Materials Science and Engineering C 35 (2014) 386–391

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Improvement of the adsorption of quaternary ammonium on polypropylene affinity membrane through the control of its surface properties Naima Hachache a,b, Youcef Bal a, Dominique Debarnot b, Fabienne Poncin-Epaillard b,⁎ a

Laboratoire de Physique Chimie Moléculaire et Macromoléculaire, Université Saad Dahleb de Blida, route Soumaa, BP 270 Blida, Algérie LUNAM Université, UMR Université du Maine – CNRS n° 6283, Institut des Molécules et Matériaux du Mans – Département Polymères, Colloïdes et Interfaces, Avenue Olivier Messiaen, 72085 Le Mans Cedex, France b

a r t i c l e

i n f o

Article history: Received 24 July 2013 Received in revised form 23 October 2013 Accepted 16 November 2013 Available online 3 December 2013 Keywords: Polypropylene Fiber Membrane Plasma Adsorption Quaternary ammonium

a b s t r a c t Polypropylene fiber meshes were plasma-treated in order to attach new chemical functions corresponding to acidic or basic groups without altering the roughness of such thin material. An almost complete wettability of these plasma-treated materials is obtained. Because of the plasma-grafting of acid or amino moieties, such surface treatment allows increasing the adsorption rate of quaternary ammonium molecule like Aliquat 336. This increase was explained by specific interactions of ammonium head of the Aliquat 336 and hydrophilic group of plasma-treated PP, followed by the adsorption of a further layer of Aliquat 336 through hydrophobic interactions of its hydrocarbon chain. These interactions between the carrier and the polymeric surface were characterized leading to physisorption mechanism. Such new material could be applied to the extraction process since no evidence of aging was given. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The occurrence of drug residues in the aquatic environment emerged as a matter of public concern since conventional sewage treatment plants do not fully degrade residues of pharmaceuticals. Thus, developing sensitive methods for extracting of bioactive molecules has reached significant proportions. With such goal, affinity membranes have been developed as potentially valuable technology for the separation and the concentration of charged or neutral species. Among several membranes types, the solid membranes such as the polymer inclusion membranes (PIM) [1,2] were shown to be efficient because of their higher stability in separation processes. Such affinity membranes involve the complexing agents, referenced as ion carriers able to quench species onto the membrane phase. However, this separation route needs to prepare a specific membrane, to control its thickness and to incorporate a large amount of the targeting molecules [3]. However, Fontas et al. [4] show a progressive reorganization of the polymeric material because of the enhancement of the preferential solvent interactions between the carrier and the plasticizer. Therefore, studies on other membrane configurations have received more attention. Due to their high packing density and their high surface area per unit module volume, the hollow fibers have found application in various membrane based extraction processes, such as supported ⁎ Corresponding author. Tel.: +33 2 43 83 26 98; fax: +33 2 43 83 35 58. E-mail address: [email protected] (F. Poncin-Epaillard). 0928-4931/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2013.11.025

liquid membrane, emulsion liquid membrane, …..[5]. The nature of the microporous membrane can be hydrophobic or hydrophilic, which means that the pores of the membrane are organic or aqueous-filled, respectively. Most of hydrophobic membranes are composed of polymeric materials thermally stable and resistant to a wide range of organic or inorganic agents, such as polyethylene (PET), polypropylene (PP) and polyvinylidene fluoride (PVDF). Since the specific surface area of the membrane is one of the most important factors for its efficiency, materials composed of meshes should have same separation behavior and will be prospected here. Complexing agents such as aliphatic amines have been widely employed as carriers. In general, the efficiency of the amine decreases in the order quaternary N tertiary N secondary N primary under the same experimental conditions. Aliquat 336, a water insoluble quaternary ammonium salt composed of a large organic cation associated with a chloride, acts as an anionic (basic) liquid exchanger that has been used for the recovery of metal ions [1,6–8], amino acids [9,10], enzymes [10,11], peptides [12], drugs [13–15]. The results obtained in those experiments fully demonstrated the usefulness of the supported liquid membrane extraction. However, the efficiency of such extraction is highly dependent on the filling and emptying kinetics of carrier solutions in or from the membrane cavities. The objective of this work is to develop PP fiber meshes as membrane and to study the impregnation Aliquat, the Aliquat-coated PP meshes then allowing the penicillin extraction. Such material is a bulky one, without any porosity but present a high specific surface

N. Hachache et al. / Materials Science and Engineering C 35 (2014) 386–391

area. Since PP is hydrophobic polymer and Aliquat a more polar molecule, PP fibers were plasma-treated in order to enhance the interfacial affinity and consequently, to attach polar chemical groups without any PP physical alteration.

