The Intercalation of a Vermiculite by Cationic Surfactants and Its Subsequent Swelling with Organic Solvents

Journal of Colloid and Interface Science 255, 303–311 (2002) doi:10.1006/jcis.2002.8673

The Intercalation of a Vermiculite by Cationic Surfactants and Its Subsequent Swelling with Organic Solvents S. Williams-Daryn and R. K. Thomas1 Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, United Kingdom Received February 15, 2002; accepted August 18, 2002

We have measured the dimensions of the interlamellar space following intercalation of a vermiculite by a range of cationic surfactants and followed the subsequent swelling of the organoclay compounds with several organic solvents. A single vermiculite (Eucatex) was used with three series of surfactants, N-alkyltrimethylammonium bromides, N,N -dialkyldimethylammonium bromides, and the gemini cationic surfactants, α,ω-bis (Nalkyldimethylammonium) alkanes. In all cases well-defined stoichiometric compounds are obtained and the amount of surfactant intercalating the layer indicates that there are two factors controlling this amount, charge neutralization of the clay and hydrophobic packing. Packing arguments are used to deduce the fraction of noncharge-neutralizing material in the interlamellar space. It is clear that by altering the surfactant charge and structure it is possible to control the degree to which adsorption beyond charge neutralization occurs in these complexes, which is important when the capacity of such complexes to sorb other materials is considered. The general pattern of swelling of the surfactant/vermiculite complex by toluene suggests that the maximum expansion of the intralamellar space is limited by the longest chain in the surfactant. In contrast to earlier results we found that these vermiculites could be swollen by alkanes as well as aromatic solvents. This is attributed to the greater hydrophobicity of the interior of an organoclay formed from a clay of higher charge density. C 2002 Elsevier Science (USA) Key Words: vermiculite; cationic surfactants; clay intercalation; X-ray diffraction; alkyl trimethylammonium bromides; dialkylammonium dimethylammonium bromides; gemini surfactants.

INTRODUCTION

Clay minerals are layered aluminosilicates in which isomorphous substitution generates a permanent negative charge, which varies from material to material and which is balanced in the natural state by inorganic cations. Under certain circumstances the inorganic cations may be replaced by larger cationic organic species with an associated increase in the spacing between the layers. Depending on its properties the cation may make the space between the layers either hydrophobic or hydrophilic. When it is made hydrophilic, usually by small cations, it may 1

To whom correspondence should be addressed. 303

become possible to swell the clay macroscopically with aqueous solution, the interlayer spacing taking values from 10 to ˚ (see, for example, (1–4)). Larger cations with a more 1000 A amphiphilic character create a hydrophobic region in the interlamellar space into which it may be possible to intercalate neutral organic species. There is therefore considerable scope for utilizing the interior of clay–organic composites for a variety of scientific and technological purposes, some examples of which are removal of organic contaminants (5, 6), use as templates for the preparation of porous materials (7, 8), or for in situ polymerization to make composite clay–polymer materials (9). Nevertheless, we do not have a clear understanding of the factors that determine the often very strong selectivity of the clay for certain ions and the propensity of the clay to intercalate species beyond the original cations. In this paper we shall be concerned with the intercalation of surfactant ions to create a hydrophobic interior followed by swelling with toluene and some other organic solvents. We first summarize the relevant previous work on the intercalation of surfactant ions, which has been well reviewed by a number of authors (10–14). Most research has been done using montmorillonite, which has a lower interlayer ion capacity (cation exchange capacity, CEC) than our vermiculite sample, and it is not clear how many of the conclusions drawn from montmorillonite are also valid for vermiculites. The adsorption isotherms of primary N-alkylammonium ions on Na+ -montmorillonite are of the Langmuir type (15–17) and alkylammonium ions are preferentially bound relative to sodium regardless of the volume fraction of organic cation in solution, a preference that increases with the molecular weight of the cation. At high concentrations many cations are taken up in excess of the CEC (15, 17–24) and this requires the intercalation of either neutral species or ion pairs. This additional intercalation seems to be restricted to cations containing eight or more carbon atoms in their chains and is primarily due to van der Waals interactions between the organic chains. Whether the excess is taken up in the form of neutral molecules or ion pairs depends on the proton-donating ability of the ion. Thus for N-dodecylammonium bromide, the equilibrium pH (∼5.6) after intercalation was found to be noticeably lower than the initial pH (∼7), suggesting that the excess takes the form of the neutral dodecylamine, but the constant 0021-9797/02 $35.00

