Miniemulsion polymerization of cyclodextrin nanospheres for water purification from organic pollutants

European Polymer Journal 46 (2010) 1671–1678

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Miniemulsion polymerization of cyclodextrin nanospheres for water purification from organic pollutants Eti Baruch-Teblum a, Yitzhak Mastai a,*, Katharina Landfester b,** b

Department of Chemistry and the Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

a r t i c l e

i n f o

Article history: Received 18 January 2010 Received in revised form 6 May 2010 Accepted 16 May 2010 Available online 1 June 2010 Keywords: Miniemulsion polymerization Water purification Cyclodextrin polymeric nanospheres Water–solid interfaces Sorption experiments

a b s t r a c t The pollution of groundwater and wells has become an environmental and economic hazard as a result of waste spillage, and industrial applications such as pesticides in agriculture. Conventional treatment techniques such as sand filtration, sedimentation, flocculation, coagulation, chlorination, and activated carbon are not very effective in reducing the concentration of the organic pollutants in the presence of dissolved organic matter. The objective of the current work is to design an efficient technology for water purification from organic contaminants by a new class of polymeric nanospheres based on cyclodextrins as building blocks. We synthesized a series of cross-linked cyclodextrin polymeric nanospheres of different sizes by a unique method, miniemulsion polymerization. These cyclodextrin nanospheres exhibit a high ability to absorb aromatic organic molecules such as toluene and phenol. Sorption experiments in solutions with high concentrations of corresponding organic molecules show a high adsorption capacity. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Water purification is essential to human health and also plays a critical feed-stock in a variety of key industries, including electronics, pharmaceuticals, and food. The contamination of water is a widespread problem as a result of industrial and agricultural pollution. The removal of organic contaminants such as chlorinated hydrocarbons [1] and gasoline additives [2] from water is an important issue. One of the most challenging tasks is how to remove from water organic molecules that typically have little affinity to most chemical compounds. Existing water purification processes use high surface area materials such as activated carbon [3,4]. Many of these materials have some affinity to organic compounds, but fail to remove contaminants to a suitable degree of low part-per-billion levels [5–8]. Zeolites [9], for example, have a well-defined porosity, but show little affinity to organic compounds in water, and * Corresponding author. Fax: +972 3 738053. ** Corresponding author. E-mail address: [email protected] (Y. Mastai). 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.05.007

therefore have been proven as ineffective. Moreover, both activated carbons and zeolites absorb moisture from air and lose their effectiveness. Other conventional water purification processes involve either filtration through activated carbons or reverse osmosis. As already mentioned, activated carbon fails to remove contaminants to a suitable degree of low part-per-billion levels. Reverse osmosis has been successfully used in many applications such as the desalination of seawater [10–14]. This technology requires high pressure, typically 20–100 bar for small molecules, in order to overcome the higher hydrodynamic resistance produced by a denser membrane. Therefore, the reverse osmosis process consumes a lot of energy in achieving separation efficiency, which is common for all membrane processes. Inexpensive and reusable thermoplastic materials that are promising in the field of chemical separation, and in particular for water purification from volatile organic compounds, were presented by Guerra et al. [15]. In this approach, semicrystalline material based on syndiotactic polystyrene comprising a nanoporous crystalline phase is used to purify water where guest molecules can be

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a

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nanospheres. The synthesis of these nanospheres is performed in a miniemulsion system by chemically modifying an a-cyclodextrin structure by a cross-linked agent (isophorone diisocyanate). It will be shown that it is possible to control the particle size by variations of the cross-linked agent and surfactant parameters. In miniemulsion polymerization, well-defined nanospheres with controlled particle size can be achieved in the presence of low surfactant amounts that are beneficial if the resulting particles are going to be used for coatings or biomedical applications. Small and narrowdistributed miniemulsion droplets are usually created

clathrated. Recently, Park et al. [16] developed a simple, robust, and efficient technology utilizing cheap and recoverable materials based on commercially-available silicone elastomer networks for removing a large variety of organic molecules, including benzene, toluene, ethylbenzene, xylene, and also crude organic oils from water. Recently [17–19], it was shown that organic nanoporous polymers made of cyclodextrin as basic building blocks of the polymer exhibit a high ability to absorb organic contaminants from water. In this paper, we report on the synthesis and characterization of a series of cross-linked cyclodextrin polymeric

