Langmuir 1994,10, 1851-1856
1851
Pyrene Sorption in Organic-Layered Double-Metal Hydroxides Prabir K. Dutta' and Daniel S. Robins Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210 Received July 8, 1993. In Final Form: February 17, 1994" Myristic, hexanoic, and succinic acids were incorporated into a lithium- and aluminum-containinglayered double-metal hydroxide (LDH), resulting in materials with interlayer spacings of 21.6, 17.8, and 7.5 A, respectively. Comparison of these spacings with the dimensions of the all-trans form of the acid molecules indicates an interlayer monolayer arrangement for myristic and succinic acids, and a bilayer arrangement for hexanoic acid. Sorption isotherms of pyrene by these organic-exchange LDH's were carried out in methanollwater mixtures, because of the limited solubility of pyrene in water. Using cosolvent theory, partition coefficients (log K,) for pyrene into myristic acid-LiAl-LDH and hexanoic acid-LiAl-LDH were determined to be 4.38 and 3.44, respectively. No sorption of pyrene was observed in succinic acidLiA1-LDH, suggesting that partitioning into the interlayer space was controlled by the width of the interlayer spacings. Infrared studies showed distortion of the acid chain packing upon incorporation of pyrene into hexanoic acid-LiAl-LDH. The conclusions of this study are that organic-exchanged LDH's exhibit novel sieving effects toward hydrophobic compounds based on interlayer spacings, and that the interlayer packing of the alkyl chains influences the extent of sorption.
Introduction Hydrophobic moleculesand, in particular, polyaromatic hydrocarbons such as pyrene are important from a photochemical' as well as an environmental perspective.2 Their photochemical properties and environmental fate are strongly influenced by their interactions with the immediate environment. Environmental scientists have noted that sorption of hydrophobic compounds by inorganic minerals is considerably enhanced if the mineral is associated with organic matter. Examples of such systems include soils: sediments? and aquifier materials5 associated with humic substances and organic-exchangedsmectite clays.e Not only does the association of organic matter with minerals vary considerably, but the uptake of hydrophobic compounds by organic-mineral composites is also dependent on the source, structure, molecular configuration, and polarity of the organic-mineral interface.%' The mechanism of solubilization and the location of hydrophobic compounds in these systems is a current area of research. Surfactants such as dodecyl sulfate also exhibit uptake of hydrophobic organic compounds comparable to organic-mineral media.s The photochemical properties of polyaromatic hydrocarbons are also strongly influenced by the nature of the microheterogeneous support, thus allowing the use of these moleculesas aprobe of the inorganic surface.' In addition, the surface can modify the photodriven electron-transfer reaction^.^ Abstract Dublished in Advance ACS Abstracts.. Mav- 1.. 1994. (1) Kamat, P.V. Chem. Rev. 1993,93,267. (2)(a)Szentpaly, L. V. J.Am. Chem. SOC.1984,106,6021.(b) Grimmer, G.H.;Brune,H.;Deutach-Wentzel,R.;Naujack,K. W.;Misfield, J.;Timm, J. Cancer Lett. (Shannon, Zrel.) 1983,211 105. (3)Lee, J. F.; Crum, J. R.; Boyd, S. A. Enuiron. Sci. Technol. 1989, 23,1365. (4)Carter, C.W.;Suffet, I. H. Enuiron. Sci. Technol. 1982,16,735. (5)(a) McCarthy, J. F. Enuiron. Contam. Toxicol. 1983,12,559.(b) Murphy, E.M.; Zachara, J. M.; Smith, S.C. Enuiron. Sci. Technol. 1990, 24,1507. (6)(a) Mortland, M. M.; Shaobai, S.; Boyd, S. A. Clays Clay Miner. 1986,34,581.(b) Wolfe, T. A,; Dermirel, T.;Baumann, E. R. Clays Clay Miner.1985,33,301. (7) Chiou, C. T.; Peter, L. J.; Frees, V. H. Science 1979,206,831. (8) Jafvert, C. T.; Heath, V. K. Enuiron. Sci. Technol. 1991,25,1031. @
