Fundamental studies of novel inorganic–organic charged zwitterionic hybrids

Journal of Membrane Science 260 (2005) 26–36

Fundamental studies of novel inorganic–organic charged zwitterionic hybrids 3. New hybrid charged mosaic membranes prepared by modified metal alkoxide and zwitterionic process Junsheng Liu a , Tongwen Xu a,∗ , Ming Gong b , Yanxun Fu a a

Laboratory of Functional Membranes, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, PR China b Laboratory for Materials Behavior and Design in USTC, Chinese Academy of Science, Hefei 230026, PR China Received 10 January 2005; received in revised form 5 March 2005; accepted 15 March 2005 Available online 7 April 2005

Abstract A series of novel hybrid charged mosaic membranes have been prepared through a coupling reaction and zwitterionic process. This kind of coupling reaction was conducted between phenylaminomethyl trimethoxysilane (PAMTMS) and Ti(O-nBu)4 modified by acetylacetone, which was proved by FTIR spectra and the conventional properties of the membranes. Ion-exchange capacity (IEC) measurements indicate that both anion-exchange capacities (an-IECs) and cation-exchange capacities (cat-IECs) of the membranes coated one to three times are in the range of 4.62 × 10−4 to 1.48 × 10−2 and 1.57 × 10−2 to 3.2 × 10−2 meq. cm−2 , respectively; while these IECs increase with the elevating Ti-content. Streaming potentials exhibit that the isoelectric points (IEPs) of the membranes coated one time are in the range of pH 6–7.5 and decrease with the increasing Ti-content; but for those coated two times, the IEPs are in the range of pH 6–7.0 and increase with the rising Ti-content. Water content demonstrates a decline tendency with the rising pH whether for the membranes coated one or two times. Pure water flux reveals a downward trend with both the increasing coating times and the ingredients of hybrid precursors. The surface morphologies of the membranes coated three times show that the membrane microstructures can be affected by the compositions of coating solutions, while cross-section SEM images suggests that the membrane thickness elevates with the increasing coating times. © 2005 Elsevier B.V. All rights reserved. Keywords: Charged zwitterionic hybrids; Hybrid mosaic membranes; Metal alkoxide; PAMTMS; ␥-Butyrolactone

1. Introduction With the rapid development of industry and population explosion throughout the world, the demand for fresh water has become increasingly urgent due to the scarcity of drinking water resource and the contamination of industry to environment. Thus, the treatment of industrial wastewater is becoming imperative; while innovative technologies, which are used to prepare fresh water such as the desalination of brackish water and to treat the industrial refuses, have ∗

Corresponding author. Tel.: +86 551 3601587; fax: +86 551 3601592. E-mail address: [email protected] (T. Xu).

0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.03.018

attracted numerous researchers. Among these novel methods, ion-exchange membrane based technologies have been regarded as both effective and economical due to its lower operation expense and secure process, etc. [1–5]. As one important branch of ion-exchange membranes, ion-exchange mosaic membrane [1], in which both anion- and cationexchange layers are arranged alternately and parallel to each other with a neutral layer, has attracted considerable attentions due to its unique molecular structure, ionic selectivity and advantages to be applied in piezodialysis for separation of inorganic salts from water-soluble organic substances [6–10]. As a result, a large number of researchers have focused on this field up to now and much work has been done

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whether in membrane syntheses or in membrane applications [1,4–7,11–12]. Recently, hybrid charged membranes (or materials) have been conducted in our lab [13–16]. To develop novel ionexchange membranes with anti-fouling, chemical stability, which are able to prevent the specific adsorption of organic molecules and can be applied to reject univalent or multivalent salt ions in some severe conditions, hybrid charged zwitterionic polymers and membranes have also been prepared and characterized in our lab [17,18]. In our previous work [18], a series of novel inorganic–organic hybrid charged zwitterionic membranes were successfully synthesized via sol–gel process of hybrid precursor that was obtained by a reaction of 3-glycidoxypropyltrimethoxysilane (GPTMS) with N-[3-(trimethoxysilyl)propyl]ethylenediamine (TMSPEDA), and then with ␥-butyrolactone (␥-BL) to create ion pairs in the polymer. It was surprisingly found that when the molar ratio of GPTMS:TMSPEDA:␥-BL = 1:1:1, the membrane B demonstrated particular molecular structure, in which only one ion pair grafted onto the main chain was arranged in parallel each other. Furthermore, the membrane behaved different ion transport phenomenon (i.e. reverse ion transport trend) when KCl solutions permeated it. Therefore, it was suggested that this trend should be ascribed to the properties of charged mosaic membranes. To continue the previous researches and have further insight into the characteristics of the above-mentioned membrane, here four types of novel hybrid charged mosaic membranes will be prepared via modified metal alkoxide and zwitterionic process of silicone and mixed silicone–titanium alkoxide. It will be demonstrated that the preparation method in this case is more simple and practical than that used in our earlier work [18].

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2. Experimental 2.1. Materials Phenylaminomethyl trimethoxysilane (PAMTMS) was purchased from Silicone New Material Co. Ltd. of Wuhan University (China) and used without further purification. ␥Butyrolactone, Ti(O-nBu)4 , acetylacetone (Acac) and other reagents were of analytical grade and used as received. Microporous alumina plates were commercially obtained and of symmetrical structure with a porosity 40–45% and average pore diameter ∼1 ␮m. Total thickness of the alumina plates was about 0.5 cm. 2.2. The preparation of sol–gel hybrid precursors Due to that both PAMTMS and Ti(O-nBu)4 are sensitive to water, for preparing a stable hybrid precursor, in our case, n-butanol (n-BuOH) was used as a solvent and acetic acid (HAc) was added to initialize hydrolysis; while Acac was used as a chelating agent to decrease the hydrolysis rate of titanium alkoxide and stabilize the sol, this process is similar to the method reported by Legrand-Buscema et al. [19]. The possible reactions for these hybrid precursors were presented in Schemes 1 and 2. The related reaction steps (in Scheme 1) included: step 1 was an alcohol-exchange reaction between PAMTMS and n-BuOH; step 2 was a cross-linking reaction of PAMTMS and Ti(O-nBu)4 , leading to the creation of silicone and titanium hybrid precursor. Scheme 2 was a zwitterionic process, in this step, ␥-BL opened the lactone ring and reacted with the NH groups, causing the generation of ion pairs grafting onto the main chain of the above-prepared polymers. The related preparation routes were illustrated as

Scheme 1. Alcoholysis and condensation reaction of phenylaminomethyl trimethoxysilane (PAMTMS) and titanium alkoxide (Ti(O-nBu)4 ).

Scheme 2. Zwitterionic process of the hybrid precursors to create both positively and negatively charged groups in the investigated membranes.

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Table 1 Composition of different coating solutionsa Coating solution

PAMTMS (mol)

Ti(O-nBu)4 (mol)

n-BuOH (mol)

Acac (mol)

HAc (mol)

Membrane number

(i) (ii) (iii) (iv)

0.1 0.1 0.1 0.1

0 0.1 0.2 0.4

4 4 4 4

0.03 0.03 0.06 0.12

0.01 0.01 0.01 0.01

A B C D

a

The molar ratio of PAMTMS:␥-BL = 1:10.

follows: firstly, Acac was added into the Ti(O-nBu)4 solution dissolved in n-BuOH, while stirred for 1 h at room temperature (about 15 ◦ C), the molar ratio of titanium alkoxide and Acac was fixed at 1:0.3 so as to obtain stable sol; then, PAMTMS was added dropwise into the above mixed solution and blended violently for 4 h; after aging for 24 h, stable precursor sols used as coating solutions were obtained; their chemical compositions were tabulated in Table 1. 2.3. The synthesis of hybrid charged mosaic membranes The membrane preparation procedure was presented in Fig. 1. To prepare a hybrid charged mosaic membrane with different properties, the microporous alumina plate was immersed into different coating solutions. After dip-coating for 15 min, the alumina plate was air-dried at room temperature for 30 min, kept at 70–80 ◦ C for 1 h and heated to 110 ◦ C and kept at 110 ◦ C for additional 1 h, respectively; and then allowed to cool to room temperature. This step might be repeated several times from the same alumina plate to increase the thickness of the membranes. Finally, the obtained membranes were dipped into ␥-BL solution (the molar ratio of PAMTMS:␥-BL was about 1:10) for 12 h and heated to 70 ◦ C and kept at 70 ◦ C for additional 1 h to produce zwitterionic charged groups, then rinsed thoroughly with deionized water, thus hybrid charged mosaic membranes can be achieved. It should be noted that in our previous paper [18], the charged mosaic membrane B was prepared by regulating the amount of ␥-BL in the reaction process; whereas in this experiment, the hybrid charged mosaic membranes were synthesized through the control of molecular structure. This is because there TMSPEDA possesses a second amine structure, thus ␥-BL can react with the NH groups of the hybrid precursor and open the lactone ring, leading to the creation of ion pairs grafting onto the two sides of the main chain of the polymer [17,18]. However, in this paper, even though PAMTMS is also a second amine, it is hard for ␥-BL to graft more ion pairs onto this nitrogen atom (as shown in Schemes 1 and 2) due to the effect of steric hindrance of phenyl group in the nitrogen element of PAMTMS and thus it provide a possibility to achieve a hybrid charged mosaic membranes by controlling the reaction of PAMTMS with ␥-BL. 2.4. Membrane characterizations FTIR spectra of the coating solutions were recorded with a Bruker Equinox-55 FTIR spectrometer. The anion-exchange

Fig. 1. Preparation procedure for hybrid charged mosaic membrane via modified metal alkoxide and zwitterionic process.

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capacity (an-IEC) of the hybrid mosaic membranes was determined by conventional Mohr method, in which the membranes were converted to Cl− ionic form, then back titrated with 0.01N AgNO3 solution and expressed as meq. cm−2 . The cation-exchange capacity (cat-IEC) of the hybrid mosaic membranes was determined by the similar method, in which the membranes were converted to Na+ ionic form, then back titrated with 0.01N HCl solution [20]. The effective membrane area for the testing of IEC was about 21.6 cm2 , and the measuring error is estimated to be within 2%. The unit for streaming potential measurements was similar to that described in detail in our previous paper [16]. To decrease the testing errors, the final results were the mean values of three times testing. Pure water flux (F) of the explored membranes was carried out in a self-made dead-end membrane module [21]. The pressure difference used in this testing was around 0–1.5 MPa, and the effective membrane area was about 18.5 cm2 ; the total volume of water across the membranes was collected at a given time interval (1–2 min). To reduce the experimental errors, the mean values of three times testing were selected as the final results. The error range of water flux testing is estimated to be within 5% and the water flux can be calculated as F = V/(AtP), where V is the total volume of water permeated during the experiment, A represents the membrane area, t denotes the operation time, and P is the pressure difference across the membranes. The determination of water content was conducted by using conventional process [20] and described in detail in our earlier work [18]. Morphologies of the considered mosaic membranes were observed by an XL30-ESEM (Philips) environmental scanning electron microscopy.

3. Results and discussion 3.1. FTIR spectra To demonstrate the reactions described in Scheme 1 and investigate the bonding coupling behavior of the samples, FTIR spectroscopy was conducted and shown in Fig. 2a–d. In Fig. 2a, the strong absorption peak at ∼1073 cm−1 is overlapped to asymmetric stretching of Si O Si and Si O C stretching bands from trimethoxysilane groups of PAMTMS [22]. The distinct absorption peaks associated with CH3 or CH2 groups of n-BuOH and PAMTMS are observed at ∼3000 cm−1 and the peak around 1220–1500 cm−1 is assigned to the C N bond in the sample [23,24]. The sharp peak at 1604 cm−1 is ascribed to the phenyl group of the above sample. The gradual disappearance of the peak situated at 1711 cm−1 (compare Fig. 2a with Fig. 2b–d hereinafter) should be attributed to the C O vibration of Acac, indicating that the coordination between Si and Acac did not occur. In Fig. 2b–d, the FTIR spectra of coating solutions (ii)–(iv) are very similar in appearance except both the

