Modification of Clay Minerals by Surfactant Agents: Structure, Properties, and New Applications

Abstract

Organically surfactant-modified clays (OC) have attracted a great deal of interest because of their wide applications in industry and environmental protection. The OC are organic–inorganic complexes synthesized through the intercalation of organic cations mainly into the interlayer space of expandable clays. Different surfactants have been used to prepare OC. These include single and dual-cationic surfactants, anionic–cationic surfactants, and nonionic surfactants. The intercalation of the surfactant cations was governed by different processes: cationic exchange and Van der Waals ‘interactions of the alkyl chains with clay surface. The structure and properties of the resultant organoclays are affected by the type of surfactant, the clay used, and the preparation method such as the conventional technique, the solid-state intercalation, and the microwave irradiation. As the result, the organoclays are characterized by hydrophobic surfaces and have attracted great interest because of their potential use in several applications, such as sorbents for organic pollutants (dyes, pharmaceutic compounds…), heavy metals and inorganic oxy-anions, clay-based nanocomposites, and in several other industries.

1. Introduction

Intercalation of organic guest species into layered inorganic solids is an interesting topic from a theoretical and applied point of view [1]. At present, there are many applications of organoclays used as sorbents in pollution prevention and environmental remediation such as the treatment of oil spills, wastewater and hazardous waste landfills, etc. Previous studies have shown that replacing the inorganic exchangeable cations of clay minerals with organic cations can greatly enhance the capacity of these materials to remove organic contaminants [2, 3]. Organoclay-based nanocomposites exhibit a remarkable improvement in surface properties when compared with untreated polymer or conventional micro- and macro-composites. These improvements include increased strength and heat resistance, decreased gas permeability and flammability, and increased biodegradability of biodegradable polymers [4, 5].

Different surfactants have been used to prepare organoclays. These include single and dual cationic surfactants [6, 7, 8], anionic–cationic surfactants [9] and nonionic surfactants [10]. The prepared organoclays, however, are structurally different even when the same surfactant was used under similar experimental conditions [11, 12, 13, 14]. Hence, the structure and properties of the resultant organoclays are affected by the type of surfactant [15], the clay mineral used and the preparation method.

2. Surfactants

2.1 Definition

Surfactants are natural or synthetic substances whose molecules have a so-called amphiphilic structure (Figure 1). These molecules thus have i) a polar head, ionized or not, capable of developing Van der Waals, Lewis acid-base, and possibly Coulomb-type interactions when an ionizable function is present: this hydrophilic part has an affinity for charged surfaces and for liquids with a strong polar character such as water; and ii) an apolar part, typically a hydrocarbon chain capable only of Van der Waals interactions and therefore having little affinity for water.

Figure 1.

Surfactant structure.

2.2 Classification of surfactants

Surfactants are historically divided according to their charge into four main classes, cationic, anionic, amphoteric and non-ionic.

• Cationic surfactants: having a positively charged polar head, belong either to the family of fatty amines such as amines, diamines and polyamines as well as their salts (quaternary ammonium salts or imidazolinium salts) or to the family nitrogen heterocycles.

• Anionic surfactants: having a negatively charged polar head, are generally alkylarylsulphonates, alkane sulphonates, olefin sulphonates, salts of carboxylic acids (soaps), salts of sulfuric acid esters (alkyl sulphates), alkyl ether sulphates, and esters of phosphoric and polyphosphoric acids.

• Amphoteric surfactants: So-called amphoteric surfactants such as alkylamino acids, alkyl betaines and sulfobetaines, have two functional groups, one anionic and the other cationic. Depending on the environmental conditions, they can ionize in aqueous solution, acquiring an anionic character at alkaline pH and a cationic character at acidic pH.

• Nonionic surfactants: are compounds whose hydrophilic group is a chain of ethylene oxide units, generally attached to a hydroxyl function: alcohols, alkylphenols, carboxylic acids and alkanolamides.

2.3 Physicochemical proprieties

2.3.1 Critical micellar concentration (CMC)

The CMC is the concentration in the solution of a surfactant above which part of the molecules dispersed within the solution come together to form micelles. Micelles are formed when the hydrophobic portions, unable to form hydrogen bonds in the aqueous phase, create a large increase in the free energy of the system. One way to lower this energy is by isolating the hydrophobic part of the water by adsorption onto organic matrices or forming micelles. Indeed, in the micelles, the hydrophobic parts group together towards the center, and the hydrophilic parts remain in contact with water. Surfactant micelles arrange in different spherical, globular or cylindrical microstructures, but spherical and irregular vesicles, tubular bilayers or lamellar structures (Figure 2) are most often encountered.

Figure 2.

Modes of aggregation and adsorption of surfactants [16].

The CMC of a surfactant varies with its structure, the temperature of the solution, the presence of electrolytes or organic compounds. The effects of electrolytes on CMC are more pronounced for ionic surfactants. The variation in the size of the hydrophobic region is an important factor and in general the CMC decreases as the hydrophobic character of the surfactant increases.

2.4 Adsorption at interfaces

Adsorption is a surface phenomenon that originates from the non-compensation in all directions of intermolecular attractions at interfaces. This results in residual inwardly directed forces, which are attenuated when amphiphilic entities attach to the surface. Information on this phenomenon is obtained by determining the surface tension of the solutions and the thermodynamic quantities of adsorption which vary according to the temperature.

3. Clays

3.1 Structures

Generally, clays are minerals composed mainly of hydrated aluminum or/and magnesium phyllosilicates. As their name suggests, they are formed by the association of layers. The latter consists of an assembly of layers of Al(OH)₆ “O” octahedra and layers of SiO₄ “T” tetrahedra. The tetrahedral layer T is formed by two planes of oxygen atoms and contains a silicon atom in IV coordination. In the tetrahedron, each silicon atom is surrounded by 4 oxygens. One tetrahedron is bonded to another through a highly covalent bond through the sharing of oxygen atoms. These are called basal or basal surface oxygens. The arrangement of these basal oxygens leads to the formation of hexagonal cavities. Opposite the basal surface are the apical oxygens which are shared between tetrahedral silicon and an octahedral cation. They establish a strong connection between the octahedra and the tetrahedra. The octahedral layer O is formed by two planes of oxygen atoms and hydroxyl groups between which aluminum or iron or magnesium cations are in VI coordination (Figure 3).

Figure 3.

Representation of silicon tetrahedra and aluminum octahedra and their layering and sheet arrangement [17].

Phyllosillicates generally present isomorphic substitutions of Al⁺³ cations by Mg⁺² and of Mg⁺² by Li⁺ in the octahedral layers and/or Si⁺⁴ by Al⁺³ or Fe⁺³ in the tetrahedral layers. These substitutions lead to a charge deficit in the sheet, which becomes negatively charged. The electroneutrality of the structure is then ensured by the presence of compensating cations (Na⁺, Ca⁺², K⁺, etc.), which take place in the interfoliar space separating two layers. The thickness of the sheets depends on their composition. A distinction is made between clays of the 1:1 or TO type (where a tetrahedral T layer is linked to an octahedral O layer) and clays of the 2:1 or TOT type (in which an O layer is inserted between two T layers). For TO sheets, the thickness is 7 Å because the interfoliar space is empty. On the other hand, for TOT sheets it is variable from 9 to 15 Å depending on the content of the interfoliar layer (relative humidity, solvation liquid, nature and hydration of the interfoliar cation). In addition, clays may have other morphologies, such as nanotubule halloysite with a diameter of 50 nm, an inner lumen of 15 nm and a length of 600–900 nm (Figure 4).

Figure 4.

Images of halloysite [18].

3.2 Classification

There are different classifications of clays. The most classic is based on the thickness and structure of the sheet. The first classification, established by the International Committee on Classification and Nomenclature of Clay Minerals in 1966, is based solely on the charge of the sheet and the number of metal atoms in the octahedral layer. The second, established by Mering and Pedro [19], takes into account the location of substitutions, their distribution and the type of compensating cations. This classification does not take into account synthetic silicates, sometimes used in the development of nanocomposites, such as fluorohectorite, fluoromica or laponite. Table 1 presents a classification derived from the work of Brindley [20] and McKenzie [21] which gives the value of the permanent load of the sheet that was used as a criterion for establishing a classification of phyllosilicates 2:1.

Type	Family	Charge of O10 (OH) 2 unity	octahedral Olayer	Exemples
T0	Kaolins	x ≈ 0	Dioctahedral	Kaolinite, dickite, nacrite
	Serpentines		Trioctahedral	Chrysotile, antigorite, berthiérine
TOT	Pyrophillites	x ≈ 0	Dioctahedral	Pyrophillite
	Talcs		Trioctahedral	Talc, willemséite
	Smectites	x from 0.2 to 0.6	Dioctahedral	Montmorillonite, beidellite, nontronite
			Trioctahedral	Saponite, stevensite
	Vermiculites	x from 0.6 to 0.9	Dioctahedral
			Trioctahedral
	Micas	x from 0.9 to 1.0	Dioctahedral	Muscovite, illite
			Trioctahedral	Phlogopite, biotite

Table 1.

Classification of phyllosilicates.

In nature there are clay minerals formed by the succession of sheets, two or more types of clay, described above. These interstratified clays may have a simple or irregular regular interstratification where no law governs the alternation of sheets.

3.3 Proprieties of clays

3.3.1 Swelling capacity

The swelling consists of a separation of the sheets up to an interfoliar distance of equilibrium. This swelling property is due to the hydrophilic nature of the surface of certain clays, due to the presence of hydratable cations in the interfoliar spaces [22]. The intensity of the swelling depends on the charge of the crystal lattice, the nature of the compensating cations, the hydration energies involved, the ionic strength of the surrounding medium and the total quantity of water. The ability to swell is therefore not a characteristic common to all phyllosilicates. In the case of micas, the location of the isomorphic substitutions in the tetrahedral layer as well as the strong charge deficit create strong bonds between the compensating ions and the sheets; which prevent the hydration of the cations. In smectites, octahedral substitutions promote swelling because interactions between sheets and compensating cations are reduced. Smectites have the best swelling properties. This property is used as an identification criterion.