The interesting issues in this area are the extent of uptake of hydrophobic compounds by organic-minerals and the mechanisms thereof. An important feature in the sorption process is the organic content of the sorbent. In systems containing replaceable inorganic ions, such as smectite clays, ion exchange with long-chain organic ions can enhance organic content and contents corresponding to 9-1096 carbon can be readily achieved.6 Since this is an ion-exchangeprocess, the maximum amount of organic ions that can be incorporated into the clay is controlled by the cation-exchange capacity, which is typically less than 1mequiv/g.lO Considerable research has been done on the swelling properties of organic-smectites as well as the packing of the long-chainorganics.lla Isotherm studies have shown that such organic-clays are effective a t removing hydrophobic compounds from air or water, though the degree of sorption depends on various structural parameters.llbsc The mechanisms proposed for organic uptake by minerals include adsorption processes12involving H-bonding, ion exchange, and ion-dipole interactions. In addition, partition mechanisms,13in which the organic matter on the mineral acts as a sorption medium for nonpolar organic solutes, are also important. In principle, the organic content as well as the arrangement of the organics in the interlayers will change for materials with different ion-exchange capacities. In this paper, we report on a layered double-metal hydroxide (LDH)14 with high ion-exchange capacities. In addition, (9)Chatterjee, P. K.; Kamoka, K.; Batteas, J. D.; Webber, S. E. J. Phys. Chem. 1991,95,960. (10)Pinnavaia, T. J. Science 1983,220,365. (11)(a) Lagaly, G.Angew. Chem., Znt. Ed. Engl. 1976,15,575. (b) Boyd, S.A.; Lee, J.-F.; Mortland, M. M. Nature 1988,333,345.(c) Wolfe, T. A.; Demirel, T.; Baumann, E. R. Clays Clay Miner.1985.33, 301. (12)Dozombak, D. A.; Luthy, R. G.Soil Sci. 1984,137,292. (13)Ainsworth,C. C.;Zachara,J. M.; Schmidt, R. L. Clays Clay Miner. 1987,35,121. (14)(a) Allmann,R. Acta Crystallogr. 1968,E24,972. (b)Miyata, S.; Okada, A. Clays Clay Miner. 1977,25,14. (c) Schutz, A.; Biloen, P. J. Solid State Chem. 1987,68,360. (d) Boehm, H.; Steinle, J.; Vieweger, C. Angew. Chem.,Znt.Ed.Engl. 1977,16,265.(e) Martin,K. J.; Pinnavaia, T. J. J.Am. Chem. SOC.1986,108,541.(0Meyn, M.; Beneke, K.; Lagaly, G.Inorg. Chem. 1990,29,5201.(g) Kwon,T.; Tsigdinoe,G. A.;Pinnavaia, T. J. J. Am. Chem. SOC.1988,110,3653. (h) Drezdzon, M. A. Inorg. Chem. 1988,27,4628.(i) Kwon, T.;Pinnavaia, T. J. Chem. Mater. 1989, 1, 381.
0743-746319412410-1851$04.50/0 0 1994 American Chemical Society
1852 Langmuir, Vol. 10, No. 6,1994
Dutta and Robins
- CATIONIC LAYER
- ANION
estimated to be 5 mequiv/g for LiAl-LDHC1. In a previous study,l’ we have shown that long-chain fatty acids are readily incorporated into the interlayersof LDH’s. These organic-LDH’s exhibit membranelike properties,with alltrans packing and characteristic phase transitions of the hydrocarbon chains. Thus, these organic-LDHs represent a new class of organic-minerals and also hold the promise of interesting behavior that has been observed with organic-smectites. We focus here on the sorption of pyrene, a hydrophobic organic molecule, into organic-LiAl-LDH. Since the solubility of pyrene in water (130pg/L) is very 10w,l8 we have obtained partition coefficientsfrom water/methanol solvent mixtures. Using cosolvent theory,lg which has found wide application in describing sorption of hydrophobic organic compounds onto soils, we have estimated the partition coefficientsof pyrene from aqueous solution onto LiA1-LDH. This theory has been described in detail in the 1iterat~re.l~A brief description follows. The volumebased sorption partition coefficient ( K D ~ mL/g) , a t a particularwater/methanolmixture i is estimated from a linear isotherm equation:
where qe is the equilibrium concentration of pyrene in the solid phase (pg/g) and Ceis the equilibriumconcentration of pyrene in solution (pg/mL). The mole-based partition coefficient K m j (mol/g) is derived from
where Vi is the molar volume of the liquid phase (mL/ mol). The molebased partition coefficients for the water/ methanol mixture ( K m j ) are related to the water-only (Km,w) coefficient by
Figure 1. (a) A schematic representation of a layered double metal-hydroxide. (b) Scanning electron micrograph of LiAlr (OH)&1. the dimensions of the interlayer space, which is the site for adsorption,are also manipulated. Figure l a shows the schematicof such a material.15 The resemblancewith the ubiquitous cationic smectite clays is obvious, with one significant difference. Unlike the clays, the layers are positively charged, with neutralizing anions in the interlayer space. Thus, ion exchange only occurs with anions, and these materials are also referred to as anionic clays. A typical composition of a LDH is [M1ll-zMH1x(OH)21A”-,/,,eH20where Mn= Mg, Zn, Fe, Co, Ni, or Cu, Mnl = Al, Cr, or Fe, and A“- is the interlayer Some of these materials, notably MgAl(OH)8+ (hydrotalcite), do occur in small quantities in nature.15 Our studies in this paper focus on a synthetic analogue,l6 LiA12(0H)6+.Cl-, where the sheets contain edge-sharing M(OH)6 octahedra with M = Li+ or Al3+, which is henceforth referred to as LiAl-LDHOC1. Figure l b is an electron micrograph of LiA1-LDH showing the characteristic layer-like morphology. The ion-exchange capacities are considerably higher than those of smectitic clays, (15) Reichle, W. T. CHEMTECH 1986,86. (16) (a) Serna, C. J.; Rendon, J. L.; Iglesias, J. E. Clays Clay Miner. 1982,30,180. (b) Dutta, P. K.; Puri,M. J. Phys. Chem. 1989,93,376. (c) Dutta, P. K.; Cooper, S. J. Phys. Chem. 1990,94,114. (d) Dutta, P. K.; Twu, J. J. Phys. Chem. 1989,93,7863. (e) Twu, J.; Dutta, P. K. J. Catal. 1990,124,503.
where cy and a,,are constanta involving solute-liquid and solutesolid interactions and f,,is the volume fraction of methanol. This allows us to obtain Kmw for an aqueous solution by extrapolation off,, to zero. Normalizing with respect to the organic carbon content of the LDH defines a Koc for the system. Using the above methodology,the partitioningof pyrene as a function of the interlayerorganicspacing is examined in this study. Information on the location of pyrene in the interlayerspace is obtained from infrared spectroscopy. There are several reasons for examining uptake of pyrene into the organic-LDH’s. First is to evaluate the usefulness of the organic-LDH’sfrom an environmental perspective for uptake of hydrophobiccompounds,with pyrene serving as a model system. Such materials may be useful as liners for containment purposes, as chromatographic supports, and in air contaminant samp1ing.m The second reason is that the photochemistry of pyrene is environment dependent, and the organic-LDH’s provide a novel environment.9,21 However, in order to examine these processes, the basic sorption behavior and the location of the organic (17) Borja, M.;Dutta, P.K.J. Phys. Chem. 1992, SS,5434. (18) Mackay, D.; Shiu, W. Y. J. Chem. Eng.Data 1977,22,399. (19) (a) Fu, J-K.; Luthy, R G. J. Enuiron. Eng. 1986,112,328. (b) Waltere, R. W.; Guiseppi-Elle, A. Enuiron. Sci. Technol. 1988,22,819. (20) (a) White,D.; Cowan, C. T. Tram. Faraday SOC.1958,54,557. (b) Boyd, S. A.; Mortland, M. M.; Chiou, C. T. Soil Sci. Soc. Am. J . 1988, 52,652. (c) Harper,M.; Purnell, C. J. Enuiron. Sci. Technol. 1990,24, 55. (21) Beck, G.; Thomas, J. K. Chem. Phys. Lett. 1985,94,553.
Pyrene Sorption in Double-Metal Hydroxides
Langmuir, Vol. 10, No. 6,1994 1853
compounds for this class of materials need to be determined, and hence this study was undertaken.
