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Effect of Dye Structure on the Interaction between Organic Flocculant PAN-DCD and Dye Ying Yu* Chemistry Department, Central China Normal University, Wuhan 430079, P.R. China
Yuan-Yi Zhuang and Ying Li College of Environmental Science and Engineering, Nankai University, Tianjin 300071, P.R. China
Ming-Qiang Qiu Riyue-Yongli Battery Ltd. Co., Wuhan 430034, P.R. China
On the basis of the experimental methods of flocculated decolorization and equilibrium dialysis, the effect of dye structure on the interaction between organic flocculant PAN-DCD and dye has been explored. It was found that, under acidic conditions, PAN-DCD is effective in the decolorization of dyes with sulfonic acid groups and carboxyl groups and that the number of these groups has something to do with decolorization effectiveness. The more acidic groups in the dye, the greater the interaction extent between the flocculant and the dye and the higher the decolorization efficiency (DE) of the dye. Meanwhile, the hydrophobic groups in dyes also affect the binding of the dyes by PAN-DCD. The more hydrophobic groups in the dye, the higher the DE of the dye. The bigger the hydrophobic group in the dye, the more intensive the interaction of the dye with the floculant. Furthermore, amino and hydroxyl groups in dyes are related to the DE of the dyes as well, according to the formation of a hydrogen bond with the groups in the macromolecule. Therefore, the process of flocculated decolorization, resulting from the interaction between dye molecules and the flocculant, is controlled by both energetic and hydrophobic interactions. Introduction One of the major problems concerning dye wastewater is colored effluent. The color is objectionable aesthetically, and it also reduces light penetration into the water, thereby decreasing the efficiency of photosynthesis in aquatic plants and, hence, has an adverse impact on their growth.1 In addition, some dyes might be toxic to various organisms.2,3 Therefore, color removal from dye wastewater is a major environmental problem. Flocculants and coagulation methods have been extensively used in industry and environmental protection for their simple operation and cheaper treatment costs. As for the dye wastewater with high color and high COD value, it is difficult for general flocculants to deal with. The novel organic flocculant, synthesized with polyacrylonitrile (PAN) and dicyandiamide (DCD), is the polymer that has several kinds of groups in its side chain4,5 and is effective in the decolorization of dye wastewater.6,7 Thus, PAN-DCD is of high value for applied practices. The phenomenon of the flocculated decolorization of dyes is essentially attributed to the interaction between the flocculant and dye. The study of the interaction between organic polymer and dyes began with the investigation of the binding of methyl orange (MO) by polyvinylpyrrolidones in the 1970s8 with the method of * To whom correspondence should be addressed. Fax: 8627-87646490. E-mail:
[email protected]. Phone: 86-2787851742.
equilibrium dialysis. During these years, the research work in this field has been gradually improved. The ways of binding of dyes by polymers include hydrophobic,9-13 energetic,14,15 and cooperation interactions.16-18 Hydrophobic interaction originates from the interaction between apolar groups. Energetic interaction results from polar bonds between molecules. When the integration of hydrophobic and energetic interaction reaches a maximum, cooperation interaction occurs. Concerning the interaction between organic flocculant PAN-DCD and dyes, there are few reports.6 We have studied the interaction in detail7 and found that there are energetic and hydrophobic interactions between them. The amino group in the flocculant reacts with the sulfonic acid in the dye molecules, generating -NH3+SO3- -, -NH2+SO3- -, or dNH+SO3-- in acidic conditions. In addition, a small amount of hydrogen bonding is formed between the molecules. Meanwhile, hydrophobic interaction is also significant in the binding of dyes by the flocculant. Through energetic and hydrophobic interactions, the dye molecules intensively bind with the flocculant,19 leading finally to color removal. Water-soluble dyes are not easily dealt with because of their high solubility. Moreover, with the development of synthesis technology, new varieties of water-soluble dyes with different structures appear continuously, which provide difficulties for the choice of a suitable flocculant. If the rule of dyes with different structure decolorized by flocculants can be found, the problem will be closed.
