- Email: [email protected]
Development of a new series of polyamine-type polymeric surfactants used for emulsion liquid membranes
Journal of Membrane Science 184 (2001) 49–57
Development of a new series of polyamine-type polymeric surfactants used for emulsion liquid membranes Yinhua Wan∗ , Xiangde Wang, Bin Zhu, Xiujuan Zhang Environmental Science Institute, South China University of Technology, Guangzhou 510641, PR China Received 21 January 2000; accepted 3 October 2000
Abstract By summarizing studies of surfactants used for emulsion liquid membrane, a new polyamine-type surfactant called LMA has been developed. This type of surfactant is composed of copolymer of isobutene and isoprene in hydrophobic site and polyethylene polyamine [NH2 (CH2 CH2 NH)n CH2 CH2 NH2 , n ≥ 1] in hydrophilic site. Experimental results show that the characteristics of this surfactant mainly depends on its mean molecular weight and its distribution of molecular weight, and the suitable surfactants are those with number-average molecular weight (Mn ) of 5000–9000 and proper molecular weight distribution (usually M w /M n = 3.0–6.0). © 2001 Elsevier Science B.V. All rights reserved. Keywords: Surfactant; Polyamine; Stability; Swelling; Emulsion liquid membrane
1. Introduction Emulsion liquid membrane(ELM) as a separation technique can be widely used in fractionation of hydrocarbons, environmental engineering, hydrometallurgy, pharmaceutical engineering and biological engineering etc. Recently, the important role of surfactant in ELM has been recognized and a growing interest has been focused on the development of new surfactants suitable for ELM separation processes [1–6]. In our previous studies [1], it was found that polymeric surfactants LMS-1, LMS-2 and LMS-3 developed in our institute were superior to typical commercial surfactants such as Span-80 and polyamine-type surfactant ENJ-3029 in strengthening ∗ Corresponding author. Present address: Department of Engineering Science, Oxford University, Parks Road, Oxford OX1 3PJ, UK. E-mail address: [email protected] (Y. Wan).
the liquid membrane and increasing the stability of the resulting emulsion, and those polymeric surfactants are characterized by their high molecular weight and proper molecular weight distribution. Further studies [5] pointed out that the excellent characteristics of these polymeric surfactants mainly resulted from their high molecular weight and proper molecular weight distribution, and these surfactants could work as mixed surfactants when used in ELMs. Moreover, it was reported [5,7] that surfactants mainly composed of nitrogen having relatively low elecrtonegativity in hydrophilic site might form more stable liquid membrane and cause less swelling ratio, and also match many kinds of mobile carriers very well. Nakashio [4], Ding [8] and Li [9] also found that surfactants with higher molecular weight demonstrated much better properties in ELMs than other surfactants with relatively lower molecular weight in their studies. All these results suggested that it might be possible to further promote the overall performances of
0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 6 1 0 - 4
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Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
surfactant, such as increasing the stability of liquid membrane and decreasing the swelling ratio of emulsion at the same time, by developing polymeric surfactants with high molecular weight, proper molecular weight distribution and hydrophilic group mainly consisting of nitrogen. From this viewpoint, a series of new polyamine-type polymeric surfactants were synthesized in this study and their performances in ELM processes were examined.
2. Design of new polyamine-type polymeric surfactant Based on the above-mentioned analysis, the desired new polyamine-type polymeric surfactant (called LMA) developed in this study have the following structure
(1) where R represents copolymer of isobutylene and isoprene with different molecular weight obtained from the heat degradation of commercial butyl rubber.A series of surfactants with above structure were synthesized according to the following scheme [2]:
kindly provided by Exxon Chemical Co. The kerosene used with a boiling point range of 200–240◦ C was obtained from the distillation of commercial product. Other chemicals were all of reagent grade. 3.2. Synthesis of new polyamine-type polymeric surfactants The synthesis of new polyamine-type polymeric surfactants LMA was conducted as follows: the commercial butyl rubber was firstly placed in an oven and nitrogen gas was employed to maintain an inert atmosphere during the degradation of butyl rubber. The degradation temperature was kept at 280–300◦ C for 1.5–2 h and then the resulting product was dissolved in kerosene with a weight proportion of 1:1. In a 3-mouth flask, the mixture of the degraded rubber solution and maleic anhydride was kept at 200–220◦ C for 6 h in mild agitation. After that, toluene was added to dilute this reaction mixture and some solid materials were removed by filtration. Then, polyamine [NH2 (CH2 CH2 NH)n CH2 CH2 NH2 , n ≥ 1] was added into this solution and refluxed at 130◦ C for 2 h, at the same time water produced in reaction was removed continuously. After residual water and toluene were removed completely under reduced pressure(0.4 bar absolute), a thick liquid with yellow-brown color of LMA product was obtained.