Experimental Section LiAl-LDHCl was synthesized according to the procedure described by Borja and Dutta.” Sodium hydroxide (0.4 mol) was dissolvedin 100mL of nanopure water that had been purged with nitrogen. Aluminum powder (0.10 mol) was slowly added to the solution until dissolved. Lithium chloride (0.50 mol) was then added to the mixture. This mixture was held in a Teflon bottle and heated for 48 h at 100 OC. After heating, the mixture was filtered, placed in 100 mL of 0.5 M sodium chloride, and shaken for 24 h. LiAl-LDHCl was then filtered, washed with water, and stored wet in stoppered vials. Myristicacid (99.5+ % ) and hexanoicacid (999% ) were obtained from Aldrich. Succinicacid (99.7 %) was obtained from Mallinckrodt. Pyrene (999% ) was obtained from Aldrich. Absoluteethanol and methanol were purchased from Midwest Grain Products and J. T. Baker, respectively. All water was obtained from a Barnstead Nanopure filtering system. The organicacids myristic acid (CHa(CH2)lzCOOH)and hexanoic acid (CHS(CH~)~COOH) were ion-exchanged into LiAl-LDHCl from pure ethanolic solutions of concentrations 0.1 and 1.0 M, respectively. Succinic acid (COOHCH2CHzCOOH) was ion-exchanged into LiAlLDHC1 from a 50% ethanol/water solution at a concentration of 0.5 M. After 2 days of shaking, the products were centrifuged and extensively washed with ethanol. All organic acid-LDHs were stored wet. All measurements were made with solvated organic-LDH. Powder diffraction patterns were carried out on a Rigaku X-ray diffractometer using Ni-filtered Cu Ka radiation. Scanning electron micrographs were taken with a JEOL JSM-820, and infrared spectra from KBr pellets were collected on a Mattson Cygnw FTIR spectrometer. The carbon content was determined by Oneida Research Services located in Whitesboro, NY. Batch sorption experiments were performed in 15-mL glass conical centrifuge tubes with Teflon-lined screw caps. The organic acid-LDH’s were dried under a flow of dry nitrogen to remove any surface-bound solvent. For each organicacid-LDH, several methanol/water ratios were utilized to determine the partition coefficient (K,)of pyrene. Each isotherm determination included a blank tube containing pyrene but no organicLDH. For each experiment, weighed amounts of organic-LDH were transferred to centrifuge tubes. Then 10 mL of the correspondingliquid phases (water/methanol)was added to the tubes including the blanks. Pyrene was added to the tubes by direct injection of a methanol solution of pyrene. The centrifuge tubes were wrapped with aluminum foil to protect them from light and placed on a wrist shaker for 24 h. The tubes were removed from the shaker, placed horizontally on a bench, and shaken oncea day for the following 2 days. Beforethe equilibrium isotherm analysis, the tubes were placed upright and allowed to remain undisturbed for 1 day. Following this period, 3-mL alliquotswere carefullywithdrawnfromthe top of each centrifuge tube for spectroscopic analysis with a Shimadzu UV-265 spectrometer. Residual pyrene concentration was determined from the 334-nm (e = 5.58 X l(r L mol-’ cm-l) absorption band after baseline correction. The pyrene solid-phase concentration was determined by the difference of the initial pyrene concentration and the residual liquid-phase concentration. The difference in absorbances before and after sorption was in the range of 0.010.5 absorbanceunit (1-cmcell),and determined the experimental accuracyof the method. Isotherm data were evaluated by linear regression. Results Packing of Interlayer Acids. The sorption of pyrene was examined on three organic-LiAl-LDH’s, chosen to provide a range of interlayer spacings. .The organic acids myristic acid (CH3(CHz)&OOH), hexanoic acid (CH3(CH2)&OOH), and succinic acid (COOHCHzCH2COOH) were ion-exchanged into LiA1-LDHCl. Figure 2 shows the powder diffraction patterns, obtained with the three acids and the starting LiAl-LDHC1. Residual reflections
a
2 b
r
f 8
C
J
J
10
20
30
40
2-9
Figure 2. Powder X-raydiffraction patternsof (a)myristicacidLiA1-LDH, (b)hexanoicacid-LiAl-LDH, (c)succinicacid-LiAlLDH, and (d) LiAl-LDH-C1. (Asterisks in (a) and (b) indicate reflection from unreacted LiAl-LDH-Cl.)