387

2.4. Specific surface measurement of PP fibers

2. Experimental

The specific surface measure of PP fibers was run thanks to the study of dye adsorption. The used dye, hexamethylpararosaniline chloride, socalled crystal violet (Aldrich ref C3886) was dissolved in water. The specific surface is calculated from:

2.1. Materials

S¼Y NA

2.1.1. Polypropylene plates and fibers Before plasma-treatment, the polypropylene plates (Exxon-Mobil) or fibers (so-called Aquatextil, Cailleton) were cleaned in a solution of ethanol stirred under ultrasound for 15 min. Then, samples were then dried in a laminar flow hood. Plates were only plasma-treated in order to improve the wettability measurement. Trioctylmethylammonium chloride, so-called Aliquat 336 (Aldrich ref 205 6130) and other reagents were used without purification.

with S, the specific area (m2/g); Y the concentration of adsorbed dye (mol/g); N, Avogadro number (6.023∗1023 molecules/mol) and A, the surface layered by one dye molecule (50 Å2). More informations were given in supplementary data files.

2.2. Cold plasma modification Experiments were performed in a RF plasma reactor. The system is pumped with a turbomolecular pump with the nominal pumping speed of 900 m3/h backed with a two-stage oil rotary pump with a pumping speed of 25 m3/h. The discharge chamber is made of aluminum and has a volume of approximately 9 L. Commercially available, highly purified CO2 (purity N 99.995 %, Air liquide), N2 and H2 (purity 99.999 %, Air liquide) or CF4 gas (purity N 99.999 %, Messer) is leaked into the discharge chamber through a precise flow controller at variable flows. The powered electrode is connected to a matching network that is in turn connected to a 13.56 MHz RF generator. The output power of the RF generator is adjustable up to 120 W. Samples (plates or fibers) are mounted on the bottom of the discharge chamber. With such apparatus, the pressure and flow parameters are bound together. With used flow, the total pressure is comprised between 1.5 and 3.0 10−4 mbar. 2.3. Adsorption protocol The adsorption kinetics was studied with specific conditions. 50 mg of Aliquat 336 (maliquat, Maliquat = 404.16 g/mol) were dissolved in 25 mL of chloroform (V, [Ci] = 0.01 mol/L), under stirring and controlled temperature (24 ± 1 °C). Then, 50 mg of virgin or plasma treated PP fibers (ms) were added and then titrated as follows. After a certain duration (t), the modified fibers are dipped into 10 mL of glacial acetic acid solution then 10 mL of mercury acetate (Aldrich ref 83352) were added. The solution was stirred during 10 min and titrated with perchloric acid solution (0.1 N in glacial acetic acid) in the presence of few added droplets of hexamethylpararosaniline chloride, so-called crystal violet (Aldrich ref C3886). The adsorbed quantity of Aliquat (Q, mmol/g) was then calculated as:  Q¼

 C i −C f V

ms  Maliquat

with Cf, the remained concentration of Aliquat in the solution and the fiber mass. When varying one of the adsorption studies, the others are kept constant as follows: - 0.01 ≤ [Ci] ≤ 0.8 mol/L; maliquat = 50 mg, V = 25 mL, T = 24 ± 1 °C, ms = 50 mg, t = 60 min. - 5° ≤ T ≤ 45°; [Ci] = 0.01 mol/L, V = 25 mL, maliquat = 50 mg, ms = 50 mg, t = 60 min. At least, three experiments were onto PP or modified-PP fibers.

