Chapter 1
Clay-Organic Interfaces for Design of Functional Hybrid Materials

Pilar Aranda1, Margarita Darder1, Bernd Wicklein1, Giora Rytwo2 and Eduardo Ruiz-Hitzky2

1Materials Science Institute of Madrid, CSIC, c/ Sor Juana Inés de la Cruz 3, 28049 Madrid,, Spain

2Environmental Physical Chemistry Laboratory, MIGAL- Galilee Research Institute of Environmental Sciences, Tel-Hai College, Upper Galilee, 12201,, Israel

1.1 Introduction

1.1.1 Clay Concepts, Definitions, and Classification

Clay minerals represent a wide family of natural silicates that are essentially related from the structural point of view with 2D solids, being arranged in a layered stacking (phyllosilicates) and showing unique colloidal and surface properties of crucial importance for the life in Earth [1]. They constitute a key reference not only as structural models and other basic aspects but also regarding their role in agronomy, industry, and environmental features influencing human activities of global concern. Clay-sphere represents a vast domain belonging to the Earth geosphere, mainly being associated with soil, which in turn can be regarded as a sort of magic skin of our planet. According to Kutílek and Nielsen [2], soil - and indeed clay minerals as one of its essential component - started its critical role when macroscopic life moved from oceans to mainland, roughly 500 million years ago. The support of and the interaction with micro- and macroorganisms is in a large extent deserved by clay mineral components that represent one of the most ample group of inorganic solids that interact with the biosphere. Even more, it has been invoked by diverse authors that clays have played an essential role in the origin of life in close interaction with organic precursors [3].

From the structural point of view, clay minerals appear mainly organized as hydrous phyllosilicates whose elemental layers have one dimension (the thickness) in the nanometer range, and therefore they can be considered as nanomaterials. According to the silicate layers nature, clay minerals can be classified as 1 : 1 (or TO) and 2 : 1 (or TOT) phyllosilicates representing the stacking of tetrahedral (T) and octahedral (O) layers, which can be considered as building blocks that combined between them lead to a huge diversity of layered silicates. Typical examples of 1 : 1 phyllosilicates are kaolinite and serpentine with octahedral layers composed of aluminum (dioctahedral silicate) and magnesium (trioctahedral silicate), respectively. Typical examples of 2 : 1 phyllosilicates are pyrophyllite and talc, with octahedral layers composed of aluminum (dioctahedral silicate) and magnesium (trioctahedral silicate), respectively. In this case, the structure of 2 : 1 phyllosilicates is composed of two tetrahedral Si sheets sandwiching the Al or Mg central octahedral sheet.

Clay minerals are often associated with smectites and particularly with montmorillonites (MMT), the main component of bentonites of great interest for diverse industrial applications. Smectites are dioctahedral aluminosilicates belonging to the 2 : 1 phyllosilicate structure with isomorphous substitutions of aluminum by magnesium ions, giving rise to negatively charged layers that are compensated with cations in the interlayer space (Figure 1.1a). These cations, named exchangeable cations, can be easily replaced by treatment with salt solutions of metal or organic cations, leading to homoionic smectites. Many other related structural 2 : 1 silicates such as beidellite, saponite, nontronite, hectorite, and so on are known, where isomorphous substitutions could also affect the cationic replacement in the tetrahedral layer (e.g., Si4+ by Al3+). Vermiculites are a family of 2 : 1 phyllosilicates with an isomorphous substitution degree higher than in smectites, therefore showing a more elevated net layer charge per formula unit (0.6-0.9) than in smectites (0.2-0.6) [4].

Figure 1.1 Schematic representation of clays structure with (a) layered habit (montmorillonite) showing on the right column the cross section of the silicate layers and (b) fibrous morphology (sepiolite) showing the organization of silicate fiber dimension.

MMT and related smectites exhibit significant characteristics/properties with particles of colloidal size, high degree of layer stacking disorder, elevated specific surface area (SSA), large cation exchange capacity (CEC), and a variable interlayer separation that depends on the nature of interlayer cation and relative humidity [1]. However, the most interesting feature of these 2D solids is definitely their capacity to intercalate many diverse species, neutral or charged, inorganic or organic, and even macromolecules of relatively high molecular mass. As will be discussed later, the nature of the interface at the interlayer space in these systems is determinant for the intercalation processes, as well as for the arrangement and stability of species in the resulting intercalation compounds.