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Fig. 2. The FTIR spectra of: (a) coating solution (i); (b) coating solution (ii); (c) coating solution (iii); (d) coating solution (iv).

peak shape situated at near 1073 cm−1 and the intensity of peak at 1711 cm−1 . The new absorption peaks at 1585 and 1525 cm−1 are clearly observed, which should be ascribed to the chelating complex of titanium and Acac [25], while their increasing intensities imply that more titanium alkoxide is coordinated by Acac with the increasing content of Ti(O-nBu)4 and Acac in the samples, but they still remain in the mixed sol solution, which is approved by the gradual disappearance of C O vibration of Acac at 1711 cm−1 . Furthermore, the peak shape at around 1073 cm−1 related to the Si O Si bonds is changed from steep shape (Fig. 2a) to broad mountain form (Fig. 2b) and then divided into several abrupt peaks (Fig. 2c and d), suggesting that the coupling reaction of silica and titanium has happened and the Si O Ti bonds have formed, which is also proved by the absorption peak in the range of 920–960 cm−1 [26]. In addition, the above changes in the peak shape are possibly ascribed to the increasing Ti-content in coating solutions (ii)–(iv) (as listed in Table 1). Moreover, it also can be seen that the broad absorption bands related to the Ti O Ti bonds in a range of 900– 450 cm−1 [27] are unnoticeable (as shown in Figs. 2b–d); the possible reason might be that the Ti(O-nBu)4 is coordinated by Acac, leading to a decrease in hydrolysis and condensation

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J. Liu et al. / Journal of Membrane Science 260 (2005) 26–36

Fig. 3. The effect of coating times on both anion-exchange capacity (an-IEC) (a) and cation-exchange capacity (cat-IEC) (b) of the mosaic membranes originate from different coating solutions. A–D correspond to the membranes listed in Table 1.

velocity of Ti(O-nBu)4 as well as the production of small amounts of titania in the coating solutions.

and the zwitterionic reaction had occurred during the membrane preparation. Moreover, it is interesting to find from Fig. 3a and b that both an-IECs and cat-IECs are highly affected by the composition of the precursor sols and the coating times: they increase not only with the rising content of Ti(O-nBu)4 but also with the increase in coating times. As shown in Table 1, the Ti-content in precursor sols increases in the order of membranes A–D; consequently, it can be concluded that for the same coating times, the IECs will increase with the rising content of Ti(O-nBu)4 in coating solutions. The reasons might be that titanium is capable of facilitating the opening of lactone ring [28] and favoring the zwitterionic reaction (as described in Scheme 2), thus elevating the content of both anionic and cationic groups in the membranes. In addition, it can also be found that for the same composition of coating solutions, the IECs increase with the coating times. This is because that both the positively and negatively charged groups enhance with the increasing coating times due to the increase in membrane thickness as proved by the cross-section SEM images in Fig. 6e–g hereinafter, leading to the promotion of ion-exchange ability of the membranes. It should be noted that from the structure of the prepared membranes (as shown in Scheme 2), the molar number of anionic and cationic groups should be equal, but the cation IEC of the prepared membranes is 2–3 times (for some points, more than 10 times) higher than the anion IEC of the prepared membranes (as shown in Fig. 3). It is hard to explain extensively this phenomenon within the authors’ present knowledge. The possible reason is related to the excess amount of ␥-BL in the membranes. As noted in Table 1, ␥-BL is in far excess (the molar ratio of PAMTMS:␥-BL = 1:10) in the experiments and this excess amount of ␥-BL may be easily transferred to butyric acid (BA) during the membrane preparation because ␥-BL is an intermediate of manufacture of BA. The produced butyric acid tends to contribute to cation IEC, resulting in great difference between the IECs of cationic and anionic groups.