2.5. Wettability measurement The contact angle was measured with ultra pure milliQ water, diiodomethane and formamide drops (3 μL). Several drops were put on a sample and the contact angle was measured with a goniometer (Ramé Hart Inc). The resolution of the device is around 1o. The surface energy of the samples was calculated from the contact angle measurements of the three different liquids using the Fowkes and Owens– Wendt method:   d d 1=2 γl ð1 þ cos ΘÞ ¼ 2 γs γ1 and γl ð1 þ cos ΘÞ     d d 1=2 nd nd 1=2 þ 2 γs γ1 ¼ 2 γs γ1 where γ stands for surface energy, Θ for contact angle and the indexes s and l indicate the solid and liquid respectively. The exponents d and nd present dispersive and polar components of the surface energy. The surface energies of different liquids are gathered as followed: Water : Diiodomethane : Glycerol :

2

γl ¼ 72:8 mJ=m 2 γl ¼ 50:8 mJ=m 2 γl ¼ 63:4 mJ=m

d

2

γ1 ¼ 21:8 mJ=m d 2 γ1 ¼ 49:5 mJ=m d 2 γ1 ¼ 37:0 mJ=m

nd

2

γ1 ¼ 51:0 mJ=m nd 2 γ1 ¼ 26:4 mJ=m nd 2 γ1 ¼ 26:4 mJ=m

2.6. Scanning Electronic Microscopy (SEM) The SEM images were obtained after gold metallization thanks to the scanning electron microscope (Hitachi type 2300). 3. Results and discussion The purpose of this work is the study of the enhancement of adsorption onto polyolefin meshes. In order to achieve such a goal, polypropylene fibers were plasma-treated for increasing their affinity towards ammonium derivative compound dissolved in organic medium (chloroform). Such molecule, here the Aliquat 336 is mainly polar, even if substituted with long aliphatic chains. 3.1. Characterization of the plasma-treated polypropylene Selected plasma treatments only modify the surface of material without altering its bulk properties, this feature is of course important when the material is formed of fibers. Such treatments also allow the attachment of new hydrophilic chemical groups, therefore having a greater affinity towards ammonium such as Aliquat molecule. CO2, N2 or N2/H2 plasmas respectively lead to the formation of oxidized groups (hydroxyl, carbonyl, acid....), amines in proportions dependent on the operating parameters [16]. Since, this type of treatment is fully described in the literature [17], the hydrophilic character of plasmatreated polypropylene is here only described as the dependence of water contact angle on energy dissipated by the phase plasma, as defined by Yasuda [18] (Fig. 1). Thus, when the W/FM increases beyond 0.3 W.sccm− 1.g− 1, the water contact angle decreases to an almost

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CO2 plasma, t = 120s

N2 plasma, t = 60 s

18

Water contact angle (°)

100

Water contact angle (°)

90 80 70 60 50 40 30

16 14 12 10 8 6 4

20

2

10

0 0

0 0

0.1

0.2

0.3

0.4

0.5

0.6

W / FM ratio (W.sccm-1.g-1) Fig. 1. Dependence of PP wettability versus injected energy in CO2 or N2 plasmas.

negligible value. It should be noted that with the chosen plasma parameters, the value of the dissipated energy is more discriminated than the plasma chemistry. The duration influence is much more important (Fig. 2). The nitrogen plasma more quickly leads to a hydrophilic surface and, compared to CO2 plasma-treatment, a gap of about 40 ° is observed. This is explained by a higher functionalization yield as shown in the literature for the same type of polymer [16,19]. Increasing the duration of CO2 plasma-treatment up to 900 s does not reach the same threshold value of few degrees of water contact angle but only to a contact angle of 27°. This is probably due to an enhanced degradation effect rather than a functionalization one since CO2 plasma produces etching species such as atomic oxygen [20]. From these results, the operating parameters of CO2 or N2 plasma requested for a significant increase in the hydrophilicity of the PP surface are: CO2 plasma: P = 50 W, F = 10 sccm and t = 150 s inducing a 10° contact angle N2 plasma: P = 90 W, F = 10 sccm and t = 90 s inducing a 4° contact angle. In order to increase the density of amino groups on the surface of polypropylene [21], hydrogen was added to the plasma phase (Fig. 3). This should promote the formation of NH radicals in the plasma phase and then, should achieve to a higher density of attached amino groups. However, with these used RF-plasma reactor and operating parameters, the addition of hydrogen less than 50% does not enhance the PP

