**3. Nanocomposite structures**

specificity of the silicate. The relatively weak Van der Waals force ensures the stacking of the sheets between them and each layer called interlayer [19–21]. Stacking of the layers leads to a regular gap between each adjacent silicate layers called the interlayer or gallery. A negative surface charge is present on the layers due to the isomorphic substitution of tetrahedral silicon or aluminum and octahedral magnesium. The charge deficit is counterbalanced by alkali

The most used layered silicates are montmorillonite, hectorite, and saponite [23]. Their chemical formulae are illustrated in the **Table 1** [24]. All these layered silicates are characterized

ion-exchange capacity (CEC) generally expressed in milliequivalents per 100 g (mequiv. per 100 g). The charge indicated by the chemical formula is an average over the whole crystal because the charge varies from layer to layer. Only a small proportion of the charges are located at the surface of the crystal, the majority being mainly in the interlayer spaces [22].

To produce the nanocomposite with the required properties, the silicate layers should be exfoliated before being dispersed inside the polymer. Indeed, the hydrophilic characteristic of the clay does not allow it to be dispersed in the polymer matrix, which is generally organophilic. To render the mixture compatible, it is necessary to modify the clay before it is dispersed. For this reason, a pretreatment process should take place to weaken the forces holding the structure [24–27]. The main goal of this process is to render the surface of the layered silicate more organophilic. This improves the dispersion process of the layers inside the polymer under well-defined experimental conditions [28]. Given the structure and properties of clay, there are several modification techniques. However, the most commonly used technique for organophilic modification of clay is the ion-exchange reactions with cationic surfactants, including primary, secondary, tertiary, and quaternary alkylammonium cations to replace the hydrated cations with the protonated amine with long-chain alkylammonium cations. Alkylammonium cations in the treated clay decrease the surface energy of the inorganic host and increase interlayer spacing, rendering the modified organoclay is more compatible with organic polymer [29–33]. The above treatment process is basically an intercalating process to increase the spacing and decrease the adhesion forces between the silicate layers. The amine has the ability to achieve such target by acting as an intercalating agent. The efficiency of the intercalating agent is mainly influenced by the agent properties such as its hydrocarbon chain length [1]. It is very important to stress that there are also other original and interesting methods for modifying 2:1 phyllosilicates, such as the use of ionomers or block copolymers, and

/g in the case of montmorillonite) and a moderate cat-

and alkaline earth cations situated inside the galleries [22].

64 Current Topics in the Utilization of Clay in Industrial and Medical Applications

by a high surface area (700–800 m2

These cations are then exchangeable in solution.

**Silicate Chemical formula Location of isomorphic** 

)Si<sup>8</sup> O20(OH)<sup>4</sup>

M = monovalent cation; x = degree of isomorphous substitution (between 0.5 and 1.3).

(Mg6−xLix )Si<sup>8</sup> O20(OH)<sup>4</sup>

(Al4−xMg<sup>x</sup>

**Table 1.** General formula and characteristic parameters of phyllosilicates 2:1.

Mg6 (Si8−xAlx )Si<sup>8</sup> O20(OH)<sup>4</sup>

Hectorite M<sup>x</sup>

Saponite M<sup>x</sup>

Montmorrillonite M<sup>x</sup>

a

**substitutions**

<sup>a</sup> Octahedral 120

<sup>a</sup> Octahedral 110

<sup>a</sup> Tetrahedral 86.6

**CEC (mequiv./100 g)**

Variations in the strength of interfacial interactions between the polymer matrix and layered silicates result in the formation of three main kinds of nanocomposites as described below [34].


Despite the impressive progress that has been reported during the last few years regarding the use of polymer-clay nanocomposites for corrosion protection, its effect on the corrosion performance has rarely been investigated. Although most organo-modified clay nanocomposites

**Figure 1.** The main types of nanocomposite. (a) Intercalated, (b) flocculated, and (c) exfoliated.

reported so far are intercalated, exfoliated structures are more desirable in property improvement of the polymeric materials. In this context, Chen and co-workers described the processing of epoxy-layered silicate nanocomposites with different properties as corrosion resistant coatings on aluminum surfaces [35]. Based on their studies, they concluded that there is a slight enhancement in anticorrosion properties for the exfoliated nanocomposites coatings and no enhancement for the intercalated nanocomposites. Finding shows that these criteria are related to the better dispersion of silicate nanosheet in some epoxy matrix than the other grades of resin matrix. However, these findings were not very conclusive, and it appears that an epoxy with a lower barrier resistance or higher permeability will help in discriminating the corrosion behavior through the introduction of clay into the matrix. Similarly, Sakai and co-workers found that the exfoliated clay structures were more fixed in polymeric matrix compared to intercalated and conventional clay structures [36]. Additionally, exfoliated clay structures exhibited better corrosion performance compared to intercalated coatings nanocomposites. This is mainly attributed to the larger clay interlayer distance, smaller clay aggregate, and uniform homogeneity of exfoliated clay structures inside the epoxy matrix. In some case, a mixture of intercalated and exfoliated nanocomposites was obtained, and it was difficult to evaluate the effect of nanocomposite structures on the corrosion performance of the prepared nanocomposites [37].
