*2.2.2 Semectite minerals*

**Smectites** are TOT phyllosilicates type. The association between smectite layers leads to a constant van der Waals break between the layers, known as "interlayer" [35].

Two smectite subgroups are existing:

i.Dioctahedral smectites, as montmorillonite, non-tronit, beidellite, etc.

*New Trends in Clay-Based Nanohybrid Applications: Essential Oil Encapsulation Strategies... DOI: http://dx.doi.org/10.5772/intechopen.106855*

ii.Trioctahedral smectites, as hectorite, saponite, sauconite, etc. [22].

The smectite clays structure is able to expand and depreciate without losing its crystallinity.

Smectite layers are typified by plentiful isomorphic substitutions in tetrahedral (Si4+ replaced by Al3+) and octahedral (Al3+ replaced by Mg2+ or Fe2+ and Mg2+ substituted by Li<sup>+</sup> ) positions generating a layer charge that is neutralized by hydrated cations in the interlayer space, named interlayer cations (Na+ , Ca2+, K+ , etc.) [36].

**Montmorillonite** (MMT) is the most important smectite clay owing to its commercial value. It emanates from the replacement of the Si4+ in the silica tetrahedral sheets by Al3+ and Al3+ in the alumina octahedral sheets by Mg2+ leading to a negative charge that is neutralized by the Na<sup>+</sup> and Ca2+ interlayer cations.

These cations can be facilely supplied by other organic or inorganic cations, due to the incomparable hydrophilicity, swelling, adsorption, and fluidity properties of montmorillonite [37].

#### *2.2.3 Palygorskite and sepiolite minerals*

**Sepiolite** (Sep) [Mg8Si12O30(OH)4(OH2)4.n(R2 + (H2O)8] and **palygorskite** (Pal) [MgAl3Si8O20(OH)3(H2O)4.n(R2 + (H2O)4] are 2:1 layer silicates being distinct from other clay minerals owing to the existence of continued two-dimensional tetrahedral sheet and broken octahedral sheets. Their form can be defined as ribbons of 2:1 phyllsilicate structure. Each ribbon is bonded to the next by inversion of SiO4 tetrahedra along a set of Si-O-Si bonds. Channels between the ribbons are generated due to these structural properties, which are larger in Sep (4 Å x 9.5 Å) in comparison with Pal (4 Å x 6 Å). Pal is dioctahedral and Sep is trioctahedral. Both minerals have elongated habits often forming bundles of lath-like or fibrous crystals. They contain two types of water, structural water coordinated to the octahedral cations and zeolitic water, which is loosely bound in the channels. Due to the existence of channels, Pal and Sep have great microporous volumes. They are also of a big interest and importance due to their large specific surface, exceptional swelling, and good absorbability [25, 38].

#### **2.3 Clay minerals modification**

Modification of clay minerals' native physicochemical and structural characteristics (porosity, CEC, acidity, surface area, etc.) is usually required to adapt and extend their use in different fields of application [20].

Several physical and chemical methods have been investigated to modify clay minerals features, such as acid activation, intercalation or adsorption of organic components, clay pillarization, synthesis of porous clays heterostructures, ultrasonication, and thermal treatment [39–42].

### **3. Essential oil: chemical composition and properties**

Essential oils (EOs) are volatile liquids extracted from numerous plants. They are hydrophobic and contain volatile aromatic compounds or vegetable essences. These aromatic compounds are volatile organic compounds (VOCs), which result from the plant secondary metabolism [43]. According to the European Pharmacopoeia, an essential oil is defined as an "odorous product, commonly with a complex

composition, obtained from a plant raw material by steam distillation, dry distillation, Hydrodistillation or other suitable methods without heating" [44].

Essential oils could be obtained from flowers (Pelargonium Rosat, Lavanduladentata), leaves (Eucalyptus globulus, Thymus vulgaris, Origanum vulgare, melaleuca alternifolia), seeds (Coriandrumsativum, Carumcarvi, Foeniculum vulgare, Pimpinellaanisum), peel (Citrus sinensis, CitrusLimon), rhizomes (Zingiberofficinale), and woods (Cinnamomum Cassia, Santalum album) [45].

Generally, EOs present less than 5% of the vegetal dry matter. Their composition may vary depending on the plant organ utilized, the soil and climatic conditions, and the harvesting season. Essential oils are insoluble in inorganic solvents, while soluble in organic ones (alcohol, ether, and oils). They are colorless, liquid, and volatile at room temperature, having a characteristic odor with a density less than water density [8].

EOs are very interesting natural products with several biological properties, such as antioxidative, anti-inflammatory, cytoprotective, antitumor, antimicrobial, antihypertensive, analgesic, larvicidal, insecticidal, antiparasitic, and other biological activities [5]. Thus, they have been largely used in agriculture, pharmaceutical field, medicine, and cosmetics. Indeed, numerous studies have shown insect-repellant and biocide activities, which are usable in agronomy and food industry [46, 47]. Due to their incredible properties, interest in EOs has massively increased in recent years. They are subsequently useful as complementary medicinal treatments, due to their availability and to synergistic therapeutic effects with conventional medicines, such as antibiotics [48]. Aspects related to the significant antimicrobial effect against multiresistant pathogens of the EOs, and their synergetic effect when one or more oils are mixed used in combination with known drugs [49, 50], are also presented in a recent review [51].

#### **3.1 Chemical composition of essential oils**

The number of known compounds present in essential oils has recently increased with the improvement in instrumental analytical chemistry. Nowadays, there are more than 300 components present in pure EOs [52, 53] that can be classified into two categories:


As mentioned before, the main component of the chemical composition of EOs is a complex mixture of hydrocarbon terpenes and terpenoids. Obviously, chemical compounds contain carbon and hydrogen as their building blocks that are isoprene (C5H8), the basic hydrocarbon unit found in EOs [46]. Moreover, terpenes are a class of natural products represented by general structural formula (C5H8)n. Containing more than 30.000 compounds, these unsaturated hydrocarbons are developed by plants, particularly conifers. The classification of terpenes is based on the number of isoprene units: Two, three, or four isoprene units are joined head to tail form monoterpenes, sesquiterpene, and diterpenes, respectively. Monoterpenes are present *New Trends in Clay-Based Nanohybrid Applications: Essential Oil Encapsulation Strategies... DOI: http://dx.doi.org/10.5772/intechopen.106855*

with more than 80% of EOs composition with 10 carbons, while sesquiterpenes and diterpenes are composed of 15 and 20 carbons, respectively. However, hemiterpenes (C5), diterpenes (C20), triterpenes (C30), and tetraterpenes (C40) are also observed in nature [55–57].

The second class of terpenes containing oxygen is called terpenoids. Those oxygenated derivatives of hydrocarbon terpenes can be aldehydes, alcohols, esters, ketones, acids, and phenols. Terpenoids also present many biological properties such as anti-inflammatory, antifungal, antiseptic, bactericidal, antiviral, sedative, and antitumor [58–60].

Furthermore, non-terpene components, defined as phenylpropanoids, are also found in EOs providing odor and a particular flavor (Eugenol and Cinnamaldehyde) [55].

#### **3.2 biological properties**

EOs have, for a long time, been recognized to possess several biological actions on humans, animals, and plants, namely [61–65]:

