9260
Langmuir 2003, 19, 9260-9265
Relationship between the Continually Expanded Interlayer Distance of Layered Silicates and Excess Intercalation of Cationic Surfactants Zhongfu Zhao, Tao Tang,* Yongxin Qin, and Baotong Huang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China Received February 13, 2003. In Final Form: July 23, 2003 Excess intercalation of cationic surfactants into Na+-montmorillonites (MMTs) was investigated in organically modified silicates (OMSs), synthesized with MMTs and octadecylammonium chloride (OAC) by systematically varying the surfactant loading level from 0.625 to 1, 1.25, 1.56, 2, and 2.5 with respect to the cation exchange capacity (CEC) of MMTs. Wide-angle X-ray diffraction and thermogravimetric analysis results indicated that the continuous increase of interlayer distances came from the entering of surfactants into the interlayer of MMTs. Excess surfactants were extracted with a Soxhlet apparatus, which showed two kinds of intercalation states of surfactants in the interlayer when the surfactant loading level was beyond the CEC. Fourier transform infrared spectroscopy and differential scanning calorimetry were used to explore the microstructures of OMSs. It was found that the surfactants arranged more orderly as the loading level increased and the excess surfactants piled up in the interlayer together with counterions, forming a sandwiched surfactant layer. On the basis of the results, the layer structures of OMSs and the mechanism by which the surfactants entered the interlayer were expounded: surfactant cations entered the interlayer through cation exchange reactions and were tightly attracted to the silicate platelet surfaces when the surfactant loading level was below the CEC; however, excess cationic surfactants entered the interlayer together with counterions through hydrophobic bonding and formed a sandwiched layer in the interlayers, leading to a continuous increase of the interlayer distance, when the surfactant loading level was beyond the CEC.
Introduction In recent years, layered silicates, modified with a broad range of organic cationic surfactants as inorganic additives for polymers, have attracted the attention of academic and industrial researchers because nanocomposites made of layered silicates and polymers frequently exhibit unexpected properties synergetically derived from their components.1-3 These polymer-layered silicate nanocomposites, in which layered silicates are nanoscopically dispersed in the polymer matrix, show dramatic improvements in mechanical, thermal, and barrier properties together with other new properties not observed in their components, although only a low weight percentage (110 wt %) of layered silicates is used, much less inorganic content than in conventional mineral-reinforced polymer composites.4-9 Layered silicates commonly used in nanocomposites structurally belong to the family known as 2:1 phyllosilicates.9-12 Their crystal lattice consists of two-dimensional * To whom correspondence should be addressed. Phone: 0086431-5262004. Fax: 0086-431-5685653. E-mail:
[email protected]. (1) Lu, S.; Melo, M. M.; Zhao, J.; Pearce, E. M.; Kwei, T. K. Macromolecules 1995, 28, 4908. (2) Novak, B. M. Adv. Mater. 1993, 5, 422. (3) Schmidt, H. In Polymer Based Molecular Composites; Schaefer, D. W., Mark, J. E., Eds.; Material Research Society: Pittsburgh, PA, 1990. (4) Usuki, A.; Kawasumi, M.; Kojima, Y. Fukushima, Y. Okada, A. Kurauchis, T.; Kamigaito, O. J. Mater. Res. 1993, 8, 1179. (5) Kato, M.; Usuki, A.; Okada, A. J. Appl. Polym. Sci. 1997, 66, 1781. (6) Hsiao, S. H.; Liou, G. S.; Chang, L. M. J. Appl. Polym. Sci. 2001, 80, 2067. (7) Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1994, 6, 1719. (8) Zilg, C.; Thomann, R.; Mu¨lhaupt, R.; Finter, J. Adv. Mater. 1999, 11, 49. (9) Wang, S. G.; Long, C. F.; Wang, X. Y.; Li, Q.; Qi, Z. N. J. Appl. Polym. Sci. 1998, 69, 1557.
