Ind. Eng. Chem. Res. 2010, 49, 5001–5005
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General Intercalation of Poly(oxyalkylene)-Amidoacids for Anionic and Cationic Layered Clays Yi-Lin Liao, Chih-Wei Chiu, and Jiang-Jen Lin* Institute of Polymer Science and Engineering, National Taiwan UniVersity, Taipei 10617, Taiwan, and Department of Materials Science and Engineering, National Chung Hsing UniVersity, Taichung 40227, Taiwan
The same poly(oxyalkylene)-amidoacids (POA-amidoacid) were incorporated into two different types of ionic clays, anionic layered-double-hydroxides (Mg-Al LDH), and cationic montmorillonite (MMT). Conventionally, the intercalation is an ionic exchange reaction, i.e. anionic organic salts for LDH and cationic salts for MMT. The successful intercalation using the neutral species of POA-amidoacid implies the existence of a new mechanism for clay intercalation. We prepared the POA-amidoacid from the reaction of maleic anhydride and poly(oxyalkylene)-diamine (POA-amine) with different hydrophobic and hydrophilic POAbackbones of 600-4200 molecular weight. The intercalation has resulted in the expansion of the clay basal spacing from 7.8 to 63 Å for LDH and from 12.4 to 51 Å for MMT, with a high Bragg’s regularity of XRD analyses. It appears that the driving force for the intercalation and organic incorporation into the silicate interlayer galleries is dissimilar to the counterion exchange but involving noncovalent bonding between amidoacid functionalities and tSiOH species in clay. The intercalation without ionic exchange reaction is effective for generating new clay hybrids with thermal responsiveness in water. 1. Introduction Layered aluminosilicate clays including montmorillonite (MMT) are abundant in nature and industrially applied for catalysts,1-4 adsorbents,5-7 metal chelating agents,8 biomaterial encapsulations,9,10 modified electrodes,11,12 multilayer selfassembling substrates,13 polymer nanocomposites,14-16 etc. Application of these inorganic layered structures oftentimes requires organic modification and intercalation with surfactants in order to disperse them in organics. In the case of MMT, which has cationic counterions in the clay structure (tSiO-M+), the organic intercalating agents are generally cationic species such as fatty quaternary ammonium or phosphonium salt.17-19 However, for oppositely charged clays such as layered double hydroxides (LDHs), anionic organics such as organic carboxylic salts are required for replacing the anionic counterions such as NO3- in the layered structures.20,21 For applications, anionic LDH is suitable for interacting and reserving DNA and drugs for delivery.22-25 Previously, we reported uses of poly(oxypropylene)-bisamidoacid sodium salts as ionic exchange agents for intercalation into Mg-Al LDH clay.26 In general, the LDH is of the generic formula [M1-x2+Mx3+(OH)2]Ax/nn- · mH2O where the metal ions can be Mg2+, Ni2+, Cu2+, or Mn2+ for divalent and Al3+, Fe3+, or Cr3+ for trivalent cations, while A- is an anion such as OH-, F-, Cl-, Br-, NO3-, CO32-, SO42-, etc.27-29 While the counteranions in an LDH are exchanged by organic anions such as the sodium salts of amido acids, the basal spacing of the multilayered structure expands. Similarly, for the cationic MMT clay, the basal spacing (XRD d spacing 92 Å) may be affected by the amine salts of poly(oxypropylene)-diamines of 4000 g/mol Mw.30 The ionic exchange reaction and basal spacing expansion can be correlated to the molecular length of intercalating organic ions. Herein, we report the intercalation of both anionic and cationic clays for expanding the layered structure without involving the conventional ionic exchange reaction. Neutral species of poly* To whom correspondence should be addressed. Tel.: +886-2-33665312. Fax: +886-2-8369-1384. E-mail:
[email protected].