from the starting material are evident in the acidexchanged LDH’s. Complete exchange of the C1- was not possible. Thus, from elemental analysis, it is difficult to determine the exact stoichiometry of the organic-LDH’s, since the fraction of the unreacted LiAl-LDHSC1 is unknown. Measureof the chloride content does not resolve this issue, for we cannot exclude the possibility that the organic-LDH particles do not retain chloride. Thus, we only present the average stoichiometry as determined solely by carbon analysis. These correspond to [LiAl2(0H)s 0.68 (myristic acid) 0.32 Cll, [LiAl2(0H)~0.52 (hexanoic acid) 0.48 Cll, and [LiA12(0H)e 0.49 (succinic acid) 0.51 Cll. The pattern of ordered (001) reflections leads to calculated values of interla er spacings (basal spacing 4.8 A) of 21.6, 17.8, and 7.5 for myristic, hexanoic, and succinic acid exchanged LDH’s, respectively. This decrease is expected, as the size of these organic spacers is estimated as 21.2,12.3,and 6.8 A, for myristic, hexanoic, and succinic acid.22 Considering the similarity of the molecular dimensions and interlayer spacings in the cases of myristic and succinic acids, a monolayer of acid groups is forming, whereas a bilayer-like arrangement exists for hexanoic acid. In a previous study on myristic acid-LiAlLDH, we have shown that the monolayer of alkyl chains within the interlayer is packed tightly with an all-trans arrangement of the -CH2 chains.17 The bilayer-type arrangement observed in hexanoic acid-LDH was observed for myristic acid in the MgAl analogue of LDH.17 It is unclear a t this point why the hexanoic acid in LiAl-LDH prefers a bilayer arrangement as opposedto the monolayer arrangement for myristic acid. Also, unlike the myristic
K
(22) The lengths were calculated using Alchemy software and include van der Waals radii of the terminal atoms.
Dutta and Robins
1854 Langmuir, Vol. 10, No. 6,1994 ---------
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myristic
(4 8.7 A
L..l../
10.7 A
(b) Figure 4. (a) Schematic of packing of the organic acids in the LDH interlayers. (b) Dimensions of the pyrene molecule.
1500
I700
1600
I500
1400
1300
Wavenumbers (cm-1)
Figure 3. Infrared spectra in the C=O stretching region for (a) myristic acid-LiAl-LDH, (b) hexanoic acid-LiAl-LDH, and (c) succinic acid-LiAl-LDH. acid- and succinic acid-LDH, the reflections in the diffraction patterns in hexanoic acid-LDH are considerably broader, indicative of disorder, as might be expected in a bilayer arrangement, if the interpenetration of the alkyl chains occurs to different degrees. In the case of succinic acid-LDH, the presence of the second COOH group favors H-bonding interactions with the metal hydroxide layer, and the presence of a monolayer arrangement is therefore expected. Infrared spectra in the C=O stretching region for the three acids are compared in Figure 3. The band at 1560 cm-l is due to the C=O stretching of the COO- group, and is indicative of ion exchange. In the case of succinic acid, there also appear bands at 1592 and 1700 cm-l due to the second COOH gr0up,~3broadened and lowered in frequency as a result of hydrogen-bonding with the OH groups of the lithium aluminate layer. Figure 4a provides a schematic of the arrangement of the three acids in the LDH layers (the COO- group in the myristic acid-LDH sample can point in either direction). Sorption of Pyrene. The sorption data for pyrene were obtained a t various methanol/HzO mixtures ranging from 20%to 75 74. Figure 5 shows a characteristic isotherm pattern for 50% methanoUH20. The r values are 0.96 for both these linear fits, indicating that partition is the main mechanism for sorption. The absence of better fits could implicate a more complex uptake mechanism. However, without better sorption data, e.g., by using fluorescence spectroscopy, it is difficult to speculate on other alternatives. The myristic acid-LiAl-LDH exhibited the highest uptake followed by the hexanoic acid sample. No pyrene uptake was found for the succinic acid-LiAl-LDH under (23) (a) Baddiel, C. B.; Cavendish, C. D.; George, W. 0.J.Mol. Struct. 1970,5,263. (b) Fukushima, K.; Adachi, K.; Watanabe, K. Bull. Chem. SOC.J p n . 1988, 61,3049.
I
0
__
.-
- '
400
1200
800
2000
1600
2400
Solution Conc ( u g i L )
Figure 5. Isotherm data for uptake of pyrene for myristic acidLiA1-LDH (0)and hexanoic acid-LiAl-LDH (+) in 50% methanol/water.
