Fatty acids in layered metal hydroxides: membrane-like structure and

J. Phys. Chem. 1992, 96, 5434-5444

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References and Notes (1) Thompson, R. C.; Barr, J.; Gillupie, R. J.; Milne, J. B.; Rothenbury, B. A. Inorg. Chem. 1965,4, 1641. (2) Commeyras, A; Olah, G. A. J. Am. Chem. SOC.1969,91,2929. (3) Olah, G. A.; DeMember, J. R.; Shen, J. J. Am. Chem. SOC.1973,95, 4952. (4) Olah, G. A.; Shen. J.: Schlosberg, R. H. J. Am. Chem. Soc. 1973,95, 4957. (5) Olah, G. A.; Halpern, Y.; Shen, J.; Mo, Y. K. J. Am. Chem. Soc. 1973, 95,4960. (6) Olah, G. A.; Kaspi, J.; Bukara, J. J. Org. Chem. 1977,42, 4187. (7) Ono, Y.; Tanabe, T.; Kitajima, N. Chem. Lett. 1978,625. ( 8 ) Tanabe, K.; Hattori, H. Chem. Lett. 1976,625. (9) Takahashi, 0.; Yamauchi, T.; Sakuhara, Y.; Hattori, H.; Tanabe, K. Bull. Chem. SOC.Jpn. 1980,53, 1807. (10) Hattori, H.; Takahashi, 0.;Takagi, M.; Tanabe, K. J. Catal. 1981, 68, 132. Takahashi, 0.; Hattori, H. J. Cafal.1981,68, 144. (11) Hino, M.; Kobayashi, S.; Arata, K. J. Am. Chem. SOC.1979,101, 6439. (12) Jin, T.; Yamaguchi, T.; Tanabe, K. J. Phys. Chem. 1986,90,4795. (13) Hatakeyama, K.; Suzuka, T.; Yamane, M. Sekiyu Gakkaishi 1991, 34. 267 (Journal of JaDan Petroleum Societv. in JaDanesel. '(14) Olah, G. A.; Felberg, J. D.; Lamm&tsma,'K. J. Am. Chem. SOC. 1983,105,6529. (1 5 ) Teo, B. K. EXAFS Basic Principles and Data Analysis; SpringerVerlaa: Berlin, 1986.

(16) Salama, T. M.; Tanaka, T.; Yamaguchi, T.; Tanabe, K. Surf.Sci. Letr. 1990,L100. (17) Bart, J. C. J. In Advances in Catalysis; Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic Press: New York, 1986; Vol. 34, p 203.

(18) Bair, R. A.; Goddard, W. A. Phys. Rev. B 1980,22, 2767. (19) There is the other hypothesis based on the photoelectron scattering

phenomenon by environmental atoms. The explanation of XANES standing on the hypothesis is pruented in the following paper: Lytle, F. W.; Greegor, R. B.; Panson, A. J. Phys. Reu. B 1988,37, 1550. (20) Yoshida, S.; Tanaka, T. Ado. X-ray Chem. Anal. Jpn. 1988,19,97. (21) McMaster, W. H.; Kerr Del Grande, N.; Mallet, J. H.; Hubell, J. H. Compilation of X-ray Cross Sections; National Technical Information SeMcc: Springfield, 1969. (22) Sayers, D. E.; Bunker, B. A. X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES; Koningsberger, D. C . , Prins, R., Eds.; John Wiley & Son: New York, 1988; p 211. (23) Tanaka, T.; Yamashita, H.; Tsuchitani, R.; Funabiki, T.; Yoshida, S. J. Chem. SOC.,Faraday Trans. I 1988,84,2987. (24) Sayers, D. E.; Stern, E. A.; Lytle, F. W. Phys. Rev. Lett. 1971,27, 1204. Lytle, F. W.; Wei, P. S. P.; Greegor, R. B.; Via, G. H.; Sinfelt, J. H. J . Chem. Phys. 1979,70, 4849. (25) Wells, A. F. Structural Inorganic Chemisty, 5th ed.; Oxford Sci. Publ.: Oxford, 1984. (26) Edwards, A. J. J. Chem. Soc A 1970,2751. (27) Moore, J. W.; Baird, H. W.; Miller, H.B. J. Am. Chem. Soc. 1968, 90,1358. (28) Nandana, W. A. S.; Passmore, J.; White, P. S. J. Chem. SOC.,Dalton Trans. 1985,9, 1623. (29) Hoffman, C . J.; Holder, B. E.; Jolly, W. L. J. Phys. Chem. 1958,62, 364. (30) Brown, I. D. Structure and Bonding in Crystals II; O'Keeffe, M., Navrotsky, A,, Eds.; Academic Press: New York, 1981; p 1. (31) Tanabe, K.; Misono, M.; Ono,Y.; Hattori, H.New Solid Acids and Bases; Kondansha-Elsevier: Tokyo-Amsterdam, 1989.

Fatty Acids in Layered Metal Hydroxides: Membrane-iike Structure and Dynamlcs Marlon Borja and Prabir K. Dutta* Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210 (Received: October 18, 1991; In Final Form: February 6, 1992)

Layered metal hydroxides consist of sheets of positively charged edge-sharing M(OH)6 octahedra as exemplified by the hydrotalcite family of materials. Anions occupy positions in the interlayers. In this study, we focus on the incorporation of long-chain fatty acids (CH,(CHJ,COOH, n = 10, 12, 14) in between these layers. These materials mimic membranes and exhibit interesting packing and dynamic characteristics. The packing is strongly influenced by the charge distribution of the metal hydroxide layer. In the case of LiA12(0H)6+(25 A2/unit charge), a monomolecular film is formed, whereas a bimolecular film is formed in Mg3AI(OH)8+(33 A2/unit charge). In the lithium aluminum hydroxide, the carboxylic acid is ion-exchanged into the layers, but in magnesium aluminate it is intercalated as the carboxylic acid form. On the basis of diffraction, calorimetry, and vibrational spectroscopy, the dynamics of the films as a function of temperature has been explored. For LiA12(0H)6+,the spacing between the layers increases with temperature and retains an all-trans packing of the alkyl chains. In the Mg,Al(OH)8+sample, the spacing between the layers decreases with temperature and the bimolecular film becomes disordered through the formation of kinks and gauche blocks at temperatures exceeding 130 OC. The control of packing density and dynamics of the alkyl chains via the charge density of the metal hydroxide layer results in the formation of novel. membrane-like materials.