3.2. Ion-exchange capacity (IEC) 3.3. Streaming potential and isoelectric point (IEP) For verifying the reaction described in Scheme 2 and determining the electrical character of the tested membranes, the determination of IECs for both positively and negatively charged groups were performed and the corresponding results are illustrated in Fig. 3a and b. It can be seen that the anion-exchange capacities of the four kinds of charged mosaic membranes are in the range of 4.62 × 10−4 to 1.48 × 10−2 meq. cm−2 , suggesting the membranes possess anion-exchange groups. Meanwhile, the cation-exchange capacities of these membranes are in the range of 1.57 × 10−2 to 3.2 × 10−2 meq. cm−2 , implying that the prepared membranes also contained cation-exchange groups. These findings clearly reveal that the above-developed membranes can exhibit the characteristics of both positively and negatively charged groups, also further corroborate the reaction illustrated in Scheme 2, i.e. the opening of lactone ring in ␥-BL

Since the mosaic membranes possesses both positively and negatively charged groups, there should have an isoelectric point in the membranes and this IEP can be determined from the measurements of streaming potential [19]. Thus, the streaming potential was measured and the corresponding results are shown in Fig. 4. It can be observed that when the membranes were coated one time, the main IEPs are in the range of pH 6–7.5 (pH 7.11, 6.93, 6.58 and 6.03 for membranes A–D, respectively) demonstrating a downward trend with the increasing content of Ti(O-nBu)4 in the membranes (as shown in Fig. 4a). However, when the membranes were coated two times, the main IEPs are in the range of pH 6–7.0 (pH 6.10, 6.24, 6.34 and 6.53 for membranes A–D, respectively), exhibiting an upward tendency with the elevating Ti-content in the membranes (as

J. Liu et al. / Journal of Membrane Science 260 (2005) 26–36

Fig. 4. Streaming potentials of the membranes coated: (a) one time; (b) two times. A–D correspond to the membranes listed in Table 1.

presented in Fig. 4b). These different change trends in IEPs of the membranes are analogous to the observations of membrane potentials against pH values in our earlier work [18], i.e. the occurrence of ion reverse transport when KCl solutions permeated through the membranes. The rational interpretation of these trends should be attributed to the properties of charged mosaic membrane because negative osmosis is a typical transport phenomenon of a charged mosaic membrane as reported by Fujimoto and co-workers [4,29], in which they investigated mixed solutions of saccharose and KCl and proved that volume flow (Jv ) showed negative osmosis. Moreover, it is interesting to find that there seems to exist second IEPs in the tested membrane samples, especially for the samples coated two times (as shown in Fig. 4b), which is similar to the outcomes of the measurements of membrane potential against pH values for membrane B in our previous work [18]. But only the IEPs located in the range of pH 4–10 are valid due to that the flux of hydrogen or hydroxide ions cannot be neglected at pH <4 or >10, which cause a decrease in absolute value of membrane potential as stated in a theoretical model proposed by Tanioka and co-workers [30], in which they suggested that this trend be a natural consequence of the behavior of the ion flux. These findings reveal

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Fig. 5. Water content of the hybrid charged mosaic membranes at different pH values: (a) dip-coating one time; (b) dip-coating two times. A–D correspond to the membranes tabulated in Table 1.

that the hybrid mosaic membranes investigated in this case also exhibit weak amphoteric performances as proved in our previous paper [18], which further corroborates the reaction described in Scheme 2. Further, combining these findings with those conclusions in our earlier work [18], it can be seen that the IEP of a hybrid charged mosaic membrane is able to be determined not only by streaming potential but also by membrane potential against pH values. 3.4. The relationships between water content and pH values To survey the hydrophilicity of the membranes in different pH values and the effect of modified titanium alkoxide on the property of the explored membranes, water content was tested and the relevant results are illustrated in Fig. 5. It can be found that the water content decreases slowly with the increasing pH values whether for the membranes coated one time (Fig. 5a) or two times (Fig. 5b), while at the same pH value, it seems that water content decreases with the elevating coating times (Fig. 5a and b). The decline trends of water content with the increasing pH values suggest that the water content is capable of being