N

A O

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HO

O HO

C

O

OH

OH

O OH

O

OH O

HO

HO

HO

OH

OH

IPDI O OH

O

OH

O OH

O

O

Polymerization OH

O

O OH

O

OH

O

O HO

OH HO

O

C

NH

O OH

OH OH

O OH

O NH

OH

O

O

OH

OH

O OH

C

O O

HO

HO

OH

O

HO

α−cyclodextrin

O

O

HO

O

OH

OH

O

HO N C O

OH O

O

O O

HO

OH O

HO HO

O

O HO

O OH

OH OH

O OH OH

O

O

OH

O

O

OH

HO

n

Cyclodextrin cross-linked polymer (Cyclodextrin-isophoronecarbamate copolymer)

B

Fig. 1. (A) Copolymerization of cyclodextrine–isophoronecarbamate copolymer. (B) Principle of the miniemulsion polymerization of cyclodextrin nanospheres.

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2. Experimental part 2.1. Materials

a-Cyclodextrin, isophorone diisocyanate (IPDI), hexadecane (HD), sodium dodecyl sulfate (SDS), toluene and phenol, were purchased from Aldrich and used as received. 2.2. Miniemulsion polymerization procedure The polymer nanospheres were synthesized by chemically modifying an a-cyclodextrin structure by a crosslinked agent in miniemulsion polymerization. The reaction conditions are given in Table 1. A typical recipe is given in the following: various amounts of IPDI, hexadecane, sodium dodecyl sulfate and distilled water were mixed together at room temperature (see Table 1). Next, 100 mg

of a-cyclodextrin, dissolved in a minimum amount of water (ca. 100 lL), was added to the two-phase solution that was obtained. The mixture was homogenized by ultrasonication for 2 min at 68% intensity (Branson sonifier W450 Digital, one half-inch tip) at 0 °C in order to prevent the polymerization of the monomer. Polymerization was carried out at 60 °C under stirring overnight. 2.3. Characterization The hydrodynamic diameter and size distribution of the polymeric nanospheres dispersed in an aqueous phase were measured with Dynamic Light Scattering (DLS) at a low concentration using a Nicomp particle sizer (model 370, PSS, Santa Barbara, CA, USA) at a fixed scattering angle of 90°. Fourier Transform Infrared (FTIR) analysis was used for obtaining information on the composition of the particles. FTIR was performed with a Jasco FT/IR-4200 spectrometer. The samples were measured as pure dried products. Liquid 1H NMR spectra were recorded on a Bruker Advance 400 spectrometer at 400 MHz in D2O as a solvent. High Resolution Scanning Electron Microscopy (HRSEM) images were used for measuring the dry size and size distribution of the particles. HR-SEM was performed with a JEOL JSM7000F field emission scanning electron microscope with an accelerating voltage of 15 kV. The powders were fixed on a copper slide and coated with a thin layer of gold under vacuum. The average size and size distribution of dry polymeric nanospheres were determined by measuring the diameter of more than 200 particles with image analysis software from AnalySIS Auto (Soft Imaging System GmbH, Germany). The particles’ surface area was determined by a Micromeritics (Gemini 2375) analyzer after outgassing the samples at 100 °C. The surface area was calculated from the linear part of the BET plot. Qualitative absorption experiments were performed by UV/vis absorption, recorded in the wavelength range of 190–350 nm at a scan rate of 40 nm/min on a JASCO UV– vis spectrophotometer (V-530) with an optical path length of 1 cm in quartz glass cuvettes. The resolution was equal to 1 nm. Background correction was done by subtracting the UV absorption of fresh deionized water measured in the same cuvette. For toluene and phenol, UV absorption was measured at kmax = 260 and 270 nm, respectively. 3. Results and discussion 3.1. Synthesis and characterization of nanospheres Cyclodextrins are a class of cyclic glucopyranose oligomers with a characteristic toroidal shape that form