(2)
(3)
3. Experimental
3.3. Operation of emulsion liquid membrane process
3.1. Materials and reagents
Experiments were conducted in a batch-type stirred glass vessel with an 11 cm inner diameter and a 20 cm depth. The bottom of the vessel was fitted with a sampling valve. Stirring was carried out by using an impeller with a 6 cm length and a 1 cm width. A
The commercial butyl rubber (mean molecular weight 520,000) was purchased from the market without further purification. Polyamine ENJ-3029 was
Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
water in oil (W/O) emulsion was prepared by mixing the kerosene containing surfactant and other additives with the internal aqueous phase with vigorous stirring (about 3000 rpm). The volume ratio of the oil phase to the internal aqueous phase (Roi ) was 1:1, and surfactant concentration in the oil phase was 4.0%(by weight). Potassium chloride (10,000 mg/l) was added into the internal aqueous phase as a tracer. The volume ratio of the emulsion to the external aqueous phase (Rew ) was 1:5, and distilled water was used as the external phase. 3.4. Measurements IR spectra were recorded in a RFX-65 type FTIR spectrometer (Amalect Company, USA). The molecular weight and molecular weight distribution were measured by Waters ALC/GPC 244 Liquid Chromatograph. The concentration of K+ was determined by atomic absorption spectrometry.
4. Results and discussion 4.1. Synthesis of surfactant LMA According to the above-mentioned synthetic route, a series of surfactants with triethylene tetramine in hydrophilic site were synthesized. Fig. 1 presents the molecular weight and molecular weight distribution of
51
a typical surfactant synthesized. From Fig. 1, it is concluded that this surfactant has the basic characteristic of polymer — molecular weight distribution, suggesting that this surfactant LMA is a mixture of surfactant molecules with different molecular weight. Therefore, it is feasible to obtain a well-compounded surfactant product by adjusting and controlling the technological conditions in LMA preparation process. In order to monitor reaction process, the IR spectra of the copolymer (degraded butyl rubber), the intermediate product (product of the copolymer reacting with maleic anhydride) and the final product LMA are presented in Figs. 2–4, respectively. Comparisons of characteristic vibrations are shown in Table 1. From Table 1, it is clear that, the synthesized intermediate resulted from the reaction of the copolymer and the maleic anhydride has the characteristic vibration bands of succinic anhydride [10], i.e. the symmetric and asymmetric C=O stretches occurred at 1867 and 1789 cm−1 , respectively, and the stretching vibration of the C–C–O group can also be found at 1070 cm−1 , indicating that the intermediate has the required molecular structure, i.e. the alkenyl succinic anhydride has been synthesized; as for the final product LMA, all the characteristic vibration bands of succinic anhydride disappear, whereas the characteristic vibration bands of succinimide, the bands at 1765 and 1705 cm−1 from the C=O stretching of cyclic imide [10], appear; besides, the bands at 3400–3200 cm−1
Fig. 1. Molecular weight distribution curves of a typical surfactant LMA M n = 6783, M w = 24803, D = M w /M n = 3.66.
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Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
Fig. 2. IR spectra of a typical copolymer (degraded butyl rubber).
from N–H stretching, at 1047 cm−1 from the primary and secondary amines C–N stretching and at 721 cm−1 from the N–H waging [10], though weeker, can also be found, suggesting that the surfactant LMA might have the desired structure listed in Eq. (1) or Eq. (3). Therefore, it could be concluded that the surfactant LMA has been successfully synthesized by following the above proposed synthesis scheme.
Table 2 lists some typical products used in this study. It can be seen in this table that these surfactants have different molecular weight and different molecular weight distribution, and all surfactants have much higher molecular weight as compared with Span-80 and ENJ-3029. It can be expected that they should have different characteristics in ELM separation processes.
Fig. 3. IR spectra of a typical intermediate product.
Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
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Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
Fig. 4. IR spectra of a typical product LMA.