10.1021/ie010745t CCC: $22.00 © 2002 American Chemical Society Published on Web 02/14/2002
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Table 1. Characteristics of the Dyes Used in This Work
name of dye (abbreviation)
classification
Reactive Brilliant Blue KN-R (KN-R) Acid Anthraquinone Blue (AAB) Acid Mordant Grey BS (BS) Reactive Violet K-3R (K-3R) Reactive Black K-BR (K-BR) Reactive Flavine Yellow K-6G (K-6G) Reactive Brilliant Red K-2BP (K-2BP) Reactive Brilliant Red K-2G (K-2G) Reactive Brilliant Orange K-GN (K-GN) Acid Turquoise Blue 2G (2G) Acid Mordant Blue B (AMB) Acid Orange II (O II) Acid Gold Yellow G (AGG) Acid Red B (AB) Acid Red 3R (3R) Acid Black 10B (10B) Acid Brilliant Red X-3B (X-3B) Reactive Red KE-3B (KE-3B) Direct Lightfast Red 4BS (4BS) Direct Orange S (DS) Direct Brown M (DM) Direct Lightfast Yellow RS (RS) Direct Copper Blue 2R (2R) Direct Lightfast Blue B2RL (B2RL) Basic Flavine Yellow O (BYO) Basic Brilliant Blue BO (BO) Basic Magenta (BM)
reactive blue 19 acid blue 25 mordant black 13 reactive violet 2 reactive black 8 reactive yellow 2 reactive red 24 reactive red 15 reactive orange 5 acid black 7 mordant blue 1 acid orange 7 acid yellow 36 acid red 14 acid red 18 acid black reactive red 2 reactive red 120 direct red 81 direct orange 26 direct brown 2 direct yellow 50 direct blue 151 direct blue 71 basic yellow 2 basic blue 7 basic violet 14
On the basis of the problem needing to be solved and the aforesaid research, the effect of groups in dye molecules on the energetic and the hydrophobic interactions between dyes and organic flocculant PAN-DCD was preliminarily investigated in this work. We hope the research will give a basis for the exploration of the rule of dyes with different structure, decolorized by flocculants. To our knowledge, there is no report to this respect. Experimental Section Materials. PAN ([η] ) 147 mL/g in dimethyl sufoxide (DMSO) at 25 °C) was supplied by Fushun Petroleum Corporation. DCD, dimethylformamide (DMFA), potassium hydrogenphthalate, and so on were all analytical reagents. The characteristics of the dyes used are shown in Table 1, and the structures are included in the Appendix. The letters in parentheses are the abbreviations of the dyes. The dyes were all obtained from commercial sources such as Dankong Industrial & Commercial Group Ltd. Co. Organic flocculant PAN-DCD was prepared with the method as described by Golhke et al.4 PAN and DCD were fully mixed with DMFA. When the temperature rose to 80 °C, 40% (w/v) sodium hydroxide solution was added. After being stirred at 100 °C for 4 h, 8% (w/v) hydrochloride acid was added, and stirring continued for an additional 30 min. Then, the orange solution was cooled, neutralized, filtered off, washed with distilled water, and dried at 50 °C. At last, a product with an intrinsic viscosity of [η] ) 56 mL/g (in DMSO at 25 °C) was obtained. Flocculated Decolorization of Dyes. The flocculation effect of PAN-DCD was measured according to the procedure in the literature.20 A 10 mL aliquot of 1% (w/v) flocculant solution was added into 12 beakers (250 mL) with the same 200 mL of 0.1 g/L dye solution. After adjusting the pH from 1∼13, settling, and measuring
color index 61 200 62 055 63 615 18 157 18 972
20 470 15 510 13 065 14 720 16 255 20 470 25 810 28 160 29 150 22 311 29 025 24 175 34 140 41 000 42 595
chemical nature
ionic nature
maximum adsorption wavelength λmax (nm)
anthraquinone anthraquinone anthraquinone metal chelate (single azo) metal chelate (single azo) single azo single azo single azo single azo triphenylmethane triphenylmethane single azo single azo single azo single azo single azo single azo double azo double azo double azo double azo double azo double azo triazo diphenylmethane triphenylmethane triphenylmethane
anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic anionic cationic cationic cationic
604 634 374 552 340 404 570 512 478 640 430 490 440 520 510 615 540 536 505 495 475 402 565 590 426 620 538
dye concentration, the maximum decolorization efficiency (DE %) of different dyes and the corresponding pH ranges were obtained. Different amounts of 1% (w/v) flocculant solution were added into 200 mL of dye solutions with different concentrations to get the effect of flocculant dose on DE % at the most appropriate pH ranges. pH values were adjusted with hydrochloride acid and sodium hydroxide solutions. After being settled for 24 h, the dye concentration in supernatant was determined from absorbance measurements by using a spectrophotometer, type 722, according to the concentration-absorbance standard curves at a respective maximum adsorption wave of different dye solutions. Then, DE % could be calculated