Introduction There is considerable interest in packing of long-chain polymethylene units and the dynamics of these chains because of their relevance to biological membranes' and also in technologies involving liquid crystalsS2 Agglomerates of such long-chain hydrocarbon molecules exhibit characteristic and collective changes in structure, dictated primarily by the fluidity and flexibility of these chain^.^ The microscopic environment around the agglomerates is of importance in this regard. Self-aggregation of biological lipid-like materials in aqueous solution is promoted by the hydrophobic effect and results in formation of bilayers and vesicles. Considerable research has been done on the structure and dynamics of these system^.^ It is of interest to explore other methods of aggregation of these systems. Layered compounds such as montmorillonite and zir*Author to whom correspondence should be addressed.

conium phosphates can intercalate long-chain polymethylene units with functionalities such as alcohols and amine^.^,^ The functional group interacts with the inorganic layer, thereby orienting the polymethylene chain in the interlayer space. The packing of these chains in the interlayers and the influence of temperature on the dynamics of these chains has been examined.'~~ Our interest has been in layered double metal hydroxides as possible hosts for aligning inorganic, organic, and organometallic species as well as studying their interlayer rea~tivity.~ These materials can be represented as [M11,,M111,(OH)2]A,,,".zH20 where MI1= Mg, Zn, Fe, Co, Ni, Cu and MI1' = Al, Cr, Fe. For example, the structure of a member of this group MgZAl(OH)&l can be pictured as derived from brucite (Mg(OH),), in which sheets of Mg2+cations octahedrally surrounded by edge-sharing OH groups exist. Upon replacement of a certain fraction of the Mg2+ions by A13+,the metal hydroxide layer becomes positively charged, necessitating the presence of anions, such as C1- in the

0022-365419212096-5434$03.00/0 0 1992 American Chemical Society

Fatty Acids in Layered Metal Hydroxides

The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5435

interlayer space. Since the anions can be replaced, these materials have been referred to as anionic clays.I0 The two layered double hydroxides (LDH) discussed in this paper are Mg3A1(OH)8-Cland LiAl2(0H)&1, chosen because the charge density of the layers are different in the two These materials are abbreviated as Mg,Al-LDH and LiAlz-LDH in this report. Besides charge densities, other factors that are not yet understood must also play a role in the ion-exchanging ability of double metal hydroxides. For example, V10028bwill ion-exchange into the magnesium-aluminum and zinc-chromium layered double hydroxides, but not in the lithium aluminum analog, even though the latter two have similar charge d e n s i t i e ~ . ~ ~ J ~ ~ Similarly, tartrate ion will ion-exchange in the Zn-Cr system but m u not in the Li-A1 system.I0g In this paper, we explore how the C differences in charge densities and other properties between these 0 0 materials influence the packing, inter- and intramolecular interactions and dynamics of polymethylene chains introduced into the layers as long-chain fatty acids. The possibility of ion-exchanging such materials in layered metal hydroxides has been established from previous studies,Ih*8although no detailed information about structure and dynamics has been reported.

Experimental Section LiA1,-LDH was synthesized by a variation of the method by Sema et ala1&Sodium hydroxide (0.2mol) was dissolved in 80 mL of freshly boiled nanopure water. To this solution was slowly added 0.05 mol of aluminum powder until dissolved. Lithium chloride (0.25mol) in 20 mL of solution was added to the mixture and heated for 48 h at 100 OC. The mixture was filtered after heating, and the residue was placed in 100 mL of 0.5 M sodium chloride and shaken for 24 h. The products were filtered and washed with water. The residue was stored wet in stoppered vials until further use. Mg3Al-LDH was prepared in a similar manner. Sodium hydroxide (0.725mol) was dissolved in 240 mL of water to which aluminum (0.0725 mol) was added slowly until dissolved. Magnesium chloride hexahydrate (0.216mol) dissolved in 60 mL water was added slowly while stirring and heated for 48 h at 100 OC. The product was filtered and washed with 0.5 M sodium chloride solution and nanopure water. Prior to reaction with long-chain carboxylic acids, both lithium and magnesium aluminum LDHs were washed with absolute ethanol. Reactions with acid were carried out with separate 0.1 M ethanolic solutions of lauric, myristic, and palmitic acids (Aldrich). The mixtures were centrifuged and the residues were extensively washed with ethanol. The resulting products were air-dried and stored in stoppered vials. X-ray powder diffraction patterns before and after ion-exchange were obtained with a Rigaku Geigerflex D/Max 2B diffractometer using Cu Ka radiation. Temperature-programmed X-ray powder diffraction patterns were obtained with Scintag XDS 2000. Infrared spectra were collected on a Mattson Cygnus FTIR spectrometer. Heating experiments were done with an IR heating cell equipped with a thermocouple, at a temperature control of f 1 OC. Excitation for the Raman spectra was done with 15 mW of 406.7-nmradiation from a Coherent Innova 100 krypton ion laser. The scattered light was collected and dispersed through a Spex 1403 double monochromator and detected with a GaAs PMT with photon counting. Differential calorimetric scans were obtained using Perkin-Elmer DSC 7 Series. X-ray fluorescence was used for C1- analysis using a Kevex 0700/7000energy dispersive instrument. Results The carboxylic acids chosen in this study are of the general formula CH3(CH2),COOH,where n = 10,12,14,the acids being lauric, myristic, and palmitic acid, respectively. The observations in all these systems were similar, so in presenting the data, we have chosen to focus on myristic acid. The ion-exchange, packing, and conformational changes as a function of temperature have been examined. Incorporation of Acids. Figure 1 shows the X-ray powder diffraction patterns for Mg3Al-LDH and LiA12-LDH after re-

X

10

5

I

2-Theta

Figure 1. X-ray powder diffraction patterns for myristic acid with (a) LiA12-LDH and (b) Mg3AI-LDH. The (001) reflections are marked by x. C1- corresponds to unreacted starting material. Ei

u

c

-

12

10

14

Number o f C a r b o n Atoms, n CHI ( CH2) ,COOH

Figure 2. Plots of interlayer spacing versus the carbon number for lauric, myristic, and palmitic acid in (a) Mg3A1-LDH and (b) LiA1,-LDH.

action with myristic acid in ethanol. The (001)reflections are marked on the figure. In both the samples, the reflection marked C1- corresponds to the unexchanged material, and represents a fraction of the starting compounds. This fraction was found to be unreactive even after repeated reaction with the myristic acid. The interlayer spacing or film thickness (calculated as basal spacing less 4.8 A) for LiA12-LDH and Mg3Al-LDH are markedly different, being 21.5 f 0.2and 39.3 f 0.2 A, respectively. The dimension of the myristic acid molecule based on an all-trans arrangement is 21.2A, including the van der Waals radii of the terminal H and 0 atoms. This dimensional analysis would suggest that a monolayer of myristic acid molecules are present in LiA12-LDH, whereas a bilayer is forming in Mg3Al-LDH. In order to confirm this, lauric and palmitic acid were exchanged into LiAl,-LDH and Mg,Al-LDH, and Figure 2 provides a plot of the interlayer spacing versus the number of carbon atoms in the fatty acid. It is clear that the slopes of the curves, which are a reflection of the change in spacing versus change in carbon length, are quite distinct for LiA12-LDH and Mg3Al-LDH, being 1.22 and 2.35 A/C, respectively. Since the C-C distance in paraffins is 1.27 A, an increase of 2.35 A/C would definitely confirm the presence of a bilayer type arrangement in Mg3AlLDH. In the case of LiA12~LDH, the slope of 1.22A/C indicates a monolayer arrangement in which the CHI chains are arranged almost perpendicularly to the horizontal metal hydroxide layer.

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5436 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992

Borja and Dutta

Al

I

1700

1600 Wavenumber (cm

1500

1400

-)

Figure 4. Infrared spectra in the 1350-1 800-cm-' region for (a) myristic acid, (b) myristic acid-Mg,Al-LDH, (c) myristic acid-LiA12-LDH, and (d) Na-myristate.

Figure 3. X-ray fluorescence analysis of (a) LiAI,-LDH-CI, (b) LiA1,-LDH-myristic acid, (c) Mg,AI-LDHCI, and (d) Mg3AI-LDHmyristic acid. (Analysis for Mg, AI was done with X-rays from a Rh source, and for C1, a secondary target of Ti was used.)

Thus, it is clear that pronounced differences exist between the arrangement of fatty acid molecules in LiA12-LDH and Mg,AI-LDH. Besides the fatty acid molecules in the interlayers, there also appears to be significant amount of ethanol in these materials. This is apparent from a weight loss of about -30 wt % upon heating both the LiA1,-LDH and the Mg3Al-LDH samples at 90 OC. We attribute this loss to that of ethanol both on the surface and in the interior of the material, although it is hard to estimate these amounts. X-ray fluorescence analysis of the samples show that in the case of LiAl,-LDH, the amount of C1 in the sample has decreased by 90%, whereas the C1 in Mg3A1-LDH remains unchanged. This is apparent from the ratio of CI/Al and Cl/Mg X-ray fluorescence signals in Figure 3. The C, H analysis for the dried (90 "C) LiA1,-LDH and Mg3Al-LDH myristic acid complexes were 39.57%, 8.62% and 35.66%, 7.70%, respectively. Since the fraction of unreacted starting material is unknown, the exact chemical composition could not be calculated. However, if we make the assumption that LiA1,-LDH ion-exchanges a monolayer and Mg3Al-LDH intercalates a bilayer of myristic acid, as is evident from the diffraction patterns and X-ray fluorescence, then the C, H analysis indicates that 85.5% of LiA1,-LDH and 55% of Mg3Al-LDH have reacted. The unreacted materials do not interfere with the spectroscopic studies discussed in this paper. These results indicate that there is a fundamental difference in the incorporation of the myristic acid in both these samples. In the LiA1,-LDH case, it occurs primarily by ion-exchange of the C1- ions, thus introducing RCOO- in the interlayer, whereas in Mg,Al-LDH, it occurs by intercalation of the RCOOH in the layers. Intercalation of intact organic acids into these layered compounds has also been previously noted for reactions in nonaqueous solvents.9b State of the Carboxylic Group. Information about the state of the carboxylic group can be obtained from the infrared spectra.

Figure 4 compares the spectra of neat myristic acid, the myristic acid intercalated Mg3A1-LDH, LiA1,-LDH, and Na-myristate. The focus will be on the 1500-1700-cm-~region, which is characteristic of the carboxyl group vibrations." The band at 1701 cm-l in myristic acid (Figure 4a) is characteristic of u(C0) of the associated carboxyl group. Upon ionization, v,,,,(CO) is observed at 1558 cm-I and the vsw(CO) at 1424 cm-I as seen in Na-myristate (Figure 4d). In the Mg3Al-LDH (Figure 4b) and LiA12-LDH (Figure 4c) samples, several bands are observed in the v,,(CO) stretching region: 1542, 1557, 1589, 1637 cm-' in Mg3AI-LDH and similar bands in LiA12-LDH, except that the 1546-cm-' band is considerably enhanced. In addition, as shown in the inset, a weak shoulder is observed at 1720 cm-I in the Mg3AI-LDH sample. This multiplicity of v,,(CO) bands indicates that there is a diversity of COO- groups existing in the interlayers. Structurally, such a distribution can be rationalized by proposing several COO- group with nonsymmetric interaction with positively charged centers. If the positive center interacts preferentially with one of the 0 atoms of COz, it will introduce anisotropy between the two C-0 bonds. This would result in the increase of the uaSym(CO) frequency. Unsymmetrical hydrogen bonding would also be expected to have a similar effect. Considering the high frequencies of the 1589-, 1637-, and 1720-cm-' bands and the X-ray fluorescence data, it is likely that the myristic acid is actually intercalating in the carboxylic acid form RCOOH, the H+beiig subsequently ionized in the interlayers but remaining strongly d a t e d with the carboxylate group as in C(0)ObH+6. The 1542-cm-l band can be assigned to the unassociated carboxylate form of the myristic acid. In the case of LiA12-LDH, the major intensity is in the 1540-cm-l region due to myristate ions replacing C1- ions in the interlayer. However, the presence of the bands at 1590 and 1640 cm-l indicates that a small amount RCOOH molecules are also present in LiAI,-LDH. The infrared spectra suggests that both ion-exchange and intercalation are indeed occurring in the mechanism of uptake of the myristic acid, however to different degrees in Mg3Al-LDH and LiA1,-LDH. Similar data were also obtained for the palmitic acid system. Packing of the Hydrocarbon Cbab. The packing of the hydrocarbon chains was examined by vibrational spectroscopy. In all cases, we have presented the spectroscopic data for myristic



1

1

2600

I 650

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Wovenumber (cm

1400

1600

7

Figure 5. Raman spectra in the 650-1600-~m-~ region for (a) myristic acid, (b) myristic acid-Mg,Al-LDH, (c) myristic acid-LiA12-LDH, and (d) Na-myristate (excitation, 406.7 nm).

acid in Mg3Al-LDH and LiA1,-LDH along with neat myristic acid and its sodium salt. A. R a m Spectroscopy. The packing of the chains and both inter- and intramolecular interactions within these chains is of interest. Raman and infrared spectra have been extensively developed as probes for chain packing and dynamics and were used to examine the long-chain fatty acids in Mg,Al-LDH and LiAl,-LDH. In the Raman spectra, the frequenciesof interest lie in the 650-1600-cm-' and 2600-3200-cm-' regions. These data for myristic acid are shown in Figures 5 and 6. The skeletal C-C stretching modes at 1060, 1100, and 1130 cm-' are characteristic of the all-trans conformer,12and these bands are evident in myristic acid and its salt (Figures 5a and 5d). These bands are preserved upon the introduction of myristic acid into the metal hydroxide layer, thus confirming a trans arrangement of the polymethylene chains in the interlayer space. The relative intensities of these bands differ in LiA12-LDH and Mg3Al-LDH, especially the increase in intensity of the 1130-cm-' band in Mg,Al-LDH (Figure 5b). The intensity of this band has been correlated with the all-trans feature of the polymethylene chain and could indicate that there is more disorder in the LA2-LDH system as compared to the Mg3A1-LDH system. However, since this band is also coupled with the methyl terminal rocking mode at 892 cm-',13 packing differences could be the reason for the alteration in intensity of the 1130-cm-' band rather than any difference in the inherent disorder between the LiA1,-LDH and Mg3Al-LDH systems. Figure 6 shows the Raman data in the C-H stretching region. The four significant bands at 2845,2880,2925, and 2968 cm-l are at frequencies appropriate for CH2 symmetric and asymmetric and CH3 symmetric and asymmetric stretches, re~pective1y.I~ The myristic acid in Mg,Al-LDH and LiA12-LDH exhibit similar frequencies for the CH2 stretching modes (Figure 6, parts b and c). However, the spectra are quite distinct in the CH3 stretching region. For example, the CH, symmetric and asymmetric stretch at 2925 and 2968 cm-l are broadened out in the LiA12-LDH sample (Figure 6c). The intensities of bands in this region are strongly influenced by Fermi resonance with overtones of the methyl deformation bands around 1400-1450 cm-I,l4 As seen in Figure 5 , two bands are observed in the 140&1500Cm-1 region, at 1430 and 1450 cm-l, and can be assigned to CHI

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2700

2800

2900

3000

Wovenumber (cm

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3200

Figure 6. Raman spectra in the 2600-3200-~m-~region for (a) myristic acid, (b) myristic acid-Mg,Al-LDH, (c) myristic acid-LiA12-LDH, and (d) Na-myristate (excitation, 406.7 nm).

scissoring and a Fermi resonance band between the methylene rocking and asymmetric methyl deformation mode. The relative intensities between these bands are different in the Mg3Al-LDH and LiA12-LDH samples which would bring about changes in intensity in the C-H stretching region. Thus, the differences that we observe in the C-H stretching region for myristic acid in Mg3Al-LDH and LiA1,-LDH can result from differences in the intermolecular chain coupling which is reflected in the C-H stretching frequency region due to Fermi resonance effects. The infrared spectra discussed below confirms this assignment. The band at 2930 cm-I arises from ordered packing in analogy with crystalline fatty acids.I4 Similar data were also obtained for the palmitic acid exchanged samples in LA2-LDH and Mg3Al-LDH. Therefore, the Raman spectra indicates that the polymethylene chains are ordered in an all-trans configuration in both LiA1,LDH and Mg,Al-LDH, though the packing of the chains may be distinct in these two cases. B. Mrared Spectrowopy. The infrared spectra of myristic acid in LiA1,-LDH and Mg3Al-LDH are compared in Figures 7-9. (Along with Figure 4 in the 1350-1800-cm-' region, these figures provide the complete mid-infrared spectrum,) In the CHI wagging region (Figure 8) a progression of bands that is characteristic of an all-trans arrangement of the polymethylene chain is observed. This is consistent with the Raman data and interlayer spacing obtained from the diffraction profiles. The increased intensity of the bands in the wagging region as compared to long-chain alkanes is due to coupling with the v(C0) of the carboxylic There is a subtle difference between the LiA1,-LDH and Mg,AI-LDH systems; the progression of bands are split into two in the latter case (Figure 8b). There are two explanations for this effect. This progression could be due to CH2 twisting modes, which occur in this region but typically have low intensities. The other possibility is that the CHI wag is splitting due to correlation effects. This will be dependent on the number of polymethylene chains per unit cell. In an orthorhombic lattice, the unit cell contains two chains and the vibrations are expected to be split because of the in-phase and out-of-phase motions of the two chains in the unit cell. For triclinic or monoclinic symmetries there is only one chain per unit cell and such splittings are not expected. These splittings are usually quite marked for the CH2 bending and rocking modes.I6 Figures 4b and 7b show that in Mg3AlLDH both the CH2 bending (1465 and 1472 cm-I) and CH2 rocking (716 and 723 cm-l) are split, whereas in LiA12-LDH, they


Borja and Dutta 1

1100

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600

Wavenumber (cm -)

Wavenumber (cm

Figure 7. Infrared spectra in the 600-1 150-cm-I region for (a) myristic acid, (b) myristic acid-Mg,Al-LDH, (c) myristic acid-LiAlrLDH, and (d) Na-myristate.

t0

I300

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1250 Wovenumber (cm

-)

Figure 8. Infrared spectra in the 1150-1350-cm-’ region for (a) myristic acid, (b) myristic acid-Mg,Al-LDH, (c) myristic acid-LiA1,-LDH, and (d) Na-myristate.

appear as single bands at 1468 (Figure 4c) and 721 (Figure 7c) cm-’, respectively. Similar observations were also made with palmitic acid sample. Thus, the symmetry of packing of the polyethylene chains in Mg,Al-LDH is in an orthorhombic unit cell,whereas it is triclinic or monoclinic in the LiA1,-LDH sample. The infrared spectra in the 2600--3200-~m-~ region is shown in Figure 9. As in the Raman spectra, the differences in the myristic acid C-H stretching region between the LiA12-LDH and Mg,-

7

for (a) myristic acid, (b) myristic acid-Mg,Al-LDH, (c) myristic acid-LiAl,-LDH, and (d) Na-myristate. Figure 9. Infrared spectra in the 2600-3100-~m-~region

AI-LDH intercalated sample lie with the frequencies that occur around the CH3 group. Bands at 2961,2938, and 2870 cm-I all decrease markedly in intensity in LiA1,-LDH (Figure 9c) as compared to Mg3Al-LDH (Figure 9b). These changes in intensities are occurring as a result of the changes in Fermi resonance between the CH3 stretches and the deformation modes since correlation splittings for the deformation modes only appear in the Mg3Al-LDH material. Thus from the infrared spectra, we can conclude that the packing of the polymethylene chains is in the all-trans configuration in both LiAl,-LDH and Mgd-LDH. However, the packing of the chains in Mg3Al-LDH leads to an orthorhombic unit cell, with two chains per unit cell, whereas in the LiA1,-LDH it is of lower symmetry with one molecule per unit cell. Based on these results, an overall arrangement of the myristic acid molecules in LiA12-LDH and Mg,AI-LDH can be proposed. These are shown schematically in Figure 10. The monomolecular films in LiA1,-LDH can be arranged in two geometries (Figure loa). In the case of Mg3Al-LDH, the C1- can H-bond with the COOH group and thereby help in orienting the acid molecules. Again, there are two ways in which the C1- ions can be distributed, neutralizing either only one layer of metal hydroxide or alternate layers, as shown in Figure lob. The present data does not distinguish between these arrangements. In our earlier study on alignment of nitrohippuricacid intercalated into LiAl,-LDH, we proposed that the C1- in one layer was interacting with the acid on the same side of the layers.9b Temperature-Dependent Dynamics hocesses of Interlayer Species. The dynamics of the polymethylene chains as a function of temperature have been examined for membranes,” polymers,18 and intercalated units in layered materials such as montmorillonite and zirconium phosphates?” In the latter system, which is of direct relevance to this study, the typical observation has been increased disorder in the polymethylene chain along with a shortening of the interlayer spacing. Below we discuss the influence of temperature on fatty acids in LiA1,-LDH and Mg3Al-LDH as determined by calorimetry, diffraction, and infrared studies. A. Calorimetry. Figure 1 l a 4 shows the differential scanning calorimetry curves between 50 and 100 OC for myristic acidLiA1,-LDH sample as synthesized, and samples heated to 100,

*



(4

9

= RCOOH

Figure 10. Schematic arrangement for myristic acid in (a) LiAI2-LDH and (b) Mg,AI-LDH.

n

I 60. 0

I

I BO. 0

I

I 100.0

I

I

120.0

1

7

7

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Iomp*ratrrP ('0

Figure 12. Differential scanning calorimetry traces for myristic acidM&AI-LDH (a) as synthesized sample and (b) heated to 120 OC. (The sample was maintained at 120 OC for 15 min in air and cooled to room temperature before recording data.)

I

50. 0

I 60. 0

I

70. 0 1e.peatUre

~~

700. 0

7 -I 90.0

100.0

(*C)

Figure 11. Differential scanning calorimetry traces for myristic acidLiA12-LDH sample (a) as synthesized and heated to (b) 100 O C , (c) 150

OC,(d) 200 O C . The samples were maintained for 15 min at the high temperature (in air) and cooled to room temperature before recording traces.

150,and 200 O C in air and then cooled to room temperaturebefore the calorimetric measurement. For the as synthesized sample (Figure 1 la), two transitions are observed at 68 and 78 OC. The second transition is not observed for the sample heated to 100 O C (Figure 1lb). As mentioned earlier, there is a considerable amount of ethanol in the as-synthesized sample, and we assign the endotherm at 80 OC to the loss of ethanol. The sample heated to 150 O C has the onset of transition shifted to 55 O C (Figure 1IC), whereas no perceptible transition is observed for the samples heated to 200 OC (Figure 1 Id). The endotherms could be repeatedly observed for any of these samples heated to a particular tem-

perature. However, none of the samples showed any exothermic peak, though it is clear that order is being restored upon cooling, until heat treatments beyond 150 OC. The transition at 68 OC for the as-synthesized sample represents a phase transition involving the packing of alkyl chains. The onset of this transition does not change for the 100 O C heated sample, though the AH shows a decrease from 53 to 49 J/g. Both the onset temperature and AHshow a considerable decrease for the 150 "C sample, being 55 OC and 15 J/g, respectively. No transition is observed for the 200 O C heated sample, indicating that the chain packing is completely disordered and the system does not relax back to an ordered assembly upon cooling. Also, at these higher temperatures, decarboxylation as well as oxidation of the hydrocarbon chains will begin to occur. Therefore, it appears that heat treatment at progressively higher temperatures introduces disorder into the system which is only partially restored on cooling, the degree of restoration decreasing at higher temperatures. Similar results were obtained with palmitic acid sample, except that the transition temperature of the alkyl chain disordering was higher by 5 OC. The data for as-synthesized myristic acid-Mg,Al-LDH is shown in Figure 12. There are two transitions at 95 O C and 100

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Borja and Dutta


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Figure 13. Hot stage X-ray powder diffraction patterns for myristic acid-LiAl,-LDH.

"C. For a sample heated to temperatures above 120 "C, no endothermic peak is observed (Figure lob). This implies that once the material goes through a transition, the ordered domains of the alkyl chains are completely destroyed and do not recover any order on cooling. This is clearly distinct from the LiA12-LDH material. Palmitic acid-Mg3Al-LDH samples also show the same effect, with the onset of transitions shifted upward by -5 "C. We assign the first transition (-90 "C) to loss of ethanol and the second one (100 "C) to alkyl chain disordering. The higher temperature of transition in Mg3A1-LDH as compared to LiAl,-LDH could be due to the more symmetric packing of the fatty acid molecules in Mg3Al-LDH. Also, the ethanol molecules are held more tightly in the Mg3Al-LDH sample. B. Diffraction. The powder diffraction data provides information about the interlayer distance and thereby indirectly probes the specific alignment of the chains in the interlayer. In order to explore the chain dynamics as a function of temperature, diffraction patterns were obtained using a hot stage diffractometer for myristic acid-LiA12-LDH and -Mg3Al-LDH. Figure 13 shows the data for the myristic acid-LiA12-LDH sample between 30 and 100 OC. The starting material has (001) reflections at 28 = 3.3,6.75, and 10.2", with a film thickness of 22 A. At 60 O C , a new set of reflections emerges at 28 = 2.64,5.33, and 8.14' (marked by x in Figure 13), indicating that the film thickness has increased to 28.64 A. With further heat treatment, the spacing continues to increase and reaches a steady state of -43 A around 100 "C ( 15-20 min). Upon cooling to room temperature, the expanded layer pattern is retained as shown in Figure 14b. This process can be reversed in the presence of ethanol, and the original as-synthesized XRD pattern is restored as shown in Figure 14c. The dynamics representing the expansion of the layers as shown in Figure 13 are novel and to the best of our knowledge have not been observed in any other layered system. Considering that each C-C distance is -1.27 A, the increase in spacing between 50 and 60 OC corresponds to a translation of -4 CH2 groups. Similar increases are observed at the higher temperatures. The onset of appearance of the expanded layer at 60 "C corresponds well with the transition observed in the calorimetry studies. However, since the sample heated to 100 OC has the increased spacing frozen in upon cooling to room temperatures, the endotherm shown in Figure 10b must relate to transitions involving