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pH in the intercalation of ethyldimethyloctadecenylammonium bromide indicates the incorporation of ion pairs (20). Greenland and Quirk (23) also found that alkylpyridinium bromides with chains of eight carbons or more are taken up as ion pairs. Basal spacings (interlamellar + clay platelet spacing) of complexes containing N-alkylammonium ions are primarily related to the carbon chain length and the layer charge of the clay mineral (19). When the area that would be occupied by the amphiphilic ion lying flat on the surface is less than the area per unit negative charge on the surface the amphiphilic ions are adsorbed in a monolayer with the molecules lying approximately parallel to the platelet surface (25). As the charge on the clay increases the amphiphilic cations first form a flat bilayer and then become increasingly tilted. Bilayer complexes for a given clay and a homologous series of N-alkylammonium cations show a regular increase in the spacing per carbon atom, with differences sometimes, but not always, occurring between even- and oddnumbered carbon chains (10, 26, 27). Weiss (11) has made the most comprehensive survey of the further swelling of surfactant/clay complexes by organic solvents and there have been more recent studies which have tended to focus on mixed solvents where one is polar or on sorption of organic species from aqueous environments, for example (28–33). In general it is agreed that many polar solvents readily swell the clay/surfactant complex and this is attributed to the amphiphilic nature of the interlamellar space. It has also been observed that nonpolar aromatic solvents often swell the complexes, and alkenes may do so under certain circumstances, but alkanes do not. Swelling by any species is generally assisted by the presence of a polar solvent. In the present work we attempt first to establish the nature and utility of the interlamellar space in the amphiphile intercalated vermiculite and then to see how this affects the uptake of organic solvents. The vermiculite used is Eucatex vermiculite from Brazil. We use X-ray diffraction to examine the intercalation by cations of different shape, charge and size, focusing particularly on the space occupied by non-charge-neutralizing species. We then use space-filling arguments to estimate the proportions of the different species present in the swollen ternary system. Such arguments necessarily involve assumptions about the packing and the molecular volumes of the different components. In subsequent papers we substantiate these assumptions by examining a selection of binary and ternary complexes more directly, using neutron diffraction and isotopic subsitution. Previous work has mainly focused on single-chain N-alkylammonium cationic surfactants. In this study, however, compounds with quaternary alkylammonium groups were chosen because they are strong electrolytes and therefore ionization of the weak amine electrolyte can play no role in the charge neutralization procedure. Any RTA+ present in the interlamellar space over and above that required to neutralize the clay surface must have a corresponding counterion. A preliminary study has already indicated that such material is present for the complex between Eucatex vermi-

culite and dodecyl- or hexadecyltrimethylammonium bromide (34). Effects of structure of the surfactant have been studied by retaining the fully alkyl-substituted amino group but in N,N dialkyldimethylammonium bromides and in the cationic gemini surfactants. EXPERIMENTAL DETAILS

The reason for choosing Eucatex vermiculite is that it can be obtained in the form of quasi-single crystals with a typical crystal size of 5 × 5 × 0.5 mm. This makes it ideal for precise measurements using X-ray or neutron diffraction. The formula is [(Si3.11 Al0.89 ) (Mg2.475 Al0.075 Fe0.305 Ti0.04 Mn0.005 ) O10 (OH)2 ]2 · X1.40 · nH2 O, where X represents the exchangeable cation and n depends on the state of hydration of the ˚ b= vermiculite. The unit cell parameters are a = 5.33 A, ˚ and c (the basal spacing) = 9.6 A. ˚ The cation ex9.18 A, change capacity (CEC) of the hydrated sodium vermiculite is 147 mequiv/100g. The Eucatex vermiculite was initially soaked in a 1.0 M solution of NaCl for a minimum of six months to ensure the homogeneity of the exchangeable cation. Sodium was chosen as the preferred cation because it is relatively easy to exchange. To prepare the organoclays the Na+ -vermiculite was immersed in excess 0.1 M solution of the appropriate surfactant and kept in solution for three weeks at 40◦ C until the exchange was complete (this could be monitored most easily by constancy of the X-ray diffraction pattern). The organically modified clay was then removed from solution and rinsed in water to remove excess salt before being air-dried at room temperature (∼20◦ C) in a dessicator. Concentrations below 0.1 M tended to result in incomplete replacement of the sodium ions. Most of the N-alkyltrimethylammonium bromides (RTAB) were obtained from Aldrich and recrystallized from acetone/ ethanol before use. The longer chain RTABs were synthesized by the direct reaction of RBr with trimethylamine in methanol and similarly recrystallized at least twice. The asymmetric RR DAB surfactants were synthesized as described elsewhere (35). The symmetric dialkyldimethylammonium bromides (n = 10, 12, 16) were obtained from Aldrich and used without further purification, and the n = 18 compound was obtained from Unilever and used as received. The gemini surfactants were synthesized and purified as described elsewhere (36). The procedure for ion exchange with the clay was exactly the same as for the single chain surfactants except for the gemini surfactant with the xylene spacer, which was done at 50◦ C because of the lower solubility of the surfactant. The X-ray diffraction patterns were recorded on a home built apparatus based on a Philips two-circle diffractometer using ˚ A special sample cell CuK α radiation of wavelength 1.54 A. was used, in which the single Eucatex crystal could be suspended in a closed, saturated solvent atmosphere. To ensure complete saturation the lower part of the crystal was kept in contact with liquid solvent. The temperature of the sample enclosure could be

