Separation of organic substances with thermoresponsive polymer hydrogel

Polymer Gels and Networks 2 (1994) 315-322 ~) 1994 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0966-7822/94/$07-00 ELSEVIER

Separation of Organic Substances with Thermoresponsive Polymer Hydrogel Hisao Ichijo, Ryoichi Kishi, Okihiko Hirasa National Institute of Materials and Chemical Research, 1-1 Higashi Tsukuba, Ibaraki 305, Japan

& Yasuhiro Takiguchi Mikasa Pharmaceuticals Co. Ltd, 2-3-1 Toyotamakita, Nerima, Tokyo 176, Japan

ABSTRACT Thermo-responsive polymer gel, poly(vinyl methyl ether) porous gel, was applied to the separation of organic substances in aqueous solution. Non-ionic surfactants were thermo-reversibly separated by the change of hydrophilic/hydrophobic balance of the gel. The separation technology may contribute to energy saving, environmental protection, etc.

INTRODUCTION There are a variety of materials and methods available for separating organic substances. However, there are some problems, for example, it is rather expensive to regenerate some materials and some methods require a great amount of energy. In the field of environmental protection and waste water treatment, the ionic surfactants in waste water are easily recovered with polymer flocculants, while it is more difficult to remove non-ionic surfactants. There have been some reports in which stimuli-responsive polymer gels have been applied to reversible separation processes. It was reported that temperature-sensitive gels could be used to extract water and low molecular weight solutes from macromolecular solutions. 1 Gel permeation chromatography of dextran was carried out by using a column packed with porous glass beads modified with temperature-responsive polymers.2 Proteins were reversibly adsorbed on the microspheres of thermo-responsive hydrogels.3 The surface properties of thermo-responsive hydrogels were found to depend on both the molecular structure of the copolymer and the thermal stimulus through adsorption of benzoic acid and phthalic acid. 4 Cell culture substrates were grafted with thermo-responsive polymers to examine the possibility of switching cell adhesion to cell detachment by changing culture temperature. 5 Slurries of different properties were dewatered quite well by applying 315

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H. Ichijo, R. Kishi, O. Hirasa, Y. Takiguchi

hydrogels swollen or shrunk reversibly in response to temperature. 6 However, little research has been carried out from the viewpoint of separating organic substances. Poly(vinyl-methyl-ether) (PVME) is one of the well known thermoresponsive polymers and undergoes phase transition at around 38°C. PVME is fully hydrated below this temperature, and becomes dehydrated, aggregates and precipitates above the temperature. PVME is easily crosslinked into hydrogel by y-ray irradiation. PVME gel is hydrated and swells at room temperature, and collapses with a temperature increase. Thus, a rise in temperature causes the increase in hydrophobicity of the gel, which leads to reversible adsorption/desorption controlled by slight temperature change. In this work, an attempt was made to develop a new separation technology by using porous PVME gel to separate organic substances, especially to recover non-ionic surfactants in aqueous solution.

EXPERIMENTAL PVME aqueous solution (30 wt %) was purchased from Tokyo Kasei Co. Ltd and was employed without further purification. Non-ionic surfactants [poly(oxyethylene nonylphenyl ether): C9H,9-~(OCH2CH2)~--OH NP - 5: n = 10, NP = 10: n = 20, N P - 20] n = 5, were purchased from Tokyo Kasei Co. and were used as an adsorbate. PVME was crosslinked by y-ray irradiation with the dosage of 150 kGy (dose rate 10-03kGy/h). During y-ray irradiation, the temperature was increased by about 5°C/h. Both cross-linking and micro phase separation simultaneously occurred under the condition, which led to the formation of a spongy structure in PVME hydrogel.7 The porous PVME gel was lyophilized for 24 h, after being washed with hot and cold water repeatedly to remove uncrosslinked polymer. About 200 mg of the lyophilized PVME gel was left in 20 ml of distilled water for 24 h for swelling. The swollen gel was cut into small square pieces which were used as a thermo-reversible adsorbent. The PVME pieces were put in the 25 ml of non-ionic surfactant solution kept at a given temperature and the surfactant concentration of the solution was measured with high performance liquid chromatography (HPLC) at given time intervals. PVME was also crosslinked in glass capillaries of 1.8 mm in inner diameter for observing the swelling behavior of PVME porous gel through a microscope. The degree of swelling was defined as the ratio of the gel diameter to the inner diameter of the glass capillary. The thermal properties of PVME gel were measured by differential scanning calorimetry (DSC). The gel sample was placed in a silver cell (70/xl) and calorimetric measurement was carried out with SSC5200H (Seiko Instrument & Electric Ltd).