DE % ) (1 - M/M0) × 100% where M and M0 denote the dye mass in the solution after and before flocculation, respectively. Equilibrium of Dialysis. The extent of the binding of the dyes by PAN-DCD was measured by an equilibrium dialysis technique21 in 100 mL of a 0.05 M potassium hydrogenphthalate buffer (pH ) 4.0). The molecular weight cutoff of dialysis membrane was 12 000. A 10 mL aliquot of 1% (w/v) polymer inside a dialysis bag was in equilibrium with 15 mL of the dye solutions of different concentrations for 24 h, after which the dye concentration outside of the dialysis bag did not change. The dialysis bag was almost submerged into the buffer, but its two sides were above the buffer surface. Control tubes contained only buffer inside the dialysis bag. Equilibrium dye concentration was determined by absorbance measurements. Experiments were carried out at 25 and 30 °C. This method is based on the following equation, rearrangement of the Langmuir isotherm, as suggested by Klotz:22
1/r ) 1/nkC + 1/n
K ) nk
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Figure 1. Relationship of flocculant dose and DE of different dyes. Table 2. MDE of the Dyes Flocculated by PAN-DCD and the Corresponding pH Ranges KN-R
AAB
BS
K-3R
K-BR
K-6G
K-2BP
K-2G
K-GN
MDE (%) pH
99.28 4.0-4.1
92.62 3.0-3.1
99.93 3.0-3.1
99.73 2.0-2.1
99.16 3.0-3.1
99.20 3.0-3.1
99.42 3.0-3.1
99.02 3.0-3.1
99.81 4.0-4.1
2G
AMB
O II
AGG
AB
3R
10B
X-3B
KE-3B
MDE (%) pH
97.28 3.0-3.1
98.59 3.0-3.1
96.38 3.0-3.1
96.25 3.0-3.1
99.55 3.0-3.1
98.72 3.0-3.1
99.67 3.0-3.1
99.28 3.0-3.1
99.86 3.0-3.1
4BS
DS
DM
RS
2R
B2RL
BYO
BO
BM
MDE (%) pH
99.57 4.0-4.1
99.78 3.0-3.1
99.34 3.0-3.1
99.92 3.0-3.1
99.75 3.0-3.1
99.93 3.0-3.1
59.16 3.0-3.1
52.43 3.0-3.1
28.39 3.0-3.1
where r is the number of mol of solute bound/105 g of polymer, which directly indicates the binding extent, k is the intrinsic binding constant, K is the first binding constant,9 which also reflects the binding extent, n is the number of binding sites/105 g of macromolecules, and C is the concentration of free dye in solution. Results and Disscusion Flocculated Decolorization of the Dyes. The decolorization effectiveness and the pH range of maximum DE (MDE) of different dyes by organic flocculant PAN-DCD are shown in Table 2. It can be seen that the dyes studied all had MDE under acidic conditions. The decolorization effectiveness of dyes with acidic groups in their molecules was much better than that with Lewis basic groups. For example, BYO, BO, and BM, without a sulfonic acid group and a carboxyl group in their molecules, were hardly decolorized by PANDCD. The reason is that, under acidic conditions, the sulfonic acid group -SO3H, which is a strong acidic group,23 reacted with the amino group in the side chain of PAN-DCD.6 A carboxyl group also could chemically act on PAN-DCD. Meanwhile, there might be hydrogen bonds between dyes and PAN-DCD. Hence, the dyes and PAN-DCD bound intensively with each other by the two kinds of energetic interactions. Moreover, the solubility of PAN-DCD was small in the pH range.5 When PANDCD sedimented, the dyes precipitated and were removed from the water phase together with the polymer, leading to flocculated decolorization. There were more reactive groups in the dyes with acidic groups so that they interacted with PAN-DCD more easily and intensively, resulting in the attainment of MDE of more than 92%. It is also found in Table 2 that the MDE of single
azo dyes was generally lower than that of double azo dyes and that the MDE of a triazo dye was the highest. Figure 1 illustrates the effect of the flocculant dose on the DE of four kinds of 0.1 g/L and 0.2 g/L dye solutions (K-GN, BS, AGG, and 2G) when they were adjusted to an optimum pH range. In Figure 1a, when the dose of PAN-DCD was 0.1 g/L, the DE of K-GN reached more than 97%, the color of the BS solution was almost all removed, but the DE of 2G and AGG was lower. When the dose was 0.4 g/L, the DE of K-GN and BS approached 100% and that of 2G and AGG also increased to 97%. With the continuous increase of the flocculant dose, the DE almost remained constant. Thus, decolorization was not an attribute to the electrostatic interaction resulting from groups having opposite charges between the dyes and the flocculant. A similar rule is shown in Figure 1b, which is the dose effect on the DE of 0.2 g/L dye solutions. The 0.4 g/L dose could entirely decolorize the dye solutions of K-GN and BS. However, entire color removal for AGG and 2G needed a flocculant dose of 1.2 g/L. Under the same conditions, the change trend of DE with the flocculant dose for the dyes was not the same, indicating that it was related to the molecular structure of dyes. It is known that K-GN has four sufonic acid groups, two benzene rings, and a naphthalene ring in its molecular form. BS has a big hydrophobic group, an anthraquinone ring, and two sufonic acid groups. AGG has only one sufonic acid group and three benzene rings, and 2G has the structure of triphenylmethane. It is inferred from Figure 1 that the DE of the dye bore some relation to the number of sufonic acid groups and to the number and the volume of the hydrophobic groups. Effect of Dye Structure on Energetic Interaction. Three dyes, DS, RS, and 2R, have similar molec-
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Figure 2. Effect of the number of anionic groups on the MDE of dyes.