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adjusted by circulation of cooling or heating fluid and maintained in the range 0–70◦ C to an accuracy of 2◦ C. RESULTS

I. Vermiculite–Surfactant Complexes Single-Chain Surfactants The intercalation of the compounds RTAB, where R is an N-alkyl group varying from C8 to C20 in steps of 2, was followed by X-ray diffraction. In each case a single compound of well-defined stoichiometry is formed and the amount of surfactant in the interlamellar space significantly exceeds that required to neutralize the charge on the platelets. The well-defined stoichiometry is indicated by the presence of an extended series of narrow basal plane reflections (001) for a given surfactant, whose positions do not change with extended soaking in surfactant solution. The X-ray diffraction patterns of the air-dried C12 TAB and C18 TAB complexes at 25◦ C are shown in Fig. 1

FIG. 1. X-ray diffraction patterns of oriented crystals of complexes of (a) dodecyltrimethyl ammonium bromide and (b) octadecyltrimethyl ammonium bromide with Eucatex vermiculite at 25◦ C. The scattering vector is aligned so that the patterns are dominated by the structure along the direction normal to the vermiculite platelets. The larger spacing in the octadecyltrimethyl ammonium bromide complex is manifested by the closer spacing of the (001) reflections.

TABLE 1 Basal Spacings of Complexes of Eucatex Vermiculite with SingleChain N-Alkyltrimethylammonium Bromides of Different Chain Length at 25◦ C Chainlength

Spacing (obs.) ❛ (A ± 0.5)

Spacing ❛ (calc.) (A ± 0.5)

Fractional filling by charge neutralizing species

8 10 12 14 16 18 20

23.9 27.0 30.0 32.4 33.5 38.2 36.8

25.6 27.7 29.8 31.9 34.0 36.1 38.2

0.77 0.72 0.69 0.68 0.72 0.65 0.74

and the basal spacings for the series (averages obtained from not less than two clay samples) are given in Table 1. For comparison a spacing calculated on the basis of the model shown in Fig. 2 is included in the table. The calculation uses ˚ and assumes interdigithe known platelet thickness of 9.6 A tated, fully stretched, all-trans N-alkyl chains tilted at an angle of 54.5◦ with respect to the platelets. This is the most favorable angle of tilt to maintain the maximum contact of the methyl groups of the headgroup with the surface. This angle of tilt has been observed by others for related compounds (10, 18). Although there are some discrepancies between the observed and calculated spacings in Table 1, the model accounts well for the increase in spacing with chainlength. Some of the discrepancies undoubtedly arise from the small but variable amount of water between the layers, which we did not attempt to control in these experiments. It is also possible to estimate the fraction of space in the layer that is occupied by charge-neutralizing species by using the unit cell parameters and standard size parameters estimated for the surfactants (37). The fraction of space filled by charge-neutralizing species is given in the last column of Table 1 and can be seen to be in the region of 0.7, showing that neutral species must be present in the layer; i.e., adsorption beyond the cation exchange capacity (CEC) takes place. Since ionization

FIG. 2. Schematic model of surfactant chains used for calculating the expected interlamellar spacings for Eucatex intercalated by N-alkyl trimethyl ammonium bromides. The interlamellar spacing is given by X = (1.265n + 1.5) × sin(54.5) where n is the number of carbon atoms in the N-alkyltrimethylammonium bromides.