Table 1 Synthetic conditions for the preparation of the miniemulsion. Sample

Dispersed phase

A B C D

100 mg 100 mg 100 mg 100 mg

of of of of

cyclodextrin, cyclodextrin, cyclodextrin, cyclodextrin,

Continuous phase 199.5 mg of IPDI, 12 mg of hexadecane 197 mg of IPDI, 13.1 mg of hexadecane 139.4 mg of IPDI, 9 mg of hexadecane 93.7 mg of IPDI, 7 mg of hexadecane

2 g of distilled water 5.6 mg of SDS 20 g of distilled water 50 mg of SDS 20 g distilled water 50 mg SDS 20 g distilled water 50 mg SDS

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from a macroemulsion with a broad droplet size distribution by the application of high shear forces, as ultrasound or high pressure homogenization. The size of the miniemulsion droplets mainly depends on the type and the amount of the emulsifier used in the particular system. In addition to the emulsifier, a costabilizer is required which acts as an osmotic pressure agent within the droplets, counteracting the Laplace pressure and suppressing effectively diffusion processes. The costabilizer has to show a good solubility in the dispersed phase, but at the same time it has to possess a lower solubility in the continuous phase than the major compound of the dispersed phase. In the case of a direct (oil-in-water) miniemulsion, this agent is an (ultra)hydrophobe, and in the case of an inverse (water-in-oil) miniemulsion it represents an (ultra)lipophobe. In Fig. 1, the general process of the miniemulsion process is schematically shown. Overall, it was shown that the miniemulsion technique can be performed with different surfactants, and that droplet sizes with a narrow size distribution in the range of 30– 500 nm can be obtained. The droplets can be regarded as individually acting nanoreactors, suitable for a wide variety of different reactions. It has been shown that organic reactions like esterification and saponification [20,21], crystallization processes [22–25] and sol–gel reactions [26], can efficiently be performed in miniemulsions. In order to obtain particles or capsules, different kinds of polymerizations, such as radical, anionic, [27] and enzymatic polymerization [28], as well as polyaddition [29] and polycondensation [30], can be carried out in the nanodroplets, which permit the formulation of a variety of polymers, copolymers, [31] or hybrid particles that previously have not been synthesized in other heterophase processes.

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well-defined, cone-shaped cavities [32]. These cavities are about 8 Å in depth and 5–9 Å in diameter, depending on the number of oligomers. Cyclodextrins are capable of including various small organic compounds that geometrically suit their tubular cavities. The inclusion relies mostly on hydrophobic interactions between the host, cyclodextrin, and the guest organic molecules. Indeed, long alkyl chains and aromatic compounds such as substituted benzenes were found to form inclusion complexes with cyclodextrins. The interactions between cyclodextrins and organic molecules can be used as a basis for the absorption or separation of various organic agents. However, cyclodextrins are soluble in water, and thus cannot be employed directly in the separation of organics in water. To overcome the solubility issues, cyclodextrins are often immobilized on solid particles as a stationary phase for enantiomer separation [33,34]. Due to these unique properties, we chose cyclodextrin as a new polymeric material that can be used for water purification from organic contaminants. A series of cross-linked cyclodextrin nanospheres (cyclodextrin–isophoronecarbamate copolymers) were prepared via miniemulsion polymerization (see Fig. 1) at various ratios of the cross-linked agent (IPDI). In order to suppress the Ostwald ripening of the droplets (i.e., the monomer diffusion from small droplets to larger ones through the continuous phase), hydrophone hexadecane was used. In addition, the effect of surfactant parameters and the amount of the cross-linked agent on the nanospheres’ size were tested. Table 1 gives a summary of the variety of synthetic conditions that were used. FTIR spectroscopy was used to monitor the polymerization of cyclodextrin–isophoronecarbamate in the miniemulsion system. In Fig. 2, the FTIR spectra of cyclodextrin monomers and of the cyclodextrin nanospheres are shown. The FTIR results show the growth of vibration bands at 3340, 1640, and 1548 cm1, corresponding to the N–H, C@O and NH–CO groups, respectively, confirm the presence of the urethane linkages for all polymeric nanospheres. The presence of absorption peaks at 2250– 2270 cm1 indicates that isocyanate groups have not been