1 ' 0 00
__ 0 10
0 20
0 30
0 40
0 50
0 60
0 70
0 80
solvent fraction
Figure 6. log K,, versus solvent fraction for pyrene uptake by myristic acid-LiAl-LDH (0) and hexanoic acid-LiAl-LDH (+I. any solvent conditions. Clearly, pyrene is being excluded from this sample. The carbon contents of the myristic acid-, hexanoic acid-, and succinic acid-LiAl-LDH were 39.6 74 , 19.173, and 9 % , respectively. Plots of the normalized log K,,L,, (normalized to organic carbon) versus solvent fraction are shown in Figure 6. The r values for the linear fits corresponding to myristic acid-LDH and hexanoic acidLDH were 0.98 and 0.99, respectively. The intercept on
Pyrene Sorption in Double-Metal Hydroxides
Langmuir, Vol. 10, No. 6, 1994 1855
Table 1. Pyrene Uptake by Organic-LiAl-LDH
A. Myristic Acid-LiAl-LDH 25% methanol KD,~ (mL/g) 723 i 0.75 0.25 fi Vi (mL/mol) 20.5 Km,i,oc (mol/g) 89.2 log Km,i,oc 1.95
50%methanol 37.9 0.50 0.50 24.1 3.97 0.599
75% methanol 9.94 0.25 0.75 29.8 0.334 -0.477
B. Hexanoic Acid-LiAl-LDH 20% methanol 35% methanol 50%methanol KD,~ (mL/g) 117 49.4 14.1 i 0.80 0.65 0.50 0.20 0.35 0.50 fi Vi (mL/mol) 20.1 21.5 24.1 Km,i,oc (mol/g) 30.4 12.0 3.06 1.48 1.08 0.485 log Kmj,oc
e. summary fraction organic carbon (%) log Km,w,oc log Koc
myristic acid
hexanoic acid
39.6
19.1
succinic acid
9.2
3.12 4.38
2.18 3.44
no sorption
the y axis is the measure of log Km,w,, from which K, is calculated (Km = Km,w,,Vw, V , being the molar volume of water). The log K , values for myristic acid- and hexanoic acid-LDH are determined to be 4.38 and 3.44, respectively. All of the relevant data in obtaining these values are listed in Table 1. Various empirical equations have been set up for computing K,’s of hydrophobic organic compounds from the octanol-water partition coefficient Using three different formalisms, log K , for pyrene has been calculated to be 5.02, 4.83, and 4.17, where log KO,for pyrene is taken to be 5.22.27 For 14 soils and sediment samples, the average log K, was found experimentally to be 4.79.25 For humic and fulvic acids, the log K,’s for pyrene were 5.2 and 4.04, respectively.26 Thus, the K , values determined for pyrene in the organic-LDH’s appear to be below the range of other soil-like systems, except for the fulvic acid sample. In addition, the K , for pyrene is higher in the myristic acidLDH compared to the hexanoic acid-LDH sample, with no uptake for the succinic acid-LDH sample. Pyrene Intercalation. In order to obtain information about the alignment of pyrene molecules among the acid chains in the interlayers, we examined the perturbation of the packing of these chains as a result of incorporation of pyrene. Infrared spectra provide a convenient probe, since bands in the CH2 wagging region (1100-1300 cm-l) as well as the C-H stretching region (2800-3200 cm-l) are sensitive to the arrangement of polyethylene chains.17 In the case of myristic acid-LDH, we find no perturbations in the infrared spectra of the myristate unit upon uptake of pyrene with loadings as high as 100 pglg. This is surprising, consideringthat if the myristate anion satisfies one equivalent charge, then it must be packed tightly with about 25 &chain. Polyethylene, which has the tightest possible packing of alkyl chains, corresponds to 19 A/chain.28 (24) (a) Karickhoff,S. W. J. HydrauLEng. 1984,110,707. (b)Kamlet, M. J.; Doherty, R. M.; Carr, P. W.; Mackay, D.; Abraham, M. H.; Taft, R. W. Enuiron. Sci. Technol. 1988,22, 503. (25) Means, J. C.; Wood, S.G.;Hasaett, J. J.; Banwart, W. L. Enuiron. Sci. Technol. 1980, 14, 1524. (26) Herbert.B. M.; Novak, J. M.Enuiron.Sci. Technol. . E.:Bertach,P. .
1993,27, 398. (27) Traina, S. J.; Onken, B. M. J. Contam. Hydrol. 1991, 7,237. (28) Alberti, G.; Constantino, U. J. Mol. Catal. 1984,27, 235.