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TABLE 2 Variation of Basal Spacings of Complexes of Eucatex Vermiculite with N-Dodecyltrimethylammonium Bromide with Temperature T/◦ C

0

10

17

27

35

45

55

65

75

TABLE 3 Basal Spacings of Complexes of Eucatex Vermiculite with Double-Chain Dodecylalkyldimethylammonium Bromides of Different Secondary Chain Length at 25◦ C

85

❛ Spacing/A ± 0.5 35.8 35.3 32.3 30.4 31.2 31.6 31.2 31.0 30.7 30.0

to the amine cannot occur in the trimethylammonium molecule the neutral species must be part of an ion pair and this result agrees with the conclusions of Grim et al. (20) and Greenland and Quirk (23) for different systems. The effects of temperature on the spacing in the dodecyltrimethylammonium (C12 TAB) vermiculite system are shown in Table 2. There is a marked change in both the spacing and the rate of change of spacing with temperature between 17 and 27◦ C. Differential scanning calorimetry showed that there is one phase change in the range −20 to 60◦ C at 18.6◦ C. This is almost certainly associated with chain melting. It seems probable that when the chains are fluid the packing will be more efficient and this is probably the explanation of the decrease in spacing. Note that these samples were not equilibrated with the bulk solution at each temperature; the composition was that fixed by the initial intercalation at 25◦ C. Double-Chain Surfactants The RTA+ cations required to neutralize the charge on the platelets account for only about 70% of the available space between the platelets. This combined with the high degree of order within the systems suggests that there is excess material in the form of ion pairs. As already noted, this contrasts with the more commonly studied N-alkylammonium surfactants, where the cation can lose a proton to give a free amine and the free amine can occupy the residual interlamellar space. Increasing the chain length in the RTAB series has no effect on the relative space-filling by charge-neutralizing species because the orientation of the alkyl chain is the same throughout the series and the chains are fully interdigitated. The only way of changing the space-filling conditions is by changing the shape of the surfactant. In order to test this hypothesis we studied the intercalation into the same vermiculite of the two series of cationic surfactants C12 C N DAB and C16 C N DAB. Just as for the single-chain RTAB the concentration behavior and X-ray diffraction patterns showed that all the RR DAB surfactants formed single welldefined compounds. The experimental spacings and fractions of space occupied by the charge-neutralizing species are given in Table 3. As expected, the percentage of space-filling by the chargeneutralizing species is greater than for the single-chain RTABs because the ratio of volume to charge has increased. On average a single-chain surfactant accounts for about 70% of the available space while for the double chain systems the average value is closer to 85%. It is also interesting that the basal spacing remains

Surfactant

Spacing (obs.) ❛ (A ± 0.5)

Fractional filling by charge-neutralizing species

C12 C2 DAB C12 C4 DAB C12 C6 DAB C12 C8 DAB C12 C10 DAB C12 C12 DAB

29.5 29.2 29.9 31.9 33.3 35.7

0.75 0.84 0.88 0.87 0.89 0.86

C16 C2 DAB C16 C4 DAB C16 C6 DAB C16 C8 DAB

34.0 33.3 34.7 35.3

0.73 0.82 0.84 0.88

C10 C10 DAB C18 C18 DAB

34.3 41.0

0.79 1.00

constant and is almost identical to that of the single-chain complex until the secondary chain length reaches the critical length of six carbon atoms in the chain (Fig. 3). The fractional spacefilling by charge-neutralizing species increases to a maximum at this point and then remains reasonably constant. This pattern suggests that for secondary chains containing up to and including six carbon atoms, the structure adopted is similar to that of the single-chain complex; i.e., the longer primary chain is in an all-trans conformation and is tilted at 54.5◦ to the clay layer, while the shorter secondary chain lies flat along the clay surface. To understand why this model is valid only for secondary chains containing six or fewer carbon atoms it is necessary to take a closer look at the area per molecule available together with the physical dimensions of surfactant. Assuming the negative charge in the clay layer is evenly distributed, the area per

FIG. 3. The basal plane spacing for complexes of Eucatex vermiculite and dialkyldimethylammonium bromides in the series C12 Cn DAB as a function of n.