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Figure 14. Room-temperature X-ray powder diffraction patterns for (a) as-synthesized myristic acid-LiA1,-LDH sample, (b) heated to 100 OC for 10 min, and (c) treated with ethanol for 3 h.

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Figure 15. Hot stage X-ray powder diffraction pattern for myristic acid-Mg,Al-LDH.

the alkyl chains in the expanded layer material. The XRD patterns of a myristic acid-Mg3Al-LDH sample obtained on a hot stage apparatus are shown in Figure 15. Around 75 O C , a (001) pattern characteristic of a material with a decreased basal spacing is observed. From a film thickness of 39 A at room temperature, a decrease to 32 A is observed at 70 "C.The complete starting material is changed to the one with 32-A spacing by 90 "C.Around 100 "C, a material with basal



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Figure 17. Infrared spectra of myristic acid-Mg,Al-LDH at temperatures of 27-130 O C with a control of & I O C .

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Figure 18. Infrared spectra taken at room temperature for (a) myristic acid-LiAl,-LDH as-synthesized sample and (b) heated to 150 O C for 15 min; (c) myristic acid-Mg,Al-LDH heated to 150 OC for 15 min.

as-synthesized sample and (d)

between 60 and 100 OC indicates that some degree of all-trans order is being maintained during the expansion of the film thickness and layer separation. The other si&icant changes in the infrared spectra (not shown) are the loss of the correlation splittings in the 712- and 1460-cm-l regions in the Mg,Al-LDH sample between 45 and 60 OC, indicating that the orthorhombic packing is being disrupted. In both LiA12-LDH and Mg,AI-LDH, the v,(C-O) stretching mode at


Borja and Dutta

1540-cm-' shifts to higher frequencies at -60 OC (1575 cm-' for LiAl,-LDH and 1584 cm-' for the Mg,Al-LDH sample). This could be due to replacement of the solvent ethanol molecules and direct interaction of the carboxylate with the metal hydroxide layer. In the C-H stretching region, the bands are increasingly broadened with temperature for both LiAl,-LDH and Mg,AlLDH, and blue shifts of -5 cm-' for the CH2 symmetric and asymmetric modes are observed only in the case of Mg,AI-LDH. This is simply indicative of increased disorder and formation of gauche block structures.'

Discussion It is clear from the data shown above that the packing and dynamics of the fatty acid chains in the LiAl,-LDH and Mg,AI-LDH are quite distinct. In this section, we review these differences in the context of the published literature and provide a model that is consistent with the data. Chain Packing. The film thicknesses (basal spacing less 4.8 A) for C,H2,+ICOOH-LiA1,-LDH complexes for n = 11, 13, and 15 are 19.6,21.4, and 23.7 A, respectively. The calculated lengths for these fatty acid molecules in their completely extended all-trans form are 18.7,21.2,and 23.7 A, respectively. The change in spacing per CH2 unit corresponds to 1.22 A, which would indicate that a monolayer of fatty acid molecules is forming between the chains, which is consistent with the film thickness that is observed. Considering that addition of each CH, unit should contribute 1.27 A (chains held perpendicular to the layers), the result of 1.22 A/C unit in LiA12-LDH leads to an almost perpendicular arrangement of the alkyl chain. The film thicknesses in the Mg3Al-LDH samples are considerably larger, being 35, 39, and 44 A for n = 11, 13, and 15, respectively. The increase in spacing per CH2 unit corresponds to 2.35 A. Both these facts indicate that a bimolecular film of the fatty acid chains is forming. The orientation of these molecules is also nearly perpendicular to the metal hydroxide layer. There is only one brief report of ion-exchange of fatty acids in double metal hydroxides, and it was done with the carboxylate salts from an aqueous solution.'@ No exchange was reported for LiAl,-LDH, but in Mg,Al-LDH and Zn2Cr-LDH, a monomolecular film was formed. Clearly, our procedure for reaction of the carboxylic acid in ethanol results in a different intercalation mechanism. The exchange of long-chain alkyl sulfates into Zn,Cr-LDH has been examined more thoro~gh1y.I~ A monomolecular film is formed from aqueous solution which expands in the presence of n-alkyl alcohols and n-alkylamines. The monomolecular films can also incorporate small organic molecules such as ethylene glycol between the end of the chain and the metal hydroxide layers. Vibrational spectra of the fatty acid molecules in the LiA1,LDH and Mg,Al-LDH samples provide clues as to the state of the COOH functionality. Bands at 1542, 1557, 1589, 1637, and -1720 cm-' are observed in the v(C-0) region whereas the presence of only COOH or COO- would result in bands at 1700 and 1558 cm-I. Clearly, the carboxylic group is present in different forms in the metal hydroxide layer. These could include COOH (1720 cm-I), COO- (H+) (1637 cm-I), COO-H+ (Cl), and COOM+ (OH) (1542-1589 cm-'). XRF analysis indicates that all of the C1- signal is still retained in Mg,Al-LDH, whereas only about 15% is retained in LiAl,-LDH. Thus, the RCOOH intercalating into Mg,Al-LDH is dissociating to a considerable degree. The intercalation into LiA12-LDH is considerably less, the primary incorporation mechanism being through ion-exchange. The progression observed in the CH, wagging region indicates that the chains are in an all-trans configuration for both the monomolecular and bimolecular films in LiA1,-LDH and Mg3AI-LDH. Such an all-trans arrangement of alkylammonium ions in montmorillonite^,^ phyllosilicates,20and zirconium phosphates' has also been reported. The most interesting result from the vibrational study is the presence of the correlation splitting observed for Mg3AI-LDH. This indicates that two alkyl chains are present per unit cell, indicating a highly symmetric arrangement such as an orthorhombic unit cell. Whereas in the LiA1,-LDH samples, a lower symmetry such as triclinic or monoclinic arrangement is present. Previous studies on alkyl chain arrangements