layers where a central octahedral sheet of alumina or magnesia is fused to two silica tetrahedra with common oxygen atoms. The layer thickness is around 1 nm, and the lateral dimensions of these layers may vary from 30 nm to several micrometers or even larger. These layers form stacks with regular van der Waals gaps called interlayers or galleries between them. Isomorphic substitution within the layers (for example, Al3+ replaced by Mg2+ or Fe2+, or Mg2+ by Li+) generates negative charges that are counterbalanced by Na+ or Ca2+ ions in the interlayer. These cations can be exchanged with cationic surfactants such as alkylammoniums or alkylphosphoniums (oniums). These layered silicates are hydrophilic and could not be intercalated with polymers without being modified with cationic surfactants. So it is important to make layered silicates organophilic by cation exchange reactions to be more compatible with organic polymers or monomers in the preparation of nanocomposites. Reichert et al.13 defined the cation exchange capacity (CEC; mequiv/g) as
CEC ) 1000(ξ¨ /M ¨)
(1)
where ξ¨ is the mean layer charge with respect to one formula unit and M ¨ is the mean molecular weight of one formula unit. It is usually believed that the amount of cationic surfactants entering an interlayer is proportional to that of metal cations naturally occurring in the interlayer and thus depends on the charge of the layers. (10) Alexandre, M.; Dubois, P. Mater. Sci. Eng. 2000, 28, 1. (11) Brindley, S. W.; Brown, G. Crystal Structure of Clay Minerals and their X-ray Diffraction; Mineralogical Society: London, 1980. (12) Giannelis, E. P.; Krishnamoorti, R.; Manias, E. Adv. Polym. Sci. 1999, 118, 108. (13) Reichert, P.; Kressler, J.; Thomann, R. Acta Polym. 1998, 49, 116.
10.1021/la030056h CCC: $25.00 © 2003 American Chemical Society Published on Web 09/24/2003
Excess Intercalation of Cationic Surfactants
Langmuir, Vol. 19, No. 22, 2003 9261
Table 1. Characterization of Octadecylammonium-Modified Layered Silicates surfactant loading (g of OA/100 CLAY) surfactant loading (normalized to the CEC) interlayer distance before extraction (nm) interlayer distance after extraction (nm) clearance distance before extraction (nm) organic content (weight loss) (wt %) surfactant adsorption level (normalized to the CEC) adsorbed percentage of surfactants (%)
OMS62.5
OMS100
OMS125
OMS156
OMS200
OMS250
19.5 0.625 1.78 1.75 0.82 8.35 0.320 51.2
32 1 2.03 2.03 1.07 15.85 0.654 65.4
39 1.25 2.36 2.18 1.40 18.07 0.805 64.4
50 1.56 2.90 2.18 1.94 26.34 1.233 79.0
64 2 3.71 2.18 2.75 28.42 1.457 72.8
78 2.5 3.87 2.20 2.91 30.56 1.743 69.7
Many previous reports believed that surfactants could not enter the interlayer any more, or the interlayer distance could not be increased any more, as soon as the cations naturally occurring in the interlayer had been completely exchanged.14-21 In their work, excess surfactants, sometimes up to 2 times the CEC, were used in the process of modifying layered silicates only to ensure complete exchange of the metal cations in the interlayer. Tseng et al. concluded that the interlayer distance increased with surfactant loading level until saturation was reached at the CEC.22 However, Yui et al.15 found that seven cationic surfactants could intercalate into the cation-exchangeable silicates in a amount above the CEC, but the sharp peak of d001 remained unchanged in the region in excess of the CEC. Previous studies suggested that the nature of the cationic surfactants was an important factor in the preparation of polymer/MMT (Na+-montmorillonite) nanocomposites.23,24 To modify MMTs, researchers treated layered silicates with various surfactants and investigated how surfactant structures affect the degree of delamination of MMT platelets. In our recent work to prepare polymer/MMT nanocomposites, it was found that cationic surfactant loadings have a great effect on the structures of organically modified silicates (OMSs) and