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including C12 C8 DAB, indicating that packing becomes more efficient as the chains become more fluid. With the exception of the C12 C10 sample, which displays an approximately constant basal spacing, all samples exhibit a decrease in basal spacing with increasing temperature. The C12 C2 sample behaves in a way similar to C12 TAB with a phase change below 20◦ C leading to a fully collapsed phase associated with more mobile chains. This observation reinforces the general conclusions reached for the single-chain complexes and seems reasonable in light of the proposed structures, since at low temperatures the chains are more rigid, making interdigitation more difficult. As the temperature is increased, the greater degree of mobility in the chains allows the chains to rearrange into a more close-packed structure with a higher degree of interchain penetration. Gemini Surfactants

FIG. 4. Possible arrangements of two chains in the interlamellar space of complexes of Eucatex vermiculite and dialkyldimethyl ammonium bromides. In (a) there is space on the surface of the clay for the shorter, secondary chains and the long chain is tilted at 54◦ , but in (b) both chains mix in the hydrophobic part of the interlamellar space and the orientation of the head group changes so that the chains are at an angle of 35◦ .

˚ 2 and 9.4 A, ˚ recharge and average charge separation are 70 A ˚ spectively. An all-trans fully stretched six-carbon chain is 9.1 A long and is therefore just able to adopt the flat arrangement depicted in Fig. 4a. As the secondary chain becomes progressively longer, the arrangement shown in Fig. 4a is no longer possible and the entire cation must rotate and adopt a new conformation. This is the most probable explanation of the observations that for both series the d-spacing remains close to the single-chain ˚ for secondary chains up to and including six value (∼34.0 A) carbons, but increases when the secondary chain is longer than C6 . This arrangement of the two chains combines the bilayer structure characteristic of high layer charge with the flat chain configuration characteristic of samples with low layer charge. The interlamellar arrangement in the longer chain complexes will involve a number of factors. As mentioned earlier, the cation may no longer be able to adopt the favored pyramidal conformation due to steric repulsion between adjacent molecules. A possible arrangement is one where the two methyl groups are on the clay surface while the two long N-alkyl chains branch out at an angle of 35.5◦ , as shown in Fig. 4b. The change in lamellar spacing from C12 C8 DAB to C12 C12 DAB is accounted for by such a model if the two layers within the lamellar space are placed end to end. However, the structure is likely to be less ordered than suggested by the diagram. In this context it is worth noting that an increase in temperature causes a steady decrease in lattice spacing for all members of the series up to and

Gemini surfactants comprise two N-alkyldimethylammonium bromide groups joined together by an alkyl spacer and with the general formula (Cn N(CH3 )2 Br)2 X (Cn Cm Cn ), where n = number of carbons in the free N-alkyl chain and X is the bridging alkyl group containing m carbon atoms. The important difference from the other surfactant series above is that the geminis carry a double charge and give the opportunity to test the effect of surfactant charge on the intercalation process. The bridging groups in this study were N-alkylene chains of varying lengths (C3 –C12 ) as well as the rigid xylyl group. All samples were studied by X-ray diffraction, following the procedure used for the single-chain samples. Diffraction patterns recorded at 25◦ C of the best and the worst of the five samples studied are shown in Fig. 5. As for the other surfactants all the samples gave a set of reflections consistent with the presence of a single phase. The C12 C4 C12 and C12 C6 C12 samples in particular have very clean diffraction patterns, while the C12 (xylyl)C12 with its rigid spacer appears to be the least ordered. The molecular parameters and basal spacings obtained at 25◦ C for each sample are listed in Table 4. None of the samples shows the spacing expected for a single˚ i.e., the structure that would chain dodecyl complex (∼30 A), correspond to a fully interdigitated monolayer arrangement with the chains fully stretched in the all-trans conformation. The ˚ can be rationalized by considering spacings higher than 30 A

TABLE 4 Basal Spacings of Complexes of Eucatex Vermiculite with Gemini Dicationic Surfactants with Different Spacers at 25◦ C Surfactant

Spacing (obs.) ❛ (A ± 0.5)

Fractional filling by charge-neutralizing species

C12 C3 C12 C12 C4 C12 C12 xylylC12 C12 C6 C12 C12 C12 C12

32.6 31.3 31.8 26.8 28.7

0.64 0.70 0.69 0.93 0.96

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FIG. 5. X-ray diffraction patterns of oriented crystals of complexes of gemini surfactants with Eucatex vermiculite at 25◦ C: (a) C12 xylylC12 and (b) C12 C6 C12 (see text for nomenclature).