(oxyalkylene)-backboned (POA) bis-amidoacids are effective for intercalating both anionic LDH and cationic MMT clays. The uses of the POA-amidoacids with various molecular weights and hydrophobic/hydrophilic backbones lead to the elucidation on the intercalation mechanism. The driving force for the organic incorporation is significantly different from the conventional ionic exchange reaction. 2. Experimental Section 2.1. Materials. Mg(NO3)2 · 6H2O, Al(NO3)3 · 9H2O, and NaOH were obtained from SHOWA Chemicals. Sodium montmorillonite (Na+-MMT), from Nanocor Co., is a Na+ form of smectite clay with a cationic exchange capacity (CEC) of 120 mequiv/100 g. A series of poly(oxyalkylene)-diamines (POA) including hydrophobic poly(oxypropylene)-diamines (POP) and hydrophilic poly(oxyethylene)-diamines (POE) of different molecular weights were purchased from Huntsman Chemical Co. The POP-diamines of 400 Mw with the POP backbone and two terminal primary amines is water-soluble, while their high Mw analogs such as 2000 and 4000 Mw are hydrophobic and water-insoluble. These POP-amines with 400, 2000, and 4000 Mw are abbreviated as POP400, POP2000, and POP4000, respectively. For hydrophilic amine, POE2000 has a watersoluble POE middle block of 2000 Mw and two terminal amines. Maleic anhydride (MA) was purchased from Aldrich Chemical Co. and purified by sublimation. 2.2. Preparation of Mg-Al-NO3 LDH. Mg2Al-NO3-LDH was prepared by a coprecipitation method according to previously reported procedures.31 A mixture of Mg(NO3)2 · 6H2O (201 g, 0.784 mol) and Al(NO3)3 · 9H2O (147 g, 0.392 mol) was dissolved in 500 mL deionized water (at the Mg/Al molar ratio of 2.0). The aqueous solution was vigorously stirred at 60 °C under nitrogen purge in order to minimize contamination with atmospheric CO2, while maintaining the pH at 10 ( 0.2 by adding 2 N NaOH. The slurry was then filtered and the precipitates were isolated and washed thoroughly with deionized water until the filtrate pH ) 7 ( 0.2. The X-ray powder diffraction pattern indicated a basal spacing of 7.8 Å.
10.1021/ie901136u 2010 American Chemical Society Published on Web 04/23/2010
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Scheme 1. Chemical Structures of Poly(Oxyalkylene)-Amidoacids Including with Poly(Oxypropylene) (POP) and Poly(Oxyethylene) (POE) Backbones
Table 1. XRD Basal Spacing of POP-MA-Intercalated LDH with Different Equivalents of NaOH Addition for Ionization intercalating agent
xa (Na /POP-MA)
d spacing (Å)b
weight fraction (w/w)c
2.0 1.0 0.8 0.4 0.2 0
7.8 63 66 64 63 62 63
41/59 86/14 90/10 90/10 92/08 92/08 92/08
+
none POP2000-2MA-xNa
a POP-MA: intercalating agent of POP2000-2MA; adding various equivalents of NaOH per molar ratio, and intercalation with LDH at a molar ratio to the anionic exchange capacity (AEC ) 300 mequiv/100 g). b The basal spacing of X-ray diffraction for n ) 1 was calculated on the Bragg’s pattern. c Weight fraction: organic composition/silicate measured by TGA (heating to 850 °C); calculated to be 87/13 based on the AEC and POP molecular weights.
2.3. Synthesis of POA-Amidoacids. The adducts of poly(oxyalkylene)-diamine and maleic anhydride at 1:2 molar ratio were prepared according to Scheme 1.26 The general procedures for preparing these POE- and POP-amidoacids are described below. To a 1 L three-necked and round-bottomed flask, equipped with a mechanical stirrer, nitrogen inlet-outlet lines, and a thermometer, was added maleic anhydride (15 g, 150 mmol) in THF (15 g) and followed by POP- or POE-amines of 75 mmol. The mixtures were stirred vigorously, and the temperature was maintained below 30 °C over 3 h. THF was removed under a reduced pressure at 60 °C. The product was obtained as a viscous light-colored liquid. The progress of the reaction was monitored by analyzing the FT-IR spectrum at 1660 cm-1 for the amide formation and 1720 cm-1 for the carboxylic acid absorbance. In addition, the amine titrations indicated a complete consumption of the amine basicity during the reaction. The products of amine to maleic anhydride at 1:2 molar ratio or NH2/MA at 1/1 were prepared and abbreviated as POP400-2MA, POP2000-2MA, POP4000-2MA, and POE2000-2MA, for various POP- and POE-diamines. Their corresponding sodium salts were prepared by treating with equivalent amounts of 0.1 N NaOH. 2.4. Intercalation of Anionic LDH. Typical procedures for the intercalation of LDH by using POA-2MA with different amounts of NaOH treatment are described. The prepared Mg2Al-NO3-LDH (1 wt %, 50 g aqueous solution, anionic exchange capacity (AEC) ) 300 mequiv/100 g)10 was placed in a 100 mL round-bottomed flask and heated until boiling. To the slurry was added POP2000-2MA, previously treated with different equivalent amounts of 0.1 N NaOH (3.3 g, 1.5 mequiv). The solution was then poured into the flask containing the swelled LDH slurry. The mixture was stirred for 8 h and cooled to room temperature. The solid products were collected, washed with water/ethanol, and dried under vacuum at 80 °C. The XRD data from POP2000-2MA-xNa intercalation are summarized in Table 1. 2.5. Intercalation of Cationic MMT. An example of intercalation experiment for preparing POA-amidoacids/layered silicate hybrid is followed. The water-swelled MMT (120 mequiv/100 g; 1 wt % of 50 g aqueous solution) was placed in a 100 mL round-bottomed flask and POP2000-2MA (1.3 g, 0.6 mequiv) was added. The slurry was stirred for 3 h and filtered at ambient temperature. The POA-MMT product was collected, washed with water/ethanol, and dried under vacuum at 80 °C.
