Competitive Intercalation of Sulfonates into Layered Double Hydroxides

Queensland 4072, Australia, and Departments of Chemistry and Materials Science, UniVersity of North Texas,. P.O. Box 305070, Denton, Texas 76203...
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J. Phys. Chem. C 2007, 111, 4021-4026

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Competitive Intercalation of Sulfonates into Layered Double Hydroxides (LDHs): the Key Role of Hydrophobic Interactions Zhi Ping Xu*,† and Paul S. Braterman‡ ARC Centre for Functional Nanomaterials, School of Engineering, UniVersity of Queensland, Brisbane, Queensland 4072, Australia, and Departments of Chemistry and Materials Science, UniVersity of North Texas, P.O. Box 305070, Denton, Texas 76203 ReceiVed: December 6, 2006; In Final Form: January 6, 2007

Layered double hydroxides (LDHs) Mg:Al 2:1 intercalated with straight-chain hydrocarbon sulfonates have been synthesized by anion exchange and characterized by powder XRD, FTIR, and elemental (C,H and metals) analysis. We find that the interlayer spacing of these LDHs can be explained by an interpenetrating packing model with the hydrocarbon chain tilting at an angle of ∼55° to the metal hydroxide layers. When two such sulfonate anions differ by only one methylene group, their simultaneous intercalation results in a single uniform LDH phase, in which the interlayer spacing varies linearly with the ratio between the two anions. In competitive intercalation experiments, the longer-chain sulfonate is preferentially or even exclusively intercalated. Semiquantitative analysis shows that the affinity of C8H17SO3- (or C7H15SO3-) for LDH is ∼6 kJ mol-1 stronger than that of C7H15SO3- (or C6H13SO3-). We attribute this increase to an increase in hydrophobic interactions between hydrocarbon chains. Hence, we estimate that the total contribution of hydrophobic interactions between hydrocarbon chains to the affinity of C8H17SO3- for Mg2Al-LDH is around 40-50 kJ mol-1.

Introduction Layered double hydroxides (LDHs), also known as anionic clays, are a special group of lamellar hydroxides, recently receiving much attention.1,2 They consist of positively charged hydroxide layers, isostructural to the brucite layer (Mg(OH)2), with anions and water molecules intercalated between these layers. The general formula can be expressed as M2+nM3+ (OH)2+2n(Am-1/m)‚xH2O, where M2+ and M3+ are divalent and trivalent cations occupying the octahedral central positions within the hydroxide layers and Am- the interlayer anions balancing the positive charges on the layers. Almost all kinds of anions have been successfully intercalated into LDH, including the common inorganic anions, polyoxometalate, polymeric anions, complex anions, carboxylates, and sulfates.1-4 However, there are relatively few reports involving alkyl- and aryl-sulfonate LDHs and their structural properties.3-8 We now describe the incorporation of straight-chain hydrocarbon sulfonates with different chain lengths into LDH, and the cooperative and competitive intercalation of two such sulfonates. We find that the longer chain sulfonate is exclusively preferred when the sulfonates differ by two or more methylene units. However, both sulfonates are incorporated to form a single LDH phase whose interlayer spacing varies linearly with the ratio of the two anions when the difference is only one methylene. We would expect the affinity of an organic anion for LDH to depend, not only on electrostatic interactions, but also on hydrophobic interactions 9a and geometric restrictions. Differences in overall affinity can be used in separations of different organic isomers. For example, as reported by O’Hare et al.,10,11 * Corresponding author. E-mail: [email protected]. Fax: 61-733463973 † University of Queensland. ‡ University of North Texas.

Valim et al.,12 Jung et al.13 and Huh et al.,5b various LDHs, such as Ca2Al-LDH and LiAl2-LDH materials, show shape selectivity for the intercalation of particular isomers of pyridinecarboxylate, pyridinedicarboxylate, phthalate, naphthalenedisulfonate and muconate anions. O’Hare et al. have also found that LiAl2-LDH, MgAl-LDH, ZnCr-LDH and NiAl-LDH preferentially intercalate cytidine-5′-monophosphate (CMP) from an equimolar solution mixture of CMP with either AMP (adenosine-5′-monophosphate) or GMP (guanosine-5′-monophosphate).14 The reaction temperature and solvent also affected the selectivity in these cases. Here we report the separabability of straight-chain hydrocarbon sulfonates using LDHs. Due to the stronger hydrophobic interactions among the longer-chain sulfonates, they are selectively incorporated into MgAl-LDHs. Furthermore, the contribution of hydrophobic interactions to the affinity of hydrocarbon sulfonates has been semiquantitatively estimated. Experimental Preparation of Precursor LDHs. The precursor LDH Mg2Al(OH)6Cl‚nH2O was prepared in the presence of excess Mg2+, as reported elsewhere.9a,15 Briefly, 50 mmol AlCl3‚6H2O (Aldrich, 99%) and 150 mmol MgCl2‚6H2O (Aldrich, 99%) were dissolved in 500 mL deionized water (18.2 MΩ cm). After being purged with N2 for around 30 min, the solution was treated with 15.7 mL 50% NaOH (300 mmol, Alfa Aesar), followed by an overnight reflux under a slow stream of N2. The precipitate (Mg2Al(OH)6Cl‚nH2O) was separated and thoroughly washed via centrifuge, and dried in vacuum over molecular sieves. Intercalation of Sulfonates. Sulfonates (sodium salts, Alfa Aesar, 97-99%) were intercalated into LDHs by refluxing the precursor LDH (ca. 2.5 mmol) with 100 mL 0.050 M sulfonate solution overnight (samples A1-A4 in Table 1), under nitrogen

10.1021/jp0683723 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/17/2007

4022 J. Phys. Chem. C, Vol. 111, No. 10, 2007

Xu and Braterman

TABLE 1: Materials Prepared, Interlayer Spacings, and Analytical Data sample A1 A2 A3 A4 C1 C2 C3 D1 D2 D3 E1 E2 F1 F2

Al:anion(s)a 1:2 1:2 1:2 1:2 1:0.25:0.75 1:0.50:0.50 1:0.75:0.25 1:0.25:0.75 1:0.50:0.50 1:0.75:0.25 1:1:1 1:1:1 1:1:1 1:1:1

anion(s) -

C5H11SO3 C6H13SO3C7H15SO3C8H17SO3C7H15SO3-:C6H13SO3C7H15SO3-:C6H13SO3C7H15SO3-:C6H13SO3C8H17SO3-:C7H15SO3C8H17SO3-:C7H15SO3C8H17SO3-:C7H15SO3C5H11SO3-:C8H17SO3C6H13SO3-:C8H17SO3C6H13SO3-:C7H15SO3C7H15SO3-:C8H17SO3-

average db (nm) 1.739 1.849 1.973 2.061 1.867 1.916 1.946 1.988 2.019 2.042 2.057 2.068 1.937 2.040

Mg%c

Al%c

14.1 13.4 12.7

7.3 7.0 6.3

C%d

H%d

14.95 18.86 21.02 15.42 17.62 18.05 17.65 20.24 21.08 18.55 20.36

4.93 5.42 6.02 4.84 5.59 5.51 4.90 5.96 6.00 5.49 5.96

a Mole ratio of Al in precursor LDH to anion(s) in the solution prior to the exchange. b From d ) (d003 + 2d006 +... + nd00(3n))/n. c By atomic absorption. d From combustion analysis of M-H-W Laboratories.