completely blocked. However, there are no bands in the studied spectra at 2130 cm1, which are representative to carbodiimide group. Therefore, these results clearly indicated that the obtained nanospheres contain cross-linked cyclodextrin polymers. In addition, 1H NMR spectroscopy was carried out on the dried nanospheres and shows the formation of cyclodextrin–isophoronecarbamate copolymer. In the 1H NMR spectrum of a-cyclodextrin (Fig. 3A) it can be seen that the signals of all C–H protons are crowded in 5.0 ppm and in the narrow range of 3.9–3.5 ppm. However, in the cyclodextrin–isophoronecarbamate copolymer spectrum (Fig. 3B), we can notice that C–H7 protons signal, which are neighbors to urethane, are presented in the 4.0 ppm range. In addition, there is a clearly change in the signals type in the 3.9–3.5 ppm range, all resulting from the polymerization process. The structural morphology of cyclodextrin nanospheres was studied with HR-SEM. All nanospheres exhibit a spherical shape and low porosity. The HR-SEM images of sample A, B, and D are shown in Fig. 4. These images illustrate that the nanospheres have different sizes, depending on their preparation conditions. In sample A prepared with a low surfactant parameter, the average size is ca. 1100 ± 14 nm. In samples B and D prepared with a high surfactant parameter, the average size of the nanospheres is ca. 225 ± 69 and 200 ± 52 nm, respectively. The cyclodextrin nanosphere hydrodynamic diameters from DLS measurements are summarized in Table 2. The hydrodynamic diameter from DLS is larger than the one measured by HR-SEM for the dry miniemulsion samples. This difference is probably due to the fact that the hydrodynamic diameter also takes into account the surface-adsorbed solvent molecules, as was already reported in a few previous publications [35,36]. The effect of the surfactant parameters can be also ascertained from these results; increasing the SDS parameter as shown in Table 2 leads to a decrease in the diameter and size distribution of the cyclodextrin nanospheres from 1600 ± 50 nm to 216 ± 5.3 nm, 199 ± 7.2 nm and 185 ± 6.6

2270

3340

1640

1548

0.08

Absorbance

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1674

0.04

A 0.00

B 1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber (cm ) Fig. 2. Absorption FTIR spectroscopy of (A) cyclodextrin monomer (B) the cyclodextrin nanospheres.

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OH O

HO

OH

O OH

O

O HO

OH HO

O

O O

O

H7

O

HO

C

NH

H7

O OH

OH OH

O OH

HN C

OH

O

O

O

OH

OH

O OH

o

H7 O

HO

HO

O

OH

O

O

H7 O HO

OH O

HO HO

O

O HO

O OH

OH OH

O OH OH

O

O

OH

OH

n

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HO

O

O

Fig. 3. 1H NMR spectra of a-cyclodextrin (A) and cyclodextrin–isophoronecarbamate copolymer (B).