the rigid xylene complex, where the tilt angle for the C12 chain increases above 54.5◦ as a result of the rigidity imposed on the system by the spacer. Taking this to be flat on the surface would increase the angle of tilt to 71◦ , which would increase the interlamellar spacing. Alternatively, a completely different structure might be adopted due at least in part to the higher charge on the surfactant. Three possible conformations are sketched in idealized form in Fig. 6. These assume that the N-alkyl chains and spacer adopt the all-trans conformation. In reality, gauche defects will probably intervene to give a less rigid and ordered structure and hence to optimize the packing. The key point about the structures shown is that, because of the presence of two cationic entities on each surfactant, the surfactant has some choice in how in interacts with the clay surface. It can attach to either one or two negative surface charges along the same surface, it can act as a bridge between two surfaces (model (c)), or, as in model (b), only one of the two charges need be directly attached to the clay with the other existing as an ion pair. ˚ and The average charge separation on the clay surface is 9.4 A the standard values for the fully extended spacer lengths (37) indicate that in C12 C3 C12 , C12 C4 C12 , and C12 (xylene)C12 they are too short to adopt the structure shown in model (a). However, a less homogeneous distribution of surface charge might allow this structure, as would the formation of an ion pair by one of the charged head groups. If each molecule carried one ion pair the fractional filling by charge-neutralizing species would be 0.5, and the value for C12 C3 C12 is not very far above this. This is the most probable structure for the short-spacer geminis. It is worth noting that when geminis are adsorbed at the air/water interface there are indications that they are often bound to one Br anion; i.e., on average they consist of one unpaired headgroup and one ion pair (36). The fractional filling indicates that only an insignificant number of ion pairs occur in the structure for the C12 C6 C12 and C12 C12 C12 complexes and these could adopt either structure (a) or (c). Since the fractional filling in these two compounds approaches 1 it is likely that there is a much higher proportion of gauche defects than in the simpler single chain

complexes and that this enables the chains to adopt conformations that maximize the space-filling. This may help to explain the remarkably small basal spacing observed for the compound with the C6 spacer.

FIG. 6. Three possible arrangements of gemini surfactant chains in the interlamellar space of complexes of geminis with Eucatex vermiculite. In (a) the spacer is lying falt on the surface to optimize the contact between the two head groups and the surface, in (b) one head group is left “floating” in the interlamellar space, and in (c) the spacer is long enough to bridge the gap between the platelets.

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II. Swelling of the Vermiculite–Surfactant Complexes with Organic Solvents Single Chain Surfactants The intake of toluene by vermiculite complexes of the singlechain N-alkyltrimethylammonium bromides series was studied with X-rays for Cn chains with n = 8–20 in steps of 2. All the samples expanded significantly in toluene and the expansion could be observed with the naked eye. The diffraction patterns for most of the toluene-swollen samples were better defined than the original patterns in the absence of toluene, suggesting that a greater degree of order exists in these systems and that the stoichiometry is unexpectedly precise. However, the longest and shortest chain compounds gave apparently less ordered compounds. Figure 7 shows the diffraction patterns obtained for the C12 and C18 TAB vermiculites swollen with toluene at 25◦ C, for which a direct comparison can be made with Fig. 1, and the values of the basal plane spacings are given for all the compounds in Table 5. The increase of basal spacing with chain length in the swollen ˚ per CH2 unit in the N-alkyl chain, samples is approximately 2 A which is similar to the increase for the unswollen systems, sug-

TABLE 5 Basal Spacings, Toluene Content, and Cn TAB Content per Unit Cell of Toluene-Swollen Complexes of Vermiculite with SingleChain Cn TABs at 25◦ C

Chainlength

Spacing (no❛ toluene) (A ± 0.5)

Spacing (toluene) ❛ (A ± 0.5)