to prevent contamination by carbonate. The materials were quickly separated via centrifuge from the hot suspension, then washed twice with deionized water and dried in vacuum over molecular sieves. Simultaneous Intercalation of Two Sulfonates. Two different sulfonates were also exchanged into the LDH under similar conditions. Their molar ratio was varied while the total molar number of two sulfonates was kept equivalent to the exchange ability of the precursor LDH (samples C1-C3 and D1-D3 in Table 1). For example, sample C1 was obtained by refluxing the precursor LDH (ca. 2.5 mmol) in a 100 mL solution containing 0.625 mmol C7H15SO3- and 1.875 mmol C6H13SO3-. The C and H elemental analyses (Table 1) show that the intercalation of two sulfonates is almost quantitative, with C1 through C3 intermediate between A2 and A3, and D1 through D3 monotonically increasing, approximately over the range between A3 and A4. For the competitive reactions, two equimolar sulfonates (sodium salt, ca. 2.5 mmol each) were dissolved in 100 mL water, followed by refluxing with the precursor LDH (ca. 2.5 mmol) overnight under N2. The solid samples (samples E1, E2, F1 and F2 in Table 1) were collected quickly from the hot suspension and washed twice with deionized water. Note that here the total amount of anions available for exchange is twice the exchange ability of the precursor LDH. Characterization of Materials. Powder X-ray diffraction (XRD) patterns were recorded using a Siemens F-series Diffractrometer with Cu KR radiation, λ)0.15418 nm), at a scanning rate of 1.2° per minute, over the range of 2-65°. Powdered CaF2 was used as an internal calibrant, and the d-spacing was calculated from the several orders of basal reflections. Infrared spectra from 4000 to 400 cm-1 were collected, using KBr discs, on a Perkin-Elmer 1760X FTIR, by scanning 40 times at a resolution of 4 cm-1. Magnesium and aluminum weight percentages were determined by atomic absorption (AAnalyst 300, Perkin-Elmer) in selected cases. The Mg:Al ratio was close to 2:1, as we have found for other LDHsurfactant nanocomposites in our earlier work.9a,16,17 Carbon and hydrogen analyses were performed by M-H-W Laboratories. The elemental weight percentages are listed in Table 1. Calculated for target composition Mg2Al(OH)6(C8H17SO3-)‚ H2O: 6.9% Al, 12.5% Mg, 24.7% C and 6.4% H with Al:Mg: C:H ) 1:2:8:25. Observed in sample A4: 6.3% Al, 12.7% Mg, 21.0% C and 6.0% H with Al:Mg:C:H ) 1:2.24:7.5:25.7, in acceptable agreement.

Figure 1. The XRD patterns of samples A1-A4. The peak marked with asterisk is ascribed to the plane (111) of the internal calibrant CaF2 and that marked with # to LDH carbonate.

Results Physical Properties. Mg2Al-sulfonate-LDHs (samples A1-A4) were characterized by XRD patterns (Figure 1) and by their IR spectra (Figure 2). As can be seen in Figure 1, the basal reflections (0,0,3n)1,2 are quite pronounced for each compound, up to the fifth order. The (0,0,9) reflection is notably weaker than its neighbors in all cases, presumably because of the phase relationships between the different scattering layers. The average interlayer spacing (average d) of each compound, as listed in Table 1, was obtained by using d ) (d003 + 2d006 + ... + nd00(3n))/n, counting in as many basal reflections as possible.16,17 The interlayer spacing on average increases by 0.11 nm per methylene group until the hydrocarbon chains extend to C14H29 (Mg2Al-C14H29SO3LDH, d ) 2.663 nm, unpublished observation), in agreement with the observations of Lagaly et al.3a and Rives at al.8 The nominal thickness of the crystallites in these LDHs is 15 to 20 nm, as estimated by using the Debye-Scherrer equation without any instrument correction.18 This corresponds to around 7 to 10 hydroxide-sulfonate alternating layers. The IR spectra show the characteristic vibrations expected for the hydroxide layer, the interlayer water, and the inserted

Competitive Intercalation of Sulfonates

Figure 2. The FTIR spectra of samples A1-A4.

J. Phys. Chem. C, Vol. 111, No. 10, 2007 4023

Figure 4. The FTIR spectra of samples A2, C1-C3, and A3 in the C-H stretching region.