nm, respectively, (Table 2). In emulsion polymerization any kind of surface-active agent plays a crucial role. The most important aspect of surfactants in the context of emulsion polymerization is their ability to adsorb at interfaces where they lower the interfacial tension and impart stability to the latex particles [37–40]. A similar effect of the surfactant parameters on the diameter and the size distribution of nanoparticles prepared by the emulsion polymerization of 2-methacryloyloxyethyl (2,3,5-triiodobenzoate) and acrylonitrile was previously reported [41,42]. A higher surfactant parameter results in more surfactant molecules adsorbed onto the monomer droplets and the formed nuclei, thereby leading to better electrostatic protection against coalescence and other growing processes. However, it seems that the amount of the cross-linked agent (IPDI)

that was used in our experiments has no significant effect on the size of the nanospheres. The surface properties of the nanospheres were studied by the BET method (see Table 2). As can be seen, cyclodextrin nanospheres have a low BET surface area (2.6– 12.5 m2/g) compared to that of activated carbon [17] (600 m2/g). This implies that organic molecules are not just adsorbed on the surface of cyclodextrin polymers, but are transported into the bulk of the nanospheres. The interactions between the polymer cavities and the organic contaminants strongly depend on their medium [17], i.e., the solvent. A hydrophilic medium (e.g., a water solution) will drive the organic guest molecules into the hydrophobic cavities, while an organic solvent such as ethanol tends to release those organic molecules that were

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Fig. 4. HR-SEM images of (a) sample A, (b) sample B, and (c) sample D.

Table 2 Data of the synthesized cyclodextrin nanospheres. Sample

Mol ratio monomer to cross-linked agent

Microsphere sizes from DLS (nm)

Surface area (m2/ g)

A B C D

1:8 1:8 1:6 1:4

1600 ± 50 216 ± 5.3 199 ± 7.2 185 ± 6.6

2.6 9.2 10.3 12.5

trapped in the nanopores. Based on this phenomenon, we have designed and synthesized cyclodextrin nanospheres that remove trace amounts of organic contamination from water and regenerate them under ethanol rinsing or washing. The mechanism of the removal of organic molecules from water by cyclodextrin nanospheres is completely different from that of activated carbon [43]. Cyclodextrin nanospheres absorb organics into their cavities, whereas activated carbons rely on their high surface area to attract organic molecules. 3.2. Sorption experiments Sorption experiments were performed by simply immersing the cyclodextrin nanospheres in an aqueous solution containing corresponding organic molecules, with stirring. The number of organic molecules absorbed by the

cyclodextrin nanospheres was determined by UV–visible spectroscopy. Sorption experiments were carried out as follows: toluene and phenol aqueous solutions (20 mL) were freshly prepared in a mixture of deionized water/ethanol (90:10). To determine the equilibrium adsorption capacity of cyclodextrin nanospheres, 10 mg of the nanospheres were placed in a flask containing solutions of several concentrations, from 10 to 100 mg L1. After 24 h the samples were centrifuged for 10 min at 5000 rpm, and the adsorbate equilibrium concentration was measured using a JASCO UV–visible spectrophotometer. The specific adsorbed amount (qe) of the organic chemical on the cyclodextrin nanospheres was calculated from the initial and equilibrium concentrations according to the following equation:

qe ¼ VðC 0  C e Þ=m

ð1Þ

where C0 and Ce are the initial and equilibrium liquidphase concentrations (mg L1) of adsorbates; V is the volume of the solution (L); and m is the amount of adsorbent (g). The amount of organic pollutant adsorbed on the sample was determined by the difference between initial and final solution concentrations using a standard calibration curve of the blank phenol and phenol solutions. The results of the adsorption experiments of organic pollutants on the cyclodextrin nanospheres samples B and D are shown in Fig. 5.

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A

Amount absorbed q e (mg/Kg)

2,800

Sample D

2,600 2,400

Sample D

A

2,000 1,500 1,000

B

500 0 0

2

4

6

8

10

12

14

16

18

20

22

24

Time (hour) Fig. 6. Kinetic measurements of adsorption of (A) toluene (B) phenol on sample D of the cyclodextrin nanospheres.

kinetics is slower in comparison to toluene and maximum adsorption is attained after 14 h. Furthermore, as mentioned above, the regeneration of the saturated cyclodextrin nanospheres is an important issue. In a series of experiments, cyclodextrin nanospheres loaded with toluene and phenol were regenerated in an ethanol solution for 24 h, after which the cyclodextrin nanospheres were collected by centrifugation, washed three times with a water–ethanol (50:50) solution, and dried for 48 h at 50 °C. A set of absorption measurements was carried out on the regenerated samples. The cyclodextrin nanospheres exhibited a slightly lower absorption loading, ca. 10%.