3.3.2 Cation exchange capacity CEC

The cation exchange capacity, denoted CEC, corresponds to the number of monovalent cations that can be substituted for the compensating cations of the clay. The latter are localized on the external surfaces of the particles as well as between the unit sheets [23]. The CEC is generally expressed in milliequivalents per 100 grams of calcined clay (meq/100 g). It varies according to the minerals, it is between 80 and 150 meq/100 g for smectites.

3.3.3 Specific surface area

In the anhydrous state, the clay sheets are joined to each other, but they move apart in the presence of water (swelling), which makes the basal surfaces initially in contact accessible. These constitute the inner surface of the mineral. The outer basal surfaces and the edge surfaces of the sheets constitute the outer surface of the clay. Table 2 presenting the surface values of some clay minerals shows that smectites have the highest total surface. They have an internal surface of around 700 m².g⁻¹, while the external surface only represents around 80 m².g⁻¹.

Clay	Internal surface (m^2 .g^−1)	External surface (m^2 .g^−1)	SSA (m^2 .g^−1)
kaolinite	0	10–30	10–30
illite	20–55	80–120	100–175
Smectite	600–700	80	700–800
chlorite	—	100–175	100–175

Table 2.

Specific surface area (SSA) of some clay minerals [24].

3.3.4 Acid-base properties of clays

Surface charge is a fundamental property of clays. The main origin of this surface charge comes from isomorphic substitutions within the crystal lattice. This negative charge is commonly denoted permanent structural charge (σ₀) [25] and is around 7.10⁻³ e/Ų. The second source of charge, which depends on the pH (denoted σ_H), comes from the presence of oxide or hydroxyl groups of the silanol (-SiOH) and aluminol (-AlOH) types. These hydroxyl groups, present at the edges of the surfaces of the clays, admit an amphoteric character according to the reactions:where ≡SOH⁺_2, ≡SOH⁰ et ≡ SO⁻ represent the hydroxide groups of the edges’ surfaces. K₁ and K₂ are the constants of protonation and group deprotonation reactions ≡SOH⁰.

The surface groups can therefore carry negative, positive or neutral charges and thus generate a charge on the surface of the particle. This charge is directly related to surface acid–base balances and/or more general surface adsorption reactions. It, therefore, depends on the composition of the liquid phase, mainly on the pH and the ionic strength. There is a particular pH value for which the proportions ≡SOH⁺₂ and ≡SO⁻ are equivalent: this is the point of zero charges (noted PCN). The point of zero charge PCN corresponds to the pH for which the global surface charge density σ is zero and therefore the activities ≡SOH⁺₂ and ≡SO⁻ are equal.

4. Clay modification by surfactant agents

The first published results on organophilic clays appeared in the early 1940s. Bradley [26] studied the adsorption of a series of polyamines and polyalcohols on certain types of montmorillonites. The advanced work of Jordan [27] showed that the intercalation of montmorillonites by quaternary ammoniums converses a hydrophilic character. In 1962, Fripiat et al. [28] used sodium, calcium and acid homoionic montmorillonites for the adsorption of certain amines (monoamines and diamines); they showed that acid montmorillonite adsorbs these products more than the other two matrices. Later, McBride et al. [29] as well as Karichoff et al. [30] showed that it was possible to use organophilic clays for the adsorption of certain aromatic compounds.

From the 1980s, many studies [31, 32, 33, 34, 35, 36, 37, 38, 39, 40] on the interactions between several surfactants and different clays were carried out by examining several parameters such as the TA/clay ratio, the pH of the environment and the nature of the intercalated surfactant. Table 3 presents the main families of surfactants used for the modification of smectites.

Family	Surfactant	Structure	Reference
Ammonium quaternaries
Cationic	Cethyltrimethylammonium (CTA)	C19H42N^+	Pospisil et al. [41]; Juang et al. [42]; Praus et al. [43]
	Hexadecyl-trimethyl-ammonium (HDPy)	C22H38N^+	Dultz and Bors [44]; Janek and Lagaly [45]; Ni et al. [46]
	Hexadecyl-trimethyl-ammonium (HDTMA)	C19H42N^+	Sheng and Boyd [47]; Lee and kim [48]; He et al. [12]
	Octadecyltrimethyl-ammonium (ODTMA)	C21H46N^+	Xi et al. [14]; Hrachová et al. [49]; Sanchez-Martin et al. [50]
	Benzyltrimethylammonium (BTMA)	C10H16N^+	Polubesova et al. [51]; Shen [52]; Park et al. [53]
	Tetrabutylammonium (TBAM)	C16H36N^+	Akcay et al. [54]
Anionic	Dodecylsulfate (SDS)	C12H25O4S^−	Permien and Lagaly [55]; Günister et al. [56]
Amphoteric	Triton X-102	C16H26O2	Lin et al. [57]
	Cocamidopropyl-betaine (CAB)	C19H38N2O3	McLauchlin and Thomas [58]
	Polyethylene-glycol (PEG-1500)	H(OCH2CH2)nOH	Zampori et al. [59]
Phosphonium quaternaries
Cationic	Tetrabutylphosphonium (TBP)	C16H37OP^+	Hedley et al. [60]
	Tetraphenylphosphonium (TPP)	(C6H5)4P^+	Patel et al. [61]; Jeschke and Meleshyn [62]
	Tributyltetradecylphosphonium	C26H56P^+	Hedley et al. [60]; Calderon et al. [63]
	Triphenyl vinylbenzyl phosphonium; Tetraoctyl phosphonium	C25H22P; C32H68P^+	Avalos et al. [64]

Table 3.