TABLE 2: Characteristic Vibrations of CH3 and CH2 and Their Relative Intensity sample νCH3 (as) νCH2 (as) IνCH3/IνCH2a ASCH3/ASCH2b SCH3/SCH2c A1 A2 A3 A4 C1 C2 C3 D1 D2 D3 E1 E2 F1 F2

Figure 3. The XRD patterns of samples A2, C1-C3, A3.

sulfonates. Besides the OH stretching vibrations (3400-3500 cm-1, very broad) and the HOH bend of water (1630 cm-1),9a there are SO3 antisymmetrical (1191-1195 cm-1, broad) and symmetrical (1051 cm-1, sharp) stretching vibrations,8,19 and metal-oxygen vibrations at 447 cm-1 (sharp, characteristic of well-ordered Mg2Al-LDHs)9,20 and 680 cm-1 (very broad).9 In addition, each LDH shows its own CH stretching pattern at 2800-3000 cm-1.19 The relative intensities of the CH2 and CH3 vibrations vary with chain length, as would be expected (Figures 2 and 4). For example, the height ratio of absorption peaks (IνCH3/IνCH2, nominal absorbance) attributed to the antisymmetrical CH3 (2958-2960 cm-1) and CH2 (2923-2936 cm-1) stretching vibration gradually decreases from 1.08 in sample A1 to 0.53 in sample A4 (Table 2).19 In addition, some samples contain a small amount of carbonate LDH, as indicated by the broad IR peak at 13601370 cm-1 in Figure 2 as well as the weak diffraction at 1112° in the XRD profile of sample A1 (Figure 1). This absorption of carbon dioxide from air is believed to occur during the separation and washing of samples.

2959.3 2959.7 2958.5 2958.7 2959.4 2959.1 2959.0 2958.5 2958.5 2958.5 2958.9 2958.7 2958.5 2958.8

2935.6 2933.3 2926.8 2923.0 2931.0 2927.2 2927.0 2926.0 2926.0 2926.5 2923.6 2925.7 2926.5 2925.5

1.08 0.84 0.70 0.53 0.77 0.74 0.73 0.62 0.61 0.56 0.55 0.57 0.70 0.58

0.85 0.50 0.43 0.26 0.48 0.45 0.44 0.33 0.31 0.30 0.30 0.29 0.30 0.32

1.75 1.08 0.87 0.56 0.94 0.92 0.79 0.76 0.70 0.68 0.60 0.69 0.74 0.71

a IνCH3/IνCH2 is the ratio of uncorrected absorbance values at peak maxima. b ASCH3/ASCH2 is the ratio of the deconvoluted area assigned for CH3 and CH2 antisymmetrical vibration. c SCH3/SCH2 is the ratio of the deconvoluted area assigned for CH3 and CH2 symmetrical vibration. Calculation of ASCH3/ASCH2 and SCH3/ACH2 can be referred to in the Supporting Information.

Incorporation of Two Sulfonates into LDH. Figure 3 displays the XRD patterns of LDHs containing both C6H13SO3and C7H15SO3- (samples C1-C3, Table 1). We note that there is only one uniform LDH phase in all samples and the interlayer spacing increases smoothly from 1.849 nm for A2, through C1, C2, and C3, to 1.973 nm for A3 (Table 1). The IR spectra of the mixed samples, especially in the CH stretching region (Figure 4 and Table 2) show a similar evolution both in intensity and in shape, varying consistently with the CH2:CH3 ratio. For example, the strongest peak at 2920-2935 cm-1 is transformed from a left-high shape in sample A2 to a left-right balanced contour in sample A3 (Figure 4), and then to a right-high shape in sample A4 (for samples D1 to D3, see Supporting Figure 1S) as the relative intensity of CH3 to CH2 stretching vibration (IνCH3/IνCH2, ASCH3/ ASCH2 and SCH3/SCH2 in Table 2) smoothly changes. Very similar phenomena were also observed for the pair C7H15SO3- and C8H17SO3- (samples D1-D3). These

