Crystalline swelling of organo-modified clays in ethanol–water solutions

https://doi.org/10.1016/j.clay.2003.12.001Get rights and content

Abstract

The swelling of organo-modified clays in organic solvents may have important consequences on bulk properties of materials based on this technology. X-ray powder diffraction was used to study the swelling of orientated films of dioctahedral smectites exchanged with benzyloctadecyldimethylammonium (BODMA) cations, when equilibrated in ethanol–water solutions. The influence of total layer charge and the location of the layer charge on swelling of two organo-modified bentonites and three organo-modified reference smectites in ethanol–water solutions were evaluated. The measured interlayer spaces of BODMA smectites equilibrated in water increased with total layer charge and with the relative amount of tetrahedral charge. At intermediate ethanol concentrations, a distinct plateau was observed in the swelling profiles of BODMA smectites having layer charges >0.8 e per O20(OH)4. When equilibrated in ethanol, all BODMA smectites expanded to similar values (34–36 Å). The magnitude of the change in crystalline swelling of organo-smectites in ethanol compared to that in water was found to be inversely proportional to total layer charge density but proportional to the ratio of octahedral to tetrahedral layer charge. Calculations using NEWMOD showed that the d-spacings measured were unlikely to be due to interstratifications of fully water-solvated with fully ethanol-solvated BODMA smectite layers. Introduction of up to 2.0 M aqueous NaCl resulted in a slight (∼0.5 Å) decrease in the interlayer spacing of BODMA montmorillonite. However, 0.02 and 0.2 M NaCl added to ethanol–water mixtures enhanced interlayer expansion in these miscible solvents. The results suggest that inorganic salt depressed the activity of water in ethanol–water mixtures and thereby increased the proportion of ethanol actually penetrating the interlayer spaces of BODMA smectites. A plausible mechanism is discussed to describe the observed interlayer expansion in ethanol–water solutions and how this might impact possible applications.

Introduction

Permeable reactive barriers offer an effective method for the control of hydrocarbon-contaminated groundwater, as they are capable of reacting physically or chemically with contaminants, thereby minimizing contaminant transport without significant reductions in the permeability of the barrier to the treated groundwater (Guerin et al., 2002). Often, the pollutants are organic solvents or organic species with limited solubilities in water but having high affinity to organo-clays as determined from sorption isotherms Jaynes and Boyd, 1991, Koh and Dixon, 2001, Bonczek et al., 2002. Alther, 1995, Alther, 2002 showed that organo-clays are cost effective as pretreatments when used ahead of activated carbon sorbents in pump-and-treat technologies. Recently, Slade and Gates (2004) showed that, when exposed to neat toluene, hexadecyltrimethylammonium (HDTMA) smectites swell to ∼33–39 Å from interstratified mono- and bimolecular interlayer arrangements (initially at ∼15–17 Å) and to 42–45 Å from bimolecular (17.7 Å) and paraffinlike (>21 Å) interlayer arrangements. Xu and Boyd (1995) and Bonczek et al. (2002) found that these materials sorb increasing amounts of organics with increasing organic carbon content. Slade and Gates (2004) showed that the organic carbon in these organo-clay intercalates consisted of both charge balancing HDTMA cations and intercalated organic salt molecules.

Organically modified clays are commonly used as emulsifiers (thinners) of water–oil systems (e.g., drilling muds) but can also be dispersed in oils (thickeners) to form lubricating greases with lower susceptibilities to heating Jordan, 1963, Van Olphen, 1963. The degree of dispersion is related to the ability of the organo-clay to sorb organic moieties and to swell to form long-range particle–particle interactions Lagaly et al., 1983, Moraru, 2001. Jordan (1963) reported that the gel volumes of bentonites saturated with various alkylammonium cations increased by as much as 27 times when dispersed in n-alkanols of varying carbon tail lengths. Weiss (1963) observed that 2:1 layer silicates, intercalated with n-alkylammonium cations and solvated with n-alkanols or n-alkylamines of like carbon tail length, resulted in layer spacing increases consistent with the formation of bimolecular layers of n-alkyl moieties within the interlayers. Lagaly (1976) further observed that various n-alkanols were capable of expanding alkylammonium 2:1 layer silicate intercalates to layer spacings as great as 70 Å and postulated that the thickness of the bimolecular layer was dependent on the most frequent end-to-end arrangement of alkanol–alkylammonium pairs. Lagaly and Witter (1982) and Lagaly et al. (1983) found that alkylammonium-exchanged clays swelled in various binary (organic-water) mixtures, that the ratio of organic–inorganic solvent sorbed by the organo-clay was constant and independent of the bathing solution mixture at intermediate composition. Furthermore, the presence of inorganic salts, the alkyl tail length (number of carbons) and the layer charge density all influenced the swelling of organo-modified clays. Moraru (2001) found that organic cation coverage up to the cation exchange capacity (CEC) of the clay enhanced gel formation in organic solvents (e.g., toluene) and observed that Bingham yield stress values of these dispersions were maximal at organic cation coverage equivalent to the CEC. Slade and Gates (2004) showed that the incorporation of HDTMA-Br above that amount required to satisfy the CEC increased the ability of HDTMA clays to absorb and disperse in toluene. The initiation of organo-clay dispersion in solvents is related to the ability of certain anions (e.g., Cl, ClO4, I) to impede the formation of solvent clusters within the interlayer pore-space Lagaly and Witter, 1982, Lagaly, 1984, Lagaly, 1987. Enhancing the expansive capacity of organo-modified clays in the presence of polar organic solvents forms the basis of various polymer–clay nanocomposite synthesis methods (e.g., Gilman et al., 2000, Moraru, 2001).