No. of Cn TAB molecules

No. of toluene molecules

8 10 12 14 16 18 20

23.9 27.0 30.0 32.4 33.5 38.2 36.8

31.7 36.0 41.0 45.0 48.0 52.1 50.4

0.71 0.67 0.64 0.63 0.64 0.63 0.71

2.9 3.9 5.2 6.0 6.4 7.3 5.5

gesting that alkyl chain orientation is the controlling factor in determining the spacing. At this stage it is not possible to determine whether the chains remain tilted at 54.5◦ to the clay surface or, as a number of authors have suggested, whether they adopt a vertical conformation (30, 33). In either cases the extent of interdigitation must be reduced to account for a large increase in the values of the basal spacing. Space-filling arguments can be used to predict the number of toluene molecules occupying the free interlamellar space but, since the X-ray data cannot discriminate between toluene and surfactant, it is not possible to determine whether or not toluene displaces any of the surfactant present as ion pairs. Table 5 contains estimates of the amount of toluene in the layer based on the assumption that no surfactant is displaced. The ion pairs are not expected to be very soluble in toluene and so it is probable that they are not removed under the conditions used for the swelling. The situation may be different when the complex is in contact with emulsion or microemulsion. The number of molecules of toluene taken up increases with chain length (apart from the C20 TAB, which seems to be anomalous) by an average of 0.4 per CH2 unit. Double-Chain Surfactants

FIG. 7. X-ray diffraction patterns of toluene-swollen crystals of complexes of (a) dodecyl trimethyl ammonium bromide and (b) octadecyltrimethyl ammonium bromide with Eucatex vermiculite at 25◦ C. The larger spacing of the swollen samples is manifested by a much closer spacing of the (001) reflections, as can be seen by comparing these patterns with those in Fig. 1.

For the double-chain surfactant–vermiculite complexes swollen by toluene the long-range order in the system is excellent, indicating an extremely well-defined composition for the composite structure. The basal spacing values for all samples are listed in Table 6 together with the number of surfactant molecules present and number of toluene molecules taken up. In keeping with the conclusions reached in previous sections it has been assumed that the toluene does not displace the excess surfactant molecules. It is clear that as the side chain increases in length the number of toluene molecules taken up decreases. An interesting way to present this data is in the form of the toluene uptake per side chain CH2 unit as a function of total number of side chain CH2 units as plotted in Fig. 8. An exponential decrease is observed for both data sets, levelling off around the six/eight carbon mark. The C16 Cn series displays a greater uptake than the corresponding C12 Cn one for

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TABLE 6 Basal Spacings, Toluene Content, and C M C N DAB Content per Unit Cell of Toluene Swollen Complexes of Vermiculite with Dialkylchain DABs at 25◦ C

Surfactant

Spacing Spacing No. of C N DAB No. of toluene (no❛ toluene) (toluene) C M ❛ (A ± 0.5) (A ± 0.5) molecules molecules

TABLE 7 Basal Spacings, Toluene Content, and Gemini Content of Toluene-Swollen Complexes of Vermiculite with Gemini Surfactants at 25◦ C

Surfactant

Spacing (no❛ toluene) (A ± 0.5)

Spacing (toluene) ❛ (A ± 0.5)

No. of gemini molecules

No. of toluene molecules

32.6 31.3 31.8 26.8 28.7

42.0 41.6 39.8 38.9 39.3

1.1 1.0 0.8 0.7 1.0

4.2 4.7 5.9 4.7 3.6

C12 C1 DAB(C12 TAB) C12 C2 DAB C12 C4 DAB C12 C6 DAB C12 C8 DAB C12 C10 DAB C12 C12 DAB

30.0 29.5 29.2 29.9 31.9 33.3 35.7

41.0 40.5 38.7 38.0 35.6 38.2 39.9

2.0 1.9 1.7 1.6 1.6 1.6 1.6

5.0 5.0 4.3 3.7 1.7 2.2 1.9

C12 C3 C12 C12 C4 C12 C12 xylylC12 C12 C6 C12 C12 C12 C12

C16 C1 DAB(C16 TAB) C16 C2 DAB C16 C4 DAB C16 C6 DAB C16 C8 DAB

33.5 34.0 33.3 34.7 35.3

48.0 47.9 45.7 44.5 45.3

2.0 1.9 1.7 1.7 1.6

6.6 6.4 5.7 4.5 4.6

The number of toluene molecules in the gemini surfactant complexes is markedly higher than that found for the diC12 DABs, indicating that the presence of the spacer unit enhances the sorptive properties. For the C12 C6 C12 and C12 C12 C12 samples the values of the spacings after swelling also eliminate any structures where the headgroups at either end of a single spacer are attached to two separate clay plates, forming a bridge as shown in Fig. 6c, since the maximum basal ˚ respecspacings for such a structure would be 26.6 and 34.1 A, tively, much smaller than the observed values. The relatively high uptake of the C12 C6 C12 sample may result from an additional ability to adopt different conformations to accommodate extra solvent molecules because its spacer length is close to the average charge separation on the clay surface. The relatively low uptake for the xylene sample may correspondingly result from the lack of spacer flexibility, which does not allow it to adjust as easily to accommodate the toluene molecules.