I
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I
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2900
2850
2
Wavenumbers (cm-1)
Figure 7. Infrared spectra in the C-H stretching region for hexanoic acid-LiAl-LDH (a) before and (b) after uptake of pyrene (33 rglg).
In the case of hexanoic acid-LDH, the infrared spectra do exhibit changes, even at loadings as low as 33 pg of pyrene/g of LDH. Figure 7 shows the data in the C-H stretching region. Four bands in this region in the starting hexanoic acid-LDH occur at 2958, 2928,2874, and 2861 cm-l. These bands are primarily due to vibrations centered on the CH3 group; only the 2861-cm-l band arises from CH2 symmetric s t r e t ~ h i n g The . ~ ~ CH2 asymmetric stretch is hidden as a broad band a t -2910-2920 cm-l. Bands involving the CH3 motion include the asymmetric stretch a t 2958 cm-l. The band a t 2928 cm-l has contributions from the CH3 symmetric stretching mode and a transition indicative of chain-chain interaction which is indicative of crystalline order. The band a t 2874 cm-l is assigned to an overtone of the CH3 asymmetric deformation in Fermi resonance with the CH3 symmetric stretch. Upon pyrene loading, the marked change is the decrease in the intensity of the 2928-cm-l band. The intensity profile in the C-H stretching region for the pyrene-loaded sample resembles that of hexanoic acid in ita liquid form. We assign the loss in intensity due to disruption in the chain-chain interaction upon incorporation of pyrene.
Discussion There are several interrelated issues that emerge from the above results that merit discussion. These include the exclusion of pyrene by succinic acid-LDH, pyrene solubilization, and the range of observed K,’s. The absence of sorption on succinic acid-LDH indicates that pyrene is neither interacting with the external surfaces of the metal hydroxide nor penetrating into the layers. Considering the hydrophobic nature of pyrene and the hydrophilic -OH groups that cover the external LDH (29) Hill, I. R.; Levin, I. W. J. Chem. Phys. 1979, 70, 842.
1856 Langmuir, Vol. 10, No. 6,1994
Dutta and Robins
not merely the amount of organic carbon that is important, but the nature of the organic-mineral interface also controls the partitioning of pyrene. This study establishes the validity of this argument. Another pos~ibility,~’ though remote, is that the lower Kw’sare a reflection of the kinetics of pyrene incorporation. The isotherms were constructed in all cases after equilibration for 4 days, thereby allowing sufficient time. However, we cannot exclude the possibility that, as the interlayer spacing decreases, the rate of intercalation may also decrease. It is important to contrast the sorption behavior of organic-LDH’s with the more extensively studied conventional organic-clays, such as those with soils and m o n t m ~ r i l l o n i t e .The ~ ~ ~principal ~ ~ ~ ~ driving forces for incorporation of hydrophobic compounds is similar in all cases, stemming from solvation and entropy gain. It has been recognized that the molecular structure of the microscopic organic phase on the inorganic mineral is important in the sorption phenomena.32 Factors such as polarity, molecular configuration, conformation, and size were considered to be critical. This study clearly illustrates the role of these factors in organic-LDH’s. Among the differences between LDH and the smectites is the organic content, which is typically lower in the latter samples, because of the smaller ion-exchangingcapacity. Also, the orientation of the alkyl chains of the ion-exchanged ammonium salts in the smectites may be different.31 Because of the lower charge density and the corresponding lower loading of organics in smectite clays, the alkyl chains can lie flat on the aluminosilicate surface rather than perpendicular to it, as in the LDH’s. Therefore, in these cases, it is likely that molecules like pyrene will be incorporated oriented parallel to the layer. Such orientations are not observed with the LDH’s. Thus, the basic mechanism of uptake may differ in these two classes of layered materials. Another suitable comparison with the organic-LDH’s is pyrene adsorption studies with micelles,* which have membrane-like structures similar to those of the organic-LDH’s. In these cases, it was found that Koc’s compare favorably with octanol-water partition coefficients, indicating that the bilayer structures are quite flexible. Thus, the rigidity imposed on the organic-LDH’s via the high interlayer charge densities contributes to the decreased Kw’s in myristic acid- versus hexanoic acidLDH, and total absence of uptake in the succinic acidLDH. Such sieving effects may be potentially useful for chromatographic applications.33 Conclusions. The following conclusions are apparent from this study. First, a sieving effect can be observed by controlling the interlayer separation. If the dimensions of the sorbed molecule are larger than the interlayer separation, no intercalation is found to occur. Since the interlayer separation can be changed with some degree of control by choosing appropriate acids, these materials have the potential to discriminate against hydrophobic materials on the basis of basal spacing. Second, we find the partition coefficients to be lower if the intercalation necessitates changes in packing of the alkyl chains, since this results in costs of energy.