Principal families of used surfactants.

4.1 Modification methods

Several elaboration strategies have been used with success to obtain organophilic clays, it is possible to distinguish the two most common modes of implementation: intercalation and grafting. In this part, we will describe in detail the first mode by presenting different experimental techniques of intercalation. This process consists of replacing the compensating cations with cations bearing alkyl chains. The most frequently used cations are alkylammonium ions. Phosphonium salts are also interesting modifying ions because their thermal stability is high but they are little used until now.

4.1.1 Batch method

This method of preparation is the most common, it consists of mixing the surfactant and clay in a common solvent and then removing it. In order to optimize this method it is necessary to use a solvent that can both swell the clay and dissolve the surfactant. The swelling of the clay facilitates the insertion of alkylammonium ions into the interfoliar spaces. After filtration of the suspension and drying of the clay, the presence of alkylammonium ions on the surface of the sheets, primary particles, and aggregates gives the clay an organophilic character. In addition, their intercalation between platelets leads to an increase in the interfoliar distance. The main disadvantage of this method is the high amount of solvent used which poses an environmental problem.

4.1.2 Solid-state reaction

This method of preparation is the most common, it consists of mixing the surfactant and the clay in a common solvent, then removing it. In order to optimize this method, it is necessary to use a solvent that can both swell the clay and dissolve the surfactant. The swelling of the clay facilitates the insertion of the alkylammonium ions within the interfoliar spaces. After filtration of the suspension and drying of the clay, the presence of alkylammonium ions on the surface of the sheets, the primary particles, and the aggregates give the clay an organophilic character. In addition, their intercalation between the platelets leads to an increase in the interfoliar distance. The main drawback of this method is the large amount of solvent used which poses an environmental problem.

4.1.3 Micro-wave irradiation

The use of microwaves in the synthesis of organophilic clays presents a new preparation technique. It offers more advantages than conventional and thermal methods listing high preparation time and energy consumption. However, there are few published works on the use of microwave irradiations to prepare organophilic clays. In 2006, Baldassari et al. synthesized organophilic clays using the conventional method (liquid state intercalation) and synthesis by microwave activation in order to compare the results obtained by the two synthesis methods under the same conditions (amounts of surfactants, mass of clay, volume of solution). The authors have shown that the intercalation of surfactants is complete after 2 hours of irradiation. However, with the classical method, the intercalation was completed after a 6-hour treatment. These results prove that microwave treatment is more effective than conventional treatment. The same conclusions are found by Li et al. [65]. They showed that the synthesis of organophilic smectites under the effect of microwave irradiation is beneficial since 5 minutes of irradiation is sufficient for the intercalation of an amount equal to 70%CEC. The advantages of synthesis under the effect of microwave irradiation are also visualized by Korichi et al. [66], Gunawan et al. [67] and Liu et al. [68]. These authors concluded that the structural and surface properties are improved.

4.2 Surfactant intercalation mechanism in clays

From an experimental point of view, it is often difficult to highlight the nature of the bonds that can be established between solutes and a system as complex as clay, especially since, for the same adsorbent-surfactant couple, several connections of different natures can be established. However, it is essential to understand the adsorption mechanisms in order to correctly interpret the partition of a solute between the aqueous liquid phase and the solid phase of the clay. The techniques used to identify adsorption mechanisms are XRD, IR, SEM, BET analysis techniques [43] or thermodynamic techniques for measuring heats of adsorption by microcalorimetry [69]. In most works on the study of adsorption mechanisms, adsorption is described based on empirical models making it possible to simulate the global phenomena observed [70]. Despite all the difficulties encountered in describing the mechanisms of adsorption of organic pollutants in soils, two main categories of mechanisms can be distinguished.

4.2.1 Ion exchange

It has an important role in the adsorption of surfactants in clay, particularly in the case of ionizable molecules. It is formed between cations or anions and, respectively, negative or positive charges located on the surface of the adsorbent. The intensity of adsorption will closely depend on the solution pH, temperature and ionic strength.

4.2.2 London: Van der Waals connections

The attractive forces involved in this type of bonds correspond to dipolar interactions between the solvent (water), the solutes, and the solid surfaces. These are electrostatic forces due to the movement of electrons which change atomic orbitals. These are low binding energy interactions. Their intensity increases with the size of the molecules due to their additive nature. These are the main forces responsible for the physical adsorption of surfactants in clays.