4024 J. Phys. Chem. C, Vol. 111, No. 10, 2007 smooth transitions in XRD patterns and IR spectra further indicate the simultaneous incorporation of both anions in these samples. Selective and Competitive Intercalation. The XRD pattern of sample E1 (Supporting Figure 2S) is very similar to that of sample A4 (LDH-C8H17SO3-), with the same interlayer spacing within the range of experimental variation (2.057 nm vs 2.061 nm, Table 1), and the C-H peak contours (2800-3000 cm-1) and positions are identical. Likewise for sample E2 (Table 1; Figures 1 and 2, Supporting Figures 1S and 2S). Thus C8H17SO3- is quantitatively selected by LDH from the solution mixture with C6H13SO3- or C5H11SO3-. In contrast, the competitive intercalation of equimolar C6H13SO3- and C7H15SO3- into the LDH results in a single uniform material (sample F1) with an interlayer spacing of 1.937 nm (Table 1 and Supporting Figure 2S), intermediate between those of A2 (1.849 nm, Mg2Al-C6H13SO3-LDH) and A3 (1.973 nm, Mg2Al-C7H15SO3-LDH). Additionally, the CH characteristic bands at 2800-3000 cm-1 are similar both in position and shape to those of sample C3 (Figure 4) which contains 25% C6H13SO3- and 75% C7H15SO3- in the interlayer. Similarly, sample F2 has an interlayer spacing of 2.040 nm, between 1.973 nm (sample A3) and 2.061 nm (sample A4), and combines the features of A3 and A4 in its C-H vibrational peaks. So in both these cases, uptake of sulfonates differing by only one CH2 unit leads to simultaneous incorporation in a single intermediate phase. The relative amounts of the two anions are estimated in the following section. Discussion Anion Packing in the Interlayer. Several packing modes for the organic hydrocarbon chains in the interlayer have been put forward by different researchers.3,9a The hydrocarbon chains, as summarized by Lagaly 3a and Carlino,3b are in general supposed to stand vertically, or tilt at an angle with respect to the hydroxide layer, to form monolayer, bilayer or antiparallel packing structures. We found for dodecylbenzenesulfonate that the antiparallel (more precisely, interpenetrating bilayer) model best described the hydrocarbon chain packing,9a and apply the same model here. This model was also proposed by Lagaly et al.,3a Rives et al.8 and Vieweger et al.23 for organic species intercalation into LDHs in some cases. The three oxygens of the sulfonate anion are equivalent so long as they are equally anchored to the hydroxide layer, as shown in Figure 5 for LDH-C8H17SO3-. This assumption is supported by the presence of a single broad peak in sample A4 (Mg2Al-C8H17SO3--LDH) at 1191 cm-1 (Figure 2), corresponding to the antisymmetrical stretching vibration(s) of SO3 group, which in the less symmetrical environment of solid C8H17SO3Na is resolved into distinct components21 which we observe at 1201 and 1182 cm-1. Such an anchor style requires the S-C1 bond to be perpendicular to the hydroxide layer, with the hydrocarbon chain tilting at an angle of around 55° (angle A in Figure 5), or half of the tetrahedral C-C bond angle B (Figure 5). The average chain cross-section is 0.24 nm2, close to the 0.20 nm2 found for close packing of fully extended hydrocarbon chains in the solid.22 This model gives predicted interlayer spacings for samples A1, A2, A3, and A4 of 1.75, 1.82, 1.97, and 2.04 nm, respectively, in good agreement with the observed values (Table 1). The interlayer water, detected by analysis and, more conclusively, by infrared, is presumably hydrogen bonded to the sulfonate oxygen as well as the oxygen in the LDH hydroxide layer.2 Two Anions in the Interlayer. When two very similar sulfonate anions, e.g., C6H13SO3- and C7H15SO3-, are interca-

Xu and Braterman

Figure 5. Packing mode of sulfonate anions in the interlayer; unit is nm.