4.2. Stability of ELM prepared with different surfactants Stability of ELM to acid and base are particularly important to the operation of ELM separation processes. In general, a tracer method is used to determine the stability of an ELM, which is evaluated by the break-up ratio of an emulsion (ε). In the present study, KCl is employed as the tracer, the break-up ratio of an emulsion (ε) is calculated from the following equation: ε=
Ve,w Ce,KCl 0 C0 Vi,w i,KCl
× 100%
(4)
where Ve,w and Ce,KCl are the volume of the external phase and the KCl concentration in the external 0 and C 0 phase, respectively; while Vi,w i,KCl represent the initial volume of the internal aqueous phase and the initial KCl concentration in the internal aqueous phase accordingly. Table 3 shows the effect of surfactants on the stability of ELM in different operating conditions. It can be seen that all the synthesized surfactants form more stable ELM than the commercial Span-80 and ENJ-3029, no matter whether, the internal aqueous phase is a strong base or a strong acid. By comparing the experimental results in Table 3, it is clear that the molecular
Table 2 Molecular weight and molecular weight distribution of surfactant products (LMA) No. of surfactant
Number-average molecular weight (Mn )
Weight-average molecular weight (Mw )
Index of molecular weight distribution (D = Mw /Mn )
1 2 3 4 5 6 7 8 9 10 11 12
9215 9150 8987 5672 5534 5492 4594 4487 4413 3360 3284 3170
24051 32488 56798 34996 25899 17959 25083 20909 10326 12163 5976 16421
2.61 3.55 6.32 6.17 4.68 3.27 5.46 4.66 2.34 3.62 1.82 5.18
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Table 3 Break-up ratio ε (%) of ELM prepared with different surfactantsa No of surfactant
1
2
3
4
5
6
7
8
9
10
11
12
Span-80 ENJ 3029
Internal aqueous phase: 5% NaOH solution 0.05 0.02 0.02 0.07 0.08 0.08 0.17 0.16 0.45 0.67 1.12 0.76 4.5 Internal aqueous phase: 5% H2 SO4 solution 0.10 0.05 0.04 0.15 0.13 0.16 0.29 0.32 1.14 1.26 2.39 1.48 1.68 a
0.90 2.45
Mixing time 40 min.
weight distribution (represented by D = M w /M n ) is the main factor affecting the stability when the mean molecular weight is nearly the same, while D is in the range of 3–6, the stability only depends on the mean molecular weight of surfactant.
prepared with most surfactants LMA are much less than those prepared with Span-80 and ENJ-3029. Besides, similar result could be found, i.e. the molecular weight distribution (represented by D = M w /M n ) is the main factor affecting the swelling when the mean molecular weight is nearly the same, while D is in the range of 3–6, the swelling only depends on the mean molecular weight of surfactant, and the suitable molecular weight is in the range of 5000–9000.
4.3. Swelling of ELM prepared with different surfactants Swelling is a common phenomenon in the operation of ELM system. Some papers have dealt with the effects of surfactants on the swelling of ELM and found that swelling ratio varies with surfactants [1–5,7–9]. An ideal surfactant must form an ELM not only exhibiting high stability but also swelling as little as possible in operation. In this study, the swelling ratio of the ELM (ηs ) is calculated by the following equation: ηs =
0 Vi,w − Vi,w 0 Vi,w
+ε
4.4. Selection of hydrophilic groups in surfactants Usually, hydrophilic group is a main factor determining the properties of a surfactant. It is of significance to explore the effect of hydrophilic group of this type surfactant. In the synthesis of this polyamine-type surfactants, their mean molecular weight and molecular weight distribution can be controlled as desired by selecting suitable technological conditions such as reaction temperature and time. A series of surfactants with different hydrophilic groups (M n = 3000–9000, D = 3–6) were synthesized and their properties determined as shown in Table 5. As can be seen in Table 5, when the molecular weight of surfactant is more than 4400 (No. 1–9), the hydrophilic part has no obvious effect on the break-up ratio and swelling ratio of ELMs; while the molecular weight of surfactant less than 4000 (No. 10–12), the break-up ratio and swelling ratio of ELMs increase
(5)
0 is the initial volume of the internal aquewhere Vi,w ous phase, Vi,w is the volume of the internal aqueous phase at time t after mixing operation and ε is the break-up ratio of emulsion. The comparison of swelling ratios of ELMs prepared with different surfactants are presented in Table 4. It is clear that the swelling ratios of ELMs
Table 4 Swelling ratio ηs (%) of ELM prepared with different surfactantsa No of surfactant
1
2
3
4
5
6
7
8
9
10
11
12
Span-80 ENJ 3029
Internal aqueous phase: 8% NaOH solution 6.4 3.6 3.7 8.6 8.5 7.5 12.3 13.5 25.3 29.7 38.4 33.6 98.6 Internal aqueous phase: 5% H2 SO4 solution 8.5 5.6 6.8 10.5 10.2 11.2 16.4 15.8 30.7 40.8 54.3 46.23 58.4 a
Mixing time 60 min.