4.3 Factors influencing intercalation

4.3.1 Influence of the clay proprieties

The quantity of modifying ions adsorbed on the surface of the layers depends on the number of accessible sites, therefore on the CEC and on the structure of the clay mineral. The isomorphic substitutions of montmorillonite are in the octahedral layers. Electrostatic interactions with compensating cations are therefore attenuated by the tetrahedral layer. It presents the most interesting compromise between a CEC large enough to allow a quality organophilic modification without sterically encumbering the interfoliar space, and small enough to allow the separation of the sheets in aqueous media.

4.3.2 Influence of exchanged cation

The inorganic compensating cation of the clay to be replaced also has an influence. This is related to its role during the dispersion of clay in aqueous solution. Larger and highly charged cations limit the opening of interfoliar spaces and are less easily exchangeable. The effect of the charge carried by the cation was also observed by McAtee [71]. He proved that for the same alkyl ammonium ion, a total exchange takes place with sodium montmorillonite while it remains limited to 70 or 80% with montmorillonites containing calcium or magnesium ions. In summary, the smaller and more mobile the compensating cation, the more the cation exchange is facilitated. The compensating cations most frequently present in clays can therefore be classified in increasing order of aid to cation exchange: Cs⁺ < Rb⁺ < Ca²⁺ < Mg²⁺ < Na⁺ < Li⁺.

4.3.3 Influence of the alkylammonium ion nature

The type of alkylammonium ion plays a considerable role in the intercalation. Indeed, the length of the carbon chain, the size and shape of the polar head, as well as the organic groups carried by the ion have significant influences on the efficiency of the exchange. The increase in the interlayer space, and therefore the improvement of the dispersion in a polymer and the properties of the final material, are related to the length of the carbon chain of the alkylammonium ion. Thus, by increasing the length of the carbon chain, we increase the entropic contribution of the adsorption energy and we develop more Van der Waals interactions. The binding of organic cations also depends on the size and shape of the polar head, according to Rowland and Weiss [72]. The results of work by McAtee [71] showed that ions from primary amines were not adsorbed in sufficient quantity to reach the CEC, unlike quaternary ammoniums. It has also been shown that the bond strength of amino derivatives decreases sharply from primary to secondary and tertiary compounds. Therefore, the groups carried by the carbon chain of the ion also affect the quality of the ion exchange. Indeed, cation exchange is favored when the ammonium cation presents a group capable of interacting with the surface oxygens of the sheets through hydrogen bonds.

4.4 Organoclay structures

It is not easy to control the structure obtained after the organophilic modification, because the chains can adopt different conformations within the interfoliar space. The type of arrangement obtained in these spaces is strongly dependent on the initial concentration of alkylammonium with respect to the CEC of montmorillonite. Indeed, the adsorption of the first layer of ions on the surface is linked to the cation exchange process, but the layers adsorbed thereafter are linked to the first by chain/chain interactions of the Van der Waals type and follow classical adsorption laws.

Lagaly, [34, 73] described the probable conformations of alkylammonium ions on the surface of the sheets. Depending on the length of the carbon chain and the charge deficit of the sheet, they can be organized in monolayers, in bilayers, according to a pseudotrimolecular or paraffinic type arrangement (Figure 5). Furthermore, Gherardi [74] described the organization of alkylammonium ions (of carbon chains with more than twelve methylene groups) in the context of adsorption above the CEC and its consequences on the multi-scale organization of montmorillonites in aqueous solution. The author observes that if the initial ion concentration is equal to the CEC of montmorillonite, the clay is completely hydrophobic and flocculates due to interactions between carbon chains. The interpretations and proposals for chain conformations made by Lagaly and Weis [73] use only X-ray diffraction measurements and are based on the assumption of the ideal case of an arrangement of the carbons of the alkyl chain in trans conformation. Vaia et al. [75] dispute these results based on infrared spectrometry measurements. Contrary to the previous case which only considers trans conformations to establish their structures, here the values of the wavelengths as well as their variations associated with the vibrations of the methyl groups which can be in left or trans conformations are studied. This work shows that the intercalated chains can be in gas, liquid, liquid crystal or solid states depending on the length of the alkyl chains (Figure 6). They show in particular that the same interfoliar distance can come from different conformations. Hackett et al. [76] confirmed these interpretations by molecular dynamics simulations. It can be noted that work based on NMR experiments has also been carried out to determine the conformation of molecules and has shown the coexistence of trans-ordered and left-disordered conformations. Moreover, these authors suggested that the charge density of the clay and the length of the aliphatic chain of the intercalated surfactant are the two interesting parameters influencing the structure of the complex obtained.

Figure 5.

Idealized structure of surfactants intercalated between the sheets of a phyllosilicate according to Lagaly [34]: (a) monolayer, (b) bilayer, (c) pseudotrimolecular, (d) paraffinic.

Figure 6.

Model conformations of chains of different lengths and intercalated according to Vaia [75]. (a) Short chains in C₆ (b) medium in C₁₂ (c) long chains in C₁₈.

5. Applications of surfactant modified-clays

All the results obtained so far concerning the different interactions of organophilic clays with organic compounds such as benzene and toluene [77] and phenol and its derivatives [78] have shown that these new materials are effective adsorbents.