Figure 6. Linear relationship between interlayer spacing and chain length in sample series A, C, and D. Solid symbols give composition as defined by method of preparation. Blank squares show measured carbon weight percentages.

lated simultaneously, a single LDH phase is formed (Figure 3), with the interlayer spacing linearly changing with the anion atomic ratio. Figure 6 plots this linear relationship as a bold straight line, where we convert the anion molar ratio (R ) C7H15SO3-/C6H13SO3-) into the average chain length (L) with a formula of L ) 6 + R for samples C1-C3. The similar linear relationship for the C7H15SO3-/C8H17SO3- pair is shown by the thin straight line in this figure. This is similar to the relationship found in layer-type VOSO4 compounds, for example, intercalated with butan-1-ol and pentan-1-ol mixtures.24 Using this linear relationship (Figure 6), we could quantitatively estimate the anion population in samples F1 and F2. Calculation thus gives about 71% C7H15SO3- and 29% C6H13SO3- in sample F1 and 75% C8H17SO3- and 25% C7H15SO3- in sample F2. As also shown by the open squares in Figure 6, there could be another linear relationship between the measured carbon weight percentage and the chain length, which gives 87% C7H15SO3- and 13% C6H13SO3- in sample F1 and 64% C8H17SO3- and 36% C7H15SO3- in sample F2. However, the linear relationship is not as good as that between the interlayer spacing and the so-called chain length (L), due to the measurement errors and the CO32- disturbance. Therefore, we prefer to use the first linear relationship to semiquantitatively estimate the anion proportion in F1 and F2.

Competitive Intercalation of Sulfonates

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We also note that the slope of the straight line is 0.13 nm/C atom on going from LDH-C6H13SO3- to LDH-C7H15SO3(even to odd) but only 0.09 nm/C atom on going from LDHC7H15SO3- to LDH-C8H17SO3- (odd to even). An effect of this sort is expected in our model, which ideally requires the C2nC2n+1 bond to be perpendicular to the metal hydroxide plane while the C2n-1-C2n bond to be tilted at 20o (refer to Figure 5). However, the observed alternation effect is much weaker than the predicted (0.15 vs 0.07), perhaps as a result of flexibility in the hydrocarbon tails. Such alternation phenomena have also been observed for VOSO4 or VOPO4 layer-type complexes intercalated with aliphatic alcohols25 and cationic clays intercalated with aliphatic amines.26 Contribution of Hydrophobic Interactions to the Affinity. The forces attaching a sulfonate headgroup to a layer will presumably be independent of chain length, while the electrostatic interaction of each headgroup with the positively charged layer on the opposite side of the gallery will be weak and, if anything, diminishing with chain length. We therefore ascribe the observed increase of affinity with chain length to hydrophobic interactions, and attempt to estimate the strength of these from our data. Since the exchange of dicarboxylates with LiAl2-Cl-LDH occurs very rapidly and is completed in less than 2 min at room temperature,27 it seems reasonable to assume that the following equilibrium is established upon refluxing overnight:

Mg2Al-C6H13SO3-LDH (s) + C7H15SO3- S Mg2AlC7H15SO3-LDH (s) + C6H13SO3- (1) Ignoring cooperative effects, the equilibrium constant, K, for the anion-exchange process can be expressed by28

K)

CC6XC7γC6fC7 CC7XC6γC7fC6

(2)

where CC6 and CC7 denote the molar concentrations of C6H13SO3and C7H15SO3- in solution, and γC6 and γC7 their activity coefficient; XC6 and XC7 denote the anion fractions of C6H13SO3and C7H15SO3- in LDH, and fC6 and fC7 the corresponding activity coefficient. Note that the sulfonate concentrations in this research are far below their critical micelle concentrations.29 To the first approximation, the quotient of activity coefficients γC6fC7/γC7fC6 is unity. Thus

K)

CC6XC7 CC7XC6

(3)