34.7 50.8
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Table 5 Effects of hydrophilic groups on the properties of surfactantsa No. of surfactant
Mn
D (Mw /Mn )
Hydrophilic part
ε (%)
ηs (%)
1 2 3 4 5 6 7 8 9 10 11 12
9063 9150 9263 5648 5492 5534 4501 4487 4887 3274 3360 3428
4.67 3.55 5.78 5.43 3.27 6.10 3.85 4.66 5.14 4.78 3.62 4.36
NCH2 CH2 NHCH2 CH2 NH2 N(CH2 CH2 NH)2 CH2 CH2 NH2 N(CH2 CH2 NH)3 CH2 CH2 NH2 NCH2 CH2 NHCH2 CH2 NH2 N(CH2 CH2 NH)2 CH2 CH2 NH2 N(CH2 CH2 NH)3 CH2 CH2 NH2 NCH2 CH2 NHCH2 CH2 NH2 N(CH2 CH2 NH)2 CH2 CH2 NH2 N(CH2 CH2 NH)3 CH2 CH2 NH2 NCH2 CH2 NHCH2 CH2 NH2 N(CH2 CH2 NH)2 CH2 CH2 NH2 N(CH2 CH2 NH)3 CH2 CH2 NH2
0.10 0.07 0.06 0.12 0.15 0.16 0.27 0.38 0.34 1.54 1.88 2.34
5.8 7.6 5.4 10.7 10.2 10.4 21.4 22.3 23.2 36.6 40.8 48.7
a
Internal aqueous phase 5% H2 SO4 solution; mixing time 60 min.
with the increasing of the hydrophilic part of surfactant, and surfactant with diethylene triamine in its hydrophilic site has the most less break-up ratio and swelling ratio. According to classic emulsion theory and practice, an ideal surfactant should form an elastic film at the water/oil interface, and the adoption of mixed surfactants favours the preparation of more stable emulsion than the sole use of a single surfactants. It has been found that the combination of different surfactants in ELM system could improve the stability of liquid membrane and extraction efficiency [5]. As mentioned above, surfactant LMA is a mixture of surfactant molecules with different molecular weight, this may be the reason why this type of surfactants has satisfactory overall performances in ELM systems. Detailed studies on the relationship between the composition, structure of surfactant and the performances of resulting liquid membranes have been carried out in our institute and will be reported later. Based on the results mentioned above, it is concluded that a series of new polyamine-type polymeric surfactants with expected structure have been synthesized and these surfactants have satisfactory overall performances in ELM systems, such as high stability, low swelling ratio, and resistance to strong acid and strong base, especially when its mean molecular weight is high up to 5000–9000, its molecular weight distribution in the range of 3.0–6.0 and the hydrophilic group mainly composed of diethylene triamine or triethylene tetramine or tetraethylene pentamine.
5. Conclusions On the basis of summarizing studies of surfactants used for emulsion liquid membranes, a new series of polyamine-type polymeric surfactants LMA were designed and then developed in order to improve the performances of ELMs to meet the requirements in practical uses. The stability and swelling of ELMs prepared with these synthesized surfactants in connection with their composition, molecular weight and molecular weight distribution were examined, and the following results were obtained. 1. The synthesized surfactants had definite molecular weight distributions, hence being mixture of surfactant molecules with different molecular weight. IR spectra analysis demonstrated that these surfactants could be synthesized in the proposed scheme and the surfactant product LMA had the desired structure. 2. As compared with commercial surfactants such as Span-80 and polyamine ENJ-3029, the synthesized surfactants, though affected by their molecular weight and molecular weight distribution, had better performances in ELM systems, such as higher stability, lower swelling ratio and resistance to strong acid and strong base. 3. The characteristics of the synthesized surfactants mainly depended on their molecular weight and molecular weight distribution, and the most suitable surfactants were those with number-average
Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
molecular weight (Mn ) of 5000–9000 and proper molecular weight distribution (usually M w /M n = 3.0–6.0). Acknowledgements This work was financially supported by the Guangdong Provincial Foundation of Natural Science and Technology (Grant No. 950137). References [1] X.J. Zhang, Q.J. Fan, X.T. Zhang, et al., Application of the new surfactant LMS-2 in phenol removal from wastewater by liquid membrane, Environ. Chem. 1 (4) (1982) 320. [2] Y.H. Wan, X.D. Wang, X.J. Zhang, A new type of surfactant used for emulsion liquid membrane and its preparation, Chinese Patent 951,214,373 (1995).