Figure 3. Effect of the aromatic ring on the MDE of dyes.
ular structure, and all have two benzene rings and two naphthalene rings. When the number of sulfonic acid groups in the dye molecules versus the MDE of the dyes is plotted, Figure 2a is obtained. Meanwhile, DM, K-2BP, K-3R, and K-GN also have similar hydrophobic structure; the relationship of the number of acid groups and MDE is presented in Figure 2b. DM has one sulfonic acid group and a carboxyl group, two acid groups in total. Figure 2 apparently indicates that the MDE of the dyes with four sulfonic acid groups was higher than that of the dye with two sulfonic acid groups (Figure 2a) when the hydrophobic structures in the dyes were similar. And, the MDE of the dyes with four acid groups was higher than that with three sulfonic acid groups and with two as well (Figure 2b). Hence, as for the dyes with similar hydrophobic structure, their decolorization effectiveness was dependent on the number of acidic groups. The greater the number of acid groups in the dyes, the higher the MDE of the dyes. Therefore, there was some relationship between the number of acid groups in the dye molecules and the decolorization effectiveness of the dyes. As can be seen from Figure 2a, under the same conditions, the MDE of DS was a little higher than that of 2R. The reason may be that DS and 2R both have -OH, -NH-, and -NH2 in their molecules, which easily form hydrogen bonds with PAN-DCD. The greater the number of groups, the more intensive the interaction coming from hydrogen bonds. The number of groups in DS is four, but that of 2R is three, leading to a 0.03% higher value of the MDE of DS than that of 2R. The similar phenomenon happened to K-3R and K-GN (in Figure 2b). The two dyes both have a similar number of sufonic acid groups and of hydrophobic groups.
However, the number of -OH, -NH-, and -NH2 in K-GN is three, and that of K-3R is two, resulting in a little higher MDE of K-GN than that of K-3R. This suggests that there was a hydrogen bond between PANDCD and the dyes and that the interaction was not predominant. Effect of Dye Structure on Hydrophobic Interaction. The dyes BS, K-BR, and 2G all have two sulfonic acid groups, but the aromatic rings in their structures are different. When different aromatic rings against the MDE of the three dyes is plotted, Figure 3a is obtained. This figure shows that the sequence of the binding of the dyes by PAN-DCD was anthraquinone ring > naphthalene ring > benzene ring. Hence, the decolorization effectiveness of dyes was related to the volume of aromatic rings in the dye structure. The bigger the volume of the aromatic rings, the higher the MDE of the dye. The dyes K-3R, K-GN, K-2G, RS, and B2RL all have four sufonic acid groups, but the number of aromatic rings is not the same. When the number of naphthalene rings in addition to that of benzene rings against MDE is plotted, Figure 3b is obtained. B2RL has four naphthalene rings in its molecular structure, so the MDE was the highest. RS has two naphthalene rings and two benzene rings, resulting in the second highest MDE. K-GN and K-3R have one naphthalene ring and three benzene rings, leading to its lower MDE. Thus, the number of naphthalene rings in the dyes was integrated with MDE, but the effect of the number of naphthalene rings on MDE was not great when the number of aromatic rings was the same. The difference of MDE for K-3R, K-GN, RS, and B2RL was within 0.4. K-2G has one naphthalene ring and two benzene rings, the
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Figure 4. Relationship of 1/r and 1/C for different anthraquinone dyes at 30 °C.