As part of an overall goal to develop clay-based reactive barriers for the remediation of hydrocarbon-contaminated groundwater, this paper aims to determine factors affecting swelling of two commercially available bentonites and three reference clay minerals, modified with the organic cation benzyloctadecyldimethylammonium (BODMA), in solutions of ethanol and water. BODMA was selected because steric effects were expected to yield greater interlayer porosities of BODMA intercalates than from di-alkyl-substituted quaternary ammonium cations. Ethanol–water solutions were used in the first instance because of the ease by which solvation reactions could be controlled. Inorganic salt (NaCl) was added to the solutions to mimic saline groundwater conditions.

Section snippets

Purification and characterization of the smectites

The smectites studied were purified by sedimentation/centrifugation methods to remove the nonswelling components, and XRD and IR were used to check their purity. The montmorillonitic fractions of two bentonites were studied. The Miles bentonite (United Minerals, Miles, Queensland) is composed of about 71% montmorillonite with the remaining materials being quartz, opal CT, feldspar and zeolite (Gates et al., 2002). Arumpo bentonite (Arumpo, Muldoora, New South Wales) is composed of randomly

BODMA smectite properties

The structural formulae and layer charge characteristics are presented in Table 1. The octahedral cation occupancies provide a range in layer charge and, in general, the tetrahedral charge increases with increasing total layer charge. Upton montmorillonite has the lowest tetrahedral charge. Of the two beidellite samples, the SBCa-1, has a very low octahedral charge compared to the De Lamar beidellite.

The %C and %N in the BODMA smectite complexes (Table 2) indicate that, while some benzyl carbon

Conclusions

Equilibration of BODMA smectites with increasing amounts of ethanol in ethanol–water solutions resulted in expansion of the interlayer spaces consistent with changes in interlayer configuration from bimolecular layers of BODMA cations, with their aliphatic carbon tails parallel to the layer plane, to bimolecular layers of BODMA cations whose aliphatic tails reside at a low angle (∼30°) with respect to the layer plane. Organo-modified clays have the capacity to swell more in organic solvents

Acknowledgements

The author acknowledges the many insightful discussions with and advice from P. Slade, G. Churchman and K. Norrish, assistance with organo-clay preparation by J. Anderson and for sample analyses by M. Fritz, G. Riley, M. Raven and S. McClure. J. Post (Vector Engineering, California) kindly provided the De Lamar beidellite.

References (28)

  • P.G. Slade et al.

    The swelling of HDTMA smectites as influenced by their preparation and layer charges

    Applied Clay Science

    (2004)
  • J.L. Bonczek et al.

    Monolayer to bilayer transitional arrangement of hexadecyltrimethylammonium cations on Na-montmorillonite

    Clays and Clay Minerals

    (2002)
  • Gates, W.P., Nefiodovas, A., Peter, P. Permeability of an organo-modified bentonite to ethanol–water solutions. Clays...
  • J.W. Gilman et al.

    Flammability properties of polymer-layered-silicate nanocomposites. Polypropylene and polystyrene nanocomposites

    Chemistry of Materials

    (2000)
  • Cited by (48)

    • Co-intercalation of organic cations/amide molecules into montmorillonite with tunable hydrophobicity and swellability

      2019, Applied Clay Science
      Citation Excerpt :

      Over the past five decades, OMt has received widespread attention and has wide applications in clay/polymer nanocomposites (Lagaly et al., 2006; Laoutid et al., 2006; Feng et al., 2012; Zhu et al., 2019), oil well drilling fluids (Mahto and Sharma, 2008), paints and emulsions (Jordan, 1963), cosmetics (Iannuccelli et al., 2018), adsorbents (McBride and Mortland, 1973; Jaynes and Boyd, 1991; Hou et al., 2015) and biosensors (An et al., 2015; Unal et al., 2018). Cationic surfactants have often been used as modifiers or intercalants to produce OMt, as they can be easily intercalated into the Mt interlayer spaces by a cation exchange reaction (Gates, 2004; He et al., 2010; Zhu et al., 2011). However, anionic surfactants have also been reported to modify Mt to produce OMt (Gu et al., 2012; Wu et al., 2014).

    View all citing articles on Scopus
    View full text