Taking X from the data for F1 and F2 and estimating C from mass balance gives the equilibrium constant K ∼9.0 and ∼6.0 for the C7H15SO3-/C8H17SO3- pair and the C6H13SO3-/C7H15SO3- pair at 100 °C, respectively. This implies that the equilibrium constant K for the C6H13SO3-/C8H17SO3- pair is ∼54, large enough to account for the virtually quantitative predominance of C8H17SO3- over C6H13SO3- (and of course C5H11SO3-) in competition experiments. Applying the standard equation

∆G°T ) -RT ln K

ites suggests that this is not a facile process.17,30 These factors are not very different from the factors between successive critical micellar concentrations.29 If we use the increase in sulfonate affinity with chain length as a direct measure of the hydrophobic interaction, this gives it a value of ∼6 kJ mol-1 per methylene group, or 36-48 kJ mol-1 for C6H13, C7H15, or C8H17 side chains. Similarly, p-dodecylbenzene sulfonate would have hydrophobic interactions of around 70 kJ mol-1, which may explain our earlier finding9 that its affinity for LDH is greater than that of sulfate and comparable to the traditional front-runner, carbonate. Application to Separation of Organic Anions with LDH. We have shown here that the affinity of sulfonates into LDH is very sensitive to the hydrocarbon chain length, and can be used for at least near-quantitative selection when the chain length differs by two or more methylene groups. For chain lengths differing by one methylene, we anticipate that elution techniques could readily be developed on the basis of the present finding. Further analytical applications could involve the separation of isomers, which would no doubt differ somewhat in their ability to pack efficiently, as we have recently shown for cis- and transunsaturated carboxylates in LDH.16 Finally, it is instructive to compare the materials we describe here with other LDHs and related materials containing two separate intercalated substances in the same material. One possibility is the formation of an inter-stratified phase, in which the two different anions occupy alternate layers. This behavior was found many years ago by Drits et al. in naturally occurring mixed sulfate-carbonate LDH,31 and had more recently been demonstrated by O’Hare et al.27 and Jones et al.32 for carboxylate-chloride materials. A second possibility, reported by O’Hare for LDH containing two different dicarboxylates, is control of the spacing by the longer chain;27 such behavior, in general, is also shown by layer-type VOSO4 containing less than 20% propanol in a predominantly hexanol interlayer.24 The third possibility, i.e., formation of a phase of intermediate spacing, is shown by VOSO4/pentanol/hexanol systems in general, and is what we find here. Conclusions Layered double hydroxides Mg2Al-Cl-LDHs readily exchange chloride for straight chain sulfonates under reflux. The interlayer spacing of LDHs intercalated with various sulfonate anions indicates an inter-penetrating bilayer packing mode for the hydrocarbon chains. The simultaneous incorporation of two similar sulfonate anions results in a uniform LDH phase whose interlayer spacing varies linearly with the molar ratio. We have also found that the Mg2Al-LDH favors incorporation of the longer chain sulfonate by a factor of 6-9 for a single methylene group difference, and overwhelmingly when the difference is greater. This corresponds to a nominal affinity difference of ∼6 kJ mol-1 for each CH2 group, assuming that the equilibrium is established at 100 °C. We attribute this effect to hydrophobic forces, and infer that such effects could dominate selectivity within other families of organo-anions, such as long chain carboxylates, sulfates, phosphates or phosphonates.

(4)

where T ) 373 K and K ) 6.0-9.0, gives a nominal value of 5.5-6.8 kJ mol-1, assuming that the observed values do reflect equilibrium at that temperature. It could reasonably be objected that the equilibrium may shift during workup, but the tight structure commonly observed for LDH-surfactant nanocompos-

Acknowledgment. The authors thank the Robert A. Welch Foundation (Grant B-1445) and the University of North Texas Faculty Research Fund, for support. Dr Xu appreciates the support from the ARC Centre for Functional Nanomaterials funded by the Australia Research Council under its Centre of Excellence Scheme.

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