Bionanocomposites of sepiolite and palygorskite and their medical issues

Among clay minerals sepiolite and palygorskite constitute a group of natural silicates of microfibrous morphology. Both silicates exhibit a crystalline structure consisting of T-O-T (T = tetrahedral; O=octahedral) structural blocks alternating with cavities (tunnels), along the fiber direction (c-axis) (Figure 9.1) (Bradley, 1940; Brauner and Pressinger, 1956; Santarén et al., 1990; Ruiz-Hitzky, 2001). The ideal unit cell formulae of sepiolite and palygorskite are Si₁₂O₃₀Mg₈(OH)₄(H2O)₄·8H₂O and Si₈O₂₀(A_l2 Mg₂)(OH)₂ (H₂O)₄·4H₂O, respectively (Suárez and García-Romero, 2011). Tunnel dimensions are 1.06 × 0.37 nm² and 0.64 × 0.37 nm² for sepiolite and palygorskite, respectively (Ruiz-Hitzky et al., 2011a). These structural micropores can be occupied only by small-size molecules such as water, methanol and acetone (Ruiz-Hitzky, 2001). Most interactions between the fibrous clays and organic or inorganic species involve the external surfaces of those silicates. The specific surface of both clays are high (ca. 300 m²/g and 200 m²/g, for sepiolites and palygorskites of different origin, respectively) being around a half of those values ascribed to the external surface (Alcântara et al., 2012). This external surface is covered by silanol (≡ Si-OH) groups regularly located at the edge of the T-O-T blocks due to the discontinuity of the silica sheets, and they can establish direct interactions with diverse organic species (Ruiz-Hitzky, 2001). These structural and textural features allow the preparation of diverse types of advanced nanostructured materials, essentially reliant on their ability to assemble inorganic species and nanoparticles (Ruiz-Hitzky and Aranda, 2014; González-Alfaro et al., 2011; Ruiz-Hitzky et al., 2012a), organic molecules (Ruiz-Hitzky et al., 2004, 2010a), as well as polymer and biopolymers (Ruiz-Hitzky et al., 2010b, 2011a, b, 2013a, b).