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[3] P.Y. Liu, Y. Chu, Z. Yan, et al., Application of polybutadienebased polymeric surfactant in liquid membrane separation, Sep. Sci. Technol. 30 (12) (1995) 2565. [4] F. Nakashio, M. Goto, M. Matsumoto, et al., Role of surfactants in the behavior of emulsion liquid membranes — development of new surfactants, J. Membr. Sci. 38 (1988) 249. [5] Y.H. Wan, Study on the stability and swelling of surfactant liquid membranes, Ph.D. dissertation, South China University of Technology, Guangzhou, May 1993. [6] Y.H. Wan, X.D. Wang, X.J. Zhang, Treatment of high concentration phenolic waste water by liquid membrane with N503 as mobile carrier, J. Membr. Sci. 135 (2) (1997) 263. [7] N. Yan, Y. Shi, Y. Fu, Permeation and entrainment swelling of W/O/W emulsions, Chem. J. Chin. Univ. 11 (7) (1990) 733. [8] X.C. Ding, Study of the swelling phenomena of liquid surfactant membranes, J. Membr. Sci. 59 (2) (1991) 183. [9] W.X. Li, Y. Shi, Water-permeation swelling of emulsion liquid membrane, Sep. Sci. Technol. 28 (1–3) (1993) 241. [10] B. Smith, IR Spectra Interpretation: A Systematic Approach, CRC Press, Boca Raton, FL, 1999.
Development of a new series of polyamine-type polymeric surfactants used for emulsion liquid membranes Yinhua Wan∗ , Xiangde Wang, Bin Zhu, Xiujuan Zhang Environmental Science Institute, South China University of Technology, Guangzhou 510641, PR China Received 21 January 2000; accepted 3 October 2000
Abstract By summarizing studies of surfactants used for emulsion liquid membrane, a new polyamine-type surfactant called LMA has been developed. This type of surfactant is composed of copolymer of isobutene and isoprene in hydrophobic site and polyethylene polyamine [NH2 (CH2 CH2 NH)n CH2 CH2 NH2 , n ≥ 1] in hydrophilic site. Experimental results show that the characteristics of this surfactant mainly depends on its mean molecular weight and its distribution of molecular weight, and the suitable surfactants are those with number-average molecular weight (Mn ) of 5000–9000 and proper molecular weight distribution (usually M w /M n = 3.0–6.0). © 2001 Elsevier Science B.V. All rights reserved. Keywords: Surfactant; Polyamine; Stability; Swelling; Emulsion liquid membrane
1. Introduction Emulsion liquid membrane(ELM) as a separation technique can be widely used in fractionation of hydrocarbons, environmental engineering, hydrometallurgy, pharmaceutical engineering and biological engineering etc. Recently, the important role of surfactant in ELM has been recognized and a growing interest has been focused on the development of new surfactants suitable for ELM separation processes [1–6]. In our previous studies [1], it was found that polymeric surfactants LMS-1, LMS-2 and LMS-3 developed in our institute were superior to typical commercial surfactants such as Span-80 and polyamine-type surfactant ENJ-3029 in strengthening ∗ Corresponding author. Present address: Department of Engineering Science, Oxford University, Parks Road, Oxford OX1 3PJ, UK. E-mail address: [email protected] (Y. Wan).
the liquid membrane and increasing the stability of the resulting emulsion, and those polymeric surfactants are characterized by their high molecular weight and proper molecular weight distribution. Further studies [5] pointed out that the excellent characteristics of these polymeric surfactants mainly resulted from their high molecular weight and proper molecular weight distribution, and these surfactants could work as mixed surfactants when used in ELMs. Moreover, it was reported [5,7] that surfactants mainly composed of nitrogen having relatively low elecrtonegativity in hydrophilic site might form more stable liquid membrane and cause less swelling ratio, and also match many kinds of mobile carriers very well. Nakashio [4], Ding [8] and Li [9] also found that surfactants with higher molecular weight demonstrated much better properties in ELMs than other surfactants with relatively lower molecular weight in their studies. All these results suggested that it might be possible to further promote the overall performances of
0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 6 1 0 - 4
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Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
surfactant, such as increasing the stability of liquid membrane and decreasing the swelling ratio of emulsion at the same time, by developing polymeric surfactants with high molecular weight, proper molecular weight distribution and hydrophilic group mainly consisting of nitrogen. From this viewpoint, a series of new polyamine-type polymeric surfactants were synthesized in this study and their performances in ELM processes were examined.
2. Design of new polyamine-type polymeric surfactant Based on the above-mentioned analysis, the desired new polyamine-type polymeric surfactant (called LMA) developed in this study have the following structure
(1) where R represents copolymer of isobutylene and isoprene with different molecular weight obtained from the heat degradation of commercial butyl rubber.A series of surfactants with above structure were synthesized according to the following scheme [2]:
kindly provided by Exxon Chemical Co. The kerosene used with a boiling point range of 200–240◦ C was obtained from the distillation of commercial product. Other chemicals were all of reagent grade. 3.2. Synthesis of new polyamine-type polymeric surfactants The synthesis of new polyamine-type polymeric surfactants LMA was conducted as follows: the commercial butyl rubber was firstly placed in an oven and nitrogen gas was employed to maintain an inert atmosphere during the degradation of butyl rubber. The degradation temperature was kept at 280–300◦ C for 1.5–2 h and then the resulting product was dissolved in kerosene with a weight proportion of 1:1. In a 3-mouth flask, the mixture of the degraded rubber solution and maleic anhydride was kept at 200–220◦ C for 6 h in mild agitation. After that, toluene was added to dilute this reaction mixture and some solid materials were removed by filtration. Then, polyamine [NH2 (CH2 CH2 NH)n CH2 CH2 NH2 , n ≥ 1] was added into this solution and refluxed at 130◦ C for 2 h, at the same time water produced in reaction was removed continuously. After residual water and toluene were removed completely under reduced pressure(0.4 bar absolute), a thick liquid with yellow-brown color of LMA product was obtained.