CO2 plasma

100

N2 plasma

40

60

80

100

Hydrogen proportion (%) Fig. 3. Dependence of PP wettability versus the hydrogen proportion in N2 plasma (FN2 = 10 sccm, P = 90 W, t = 90s) plasmas.

wettability, as the water contact angle is of the same order of magnitude as that one obtained with pure N2 plasma. Beyond that proportion, the water contact angle increases, the plasma phase being probably too poor in nitrogen species. Therefore, PP meshes will be N2/H2 plasmatreated in equivalent proportions of gasses. In Table 1, the dispersive–non dispersive and acid–base properties of CO2, N2 and N2/H2 surfaces plasma-treated PP surfaces are compared according to Owens–Wends and Lewis acid–base character theories. Whatever the chemistry of the plasma treatment, a strong nondispersive (polar) energy appears related to the grafting of amino or oxidized groups. With the Lewis acid–base prediction, an electron donor component is noticed showing the basic character of the plasmatreated surfaces as expected for N2 or N2/H2 plasma-treatments, but more surprising also for the CO2 plasma-treatment. The latter plasma phase should contain some N2 impurities. Alumina deposition, almost 3% as determined by XPS quantification [22] and assigned to Al sputtering from the electrodes, was also identified by XPS analysis. These both plasma and solid phases impurities may explain the basic energetic component for the CO2 plasma-treatment. The adsorption of organic molecule onto specific surface is not only linked by their respective chemical affinities but is also controlled by the surface roughness that may act as potential mechanical anchoring site. The SEM analysis (Fig. 4) of fibers plasma-treated under the conditions described above shows that the topography is not or little altered by the plasma treatment in comparison with virgin fiber topography. These plasma phases therefore contain few etching species. This result is confirmed by measurements of hysteresis of water contact angle. Only a hysteresis of ten degrees is measured, a similar value of that obtained for the control and showing slight chemical or topographic heterogeneities. 3.2. Study of the Aliquat 336 adsorption onto the plasma-treated polypropylene

90

Water contact angle (°)

20

0.7

80

3.2.1. Kinetics study Kinetics study of the adsorption of the Aliquat on different plasmatreated fibers (Fig. 5) shows different rates depending on the chemistry

70 60 50 40 30

Table 1 Surface energies (mJ m−2) of virgin and plasma-treated PP calculated with Fowkes or Lewis acid–base theories.

20 10 0 0

20

40

60

80

100

plasma duration (s) Fig. 2. Dependence of PP wettability versus plasma duration in CO2 (FCO2 = 50 sccm, P = 90 W) or N2 (FN2 = 10 sccm, P = 90 W) plasmas.

Fowkes

PP

PP-CO2

PP-N2

PP-N2/H2

γnd Lewis acid–base γ− γ+

0.5 PP 0.1 0.3

42.5 PP-CO2 44.9 1.8

41.8 PP-N2 44.7 1.7

42.4 PP-N2/H2 44.5 1.8

N. Hachache et al. / Materials Science and Engineering C 35 (2014) 386–391

389

V-PP

PP-CO2

PP-N2

PP-N2/H2

Fig. 4. SEM pictures of different plasma-treated PP fibers.

of the applied plasma-treatment. An adsorption order can be established as: PP−CO2 NPP−N2 NPP−N2 =H2 N N NPP≈PP−CF4 ≈PP−He: The adsorption of the Aliquat 336 onto the PP-CO2 is maximum after 45 min, reaching the maximum rate of 0.74 mmol/g, higher than one with virgin polypropylene fibers. Beyond this value, the saturation appears. With PP-N2 and PP-N2/H2, the saturation plateau is observed at 0.6 mmol/g after 60 min. Virgin fibers or treated ones in the other plasmas (He and CF4) show an adsorption capacity much lower