2,200 2,000

4. Conclusions

1,800 1,600 1,400 1,200 1,000 800 600 0

Amount absorbed qe (mg/Kg)

2,500

1,300 1,250 1,200 1,150 1,100 1,050 1,000 950 900 850 800 750 700 650 600 550 500

10

20 30 40 50 60 70 80 90 100 110 120 Equilibrium concentration of Toluene solution (mg/L)

Sample D

B

Sample B

10

20

30

40

50

60

70

80

90 100 110 120 130

Equilibrium concentration of Phenol solution (mg/L) Fig. 5. Equilibrium data of adsorption of (A) for toluene and (B) for phenol by the synthesized cyclodextrin nanospheres.

In this work, we synthesized a series of cyclodextrin nanospheres with different sizes by miniemulsion polymerization. These nanosphere cyclodextrins exhibit a high ability to absorb aromatic organic molecules from water. The cyclodextrin cavities provide a hydrophobic environment, and hence, generate the strong affinity of the organic molecules at water–solid interfaces. The absorption process at water–nanosphere–water interfaces effectively separates organic components from water by forming inclusion complexes. These particles are suitable for the removal of a large variety of organic contaminants, mainly aromatic rings such as toluene and phenol. The regeneration of the absorbing particles is achieved by treating them with organic solvents such as ethanol. The importance of these results is that hazardous organic contaminants may be reduced to very low levels in water by these nanosphere particles. The advantage that polymeric nanospheres have over inorganic materials is that they offer flexibility in processing, and can be fabricated into granular solids, powders, thin films, and possibly smart membranes. Such flexibility enables these materials to be used for multiple applications and formats, thereby accommodating different water-treatment configurations and needs. We have to mention that the overall sorption performance of our cyclodextrin nanospheres is

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3,000

3,000

Amount absorbed q e (mg/Kg)

The results of these measurements show that the sorption behavior is approximately linear, namely, the sorption capacities increased with increasing equilibrium concentration. This increase in the loading capacities is due to the interaction between organic compounds and cyclodextrin nanospheres. A comparison between the adsorption isotherms shows that there is no significant difference in performance between the sorption of samples B and D. However, it should be mentioned that for phenol the removal efficiency is low in relation to the relatively high solubility of phenol in water, in comparison with toluene. For evaluating the applicability of these cyclodextrin nanospheres as water purification materials, two more parameters should be considered, the kinetics of the sorption process and the possibility of regenerating saturated materials. For kinetics, the measurement of identical experiments on sorption samples, as described above, was performed for sample D at maximum equilibrium concentrations of the organic pollutants (100 mg L1). In these experiments, 1 mL from the solutions was collected, centrifuged (5000 rpm, 10 min), and the concentrations of the organic pollutants in the solutions were measured. Samples were collected at time intervals of 2 h. The kinetics of cyclodextrin nanosphere adsorption is illustrated in Fig. 6. As can be seen from Fig. 6, the initial adsorption rate for toluene is rapid, and within 8 h maximum adsorption loading is achieved. For phenol solutions, the adsorption

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low in comparison to other materials such as activated carbon or zeolites. But on the other hand our cyclodextrin nanospheres have few advantages compared to activated carbon such as: they do not absorb moisture from air and lose their effectiveness and can be easily regeneration. Moreover, the success of this method may lead to the development of generic technologies which could further be a part of industrial usage. Acknowledgments The authors wish to thank the German-Israeli Cooperation in Water Technology Research for their financial support. E. Baruch-Teblum would like to acknowledge the Bar-Ilan President’s Ph.D. Scholarship Foundation.

MACROMOLECULAR NANOTECHNOLOGY

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