Moreover, certain other recent works on bridged clay-pesticide interactions (in particular herbicides) have confirmed the hydrophobic nature of these complexes. This is how Nahhal et al. [79] and Carrizosa et al. [80] have shown through the various results obtained that there are indeed strong pesticide-organophilic clay interactions.

In the field of discoloration of textile industry effluents, Elguendi [81], Choi and Cho, [82] were interested in the adsorption of certain dyes such as basic blue 69 (Artrozone blue) and basic red 22 (Maxillon red; C₂₄H₁₈N₄O₄) on montmorillonites and vermiculites inserted by surfactants. Overall, they observed strong adsorbent-adsorbate affinities attributed to the hydrophobic nature of the surfactant molecules inserted into these clays.

The organo-clays were attractive for use as selective sorbents due to the organic layer such as dyes like Acid Blue 193 [83], acid dyes [84], and basic dyes [85], Congo red [86], Acid Orange 7 and Acid Blue 9, Malachite green [87], and Basic Red 18 [88]. According to the results obtained by these authors, the organo-clay was more effective than raw clay in the removal of reactive, basic, or anionic dyes. This result is due to the fact that after modification, the surface area and its porosity also increase.

Recent studies by Oyanedel-Craver and Smith [89], Oyanedel-Craver et al. [90], Guimaraes et al. [91] on the treatment of water polluted by heavy metals using montmorillonites intercalated with cationic surfactants confirmed the organophilic nature of these matrices and made it possible to classify their adsorption capacities.

Jiang et al. [92] modified an MMT montmorillonite with solutions of Fe, Al, Fe/Al, Fe-HDTMA, Al-HDTMA and Fe/Al-HDTMA hydroxides. The X-ray analyzes of different modified clays showed an increase in the basal distance d₀₀₁ from 15 to 18 Å, of which the Fe/Al-HDTMA-MMT clay presents the highest distance. The authors then used modified clays for the adsorption of phenol. The adsorption isotherms obtained show that the MMT treated with HDTMA is more effective for the retention of phenol molecules than the natural MMT and treated with Fe, Al, Fe/Al.

Oyanedel-Craver et al. [90] studied the simultaneous retention of benzene and heavy metals (Pb, Cd, Zn and Hg) by MMTs modified with HDTMA and BTEA. They showed that metal retention is achieved by the formation of complexes with silanol and aluminol groups. By comparing the results of benzene adsorption by HDTMA-MMT and BTEA-MMT, the authors concluded that the mechanism of retention differs according to the length of the carbon chain of the intercalated surfactant: if the chain is short, the retention takes place by interaction between the surface of the clay and the organic molecule whereas if the chain is long, the retention is done by interaction between the surfactant chain and the organic molecule.

Szanto et al. [93] examined the adsorption of methanol and benzene by two montmorillonites sodium and calcium modified by different amounts of surfactant HDPy (20–90 mmol/100 g). They showed that the methanol and benzene retention capacity is higher for organo-clays than for natural clays. The fixed amounts of methanol and benzene on natural and modified sodium clay are 3.44; 1.8; 8.25 and 4.35 mmol.g⁻¹.

Tvardovski et al. [94] studied the retention of benzene and hexane vapors by a natural Montmorillonite clay (CEC = 100 meq/100 g, SAA = 34 m²/g) and a synthetic fluorohectorite clay (CEC = 82 meq/100 g, SAA = 12 m²/g) modified by HDPy. The authors showed that the retention of benzene (Q_ads = 5.2 mmol.g⁻¹ by HDPy-MMt, Q_ads = 2.6 mmol.g⁻¹ by HDPy-fluorohect) is greater than that obtained for hexane (Q_ads = 0.72 mmol.g⁻¹ by HDPy-MMt, Q_ads = 0.78 mmol.g⁻¹ by HDPy-fluorohect). By comparing the adsorption results of benzene and hexane, the authors concluded that the retention mechanisms differ according to the microstructure and the crystallite of the organo-clays prepared.

Bors et al. [95] used a sodium bentonite (CEC = 76 meq/100 g) modified by quantities of HDPy varying from 0.0 to 2.0 CEC as an adsorbent of iodine (I), technetium (Tc), cesium (Cs) and strontium radionuclides (Sr). They showed that the quantities of radionuclides adsorbed by organophilic bentonite exceed those adsorbed by natural bentonite and also that the iodide ions are more adsorbed (I˃Te˃Ce˃Sr). This adsorption is influenced by the structure of the modified clay and the intercalated quantity of the surfactant. They found that these radionuclides are retained primarily by ion exchange.