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J. Liu et al. / Journal of Membrane Science 260 (2005) 26–36

highly influenced by the pH values. Since hybrid charged mosaic membranes contain both positively and negatively charged groups that can be affected significantly by the pH values (as shown in Scheme 2) [21], they are able to exhibit amphiphilic performances in aqueous solutions. As discussed in our previous paper [18], in acidic conditions, the carboxyl groups tend to easily combine with hydrous ions and thus will increase the weight of wet membrane with the decline of pH in external solution; but in basic environments, the hydroxyl ion may be more difficult to combine with positively charge group compared with proton combination with negatively group in acidic conditions, thus the weight of wet membranes decreases with an increase in pH value and correspondingly give rise to a decrease in water content of the membrane. Therefore, the influence of pH value on water content can be explained as the interaction between ionic charge sites of the membranes as well as the change in the amount of acidic or basic ions in aqueous solutions. Further, the effect of chemical composition of the membranes on water content can be obtained by comparing membranes A–D coated one or two times. It is shown that the membrane A (free Ti-content) exhibits the minimum water content among the investigated membranes whether coated one or two times (as presented in Fig. 5a and b). But the water content in membranes B–D (doped with Ti) is largely dependent on the coating times. As shown in Fig. 5a, when the membrane was coated one time, water content decreased with the order membranes B > C > D, i.e. the water content promoted with the increasing Ti-content; while in the case of two times coating, water content increased with the order membranes B < D < C (as shown in Fig. 5b). The hydrophobic property of organic compound might be responsible for the minimum water content in membrane A due to small amounts of carboxyl groups being grafting onto it compared with membranes B–D. The possible reason may be related to the instability of Si O Ti bonds in the membranes. When the membranes were coated one time and heated up to 110 ◦ C, the unhydrolyzed metal alkoxide molecules (as proved by the FTIR spectra in Fig. 2) may continue to hydrolyze and condense with the elevated curing temperature, resulting in the rise of both Si O Si bonds and Ti O Ti bonds and the formation of silica and titania, especially for the high Ticontent membranes [31]. As a result, the hydrophilicity of the membranes is highly weakened with the increasing Ticontent. But when the membranes were coated two times, due to the increase in coating thickness and the denser in membrane surface, the influence of curing temperature on the stability of Si O Ti bonds in the membranes, particu-

larly inside the membranes, is reduced; consequently, the hydrophilic property increases a little with the increasing content of carboxyl groups in the membranes, which conformed to the theoretical trends in our previous work: the increasing amount of carboxyl groups in the membranes with the increase of ␥-BL in polymers [17] will give rise to the enhance of hydrophilicity in the charged membranes as discussed above. Moreover, the particular attention should be paid to the anomalous behavior of water content in membrane D, which did not show the maximum value as theoretically expected whether coated one or two times, though it contained the highest amount of carboxyl groups. It is hard to extensively clarify this abnormal phenomenon within the authors’ present knowledge. The possible reason is related to the microstructure of membrane D. As explained hereinbefore, high Ticontent will cause the formation of silica and titania network and lead the membrane to embrittle and crackle (as shown by SEM images in Fig. 6 hereinafter), resulting in the reduce of hydrophilicity in membrane D. 3.5. Pure water flux In order to examine the microstructure of the membranes and inspect the influences of modified titanium alkoxide on membrane performances, pure water flux was measured and the corresponding results are summarized in Table 2. It is interesting to find that both the ingredient of the coating solutions and the coating times can highly affect pure water flux. For example, for the same composition of coating solutions (i)–(iv), pure water flux decreased significantly with the increasing coating times up to two times. The main reason of this trend might be related to the textural properties of the membranes. After the alcoholysis reaction, the hydrolysis and condensation of the modified titanium alkoxide and ethyloxy group in the PAMTMS increase with the proceeding of esterification reaction induced by acetic acid [32], leading to high Ti dispersion in the xerogels, which favors the formation of Si O Ti bonds [33,34]. Another reason is attributed to both the enhance in membrane thickness and the shrinkage of membrane pore size with the increasing coating times as demonstrated by the cross-section SEM images in Fig. 6e–g hereinafter. Nevertheless, for the same coating times of the membranes with different compositions, water flux decreases with the elevating Ti-content in the coating solutions. The cause is that the concentration of the coating solutions enhances with the increasing Ti-content and thus decreases the pore size, leading to the reduction in water flux.