(2)
(3)
3. Experimental
3.3. Operation of emulsion liquid membrane process
3.1. Materials and reagents
Experiments were conducted in a batch-type stirred glass vessel with an 11 cm inner diameter and a 20 cm depth. The bottom of the vessel was fitted with a sampling valve. Stirring was carried out by using an impeller with a 6 cm length and a 1 cm width. A
The commercial butyl rubber (mean molecular weight 520,000) was purchased from the market without further purification. Polyamine ENJ-3029 was
Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
water in oil (W/O) emulsion was prepared by mixing the kerosene containing surfactant and other additives with the internal aqueous phase with vigorous stirring (about 3000 rpm). The volume ratio of the oil phase to the internal aqueous phase (Roi ) was 1:1, and surfactant concentration in the oil phase was 4.0%(by weight). Potassium chloride (10,000 mg/l) was added into the internal aqueous phase as a tracer. The volume ratio of the emulsion to the external aqueous phase (Rew ) was 1:5, and distilled water was used as the external phase. 3.4. Measurements IR spectra were recorded in a RFX-65 type FTIR spectrometer (Amalect Company, USA). The molecular weight and molecular weight distribution were measured by Waters ALC/GPC 244 Liquid Chromatograph. The concentration of K+ was determined by atomic absorption spectrometry.
4. Results and discussion 4.1. Synthesis of surfactant LMA According to the above-mentioned synthetic route, a series of surfactants with triethylene tetramine in hydrophilic site were synthesized. Fig. 1 presents the molecular weight and molecular weight distribution of
51
a typical surfactant synthesized. From Fig. 1, it is concluded that this surfactant has the basic characteristic of polymer — molecular weight distribution, suggesting that this surfactant LMA is a mixture of surfactant molecules with different molecular weight. Therefore, it is feasible to obtain a well-compounded surfactant product by adjusting and controlling the technological conditions in LMA preparation process. In order to monitor reaction process, the IR spectra of the copolymer (degraded butyl rubber), the intermediate product (product of the copolymer reacting with maleic anhydride) and the final product LMA are presented in Figs. 2–4, respectively. Comparisons of characteristic vibrations are shown in Table 1. From Table 1, it is clear that, the synthesized intermediate resulted from the reaction of the copolymer and the maleic anhydride has the characteristic vibration bands of succinic anhydride [10], i.e. the symmetric and asymmetric C=O stretches occurred at 1867 and 1789 cm−1 , respectively, and the stretching vibration of the C–C–O group can also be found at 1070 cm−1 , indicating that the intermediate has the required molecular structure, i.e. the alkenyl succinic anhydride has been synthesized; as for the final product LMA, all the characteristic vibration bands of succinic anhydride disappear, whereas the characteristic vibration bands of succinimide, the bands at 1765 and 1705 cm−1 from the C=O stretching of cyclic imide [10], appear; besides, the bands at 3400–3200 cm−1
Fig. 1. Molecular weight distribution curves of a typical surfactant LMA M n = 6783, M w = 24803, D = M w /M n = 3.66.
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Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
Fig. 2. IR spectra of a typical copolymer (degraded butyl rubber).
from N–H stretching, at 1047 cm−1 from the primary and secondary amines C–N stretching and at 721 cm−1 from the N–H waging [10], though weeker, can also be found, suggesting that the surfactant LMA might have the desired structure listed in Eq. (1) or Eq. (3). Therefore, it could be concluded that the surfactant LMA has been successfully synthesized by following the above proposed synthesis scheme.
Table 2 lists some typical products used in this study. It can be seen in this table that these surfactants have different molecular weight and different molecular weight distribution, and all surfactants have much higher molecular weight as compared with Span-80 and ENJ-3029. It can be expected that they should have different characteristics in ELM separation processes.
Fig. 3. IR spectra of a typical intermediate product.
Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
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Fig. 4. IR spectra of a typical product LMA.