Behnsen and Reibe [96] modified a montmorillonite of CEC = 89 meq/100 g by three cationic surfactants HDTMA, BE and HDPy. The adsorption isotherms of I⁻, NO₃⁻, ReO₄⁻, SeO₃⁻² and SO₄⁻² anions with an initial concentration of 1–10 mmol.L⁻¹ show good efficiency of these organo-clays in the retention of anions. The authors concluded that the anions are retained by anion exchange. The anion selective order is I⁻ > ReO₄⁻ > NO₃⁻ > Cl⁻ > SO₄⁻² > SeO₃⁻² for the three organo-clays. Comparatively, the fixed amounts of I⁻, ReO₄⁻, NO₃⁻, Cl⁻, SO₄⁻² and SeO₃⁻² on the HDPy-MMt clay are the highest, they are respectively 452, 431, 393, 276, 88 and 75 mmol.Kg¹.

Yab et al. [97] were prepared inorganic tubular micelle. Selective modification of aluminosilicate clay nanotube inner lumen with octadecyl phosphonic acid and dopamine was demonstrated. The adsorption study showed that halloysite with organosilane adsorbs more ferrocene than its hydrophilic derivative (ferrocene carboxylic acid). Therefore, like in organic micelle, the octadecyl phosphonic acid immobilized in the halloysite lumen may behave as sponge for physisorption increasing adsorption capacity for hydrophobic molecules.

Owoseni et al. [98] proposed a new technology for the treatment of oil spills using organo-modified clay. The method is based on principles of emulsification using naturally occurring halloysite clay with structure Al₂[Si₂O₅(OH)₄].2H₂O and the Dioctyl sulfosuccinate surfactants from the halloysite nanotubes to the oil-water interface. At appropriate surfactant compositions and loadings in halloysite nanotubes, the crude oil-saline water interfacial tension is effectively lowered to levels appropriate for the dispersion of oil (Table 4).