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F i 19. Idealized arrangement of metals in the metal hvdroxide laver for (a) LiAI,-LDH, (b) hexagonal Mg3A1-LDH, and (c) orthorhombic Mg3AkLDH. in layered systems have only noted the presence of lower symmetry unit cells. The origin of the different packing in LiA1,-LDH and Mg,AI-LDH must lie in the charge distribution of the metal hydroxide layer as well as the arrangement of the cations, since these factors influence the packing of the fatty acid chains. Previous studies have hinted at the anomaly of the LiAI,-LDH system as compared to the M2+M3+type double hydroxide.'@ For example, secondary alkanesulfonates exhibited a higher spacing (1-2 A) in LiA12LDH as compared to Z n x r , Zn-Al, Mg-AI, and Ca-AI layered double hydroxides and showed an unusual effect of increased interlayer space upon drying by 1-2 A. This was attributed to a different charge distribution in the Li+-A13+ case as compared to M2+-M3+ hydroxides. The present study, however, shows a considerably greater difference in the packing of alkyl chains between LiAl,-LDH and Mg,Al-LDH systems. The major difference between this study and previous studies is that the reaction with the fatty acid is being carried out in an ethanolic medium as compared to previous studies in aqueous solution. The lower polarity of the solvent medium markedly emphasizes the differences in the charge density and structure between the LiA1,-LDH and Mg,Al-LDH systems. The arrangement of the cations in the octahedral layer has been examined in the literature'"J' and is compared in Figure 19. The distances between the closest Li+ ions are 5.32 A in the LiA1,LDH, whereas in Mg,AI-LDH, the positive charges are 6 A apart. The area per cationic charge in LiA1,-LDH is 24.5 A2,whereas in Mg,AI-LDH it is 33 A*. In the closest possible packing of alkyl chains, as in polyethylene crystals,6 the unit area per chain is 19 A,. So, in LiAl,-LDH the fatty acid chains can be expected to be more densely packed than in Mg3AI-LDH. The crystal packing in the layers, as shown in Figure 19, are idealized projections. This is evident from the low symmetry of packing (Le. triclinic or monoclinic) of the alkyl chains in LiA1,-LDH, which would indicate some disorder in the distribution of Li+ in the vacancies of the gibbsite structure. This was also noticed in an earlier spectroscopic study of LiA12(0H)6.C1.9aIn the case of Mg,AlLDH, the ordering as shown above for a hexagonal supercell or an orthorhombic supercell (Figure 19b,c) is consistent with the high symmetry of packing of the fatty acid chains.


Fatty Acids in Layered Metal Hydroxides The difference in charge density of LiA12-LDH and Mg3AlLDH is not only reflected in the monomolecular versus bimolecular film but also the fact that in LiA12-LDH the process of incorporation of the fatty and molecules is primarily by ion-exchange, whereas it occurs by intercalation in Mg3Al-LDH. The charge density of the layers must determine the balance in energetics between the increase in interlayer spacing and the mutual attraction/repulsion between the layers. Since the mechanism of entry of these molecules is not yet understood, any detailed proposal of how the charge densities of LiA12-LDH and Mg3A1-LDH influence the ion-exchange/intercalationprocess will have to be speculative. Temperature-Dependent Dynamic Effects. The differences in the packing of chains in LiA12-LDH and Mg3Al-LDH are also reflected in the dynamics of these chains as a function of temperature. The transition temperature for myristate-LiA12-LDH (65 "C) observed in DSC is about 35 "C lower than in Mg3AlLDH (100 "C). This is probably a reflection of the bimolecular packing in Mg3Al-LDH. Typically, transition temperatures rise with increasing length of the alkyl chain. The transitions observed in DSC for the as synthesized fatty acid-metal hydroxide samples lead to a change in interlayer packing of the fatty acid chains. The most obvious effect is the shortening of the interlayer spacing in Mg3Al-LDH and an extension in the case of LiA12-LDH. In myristate-Mg3Al-LDH, the film thickness decreases from 39 A at room temperature to 24 A at 140 O C . For a myristate-LiA12-LDH sample, the film thickness increases from 22 A at room temperature to 43 A at 100 OC. Thus, it appears that in the Mg3Al-LDH the bimolecular layer is becoming disordered as a function of temperature. No studies have been reported for the temperaturedependent behavior of alkyl chains in layered metal hydroxides, but considerable work has been done for similar systems in montmorillonites, layered silicates, and zirconium phosphate^.^^^ In all these cases, the interlayer spacing is found to decrease with temperature and proceed through the development of a kink in the chain followed by gauche blocks. This is indeed what appears to be happening in the myristate-Mg3Al-LDH sample. The first kink is manifested in the XRD pattern at 70 OC and involves a decrease in basal spacing of 1.3 A. This decrease is characteristic of a 2gl kink (...tgtg t...). Between 70 and 75 OC, a marked decrease of 3.6 A, corresponding to a 2g3 kink, is observed. The next major transition occurs around 100 OC, coincident with the DSC transition, and involves a decrease in interlayer spacing of -7 A. This corresponds to the formation of gauche blocks. The observation that various pretransitional phenomena are occurring well before the maximum in the DSC curve has also been made with alkylammonium ions in zirconium phosphate.' The gauche blocks are retained upon cooling, and the conformational changes are not reversible. Treatment with ethanol also does not have any effect on the heated samples. Vibrational spectra also provide information about the structural changes that occur. At temperatures as low as 60 "C, the correlation splitting of the 1460and 720-cm-' bands due to packing of two chains in the unit cell is lost.24 The 2-5-cm-l blue shifts of C-H stretches of the CH2 groups is observed with the formation of the gauche blocks around 100 "C. The presence of kinks (gtg) upon heating is also confirmed by the increase in intensity of the band at 1310 cm-l (Figure 17). The tailing of the intensity toward the lower frequency (- 1250 cm-l) is indicative of distribution of kink sites and resembles the spectra observed in liquid alkanes.25 The presence of end-gauche (- 1340 cm-l) or double-gauche (1 353 an-') structures could not be definitively deduced from the infrared spectra. That these kink defects are retained upon cooling the myristateMg3Al-LDH sample is obvious from Figure 18d which shows the prominent kink defect band at -1310 cm-l. The observations with thz myristic acid-Mg3Al-LDH are in reasonable agreement with previous studies on other layered supports. A schematic of the dynamics of the chains is shown in Figure 20. However, the behavior of the LiA12-LDH system is completely anomalous and has not been reported to date. The interlayer film thickness indicates an expansion from a monomolecular to a