4.2. Stability of ELM prepared with different surfactants Stability of ELM to acid and base are particularly important to the operation of ELM separation processes. In general, a tracer method is used to determine the stability of an ELM, which is evaluated by the break-up ratio of an emulsion (ε). In the present study, KCl is employed as the tracer, the break-up ratio of an emulsion (ε) is calculated from the following equation: ε=
Ve,w Ce,KCl 0 C0 Vi,w i,KCl
× 100%
(4)
where Ve,w and Ce,KCl are the volume of the external phase and the KCl concentration in the external 0 and C 0 phase, respectively; while Vi,w i,KCl represent the initial volume of the internal aqueous phase and the initial KCl concentration in the internal aqueous phase accordingly. Table 3 shows the effect of surfactants on the stability of ELM in different operating conditions. It can be seen that all the synthesized surfactants form more stable ELM than the commercial Span-80 and ENJ-3029, no matter whether, the internal aqueous phase is a strong base or a strong acid. By comparing the experimental results in Table 3, it is clear that the molecular
Table 2 Molecular weight and molecular weight distribution of surfactant products (LMA) No. of surfactant
Number-average molecular weight (Mn )
Weight-average molecular weight (Mw )
Index of molecular weight distribution (D = Mw /Mn )
1 2 3 4 5 6 7 8 9 10 11 12
9215 9150 8987 5672 5534 5492 4594 4487 4413 3360 3284 3170
24051 32488 56798 34996 25899 17959 25083 20909 10326 12163 5976 16421
2.61 3.55 6.32 6.17 4.68 3.27 5.46 4.66 2.34 3.62 1.82 5.18
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Table 3 Break-up ratio ε (%) of ELM prepared with different surfactantsa No of surfactant
1
2
3
4
5
6
7
8
9
10
11
12
Span-80 ENJ 3029
Internal aqueous phase: 5% NaOH solution 0.05 0.02 0.02 0.07 0.08 0.08 0.17 0.16 0.45 0.67 1.12 0.76 4.5 Internal aqueous phase: 5% H2 SO4 solution 0.10 0.05 0.04 0.15 0.13 0.16 0.29 0.32 1.14 1.26 2.39 1.48 1.68 a
0.90 2.45
Mixing time 40 min.
weight distribution (represented by D = M w /M n ) is the main factor affecting the stability when the mean molecular weight is nearly the same, while D is in the range of 3–6, the stability only depends on the mean molecular weight of surfactant.
prepared with most surfactants LMA are much less than those prepared with Span-80 and ENJ-3029. Besides, similar result could be found, i.e. the molecular weight distribution (represented by D = M w /M n ) is the main factor affecting the swelling when the mean molecular weight is nearly the same, while D is in the range of 3–6, the swelling only depends on the mean molecular weight of surfactant, and the suitable molecular weight is in the range of 5000–9000.
4.3. Swelling of ELM prepared with different surfactants Swelling is a common phenomenon in the operation of ELM system. Some papers have dealt with the effects of surfactants on the swelling of ELM and found that swelling ratio varies with surfactants [1–5,7–9]. An ideal surfactant must form an ELM not only exhibiting high stability but also swelling as little as possible in operation. In this study, the swelling ratio of the ELM (ηs ) is calculated by the following equation: ηs =
0 Vi,w − Vi,w 0 Vi,w
+ε
4.4. Selection of hydrophilic groups in surfactants Usually, hydrophilic group is a main factor determining the properties of a surfactant. It is of significance to explore the effect of hydrophilic group of this type surfactant. In the synthesis of this polyamine-type surfactants, their mean molecular weight and molecular weight distribution can be controlled as desired by selecting suitable technological conditions such as reaction temperature and time. A series of surfactants with different hydrophilic groups (M n = 3000–9000, D = 3–6) were synthesized and their properties determined as shown in Table 5. As can be seen in Table 5, when the molecular weight of surfactant is more than 4400 (No. 1–9), the hydrophilic part has no obvious effect on the break-up ratio and swelling ratio of ELMs; while the molecular weight of surfactant less than 4000 (No. 10–12), the break-up ratio and swelling ratio of ELMs increase
(5)
0 is the initial volume of the internal aquewhere Vi,w ous phase, Vi,w is the volume of the internal aqueous phase at time t after mixing operation and ε is the break-up ratio of emulsion. The comparison of swelling ratios of ELMs prepared with different surfactants are presented in Table 4. It is clear that the swelling ratios of ELMs
Table 4 Swelling ratio ηs (%) of ELM prepared with different surfactantsa No of surfactant
1
2
3
4
5
6
7
8
9
10
11
12
Span-80 ENJ 3029
Internal aqueous phase: 8% NaOH solution 6.4 3.6 3.7 8.6 8.5 7.5 12.3 13.5 25.3 29.7 38.4 33.6 98.6 Internal aqueous phase: 5% H2 SO4 solution 8.5 5.6 6.8 10.5 10.2 11.2 16.4 15.8 30.7 40.8 54.3 46.23 58.4 a
Mixing time 60 min.
