ABSTRACT
Table 3.1 Mechanisms of clay-organic interactions Nature of the interactions CharacteristicsElectrostatic Ion exchange of interlayer cations with organic cationsVan der Waals forcesHydrogen bonding and water bridgesIon dipole and coordination Proton transferElectron transfer
Adsorption of neutral molecules by interactions with external or internal (intracrystalline region of silicates) surfaces Covalent bonding Grafting reactions of organic groups To modify clays and clay minerals, as mentioned in Chapter 2, several routes have been employed, such as adsorption, ion exchange with inorganic cations and organic cations, binding of inorganic and organic anions (mainly at the edges), grafting of organic compounds, reaction with acids, pillaring by different types of poly(hydroxo metal) cations, intraparticle and interparticle polymerization, dehydroxylation and calcination, delamination and reaggregation of smectites, and lyophilization, ultrasound, and plasma (Bergaya and Lagaly, 2001). Ion exchange with alkylammonium ions is well known and the preferential method to prepare organoclays. Generally, experimental studies describe the preparation of organoclays at the laboratory scale, with different experimental conditions, clays from several regions and suppliers, and several kinds of organic compounds (de Paiva et al., 2008). Moreover, the importance of clay-organic interactions arises because such compounds are used in industrial applications, such as additives, fillers, rheological agents, and specific sorbents, and, recently, in technological applications as advanced materials (Ruiz-Hitzky and Meerbeek, 2006). On the other hand, clay-organic interactions are of crucial importance regarding agricultural production, the origin and exploitation of oil resources, and the origin of life. The current relevance and vitality of the field can be inferred from the exponential increase in scientific works appearing during the last few decades. This chapter has been devoted to review and summarize the main mechanisms governing clay-organic interactions, from discrete molecules or cations to
large entities, such as polymers. Only a few detailed examples will be presented in trying to group these interactions into several representative systems. In addition, the characterization of organic-inorganic solids derived from various common clays, as well as a brief discussion of current research and trends in the development of new hybrid compounds will be addressed. Consequently, a general overview associated with the application perspectives of these hybrids compounds as advanced materials will be presented. Milestones concerning clay-organic interactions and relevant research achievements are given in Table 3.2. Table 3.2 Milestones in clay-organic interactions Date Milestones Examples
8th centuryAD Clay-dye hybrid compounds Palygorskite/indigo mixtures (Maya blue)1911 Catalytic transformation of organic compounds activated by clays Pinene to camphene over palygorskite1939-41 Intercalation of organic cations in smectites Montmorillonite/aliphatic and aromatic ammonium cations1944 Intercalation of neutral species in clays Montmorillonite/glycerol1945 Structural features by XRD, one-dimensional Fourier analysis of clay-organic compounds Montmorillonite/diamines and glycols1949 Application of thermal analysis in organoclay characterization DTA of montmorillonite/alkylammonium1954 Selective sorption of hydrocarbons by palygorskite and sepiolite Palygorskite/n-heptane iso-octane1961 Intercalation of salts in kaolinite Kaolinite/K-acetate1961 Intercalation of neutral molecules in kaolinite Kaolinite/urea1961 UV-Vis application to study clay-organic systems Montmorillonite/benzidine1961 Polymer-clay intercalation compounds Montmorillonite/polyacrylonitrile
(Continued)
Date Milestones Examples1965 Orientation of organic molecules in the interlayer space by IR spectroscopy Montmorillonite and vermiculite/pyridine and pyridinium 1968 Organic derivatives of clays through covalent bonds (grafting) Vinyl derivatives of chrysotile1969 π bonds in clay-aromatic compounds Cu-montmorillonite/benzene1974 Organic reactions in the interlayer space of clays Intracrystalline sorption of organic compounds in sepiolite
Vermiculite/l-ornithine peptide formation Sepiolite/hexane1976 Organic pillared clays Montmorillonite/diprotonated triethylenediamine
1980 Application of 13C-NMR to characterize clay-organic systems Ag-hectorite/benzene 1983 Catalysts based on organometallic-clay complexes Montmorillonite/[Rh(PPh3)3] 1984 LMMS application to characterize clay-organic systems Sepiolite/organosilanes 1985 Photostabilization of co-adsorbed labile bioactive species Montmorillonite/methyl green/bioresmethrin 1989 Microwave activation of organic reactions on clay-adsorbed compounds Rearrangement of pinacol to pinacolone intercalated in montmorillonite1990 Ion-conducting polymers-2D intercalated materials Montmorillonite/PEO1993 Polymer melt intercalation in organosmectites Montmorillonite/polystyrene1993 Grafting of organic groups in the interlayer space of kaolinite Methoxy derivatives of kaolinite 1998 Templated synthesis of polymer-clay nanocomposites Synthetic fluorosmectite/polyvinylpyrrolidone
Table 3.2 (Continued)