MDE of which was much lower than that of the other four dyes. It is indicated from this data that the number of aromatic rings did something to the MDE of the dyes. The greater the number of aromatic rings, the higher the MDE of the dyes. Therefore, the volume and the number of hydrophobic groups in dyes both affect the binding extent of the dyes by PAN-DCD when dyes have same acid groups in their molecules. Effect of Dye Structure on Interactions. Structure Effect of Anthraquinone Dyes. The relationship of 1/r and 1/C at 30 °C for three anthraquinone dyes, KN-R, BS, and AAB, which have different numbers of sulfonic acid groups and benzene rings, is displayed in Figure 4. Calculating from the slope, the obtained values of the binding constant K8 were 12.2 × 105 for KN-R, 42.3 × 105 for BS, and 1.13 × 105 for AAB, which shows that the binding extent of the three dyes by the flocculant was in the order BS > KN-R > AAB. The result was similar to that obtained in the flocculation experiment. AAB only has one sulfonic acid group, the binding of which was less than that of BS and KN-R, both having two sulfonic acid groups (the reactivity of sulfate vinyl sulfone is similar to that of the sulfonic acid group). This further proves that there was a certain relationship between the decolorization effectiveness of the dyes and the number of sulfonic acid groups in the dyes. The interaction between KN-R and the flocculant was less than that of BS, which was related to the hydrophobic interaction. BS has one more benzene ring than KN-R, resulting in KBS > KKN-R. Thus, for anthraquinone dyes, the binding of them by the flocculant was not only related to the number of sulfonic acid groups but also to the number of hydrophobic groups in the dye molecules. Structure Effect of Different Dyes. The relationship of the binding extent of AGG, 2G, K-GN, and BS by PAN-DCD and free dye concentration at 25 °C is presented in Figure 5. The Y ordinate of the broken line corresponding to BS’s plot is on the right, and that of the solid lines corresponding to AGG, 2G, and K-GNs is on the left. When free dye concentration was the same, the binding extent of the four dyes by the flocculant was different. The order was BS > K-GN > AGG > 2G. Though the four dyes all have the hydrophobic group -C6H5, BS additionally has an anthraquinone ring in its molecular structure, which enhanced hydrophobic interactions. Meanwhile, the sulfonic acid
Figure 5. Relationship of r and log C for different dyes at 25 °C.
group formed a weak chemical bond with the macromolecule. The two kinds of interactions together led to the highest of the binding extent of BS by PAN-DCD. That was why the decolorization effectiveness of BS was the best in the flocculation experiment. Furthermore, K-GN has a naphthalene ring, two benzene rings, and several sulfonic acid groups, which bound more intensive than AGG with PAN-DCD by way of hydrophobic and energetic interactions. 2G and AGG both have benzene rings and two sulfonic acid groups, but the quaternary ammonium ion and -SO3- in the 2G molecule could form an inner salt, which weakened the interaction between PAN-DCD and 2G. The sulfonic acid group in AGG was more active, leading to smaller steric hindrance when acting on the flocculant, resulting in a little greater binding extent than that of 2G. Anyway, the binding extent of AGG and 2G was lower, which reflected that the maximum DE of them should be lower than that of K-GN and BS in accordance with the results in Table 2. Conclusions (1) The MDEs of dyes with sulfonic acid groups and carboxyl groups are higher, while the dyes without an acidic group are hardly decolorized. (2) The energetic interaction is related to the number of acidic groups in dye structure. The greater the number of sulfonic acid groups and carboxyl groups, the greater the binding extent of the dye by the flocculant and the better the decolorization effectiveness of the dye. (3) The hydrophobic interaction depends principally on the number and the volume of the hydrophobic groups in dyes. The greater the number of hydrophobic groups, the greater the interaction between the dyes and PAN-DCD. The bigger the volume, the higher the MDE of the dyes. (4) The process of flocculated decolorization resulted from the interaction between dye molecules, and the flocculant macromolecule is controlled by both energetic and hydrophobic interactions. Acknowledgment The authors thank Dankong Industrial & Commercial Group Ltd. Co. for its financial support of this research. Appendix Structure of Dyes Studied.
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Received for review September 6, 2001 Revised manuscript received December 10, 2001 Accepted December 12, 2001 IE010745T