2.6. Characterization. The interlayer basal spacing or d spacing was analyzed by using an X-ray powder diffractometer (Rigaku D/DAX-II B using a Cu target, λ ) 1.5418 Å at 30 kV, 20 mA) with a scanning rate of 2°/min from 2θ ) 2-14°. The POA-LDH and POA-MMT organoclays generally exhibited a series of multiple peaks with a pattern following Bragg’s equation. The value of d spacing for n ) 1 was calculated from the observed values of n ) 2, 3, 4, etc., according to the equation (nλ ) 2d sin θ). The thermal stability was analyzed by using thermal gravimetric analyzer (TGA), on a Perkin-Elmer Pyris 1 model. The organic weight was estimated from the weight losses by ramping the temperature from 100 to 850 °C at the rate of 20 °C/min in air. Fourier transform infrared spectroscopy (FT-IR) was carried out using a PerkinElmer Spectrum One FT-IR spectrometer in the range of 400-4000 cm-1. Samples were prepared by dissolving in THF and evaporated into thin film on a KBr plate. The lower critical solution temperature (LCST) was measured using a UV-visible spectrophotometer (Shimadze UV mini 1240) at 550 nm wavelengths at varying temperatures. The LCST was defined as the temperature at which the transmittance was altered by 50% at 550 nm. 3. Results and Discussion 3.1. Intercalation without Ionic Exchange Reaction. For both ionic clays including anionic LDH and cationic MMT, the organic modification is conventionally performed by an ionic exchange reaction. It requires that the organic intercalating agents to be ionic species. For example, the sodium salts of poly(oxypropylene)-amidoacid (POP2000-2MA-2Na+), prepared from the reaction of POP-diamine 2000 g/mol Mw and 2 equiv of maleic anhydride, were reported previously for effectively interacting with the anionic LDH.26 The anionic LDH (e.g., NO3- counterions in the inorganic Mg-Al layered double hydroxide clay) and cationic MMT (e.g., Na+ in montmorillonite) were intercalated by using organic surfactants with cationic and anionic species, respectively. In a systematic approach for understanding the ionic exchange capacity for LDH, different equivalents of NaOH was added to convert -COOH into -COO-Na+. As shown in Table 1, various sodium equivalents of POP2000-2MA-xNa+ were allowed to interact with the LDH, which expanded the basal spacing from the pristine 7.8 to 62-66 Å. The expansion was accompanied by an increase amount of organic incorporation. Unexpectedly, it was observed that the intercalations generated the same LDH d-spacing regardless of the sodium-salt equivalents in the structures of these intercalating agents. It also implies
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Scheme 2. Conceptual Illustration of the Conventional Ionic Exchanging Intercalation and the Hydrogen Bonding Driven Mechanism
Figure 1. XRD Bragg’s pattern by POP2000-2MA treated with different NaOH equivalents: (a) 2.0, (b) 1.0, and (c) none of NaOH. Table 2. Intercalation of LDH and MMT by POP-2MA of Different Molecular Weights from 400 to 4000 POP Backbones weight fraction (w/w)c a
intercalation agent LDH POP400-2MA POP2000-2MA POP4000-2MA MMT POP400-2MA POP2000-2MA POP4000-2MA
d spacing (Å) 7.8 32 63 7.8 12.4 16 51 16
b
calc by AEC or CECd 64/37 86/14 92/8 41/59 72/28 83/17
TGAe 41/59 59/41 83/17 63/37 6/94 21/79 71/29 28/72
a POA-amidoacids/clay ) 1/1 equivalent ratio. b Basal spacing of X-ray diffraction calculated by Bragg equation (n ) 1). c Weight fraction: organic composition/silicates. d Calculated by anionic or cationic exchange capacity (AEC or CEC). e Measured by TGA.