the structures and properties of the corresponding polymer/MMT nanocomposites. However, there are no reports about the above phenomena. In this paper, OMSs are synthesized by systematically varying the surfactant loading level, and the relationship between the expanded interlayer distances of OMS and excess intercalation of cationic surfactants will be explored by investigating the effects of surfactant intercalation on the structures of OMSs. To the best of our knowledge, there have previously been no such reports. On the basis of the results, we expound the structure of OMSs and propose a mechanism about how surfactants enter the interlayer. It will benefit our understanding of the (14) Ogawa, M.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2593. (15) Yui, T.; Yoshida, H.; Tachibana, H.; Tryk, D. A.; Inoue, H. Langmuir 2002, 18, 891. (16) Hsu, S. L. C.; Chang, K. C. Polymer 2002, 43, 4097. (17) Kaempfer, D.; Thomann, R.; Mu¨lhaupt, R. Polymer 2002, 43, 2909. (18) Hsiao, S. H.; Liou, G. S.; Chang, L. M. J. Appl. Polym. Sci. 2001, 80, 2067. (19) Philip, B. M.; Emmanuel, P. G. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 1047. (20) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Chem. Mater. 1994, 6, 1017. (21) Yano, K.; Usuki, A.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2493. (22) Tseng, C. R.; Lee, H. Y.; Chang, F. C. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 2097. (23) (a) Fornes, T. D.; Yoon, P. J.; Hunter, D. L.; Keskkula, H.; Paul, D. R. Polymer 2002, 43, 5915. (b) Dennis, H. R.; Hunter, D. L.; Chang, D.; Kim, S.; White, J. L.; Cho, J. W.; Paul, D. R. Polymer 2001, 42, 9513. (c) Reichert, P.; Nitz, H.; Klinke, S.; Brandsch, R.; Thomann, R.; Mulhaupt, R. Macromol. Mater. Eng. 2002, 275, 8. (24) (a) Usuki, A.; Kawasumi, M.; Kojima, Y.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Mater Res. 1993, 8, 1174. (b) Okada, A.; Usuki, A. Mater. Sci. Eng. 1995, C3, 109. (c) Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater. 1995, 7, 2144.
dispersion of MMT platelets in a polymer matrix to learn more about OMSs. Experimental Section The materials used in this paper are layered silicates (Na+montmorillonites) with a CEC of 119 mequiv/100 g from Kunimine Co. and octadecylamine from Wako Pure Chemical Industries Co., Osaka, Japan. OMSs were synthesized by cation exchange reaction between MMTs and octadecylammonium. Octadecylamine, first protonated with equimolar concentrated HCl in 1000 mL of hot deionized water (about 80 °C), was poured into a hot dispersion of MMTs (16 g) in 1000 mL of hot deionized water (about 80 °C). The mixtures were stirred vigorously for some time, giving white precipitates. Then, the precipitates were filtered with a nylon cloth, washed with hot deionized water three times (about 80 °C), and dried in the air. At last, they were crushed with a pulverizer and dried again in a vacuum drier at 60 °C for 24 h. Wide-angle X-ray diffraction (WAXD) was carried out with a Rigaku model Dmax 2500 with Cu KR radiation (λ ) 1.54 Å), and the interlayer distance (d001) of OMSs was estimated from the (001) peak in the WAXD pattern with the Bragg formula. To obtain the cationic surfactant content in OMSs, thermogravimetric analysis (TGA) was conducted with a Perkin-Elmer TGA7 in the air, with heating from room temperature to 700 °C and a heating rate of 10 °C/min. Fourier transform infrared spectroscopy (FTIR) spectra were collected using a Y-Zoom Scroll spectrometer with a nominal resolution of 4 cm-1. Spectra were obtained from KBr pellets. Differential scanning calorimetry (DSC) was performed using a Perkin-Elmer thermal analyzer with a heating speed of 10 °C/min under N2. The samples first underwent a heating from -20 to +120 °C, then a cooling, and finally a second heating. Electron spectroscopy for chemical analysis (X-ray photoelectron spectroscopy, XPS) was also used to investigate N and Cl in the extraction on a VG ESCA Lab MK II.