3.2 Synthesis of OrganoclaysThe synthesis of organoclays is based on the mechanisms of the interactions between clay minerals and organic compounds. A dis-placement process occurs when water molecules in the interlayer space of smectites and vermiculites are displaced by polar molecules (Bergaya et al., 2006). Neutral organic compounds can form complexes with the interlayer cations (de Paiva et al., 2008). There are different ways to modify 2:1-type clay minerals: hydrogen bond-ing; ion-dipole interactions; coordination bonds; acid-base reac-tions; charge transfer; van der Waals interactions; ion exchange with organic and inorganic ions and cationic complexes; grafting of organic compounds; pillaring with the different types of poly(hydroxy metal) cations; interlamellar or intraparticle polym-erization; and the delamination and reaggregation of smectitic clay minerals, with the thermal processes and some physical techniques, such as dehydroxylation, calcination, lyophilization, ultrasound, and plasma (see Chapter 2). The organoclays are generally prepared in solutions by cation exchange and solid-state interaction (de Paiva et al., 2008). However, to increase the effectiveness of these meth-ods, new efforts are conducted. For example, Baldassari et al. (2006) synthesized six organo derivatives with each of the lower-charged fluorophlogopite-type clays by a microwave-assisted procedure. They found that using natural sodium montmorillonite (Na-MMT) or low-charged micas as precursors, complete intercalation can be easily achieved with conventional methods under mild temperature conditions (~60°C), but a microwave-assisted process is more effec-tive than the conventional process when applied to the intercalation of higher-charged clays. 3.2.1 Cation Exchange Clay minerals consist of small crystalline particles with silica oxygen tetrahedral sheets and aluminum or magnesium octahedral sheets where an aluminum or magnesium ion is octahedrally coordinated to six oxygens or hydroxyls. The octahedral sheet is located between two Si tetrahedral sheets. Organoclays or organomontmorillonites are clays that have been modified with organic surfactants with single and dual cationic surfactants, anionic-cationic surfactants,
and nonionic surfactants. Cation exchange has been used for nearly 50 years. This method is based on the displacement with cations in the interlayer region of clay minerals of quaternary alkylammonium cations in aqueous solution (Mandalia and Bergaya, 2006). Generally, this can be done by ion exchange with cationic surfactants, including primary, secondary, tertiary, and quaternary alkylammonium or alkylphosphonium cations. Alkylammonium or alkylphosphonium cations in the organosilicates lower the surface energy of the inorganic host and improve the wetting characteristics of the polymer matrix and result in a larger interlayer spacing. The structure and properties of the resultant organoclays are affected by the type of both surfactant and clay minerals. From the ion exchange, the interlayer spacing between the single sheets is broadened. This enables the adsorption of organic cation chains and changes the surface properties of each single sheet from hydrophilic to hydrophobic or organophilic. The interlayer cation density or packing density of the alkylammonium ions of the clay minerals and the chain length of the organic ion are important factors to determine the arrangement of organic molecules between the layers. The formation of monolayers, bilayers, and pseudotrimolecular layers of alkylammonium ions in the interlayer spaces of montmorillonite was characterized by the basal spacings (Park et al., 2011). Additionally, the alkylammonium or alkylphosphonium cations can provide functional groups that can react with the polymer matrix or in some cases initiate the polymerization of monomers to improve the strength of the interface between the inorganic and the polymer matrix (Sinha Ray and Okamoto, 2003). Schematic representation of a cation exchange mechanism between the silicate and an alkylammonium salt has been shown in Fig. 3.1. Mallakpour and Mohammad (2013) prepared organomodified clays by a cation exchange method, which is a displacement of the sodium cations of Cloisite-Na+ with the protonated amino acids. Vazquez et al. (2008) prepared an organomontmorillonite according to a standard ion exchange procedure. Cloisite-Na+ was dried at 110°C for 48 h and washed with distilled water (10 wt.%) with vigorous stirring for 24 h to cause the delamination of the montmorillonite. The amount of surfactant added was about 1.5 cation exchange capacity (CEC). The suspension was stirred at
room temperature for 15 min. The montmorillonite dispersion was introduced in a 500 mL flask and vigorously stirred in reflux for 6 h, filtered in vacuum, and washed with water and in some cases subsequently with 50/50 vol.% ethanol/water mixture. Two indicators were used to point out the removal of the anions and the excess of organic cations: on the one hand, a chloride and bromide test with a few drops of 1 M AgNO3 solution up to no precipitate was observed, and on the other hand, the absence of lather in the filtered solution after stirring was observed. Several treatments were tested to improve the amount of intercalated surfactant as follows: (1) Water. After modification, the organomontmorillonite was
washed only with distilled water in reflux. The water volume depended on the type of surfactant. (2) EtOH/water. After washing with distilled water, a 50/50 mixture of ethanol/water in reflux was used. (3) Sonication. Sonication was used before the modification to promote delamination of the montmorillonite particles. The time was 6 h, and the organically modified montmorillonite was washed with the EtOH/water method.