the existence of a different mechanism from the conventional ionic exchange reaction. Further analysis by X-ray diffraction indicated that the conventional intercalation with POP2000-2MA-xNa+ afforded silicates with a less ordered Bragg’s pattern. As compared in Figure 1, the X-ray diffraction pattern shows a series of Bragg’s peaks in a pattern from n ) 2 to 7 when the POP2000-2MA was treated with 1 equiv of NaOH. A similar basal spacing but with a highly ordered gallery structure was obtained with less NaOH addition. A highly ordered pattern was similarly obtained for the POP-MA without Na+. This observation implies the existence of an entirely different mechanism from the ionic exchanging for LDH intercalation. 3.2. Intercalation for Anionic LDH and Cationic MMT by the Same POA-Amidoacid. Previously, we developed a nonconventional intercalation mechanism for cationic clays by using amidoacids and carboxylic acid salts.32-34 The mechanism involves a beta-amido acid chelation intermediate with sodium ions in the silicate interlayer confinement. The driving force by the formation of 5- or 7-membered ring intermediates was considered for organic access into the clay galleries. However, this hypothesis can not be extended to the anionic LDH clays because of the negatively charged species. In Table 2, it is demonstrated that both MMT and LDH clays are affected by thePOA-2MAamidoacidsinasimilarmanner.BothPOP400-2MA (ca. 400 Mw) and POP2000-2MA (ca. 2000 Mw) are effective for incorporation into the LDH and MMT clays. The XRD analyses indicate an increase in basal spacing from 7.8 to 63 Å for LDH and 12.4 to 51 Å for MMT when using POP2000-2MA as the intercalating agent. However, the analogous POP4000-2MA
with a high molecular weight backbone (4000 Mw) failed to expand both clays as measured by XRD. Actually, the lengthy POP backbone reduced the intercalation ability of the amidoacid and rendered the compound insoluble and nondispersible in water. When comparing the basal spacing, organo-MMT was expanded in a less degree than organo-LDH, perhaps due to the differences between tSisOH (in MMT) and dAlsOH (in LDH). Furthermore, the organic incorporation into the confinement can be analyzed using thermal gravimetric analysis (TGA). The POP2000-2MA intercalation resulted in hybrids with 83 wt % organics for LDH and 71 wt % for MMT. The analyses are consistent with the theoretical calculation of 86 and 72 wt % based on the clay cationic exchange capacity (Table 2). These amounts of organic incorporation correspond with the basal spacing of 63 Å for LDH and 51 Å for MMT. 3.3. General Mechanism for Anionic and Cationic Clay Intercalation. A general mechanism is proposed to account for the MMT and LDH intercalation by the same intercalating agent. In Scheme 2a, the conventional ionic exchange reaction is illustrated, showing NO3- in the anionic LDH is exchanged producing NaNO3 as the leaving salt while Na+-MMT produces NaCl. In both cases, the oppositely charged organics are driven into the clay galleries through an ionic exchange mechanism.ToexplaintheeffectiveintercalationbyPOP2000-2MA in the neutral form, the hydrogen-bond driven mechanism is proposed in Scheme 2b. It is suggested that, due to the nature of oxypropylene and amidoacid moieties favoring the formation
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action of organics with the counterions in clays. The organoclays generating from the POA-amidoacids have a thermal responsive property in water and potential applications in the areas of drug encapsulations, biomaterial interaction, and sensor devices. Acknowledgment We acknowledge financial support from the Ministry of Economic Affairs and the National Science Council (NSC) of Taiwan. Literature Cited
Figure 2. Thermal responsiveness of POP-MA, POE-MA, and their derived organoclays, POP-MA/LDH and POE-MA/LDH, for dispersing in water.
of hydrogen bonds, the clay surface with tSisOH (in MMT) and dAlsOH (in LDH) functionalities may be accessible. The electrostatic interaction and hydrogen bonding as depicted in the inset diagram in Scheme 2b could well serve as the weak force for the general mechanism. 3.4. Property of Thermal Responsiveness for Dispersing the POP-Intercalated Clays in Water. It is known that the starting reactant of POP-diamine (2000 g/mol Mw) for POP-MA amidoacid synthesis is originally insoluble in water at ambient temperature but possessing a lower critical solution temperature (LCST) property below 10-15 °C.9,35 This implies that the hydrogen bonding is the main factor for the dispersing ability in water. Similarly, the corresponding POP2000-2MA amidoacid is soluble in water only at temperatures less than 10 °C. By comparison, the hydrophilic poly(oxyethylene)-backboned analog, POE2000-2MA amido acid is highly water-soluble through higher temperatures over 50 °C, as measured by optical transparency at 550 nm absorption spectroscopy and shown in Figure 2. The LCST phenomenon is attributed to thermal energy breaking hydrogen bonds between the POP or POE backbones and the water solvent. For the intercalated organoclays, POP2000-2MA/LDH exhibited a dispersing behavior at a low temperature (