Results and Discussion Intercalation of Cationic Surfactants into Na+montmorillonites. MMTs were well dispersed in hot deionized water by vigorous stirring. On pouring the aqueous solution of the surfactant into an aqueous MMT suspension with vigorous stirring for 30 min, white precipitates were formed under conditions described in the Experimental Section. Under the same conditions, the loading levels of surfactants with respect to the CEC of MMTs were varied systematically from 62.5% to 100%, 125%, 156%, 200%, and 250%. OMSs with different surfactant adsorption levels were obtained, described as OMS62.5, OMS100, OMS125, OMS156, OMS200, and OMS250, respectively. The characterization results are summarized in Table 1. To find the adsorption level of organic surfactants, OMSs were characterized by TGA (Figure 1), and according to weight loss from 200 to 500 °C,25 the adsorbed level and adsorbed percentage of surfactants could be obtained (Table 1). Obviously the surfactants are incorporated into the layered silicates, and the adsorption level of the (25) Xie, W.; Gao, Z. M.; Pan, W. P.; Hunter, D.; Singh, A.; Vaia, R. Chem. Mater. 2001, 13, 2979.
9262
Langmuir, Vol. 19, No. 22, 2003
Zhao et al.
Figure 1. TGA curves of OMSs. Figure 3. Change of the adsorbed level of surfactants and the clearance distance as a function of the surfactant loading level.
Figure 2. WAXD profiles of various OMSs as a function of the surfactant loading level.
surfactants increases monotonically with the surfactant loading level. It is more important that this tendency does not stop even at a surfactant loading level of 2.5 times the CEC. To study the intercalated structure of OMSs, WAXD measurements were carried out (Figure 2). The interlayer distances, as expanded by surfactant intercalation, were estimated from the position of the (001) diffraction peak and are summarized in Table 1 together with a “clearance distance”, defined as the value obtained by subtracting the intrinsic layer thickness (0.96 nm)26 from the observed interlayer distance. In Figure 2, the (001) peak (2θ ) 7.02°; d ) 1.258 nm) of the original MMTs undergoes an obvious shift to a smaller angle as the surfactant loading level increases, implying that the interlayer distance continually increases with the surfactant loading level. To clarify the fact that the interlayer distance increases with surfactant loading level beyond the CEC, a phenomenon not reported before, the relationship between the clearance distance and the adsorption level of surfactants was sought. From TGA and WAXD data, both the clearance distance and the adsorption level of the surfactants depend on the surfactant loading level (Figure 3). The continual increase of surfactant adsorption level with surfactant loading level testifies that the excess surfactants indeed could enter and reside inside the interlayer, in agreement with literature reports.15,26 These authors had proved that the majority of the excess surfactants resided in the interlayer, but not physioadsorbed to the exterior of the crystallites or contained in voids within the OMS aggregates, and overexchanged OMSs did not have a substantially greater (26) Habti, A.; Keravis, D.; Levitz, P.; Damme, H. V. J. Chem. Soc., Faraday Trans. 1984, 80, 67.
Figure 4. Comparison of the interlayer distance of OMSs before and after extraction.
fraction of surfactants residing outside the interlayer than that exchanged at equivalence. It is interesting that the clearance distance also monotonically increases with the surfactant loading level. The results strongly indicate that the increase of the interlayer distance comes from entering of the surfactants into the interlayer. To further prove the above conclusion, OMSs were extracted with tetrahydrofuran in a Soxhlet apparatus and then characterized with WAXD. The interlayer distances of OMSs after extraction (Table 1) are compared with those of the corresponding OMSs (Figure 4). Their interlayer distances are found to decrease after extraction except for those of OMS62.5 and OMS100. Thus, it is demonstrated that the enlargement of interlayer distances is due to the entering of excess surfactants into the interlayer beyond the CEC, while the interlayer distances of OMS62.5 (