34.7 50.8
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Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
Table 5 Effects of hydrophilic groups on the properties of surfactantsa No. of surfactant
Mn
D (Mw /Mn )
Hydrophilic part
ε (%)
ηs (%)
1 2 3 4 5 6 7 8 9 10 11 12
9063 9150 9263 5648 5492 5534 4501 4487 4887 3274 3360 3428
4.67 3.55 5.78 5.43 3.27 6.10 3.85 4.66 5.14 4.78 3.62 4.36
NCH2 CH2 NHCH2 CH2 NH2 N(CH2 CH2 NH)2 CH2 CH2 NH2 N(CH2 CH2 NH)3 CH2 CH2 NH2 NCH2 CH2 NHCH2 CH2 NH2 N(CH2 CH2 NH)2 CH2 CH2 NH2 N(CH2 CH2 NH)3 CH2 CH2 NH2 NCH2 CH2 NHCH2 CH2 NH2 N(CH2 CH2 NH)2 CH2 CH2 NH2 N(CH2 CH2 NH)3 CH2 CH2 NH2 NCH2 CH2 NHCH2 CH2 NH2 N(CH2 CH2 NH)2 CH2 CH2 NH2 N(CH2 CH2 NH)3 CH2 CH2 NH2
0.10 0.07 0.06 0.12 0.15 0.16 0.27 0.38 0.34 1.54 1.88 2.34
5.8 7.6 5.4 10.7 10.2 10.4 21.4 22.3 23.2 36.6 40.8 48.7
a
Internal aqueous phase 5% H2 SO4 solution; mixing time 60 min.
with the increasing of the hydrophilic part of surfactant, and surfactant with diethylene triamine in its hydrophilic site has the most less break-up ratio and swelling ratio. According to classic emulsion theory and practice, an ideal surfactant should form an elastic film at the water/oil interface, and the adoption of mixed surfactants favours the preparation of more stable emulsion than the sole use of a single surfactants. It has been found that the combination of different surfactants in ELM system could improve the stability of liquid membrane and extraction efficiency [5]. As mentioned above, surfactant LMA is a mixture of surfactant molecules with different molecular weight, this may be the reason why this type of surfactants has satisfactory overall performances in ELM systems. Detailed studies on the relationship between the composition, structure of surfactant and the performances of resulting liquid membranes have been carried out in our institute and will be reported later. Based on the results mentioned above, it is concluded that a series of new polyamine-type polymeric surfactants with expected structure have been synthesized and these surfactants have satisfactory overall performances in ELM systems, such as high stability, low swelling ratio, and resistance to strong acid and strong base, especially when its mean molecular weight is high up to 5000–9000, its molecular weight distribution in the range of 3.0–6.0 and the hydrophilic group mainly composed of diethylene triamine or triethylene tetramine or tetraethylene pentamine.
5. Conclusions On the basis of summarizing studies of surfactants used for emulsion liquid membranes, a new series of polyamine-type polymeric surfactants LMA were designed and then developed in order to improve the performances of ELMs to meet the requirements in practical uses. The stability and swelling of ELMs prepared with these synthesized surfactants in connection with their composition, molecular weight and molecular weight distribution were examined, and the following results were obtained. 1. The synthesized surfactants had definite molecular weight distributions, hence being mixture of surfactant molecules with different molecular weight. IR spectra analysis demonstrated that these surfactants could be synthesized in the proposed scheme and the surfactant product LMA had the desired structure. 2. As compared with commercial surfactants such as Span-80 and polyamine ENJ-3029, the synthesized surfactants, though affected by their molecular weight and molecular weight distribution, had better performances in ELM systems, such as higher stability, lower swelling ratio and resistance to strong acid and strong base. 3. The characteristics of the synthesized surfactants mainly depended on their molecular weight and molecular weight distribution, and the most suitable surfactants were those with number-average
Y. Wan et al. / Journal of Membrane Science 184 (2001) 49–57
molecular weight (Mn ) of 5000–9000 and proper molecular weight distribution (usually M w /M n = 3.0–6.0). Acknowledgements This work was financially supported by the Guangdong Provincial Foundation of Natural Science and Technology (Grant No. 950137). References [1] X.J. Zhang, Q.J. Fan, X.T. Zhang, et al., Application of the new surfactant LMS-2 in phenol removal from wastewater by liquid membrane, Environ. Chem. 1 (4) (1982) 320. [2] Y.H. Wan, X.D. Wang, X.J. Zhang, A new type of surfactant used for emulsion liquid membrane and its preparation, Chinese Patent 951,214,373 (1995).
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