surface, the lack of interaction with the external surface is not unexpected. Retention of hydrophobic compounds by low-organic-content soils and clays is also minimal for the same reasons, as these nonpolar compounds cannot compete with polar solvent molecules at the external adsorption sites on the mineral surfaces.27 As far as the penetration of the pyrene into the interlayer space is concerned, two orientations are possible. These include the plane of the aromatic unit being parallel or perpendicular to the metal hydroxide layer. The dimensions of pyrene along the long and short axes are 10.7 and 8.7 A, respectively (Figure 4b).22 Therefore, for it to penetrate into the succinic acid-LDH with an interlayer spacing of 7.5 A, the pyrene must enter parallel to the metal hydroxide layer. In order for such an arrangement to occur, considerable disruption of the succinic acid molecules in the interlayers has to occur, including breaking of the hydrogen bonds between the acid and the layer hydroxide, and will necessitate further separation of the layers. The energetics of this process make it unfavorable for sorption of pyrene, and is the reason that no uptake is observed. In the case of myristic acid- and hexanoic acid-LDH, the interlayer spacings are considerably larger, 21.6 and 17.8A, and pyrene can penetrate with its planar ring system perpendicular to the metal hydroxide layer, thus minimizing the perturbation on the chain packing. Two possibilities exist for the orientation of pyrene inside the interlayer, along its short or long axis. It is difficult to predict from these studies which of these arrangements occur, except to state that the orientation along the long axis will have smaller perturbations on the immediate environment. Since the interlayer spacing in myristic acidLDH is larger (21.6 A) than that in hexanoic acid-LDH (17.8 A), the pyrene molecules have greater flexibility for orientation along the long axis (10.7 A) in the myristic acid-LDH. The lack of perturbation in the chain packing upon incorporation of pyrene into the myristic acid-LDH as evidenced from the infrared spectra provides insight into the interlayer geometry. In a previous study,17the myristic acid-LDH was shown to have an all-trans packing of the alkyl chains, on the basis of calorimetry and spectroscopic studies. There must be sufficient flexibility of the chains in the monolayer microscopic organic phase to create an effective partition medium, which accommodates the pyrene molecules between the chains, thereby avoiding chain disordering. However, in the case of the pyreneincorporated hexanoic acid-LDH sample, interchain perturbations, indicative of increased disordering, are observed from the infrared studies. Hexanoic acid forms a bilayer arrangement in which the opposing chains interpenetrate to some extent since the basal spacing is less than twice the length of the hexanoic acid chain. This makes the interlayer region more rigid, and the necessary penetration of the interchain region by pyrene will lead to squeezing of the alkyl chains and chain distortions. This also serves to explain why the KO,in hexanoic acid-LDH is lower than in myristic acid-LDH. In the latter, the pyrene can be accommodated in the interstices between myristic acid chains, with minimal perturbation on the chain packing. In the case of hexanoic acid, chain reorientation needs to occur to accommodate the pyrene. Differences in Ko,’s of pyrene have also been noted in different aquifier materials.3O It was concluded that it is
Acknowledgment. We gratefully acknowledge support from the Division of Chemical Sciences, Office of Basic Energy Sciences, Department of Energy, under Contract DE-FG02-90ER14105. We also thank the reviewers for helpful comments.
(30) (a) Herbert, B. E.; Bertsch, P. M.; Novak, J. M. Enuiron. Sci. Technol. 1993,27,398. (b) Murphy, E. M.; Zachara, J. M.; Smith, S. C. Enuiron. S C LTechnol. 1990, 24, 1507.
(31) Barrer, R. M. Clays Clay Miner. 1989, 37, 385. (32) Smith, J. A.; Jaffe, P. R. Enuiron. Sci. Technol. 1991, 25, 2056. (33) Eiceman, G. A.; Lara, A. S. J. Chromatogr. 1991,549, 273.