Typical morphology of clay minerals corresponds to platelike crystallites of micrometer size. However, in certain circumstances special structural arrangements determine other conformations, as it is the case of halloysite that can exhibit a nanotubular morphology due to the rolling of 1 : 1 aluminosilicates such as kaolinite, leading to a multiwalled system. Halloysite nanotubes have characteristic inner diameter in the order of 15 nm, their lumen facilitating the access of organic species with the formation of hybrid organic-inorganic materials [5]. Certain 2 : 1 phyllosilicates such as sepiolite (Figure 1.1b) and palygorskite show a fibrous habit with a continuous tetrahedral silica layer and a discontinuity of the octahedral sheet due to the periodic inversion of the apical oxygen atoms of the tetrahedra in every two (palygorskite) or three (sepiolite) silicate chains [1, 6]. As a consequence of this structural arrangement [7], both types of silicates show an alternation of blocks and cavities named tunnels [8], which are oriented along the c-axis, that is, in the fiber direction (Figure 1.1b). The fiber length varies depending on the origin of the fibrous silicates, typically the fibers of sepiolite from Vallecas-Vicálvaro deposits in Spain being between 1 and 5 µm. The structural blocks are composed of two tetrahedral silica sheets sandwiching a central sheet of magnesium oxide-hydroxide in the case of sepiolite and of magnesium and aluminum oxide-hydroxide in the case of palygorskite. Due to the periodic discontinuity of the silica sheets, silanol groups (?Si-OH) are covering the "external surface" of the silicate fibers [8, 9]. The tunnel dimensions are in the nanometer range (0.37 nm × 1.06 nm, in sepiolite and 0.37 nm × 0.64 nm in palygorskite), allowing the entrance of small molecules such as N2, H2O, NH3, CH3OH, and others to the interior of the silicates, that is, these molecules can be adsorbed at the internal surface of sepiolite and palygorskite [8].

The surface and the interface chemistry of the different types of clay minerals - and indeed essential properties inherent to these silicates - are determined by various factors such as:

1. 1. The chemical composition and the nature of the surface (mainly oxygen atoms, water molecules, and hydroxyl groups)
2. 2. The type, extent, and localization of the electrical charge in the silicate
3. 3. The nature and CEC in the external and intracrystalline regions
4. 4. The extent (SSA) and reactivity of the planar external surface, the internal surface (interlayer space), and the edge of the clay particles

In this way, the surface activity toward adsorption and other interactions of diverse compounds with clay minerals can be modulated by the aforementioned features.

1.1.2 Clay-Organic Interactions: Clay-Organic Interfaces and Functionalization of Clay Minerals

The history of well-defined organic-inorganic hybrid materials starts with the assembling between organic compounds and clay minerals, which took place in the intercalation of organic cations in smectite phyllosilicates by ion-exchange processes replacing interlayer cations by alkylammonium species, as reported by the first time by Gieseking [10]) and Hendricks [11]. Since these early reports, many different types of organic cations, such as alkyl- and arylammonium species, cationic dyes, amino acids, charged peptides and proteins, cationic pesticides and surfactants, and so on, have been intercalated in smectites and vermiculites [12]. Clearly, electrostatic bonding mechanisms between the organic cations and the charged clay layers can be here invoked, although, strictly speaking, non-coulombic attractions such as van der Waals forces must be also considered. Nowadays, long-chain alkylammonium species belonging to the cationic surfactants group represent from far the most remarkable organic compound used for the preparation of so-called organoclays of great importance in diverse applications. As initially reported by MacEwan, non-charged, that is, neutral, molecules can also be accommodated in the interlayer region of smectite and vermiculite layered silicates in interaction with interlayer cations and oxygens belonging to the internal surface of these solids [13]. Since this last discovery, a huge number of neutral compounds of different functionality have been intercalated in layered clays involving diverse host-guest mechanisms in those clay-organic interactions: amines...