Retention of organic compounds
adsorbate	Surfactant modified clay	Results	Reference
Phenol and its derivatives
Phenol	MMT modified by Fe/Al-HDTMA	Qphenol = 6 mg.g^−1	Jiang et al. [92]
	Bentonite modified by HDTMA	* Qphenol = 180 mol.g^−1	Yapar et al. [99]
	Bentonite modified by HDTMA	oxidation 100	Mojovic et al. [100]
Pentachlorophenol PCP	Bentonite modified by (DODMA, HDTMA, HDPy, TMA)	QPCP(bent-HDTMA, DODMA) = 0.12 mg.g^−1; QPCP(bent-TMPA,TMA) = 0.02 mg.g^−1	Boyd et al. [101]
	Na-MTM modified by HDTMA	QPCP max = 0.6 mol/kg	Stapleton et al. [102]
A = 3, 4, 5- trichlorophénol,; B = 3, 5-dichlorophénol; C = 3-chlorophénol	Bentonite modified by HDTMA	QadsA = 2 mmole/g; QadsB = 3 mmole/g; QadsC = 0.9mmole/g	Mortland et al. [103]
Tanin	Bentonite modified by HDTMA	QTanin max = 2.5
para-nitrophénol PNP	Ca-MMT modified by HDTMA	—	Zhou et al. [78]
2,4,5-trichlorophénol	Bentonite modified by HDTMA	QadsA = 374.9 mg/g	Zaghouane-Boudiaf and Boutahala [104]
Benzene and its derivatives
Benzene BZ	Smectites modified by HDPy	QBZ = 5.5 mmol.g^−1	Stul et al. [38]
Methanol M; Benzene BZ	Na-MMT modified by HDPy	QM = 8.25 mmol.g^−1; QB = 4.35 mmol.g^−1	Szanto et al. [93]
Nitrobenzène NB Trichloréthylène TCE	Smectite modified by HDTMA	QNB = 2 mmol.g^−1; QTCE = 4 mmol.g^−1
Benzene BZ; Trichloro-ethane TCE; 1,2-dichloro-benzene DCB	Bentonite modified by HDTMA	QDCB = 700 mg.kg^−1; QBZ = 600 mg.kg^−1; QTCE = 400 mg.kg^−1	Bartelt-Hunt et al. [105]
Nitrobenzène	Na-MMT modified by HDTMA	QNB > 100 mmol.kg^−1	Borisover et al. [106]
Pesticides
Triadimefon TDF	MMT (SW, SA); modified by HDTMA	QTDF = 6	Celis et al. [107]
Sulfometuron SFM	Na-MMT modified by HDTMA	Taux(%) = 60.0%	Mishael et al. [108]
Fluridone FD	Na-MMT modified by HDTMA	QFD(MMT HD-0,8CEC) = 35 μmol.g^−1	Yaron-Marcovich et al. [109]
allelochemical scopoletin	commercial organoclay	—	Galán-Pérez et al. [110]
Pharmaceutical wastes
lactam antibiotics	MMT modified by DDAB	Removal 97%	Saitoha and Shibayama [111]
diclofenac sodium	Bentonite modified DMA	Qmax = 0.115(mmol/g)	Maia et al. [112]
codeine,; oxazepam,; ibuprofen	MMT modified by BDTA and Brij-O20	R (BDTA-MMT) = 12.74%; R (Brij-O20-MMT) = 59.38%; R (BDTA-MMT) = 81.99%; R (Brij-O20-MMT) = 48.98%; R (BDTA-MMT) = 92.74%; R (Brij-O20-MMT) = 47.64%	De Oliveira et al. [113]
AZM antibiotic	L-methionine modified montmorillonite	Qmax = 298.78 mg/g	Imanipoor et al. [114]
Dyes and pigments
IC dye	PDADMA-bentonite	Qmax = 149.2 mg/g	Shen et al. [115]
	HDPy^+-clay	Qmax = 326.31 mg/g	Gamoudi and Srasra [116]
MO dye	HDTMA-bentonite	Qmax = 141.0 mg/g	Belhouchat et al. [117]
	HDPy^+-clay	Qmax = 277.27 mg/g	Gamoudi and Srasra [116]
RP dye	CP-montmorillonite	Qmax = 19.78 mg/g	Lee et al. [118]
	HDPy^+-clay	Qmax = 344.82 mg/g	Gamoudi and Srasra [116]
CV dye	HDTMA-bentonite	Qmax = 162.5 mg/g	Anirudhan and Ramachandran [85]
	HDPy^+-clay	Qmax = 65.78 mg/g	Gamoudi and Srasra [119]
b-carotene	Bentonite modified by RSB	Qmax = 97.58 mg/g	Santoso et al. [120]
Retention of inorganic compounds
Retention of anions
iodure (I), technetium (Tc), cesium (Cs), strontium (Sr)	Na-Bent modified by HDTMA	Selectivity: I˃Te˃Ce˃Sr	Bors et al. [95, 121]
iodure I	Na-bent modified by HDPy	Kd = 297 L.Kg^−1	Dultz et al. [122]
chromate	Am-Bent by HDTMA	*Qmax = 795 mmol.Kg^−1	Krishna et al. [123]
	MMT by HDTMA HDP	* Qads = 18 mg.g^−1	Brum et al. [124]
I^−, ReO4^−, NO3^−, Br^−, SO4^−2, SeO3^−2	Bent modified by HDTMA	Qads (ReO4^−) = Qads (I^−) = 1,5Qads(NO3^−) = 2 Qads (Br^−) = 8Qads (SO4^−2) = 8Qads (SeO3^−2) = 450 mmol.kg^−1; Selectivity: ReO4^−˃I^− ˃NO3^−˃Br^−˃Cl^−˃ SO4^−2 ˃ SeO3^−2	Behnsen and Riebe [96]
Nitrate	Bent modified by HDTMA	Qads = 14,76 mg.g^−1	Xi et al. [125]
NO3-	Smectite/illite modified by HDTMA	QNO3- = 15.38 mg.g^−1	Gamoudi et al. [126]
F-	Smectite modified by HDPy	QF- = 19.34 mg.g^−1	Gamoudi et al. [127]
I^−, IO3^−	Organoclay	Qads (I^−) = 12.1 mg.g^−1	Li et al. [128]
Adsorption of metallic ions
Pb, Cd, Zn et Hg	Ca-bent modified by BTEA	QCd = 7.82 mg.g^−1; QPb = 87,8 mg.g^−1	Oyanedel-Craver, [89, 90]
Sb (III)	Montmorillonite modified by HDPy	QSb = 108.7 mg.g^−1	Bagherifam et al. [129]
Retention of gases
CO, CH4, SO2	Bent modified by BTEA	QSO2 = 1.65 mmol.g^−1	Volzone et al. [130]
O2, N2, CO, CH4, C2H2, CO2 and SO2	Na^+-MMT modified by HDTMA	QSO2 = 1.7 mmol.g^−1	Volzone [131]
NH3 and CO2	Laponite modified by PAMAM	Selectivity CO2 > NH3	Kinjal et al. [132]
CO2	Halloysite d by HKUST-1	—	Park et al. [133]
Antimicrobial activity
Salmonella enteritidis	SAz and SWy MMTs modified byHDPy	Qads(SAz-1-HDPy) = 97%; Qads(SWy-2HDPy) = 95%	Herrera et al. [134, 135]
Escherichia coli	Montmorillonite modified by HDPy	Qads (SAz-1-HDPy) = 95%	Ma et al. [136]
Staphylococcus aureus; Escherichia coli	Montmorillonite modified by TBA, DDA and TBP		Bujdáková et al. [137]
Bacillus subtilis	Hallyosite modified by CTAB and SDS		Abhinayaa and Mangalaraj [138]

Table 4.

Applications of surfactant modified clay.

6. Conclusion

The OC are organic–inorganic complexes synthesized through the intercalation of organic cations mainly into the interlayer space of expandable clays. Different surfactants have been used to prepare OC. These include single and dual cationic surfactants, anionic–cationic surfactants and nonionic surfactants. The intercalation of the surfactants cations was governed by different processes: cationic exchange and Van der Waals ‘interactions of the alkyl chains with clay surface. The structure and properties of the resultant organoclays are affected by the type of surfactant, the clay used and the preparation method such as the conventional technique, the solid-state intercalation, and microwave irradiation. As a result, the organoclays are characterized by hydrophobic surfaces and have attracted great interest because of their potential use in several applications, such as sorbents for organic pollutants, heavy metals and inorganic oxy-anions, in clay-based nanocomposites, and in several other industries.