others, which might be due to the presence of fluorides in the structure of FS–PP.

Sorption capacity, recyclability and oil–water separation

The absorption capacities of the membranes for oils such as petrol, hexadecane, crude oil, silicon oil, chloroform, DCM and toluene were determined. A recycling test in which the absorption capacity of the samples was measured over 10 cycles was also carried out to evaluate their reusability. As shown in Fig. 5A, all the membranes exhibited high absorption capacity of oils of about 70 to 568% depending upon the type of oil, but no absorption of water.

Fig. 5 Oil (organic) absorption capacities for the OS–PP, DS–PP and FS–PP and recyclability of the OS–PP membrane.

The highest oil absorption capacity was found in the cases of silicon oil and crude oil. The oil absorption capacities for silicon oil and crude oil were around 558 and 450% respectively, while those for chloroform, hexadecane, DCM and toluene were shown to be around 100–180%. The OS–PP membrane showed higher oil absorption capacity than the DS–PP and FS–PP membrane. The order of absorption capacity for membranes found was OS–PP > DS–PP > FS–PP. The reason behind this behaviour is the long alkyl chain of OS–PP which absorbed more oil than the shorter alkyl chain organosilane. Oil absorption capacity remained constant even after 10 consecutive cycles of oil absorption as revealed in Fig. 5, indicating that the membranes are stable and reusable. The membranes were then tested for oil–water separation in a simple gravity-driven process and in an emulsion oil–water separation wherein the simple process feed oil–water mixture (1 : 1, v/v) was in contact with the top membrane surface without any external driving force, either positive pressure on the feed side or suction (reduced pressure) on the permeate side, being applied. For clear visibility the water was coloured blue with methylene blue. The oil was quickly absorbed by the membranes and flowed down through the membranes within seconds to the permeate side whereas water was retained on the feed side. As shown in Videos S1 and S2† and in Fig. 6A–G the PP membrane easily separated the organics from water mixture. The water-in-oil emulsion mixture of hexadecane and water (oil/water 9 : 1, v/v) was poured on top of the membrane surface. After pouring, hexadecane penetrated the membrane and flowed down into the cylinder underneath whereas the water was retained on the surface of the membrane. After filtration, no water was found in the cylinder; only clear and transparent oil was visible, which indicates the high oil–water separation efficiency of the membranes. The droplet size of the emulsion measured by dynamic light scattering was about 100–1000 nm (Fig. 7). Such emulsion droplets were not observed in the retentate by dynamic light scattering, indicating oil was separated out by the filtration. The organic flux data for the membranes for simple and emulsion filtration by gravity-driven process are given in Table 2.

Fig. 6 Oil–water separation by emulsion separation and simple separation processes. Emulsion separation (A–C) from 9 : 1 oil/water feed mixture by using span-80 surfactant. (B) and (C) are optical images of emulsion and product, respectively; simple filtration (D–G) was carried out by 50 : 50 v/v mixture of oil/water.

Fig. 7 Plot of autocorrelation function for emulsion (A) and droplet size of emulsion (B) obtained from dynamic light scattering measurement.

Table 2 Permeate oil flux values of the membranes during oil–water separation by simple filtration and emulsion separation processes

Oil/Organic	Flux (L m^−2 h^−1)
	Simple filtration			Emulsion filtration
	FS–PP	DS–PP	OS–PP	FS–PP	DS–PP	OS–PP
DCM	531 ± 9	559 ± 9	685 ± 8	180 ± 6	200 ± 10	238 ± 5
Chloro-form	139 ± 4	155 ± 3	195 ± 4	73 ± 2	77 ± 3	102 ± 3
Toluene	106 ± 2	118 ± 3	231 ± 3	66 ± 2	70 ± 1	122 ± 2
Petrol	78 ± 3	76 ± 1	163 ± 5	38 ± 4	40 ± 3	85 ± 5
Hexadecane	18 ± 1	20 ± 2	35 ± 2	10 ± 1	11 ± 1	15 ± 2

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The performances of the PP membranes as shown in Table 2 indicated excellent separation of oil, with >99% oil purity, from an oil–water mixture (Fig. 8). The oil separation efficiency for the membranes was in the order OS–PP > DS–PP > FS–PP, which is in agreement with their organic absorption capacities. The comparison of the PP membranes with other reported materials in terms of organic absorption capacity and productivity flux is given in Table 3.

Fig. 8 Oil (organic) separation efficiency for the OS–PP, DS–PP and FS–PP respectively.

Table 3 Comparison of the PP membrane with the other reported materials

Material	CA (°)	Absorption capacity, %	No. cycles	Feed organic	Flux (L m^−2 h^−1)	Reference
Superhydrophobic polyester material	>150	Crude oil = 300, diesel = 250, petrol = 225	10	—	—	22
Porous polylactic acid film	151.2	Hexane = 250, diesel = 380, gasoline = 450	5	—	—	29
Superhydrophobic composite filter paper	157 ± 2	Diesel oil = 340, dodecane = 280, octane = 260	5	Diesel oil/water	—	30
F-PBZ/SiO2 NP-modified cellulose acetate nanofibrous membrane	161	—	—	DCM/water	33.33	7
Superhydrophobic OS–PP membrane	153.5	Silicon oil = 558, crude oil = 458, hexadecane = 165	10	DCM/water	685 ± 8	Our study
Superhydrophobic DS–PP membrane	153.8	Silicon oil = 450, crude oil = 453, hexadecane = 150	10	DCM/water	559 ± 12	Our study
Superhydrophobic FS–PP membrane	154.3	Silicon oil = 450, crude oil = 451, hexadecane = 152	10	DCM/water	531 ± 10	Our study

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Conclusions

Superhydrophobic PP membrane was prepared by a simple dip-coating process using a colloidal coating solution of alkylsiloxane-networked hydrophobic silica nanoparticles. The alkylsiloxane-networked silica colloidal solution was derived from the reaction between trichloro(alkyl)silane and fumed silica nanoparticles in toluene under inert environment. The alkyl chain type in alkylsiloxane groups was varied as n-octadecyl (C₁₈), dodecyl (C₁₂) and 1H,1H,2H,2H-perflourooctyl (C₈) to prepare coating solutions with different properties. All the PP membranes showed superhydrophobic characteristics with contact angle (water) > 150° and they effectively separated oil with >99% oil purity from the oil/water mixture using a gravity-driven process. The PP membrane coated with longer alkyl chain octadecylsiloxane–silica showed the best result in terms of oil absorption capacity and oil flux in oil–water separation under gravity-assisted separation. Furthermore, the membranes were recyclable and promising for large-scale application in cleaning up oil spills.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11008h

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