Table 2 Pure water flux (l m−2 Pa−1 h−1 ) and the estimated pore diameter (␮m, listed in parenthesis) of the membranes A–D coated for different times Coating times

Membrane A

Without coating 1 2 3

185.69 × 10−5 163.28 × 10−5 96.05 × 10−5 33.62 × 10−5

Membrane B (0.025) (0.022) (0.013) (0.0045)

201.70 × 10−5 83.24 × 10−5 32.02 × 10−5 16.81 × 10−5

Membrane C (0.027) (0.011) (0.0043) (0.0022)

166.48 × 10−5 48.02 × 10−5 10.01 × 10−5 24.01 × 10−5

Membrane D (0.022) (0.0064) (0.0013) (0.0032)

102.45 × 10−5 18.25 × 10−5 4.57 × 10−5 64.03 × 10−5

(0.014) (0.0024) (0.00061) (0.0085)

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Fig. 6. (a–d) The surface SEM images of membranes A–D coated three times, respectively. The cross-section SEM images of: (e) alumina plate (without coating); (f) membrane A coated one time; (g) membrane A coated three times.

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Moreover, it also can be detected that when the coating times increased up to three times, pure water flux for higher Ti-content membrane samples increased a little (e.g. membranes C and D as shown in Table 2). The theoretical explanation of this trend is that low titanium doping favors the formation of microporosity and the incorporation of titanium into silica matrix, but high Ti-content will facilitate the formation of Ti O Ti structural units, causing a rise in mesoporosity of the membranes [32,33,35]. While the concentration of the coating solutions increases and the unhydrolyzed alkoxide molecules might continue to hydrolyze and condense during the aging, resulting in the rise of both Si O Si bonds and Ti O Ti bonds due to the reduce of stability of Si O Ti bonds in the membranes [31]. As a result, the phase separation occurs and the membranes begin embrittle and crackle (as shown by SEM images in Fig. 6d hereinafter), accordingly causing the increase in pure water flux for high Ti-content membranes (as tabulated in Table 2). Another origin is possibly related to the disfigurement of the explored membranes during the testing. Although we have not directly detected the pore size of the membranes and alumina plate (without coating), they can be estimated from pure water flux by comparing with a standard alumina ceramic with known average pore diameter (0.2 ␮m) and pure water flux (1500 × 10−5 l m−2 Pa−1 h−1 ) [15,16]. The average pore diameter for the membranes and alumina substrate estimated in this manner were listed in Table 2. Obviously, the pore size of the membranes decreases with the increasing coating times for the same coating solutions. Whereas for the same coating times, the pore size of the membranes decreases with an increase in Ti-content in the coating solutions. These change trends in membrane pore size are in well agreement with those of pure water flux as discussed above and coincided with cross-section images shown in Fig. 6e–g hereinafter. Therefore, the shrinkage of membrane pore size could be regarded as one of the reasons responsible for the decline in pure water flux. 3.6. Morphology of the hybrid charged mosaic membranes For detecting the effect of modified metal alkoxide on the microstructure of the membranes, the surface SEM observations of the membranes coated three times were carried out and the corresponding SEM images are presented in Fig. 6a–d. To investigate the difference of membrane thickness between the alumina plate (without coating) and the membranes with different coating times, cross-section SEM observations were also performed. It seems that the crosssection SEM images of the four kinds of membranes exhibit similar change trends with the increasing coating times; therefore, as an example, only the cross-section SEM images of alumina plate and membrane A coated one and three times, respectively, are illustrated in Fig. 6e–g. It can be seen that the surface micrograph of membrane A (free Ti) exhibits some large pores and voids (Fig. 6a);