a given n but this difference decreases with increasing n. This is consistent with the result above that the fractional space-filling in these samples increases up to n = 6, remaining relatively constant thereafter. The structural explanation of what is happening appears to be that the toluene allows the bilayer to become less interdigitated, filling the newly created space, but the extent to which the complex can expand is limited by the need for the longer chains to remain in contact across the bilayer. Gemini Surfactants The swelling of the gemini surfactant–vermiculite complexes with toluene gave swollen samples with well-defined stoichiometries and good diffraction patterns, just as the other compounds so far considered. The results are given in Table 7.

III. Organic Material Other Than Toluene So far only the swelling behavior in toluene has been studied in order to understand the role played by the cationic surfactant. However, the strange observation of Weiss that aromatic solvents would swell the complexes but alkanes would not seemed to be worth following up. All the solvents we tried were taken up, but to different extents, and the values of the basal spacings for the different systems are given in Table 8. Surprisingly, in view of Weiss’s results, the two nonpolar alkanes not only are taken up, but lead to a degree of swelling TABLE 8 Basal Spacings after Swelling of C16 TAB/Vermiculite Complexes with Different Organic Solvents at 25◦ C

FIG. 8. The uptake of toluene molecules by complexes of Eucatex vermiculite with C12 Cn DAB () and C16 Cn DAB () as a function of the number of carbons, n, in the secondary chain.

Organic solvent

❛ Spacing (A ± 0.5)

Toluene n-Hexane Cyclohexane Ethanol Propylene carbonate No solvent

48.0 49.3 48.0 37.2 35.6 33.5

311

VERMICULITE INTERCALATION AND SWELLING

equal to and even greater than that observed for the toluene. A possible explanation for this lies in the nature of the headgroup as well as the clay layer charge. Weiss’s observations that alkanes do not cause swelling of organoclays were based on samples of dodecylammonium-modified beidellite, whose charge density of 0.54 per (Si, Al)4 O10 unit is lower than for the vermiculite used here (0.7), implying that there would be a smaller number of surfactant molecules in the interlamellar space of the beidellite. The hydrophobicity of the interior of the interlamellar space of vermiculite complexes may therefore be higher and, in addition, the trimethylammonium headgroup of the surfactants used in our study may further shield the hydrophilic clay surface and hence enhance the organophilicity of the system to a point where alkanes can be sorbed. The polar solvents do not give rise extensive swelling and are almost certainly located around the headgroup/clay interface. A general conclusion found by previous researchers (28, 38) is that polar solvents are taken up to a greater extent by lower charge clays where fewer surfactant molecules are present. Thus the lower uptake of polar solvents into these materials is consistent with their extra ability to sorb alkanes. CONCLUSIONS

It is clear that by altering the surfactant charge and structure it is possible to control the degree to which adsorption beyond charge neutralization occurs in these complexes, which is important when the capacity of such complexes to sorb other materials is considered. It is plausible that minimizing the fractional filling by charge neutralizing species helps to maximize the sorptive capacity. There would appear to be little scope for varying the fractional filling for the single-chain cationics, but for the doublechain cationics variation of the secondary (shorter) chain length allows significant variation. A short secondary chain tends to lie flat on the surface and reduce the fractional filling by chargeneutralizing species. The chain length at which this occurs suggests that this could be optimized by choice of charge density on the clay. For the gemini surfactants the picture is less clear but there is once again considerable scope for varying the fractional filling. The general pattern of swelling of the surfactant/ vermiculite complex by toluene suggests that the maximum expansion of the intralamellar space is limited by the longest chain in the surfactant. Presumably the chains from the surfactants attached to opposite plates must retain some degree of interdigitation for the structure to remain stable. This cannot be estimated from the present data because we cannot distinguish expansion of the intralamellar space by changes in the tilt angle of the chains from simple expansion with no change in tilt angle. We will address this in a more detailed structural study in a subsequent paper. ACKNOWLEDGMENTS We thank Dr. M. A. Castro and Dr. A. Becerro (Departmento de Quimica Inorganica, Instituto de Ciencia de Materiales, Universidad de Sevilla) for help-

ful discussions. We also thank the Engineering and Physical Sciences Research Council for support of this project.

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