Separation of organic substances with polymer hydrogel

317

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O

0

1

0.5

A

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20

30

40

50

60

70

Temperature / °C

Fig. 1. Swelling behavior of PVME gel in NP-10 solution. NP-10 conc.: O, 0 mg/l:A, 20 mg/l:rq, 400 mg/l.

RESULTS AND DISCUSSION

Swelling behavior of PVME gel in non-ionic surfactant solution PVME hydrogel in water collapses with a temperature increase and the gel size is reduced to the minimum at 38°C. The swelling behavior of PVME gel in non-ionic surfactant solution (NP-10) is almost the same as that in water, regardless of its concentration (see Fig. 1). The temperature corresponding to an endothermic peak of DSC is 38°C and is also independent of the NP-10 concentration. These results are in fair agreement with the data from microscopic observation of gel size.

Thermo-reversible adsorption of non-ionic surfactants on PVME gel Since PVME gel is dehydrated and becomes hydrophobic with a rise in temperature, the hydrophobic interaction between PVME and organic substances (non-ionic surfactant) also increases with temperature. The interaction greatly depends on temperature. NP-10 is adsorbed slightly on PVME gel at a low temperature and is well adsorbed at high temperature (see Fig. 2). But, if the temperature is lowered again, the adsorbed surfactant is released from the gel into solution. The adsorption is hence thermo-reversible. When PVME aqueous solution (1.0wt %) is heated, the transmittance of the solution is

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H. Ichijo, R. Kishi, O. Hirasa, Y. Takiguchi 60

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reduced to 50 % at around 32°C. It was reported that the relaxation time of water for 30 wt % PVME aqueous solution measured by dielectric relaxation was shorter than the time for pure water at low temperature. 8 However, the relaxation time for the PVME greatly increases to the value for pure water, when temperature is raised over 30°C. This suggests that the hydrophilic/hydrophobic balance of PVME would be shifted to the hydrophobic side at around 30°C. In the course of dehydration, the water molecules attracted to a methyl group of PVME by hydrophobic interaction are first released from the polymer by thermal energy, then the water bound to oxygen in the ether bond is released. 8 Since PVME is dehydrated and becomes hydrophobic at a temperature lower than the transition point where PVME gel collapses completely, NP-10 would be greatly adsorbed on the gel at around 30°C. PVME adsorbs NP-10 with a temperature increase as shown in Fig. 2. The data qualitatively agree with the shrinkage of PVME gel shown in Fig. 1. Both the adsorption and the size change are due to the dehydration and hydrophobicity of PVME. The amount of NP-10 adsorbed on PVME gel increases a little with temperature between 10 and 20°C, then increases significantly at around 30°C. The value is kept almost the same over the transition temperature. The amount of NP-10 adsorbed on the gel increases with initial NP-10 concentration. When the initial concentration of NP-10 is increased by about 10 times, the NP-10 adsorbed at 10 and 20°C is doubled at equilibrium. But the amount of adsorption at a temperature higher than 30°C is increased by six times. The adsorption is not caused by the pumping effect associated with the swelling and shrinking of the gel, because the swelling behavior of the gel is not dependent upon NP-10 concentration as shown in Fig. 1. A plot of NP-10 adsorbed on PVME gel against NP-10 concentration at