(≈0.08 mmol/g). These curves clearly show the affinity of the quaternary ammonium towards fibers bearing with hydrophilic and/or basic groups. We can therefore assume that the first adsorbed layer of the Aliquat is established via the ammonium functions of the Aliquat and these surface groups. The second layer of Aliquat can then grow through the interactions between the hydrocarbon chains of this molecule, positioning ammonium heads to the external interface, substrate water. This corresponds to the self-assembled monolayers with an organization

0,6 PP-N2/H2

PP-CO2

PP

PP-CF4

0.9

Adsorption rate (mmol/g)

PP-N2, film

PP, fiber

PP-N2, fiber

PP-He

Adsorption rate (mmol/g)

PP-N2

PP, film

0.8 0.7 0.6 0.5 0.4 0.3

0,5 0,4

0,3

0,2 0,1

0.2 0.1

0,0

0 0

10

20

30

40

50

60

Time (min) Fig. 5. Kinetics of the Aliquat adsorption onto virgin and plasma-treated PP fibers.

0

10

20

30

40

50

60

Time (min) Fig. 6. Kinetics of the Aliquat adsorption onto virgin, plasma-treated PP fibers or films.

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Table 2 Determination of specific surface of virgin, plasma-treated PP fibers.

Table 3 Repeatability of the Aliquat adsorption onto N2 plasma-treated PP fibers.

Sample

Y (mmol/g)

A (cm2/g)

PP PP-N2 PP-N2/H2 PP-CO2

5.42 ∗ 10−4 11.4 ∗ 10−4 8.46 ∗ 10−4 11 ∗ 10−4

1650 3560 2640 3450

head–tail–tail–head. This scheme is confirmed by the higher efficiency of PP-N2 plasma-treated meshes during the water depollution tests. Since this Aliquat molecule bears long hydrocarbon chains, hydrophobic interactions may enhance the membrane efficiency. Therefore, the fibers have been also plasma-treated in order to increase or at least to preserve the PP hydrophobicity. Tetrafluoromethane plasma allows the grafting of hydrophobic groups such CFx groups onto PP surface while He plasma induces a low hydrophilic functionnalization. The plasma parameters are chosen based on previous work [23]: Plasma CF4: P = 50 W, F = 40 sccm, t = 3 min. Plasma He: P = 70 W, F = 80 sccm, t = 2 min.

PP-CO2

PP-N2

PP-N2/H2

PP

Adsorption rate (mmol / g)

0.6 0.5 0.4 0.3 0.2 0.1

0.60 1

0.55 2

0.60 3

0.60 4

0.60 5

0.44 6

capacity will linearly decrease with the increase of the water contact angle. However, at the beginning of the aging (Fig. 7), a plateau is observed until 30° and 50° respectively for N2, CO2 and N2/H2 plasmatreated PP. Then, the adsorption rate falls down. N2/H2 plasma seems to induce less degraded solid fragments. In the same order of idea, the repeatability of the adsorption capacity of PP-N2 fibers was studied (Table 3). Here, same fibers were used several times for the Aliquat adsorption and it is only after 5 successive assays that the Aliquat adsorption starts to decrease. 3.2.2. Thermodynamic study In the domain of the studied concentrations, the Aliquat adsorption capacity onto the fibers treated in N2 plasma is dependent on its initial concentration and varies substantially linearly without the occurrence of a saturation plateau (Fig. 8). The kinetics of the absorption seems to follow a law in first order. Adsorption studies have been conducted with different initial Aliquat concentrations with fixed adsorbent fibers. The adsorption increases with the concentration as expected due to the increased flux of surfactant to the surface. However, the rate of increase in the adsorption rate with increasing concentration reveals details of the adsorption process [13]. The adsorption isotherm of Aliquat onto plasma-treated PP fibers in non-aqueous solution, not shown here, is typical S shaped. In the low concentration region, the adsorption increases linearly, confirming monolayer adsorption capacity of the PP fibers while at the higher concentration, the multilayer adsorption is enhanced [26,27]. The Aliquat adsorption rate is also strongly dependent on the temperature (Fig. 9), passing through a maximum when chloroform temperature reaches 25°. Beyond this value, the adsorption rate decreases sharply. This result indicates that the adsorption corresponds to physisorption one as at high temperature, Van der Walls interactions between the Aliquat and the material are diminished. 4. Conclusions The plasma treatment can modify the physicochemical properties of polypropylene fibers as the grafting of amine, oxidized or fluorinated