and the surface microstructure of membranes B and C (with increasing Ti-content) become smoother and denser, while small amounts of chasm are also spotted on the surface of these two membranes (cf. Fig. 6b and c); however, remarkable crackles are detected on the surface of membrane D (containing excessive Ti-content) (cf. Fig. 6d). The above SEM observations indicate that hybrid thin films actually formed on the surface of the alumina plates. These changes in surface morphologies of the membranes should be ascribed to the difference in compositions of coating solutions. For membrane A, due to its dilute concentration of coating solution and undoped titanium alkoxide (as listed in Table 1), thus small amount of film is generated on the surface of alumina plate even if the coating times increase up to three times, this thinner film is insufficiently covered the pores and voids on the surface of alumina plate, hence its SEM image indicates the original morphology of alumina plate (cf. Fig. 6a). Whereas for membranes B and C, the concentrations of coating solutions increase with the elevating Ti-content, as a result, thicker film is produced and the thickness is enhanced with the increase of coating times, accordingly the surface of membrane become smoother and denser. Further, the hydrolysis and condensation of silicone and modified titanium alkoxide maybe continue during the aging of the membranes and this hydrolysis can facilitate the formation of Si O Ti bonds [31], resulting in the shrinkage of pore size inside membranes. But if Ti-content in the coating solution increases further, the stability of Si O Ti bonds will be decreased due to the increasing formation of Si O Si and Ti O Ti networks, leading to the phase separation and chasm in the membranes [31]. This might be one of the reasons why membrane D with high Ti-content in coating solution has shown marked crackles (cf. Fig. 6d), especially for the membranes coated several times, which is also confirmed by the measurements of pure water flux of the membranes (as listed in Table 2 for membranes C and D coated three times). Another reason is possibly linked to the formation of higher cross-linking density in the inorganic and organic networks due to that the formation of one network has a significant effect on the formation of the other [36]. As we know, Ti(O-nBu)4 is a cross-linking agent to build the cross-linking network of PAMTMS by the condensation between Si (OH)3 from PAMTMS and Ti (OH)4 from Ti(O-nBu)4 , an increase in Ti(O-nBu)4 amount in coating solutions will result in higher cross-linking density in the coating layer. Consequently, the materials become brittle and develop more cracks on the coating layer (as shown in Fig. 6b–d). From the cross-section SEM images of membrane A coated one and three times shown in Fig. 6f–g, it can be found that the holes in the cross-section become smaller and smaller with the increasing coating times. Compared with some large cavities observed in the alumina plate (without coating) (as shown in Fig. 6e), it can be suggested that the prepared membranes have formed in the holes of alumina plates and the membrane thickness increases with the increasing coating times. These observations further confirm

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the conjectures made in above: water flux decreases with the increasing coating times due to an increase in membrane thickness (as listed in Table 2) and the change trends of IECs are also connected with the alternation of membrane thickness (as illustrated in Fig. 3a and b).

4. Conclusions Four types of novel inorganic–organic hybrid charged mosaic membranes have been prepared via coupling reaction of mixed PAMTMS/Ti(O-nBu)4 modified by Acac as well as zwitterionic process. It exhibits that both the anIECs and cat-IECs of the four kinds of membranes are in the range of 4.62 × 10−4 to 1.48 × 10−2 and 1.57 × 10−2 to 3.2 × 10−2 meq. cm−2 , while these IECs increase not only with the rising Ti-content but also with the increasing coating times. The measurement of streaming potentials indicates that when the membranes are coated one time, the IEPs are in the range of pH 6–7.5 and decrease with the increasing Ti-content; but when the membranes are coated two times, the IEPs are in the range of pH 6–7.0 and increase with the elevating Ti-content in the membranes, demonstrating the reverse transport of ion. Pure water flux exhibits that it can be highly affected by both the ingredient of the coating solutions and the coating times, while it decreases not only with the increasing coating times but also with the elevating content of Ti(O-nBu)4 in the coating solutions. Surface morphologies of the membranes coated three times shows that the membranes become thicker and denser with the increasing Ti-content and coating times, but excessive Ti-content will decrease the stability of Si O Ti bonds and lead the membranes to fracture. The cross-section SEM images of the membranes imply that the thickness of the prepared membranes increases with the increasing coating times.

Acknowledgements This job was supported in part by Program for New Century Excellent Talents in University, National Basic Research Program of China (973 program, No. 2003CB615700) and the Natural Science Foundation of China (No. 20376079).

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