Separation of organic substances with polymer hydrogel

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equilibrium is found to be linear (see Fig. 3). The curve in Fig. 3 has a shape characteristic of Freundlich adsorption isotherms. The amount of NP-10 adsorbed on porous PVME gel increases and decreases in response to temperature change and reaches an equilibrium state as the change of hydrophilic/hydrophobic balance of PVME is thermoreversible. PVME gel repeats the adsorbing and desorbing of NP-10 in response to stepwise temperature change between 20 and 50°C (see Fig. 4). Water molecules are released from PVME with a temperature increase, at the same time the polymer molecules are associated and change their conformation. Because NP-20 carries a long polyoxyethylene unit, it is more hydrophilic than NP-10 and would be less accessible to PVME molecules. On the other hand, NP-5 has a short polyoxyethylene unit, and is more hydrophobic and more accessible to PVME molecules than NP-10. PVME becomes hydrophobic and adsorbs NP-10 well over 30°C (see Fig. 2). Therefore, NP-20 is adsorbed a little even at a temperature higher than 30°C. NP-5 is more easily adsorbed on PVME gel at a higher temperature than NP-10; all the surfactants are adsorbed a little at low temperature. Finally, NP-5 is adsorbed the most, NP-10 the second and NP-20 is the least adsorbed on the gel (see Fig. 5). When PVME gel is added to the mixture of NP-10 and NP-20 solutions kept at 50°C, NP-10 is selectively adsorbed on the gel (see Fig. 6). Before adsorption starts, two peaks on the chromatogram for the mixture indicate NP-10 and NP-20, respectively. After adsorption for 24 h, the peak for NP-10 in the mixture is reduced more than that for NP-20 is. The gel is put into distilled water at 10°C, then the adsorbed NP-10 is released from the gel. Therefore, the chromatogram for the aqueous solution shows only one peak for NP-10, which indicates that NP-10 is separated from NP-20 with a

H. Ichijo, R. Kishi, O. Hirasa, Y. Takiguchi

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Fig. 4. NP-10 adsorbed on PVME gel in response to stepwise temperature change between 20 and 50°C. NP-10 conc.: 160 mg/l.

2

0

=

0

l

10

=

I

20

,

I

I

I

30

40

50

60

Temperature / °C

Fig. 5. Dependence of adsorption of three surfactants (NP-5, NP-10, NP-20) on temperature. Surfactant cone.: 20 rag/l; 1"7,NP-5; O, NP-10; A, NP-20.

Separation of organic substances with polymer hydrogel

321

O

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Q. Z

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Mixture of NP-10 and NP-20 before adsorption

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thermo-responsive hydrogel. When PVME gel is left in the mixture of NP-5 and NP-20 solutions, NP-5 is selectively adsorbed on the gel and separated from NP-20. It is concluded that the thermo-responsive polymer gel, porous PVME hydrogel, can be applied to the separation or recovery of organic substances in aqueous solution. Hot water at 40-50°C is currently drained off as a waste product in many

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factories and offices. Separation with the thermo-responsive gel therefore makes efficient use of unused waste energy and may contribute to environmental protection. REFERENCES 1..Freitas, R. F. S. & Cussler, E. L., Temperature sensitive gels as extraction solvent. Chem. Engng Sci., 42(1) (1987) 97-103. 2. Gewehr, M., Nakamura, K., Ise, N. & Kitano, H., Gel permeation chromatography using porous glass beads modified with temperature-responsive polymers. Makromol. Chem., 193 (1992) 249-56. 3. Kawaguchi, H., Fujimoto, K. & Mizuhara, Y., Hydrogel microspheres 3. Temperature-dependent adsorption of proteins on poly-N-isopropylacrylamide hydrogen microspheres. Coll. Polym. Sci., 270 (1992) 53-7. 4. Seida, Y., Nakano, Y. & Ichida, H., Surface properties of temperature-sensitive N-isopropylacrylamide-copolymer gels. Kagakukohgaku Ronbunsyu, 18(3) (1992) 346-52. 5. Yamada, N., Okano, T., Sakai, H., Karikusa, F., Sawasaki, Y. & Sakurai, Y., Thermo-responsive polymeric surfaces; control of attachment and detachment of cultured cells. Makromol. Chem. Rapid Commun., U (1990) 571-6. 6. Huang, X., Akehata, T., Unno, H. & Hirasa, O., Dewatering of biological slurry by using water-absorbent polymer gel. Biotechnol. Bioengng, 34 (1989) 102-9. 7. Hirasa, O., Ito, S., Yamauchi, A., Fujishige, S. & Ichijo, H., Thermoresponsive polymer hydrogel. In Polymer Gels, ed. D. DeRossi, K. Kajiwara, Y. Osada & A. Yamauchi. Plenum Press, New York, 1991, pp. 247-56. 8. Maeda, H., Phase transition of thermo-reversible polymer in solution. Polym. Prepr. Jpn, 38 (1989) 3732-4.