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+ Figure 20. Schematic description of temperature-dependent changes in myristic acid-Mg,Al-LDH

sample.

bimolecular film. Another explanation for the increased spacing could be rearrangement of the myristic acid molecules within the layer such that four metal hydroxide layers are required per unit cell. However, the observed change in interlayer spacing varies almost in a continuous fashion upon heating, whereas this model would have predicted a stepwise jump. The appearance of bands in the infrared spectra (Figure 16) at 1315, 1340, and 1362 cm-l (not shown) in the myristate-LiA12-LDH samples at 130 "C indicates that both kinks (gtg) and end-gauche (gt,) defects are forming at these elevated temperature^.^^^^^ Upon cooling the samples (Figure 18b), the characteristic all-trans CH, wag progression is observed. The trans order of the chains as evident from the CH2 wagging progression appears to be maintained at higher temperatures and is also evident on cooling the samples. Thus the transitions can be repeatedly observed in DSC, since the conformational changes are reversed on cooling. The increased bimolecular type spacing observed at high temperature is also retained upon cooling. The differences between the LiA12-LDH and Mg3A1-LDH samples include (a) formation of monolayer in LiA12-LDH versus bilayer in Mg3Al-LDH with orthorhombicpacking of chains; (b) expansion of the interlayer spacing in LiA1,-LDH upon heat treatment as compared to contraction in Mg3A1-LDH; (c) reversibility of the chain phase transformations upon heating and cooling in the LiA12-LDH as compared to irreversible transformation in Mg3Al-LDH. In our analysis above on the packing of fatty acid chains in LiA12-LDH, it was mentioned that a monomolecular film results because of the balance between the interaction between the positively charged layers and the increased spacing. Also,the high charge density of the metal hydroxide layer will promote dense packing of the fatty acid chains. Bilayers in lipids are known to undergo a large lateral expansion along with a longitudinal contraction on heating, due to the formation of nonplanar gauche structures.26 A similar effect is occurring in the Mg,Al-LDH sample, which is reflected in the decreased interlayer space and the appearance of the band at 1310 cm-l due to gtg defects. We propose that ethanol molecules occupy space in between the chains. Loss of these solvent molecules upon heating leads to considerable space around the chains, thus promoting conformational disorder. Since the carboxylate ends of the molecules are anchored to the metal hydroxide layer, the disordering must begin and proceed from the CH3 end of the molecule. In this respect, these systems resemble lipid bilayers rather than alkanes.25 The expansion of the layers upon heating the myristateLiA12-LDH is puzzling and more difficult to explain. The packing of the myristic acid molecules is more dense because of the higher charge density. The ethanol can therefore be expected to be between the layer and the carboxylategroup rather than between the chains. Heating this material results in loss of ethanol, expansion of interlayer space, and the appearance of end-gauche and kink defects indicative of conformational disorder. Cooling the sample results in reordering of the carboxylic acid chains in an all-trans arrangement, but the expansion of interlayer remains frozen in. Treatment with ethanol brings about a decrease of the spacing and resembles the starting material. Therefore, it appears

Borja and Dutta

5444 The Journal of Physical Chemistry. Vol. 96, No. 13, 1992

all-trans conformation and exhibit repeatable phase transitions. The nonexpanded state can be reformed by treating the sample with ethanol. In Mg3Al-LDH, the layer spacing decreases through the intermediate formation of kink blocks and finally gauche blocks. Once the chains are in this state, the system cannot be reversed by cooling or ethanol, unlike the LiA12-LDH case. (d) Layered double metal hydroxides provide an interesting and controllable environment for generating membrane-like materials.

Acknowledgment. We gratefully acknowledge support from the Division of Chemical Sciences, Office of Basic Energy Sciences, Department of Energy, under contract DE-FG0290ER14105. We would also like to thank Professor Patrick Gallagher for help with the thermal analysis experiments and interpretation. References and Notes

Figure 21. Schematic description of temperature-dependent changes in myristic acid-LiA1,-LDH sample.

that the expansion of the layers is linked to the loss of ethanol. A possibility could be that the ethanol acts as a dielectric, shielding the positively charged layers from each other, and its loss leads to repulsion between layers. The change in interlayer spacing during heating also supports this view since it changes in an almost continuous fashion and does not have the stepwise changes more characteristic of kink defect block formation. Based on the infrared data which shows the formation of end-gauche and kink defects, we present a schematic of the chain disorder in LiA1,,LDH as a function of temperature in Figure 21. The denser packing in LiAl,-LDH leads to a system in which the chains do order upon cooling, whereas in Mg3Al-LDH, with the lower packing density the mutual interaction between chains is diminished, and the system does not reorder upon cooling. Conclusions This study has examined the packing and dynamics of longchain saturated fatty acid molecules in layered double metal hydroxides. The two hydroxides that form the focus are LiA1,(OH),' and Mg3A1(OH),+ chosen because of their different charge densities, being 0.25 and 0.33 m-2, respectively. The following conclusions can be drawn from this study: (a) A monomolecular film of fatty acid chains are packed with high density in an all-trans and in an almost perpendicular arrangement to the layer surface in LiA1,-LDH. Ion-exchange is the primary means of incorporation of the acid molecules. The symmetry of the chain packing is triclinic or of lower symmetry. Ethanol, the solvent used for the reaction of fatty acid with LiA12-LDH, is also incorporated between the metal hydroxide layer. (b) A bimolecular and almost perpendicular film of myristic acid is formed in Mg3Al-LDH. The chains are packed in orthorhombic or higher symmetry in an all-trans form. The primary method of incorporation of acid is via intercalation. The solvent ethanol is accommodated between the chains, for the packing is not as dense. (c) In both cases, increase in temperature brings about major changes in the packing of chains. In LiAl,-LDH, this results in expansion of the layers while the alkyl chains still maintain an

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Fatty acids in layered metal hydroxides: membrane-like structure and

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