1,2

Adsorption rate (mmol/g)

The rates of the adsorption kinetics of the Aliquat onto such plasmatreated fibers are low, similar to those obtained with the control fiber (Fig. 5). These results give an idea of the organization of the adsorbed layer that is not due to hydrophobic interactions with its alkyl chains. Even if improving the adsorption kinetics onto N2 plasma-treated meshes or film (Fig. 6), the phenomenon is emphasized when the substrate material has a high surface area. This data, given in Table 2 [24,25], is doubling in a similar manner either in case of N2 plasma or CO2 plasma. N2/H2 plasma-treatment gives a lower specific surface area but nonetheless larger than that of the virgin PP. For the targeted applications with this type of material, aging and recovery of the hydrophobicity are the key-parameters. These are understood through the water contact angle measures for different periods of storage of PP, PP-N2, PP-CO2 and PP-N2/H2 up to 60 days in air atmosphere. Rapid recovery (results not shown here) is generally observed with these polypropylenes, reaching a plateau at 70–80° after a month. CO2 plasma-treated shows the faster decay. In any case, the almost complete recovery is associated to the turn-over phenomenon of surface functional groups inside the material bulk. However, the aging may occur if any weak boundary layer is formed during the plasmatreatment. In that case, storage duration and dipping the fibers in the Aliquat solution will removed this layer and therefore, will decrease their adsorption capacity. Indeed, after 20 days of storage, their adsorption capacity was decreased and reached the value of virgin PP one. If the weak boundary layer formation is negligible, the adsorption

Adsorption rate (mmol/g) Number of assays

1,0 0,8 0,6 0,4 0,2 0,0

0 0

10

20

30

40

50

60

70

80

90

100

water contact angle (°) Fig. 7. Dependence of the Aliquat adsorption rate versus water contact angle onto virgin, plasma-treated PP fibers.

0,00

0,02

0,04

0,06

0,08

initial concentration (mol/L) Fig. 8. Dependence of the Aliquat adsorption rate onto virgin, N2 plasma-treated PP fibers versus its concentration.

N. Hachache et al. / Materials Science and Engineering C 35 (2014) 386–391

Adsorption rate (mmol/g)

0,55

0,50

0,45

0,40

0,35 0

10

20

30

40

50

Temperature (°) Fig. 9. Dependence of the Aliquat adsorption rate onto virgin, N2 plasma-treated PP fibers versus the temperature.

groups is effective. These plasma-modified fibers then have a more substantial adsorption capacity, particularly those modified with N2, N2/H2 and CO2 plasma. The adsorption of quaternary ammonium such as the Aliquat 336 seems to be governed by the affinity between the hydrophylic groups of both the adsorbate and the substrate; but remains highly sensitive to temperature proving there a physisorption rather chemisorption. Although the surface properties of the plasmatreated fibers are altered over storage time, the adsorption capacities remain over a period of over 20 days and the same fiber can be used several times. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2013.11.025. References [1] O. Kebiche-Senhadji, S. Tingry, P. Seta, M. Benamor, Selective extraction of Cr(VI) over metallic species by polymer inclusion membrane (PIM) using anion (Aliquat 336) as carrier, Desalination 258 (2010) 59–65. [2] A.H. Blitz-Raith, R. Paimin, R.W. Cattrall, S.D. Kolev, Separation of cobalt(II) from nickel(II) by solid-phase extraction into Aliquat 336 chloride immobilized in poly(vinyl chloride), Talanta 71 (2007) 419–423. [3] J. Konczyk, C. Kozlowski, W. Walkowiak, Removal of chromium(III) from acidic aqueous solution by polymer inclusion membranes with D2EHPA and Aliquat 336, Desalination 263 (2010) 211–216.

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Improvement of the adsorption of quaternary ammonium on polypropylene affinity membrane through the control of its surface properties.

Polypropylene fiber meshes were plasma-treated in order to attach new chemical functions corresponding to acidic or basic groups without altering the ...
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