## 2 Sepiolite as a Reinforcing Filler  
Sepiolite is an Mg-silicate belonging to the class of phyllosilicate, differing from the others by the fact that the typical T-O-T structure is prominently elongated into one of the two directions as represented in Figure 2. These elongated platelets of TOT silicates are then fused by the edges into needle-like arrays of approx. 20 nm \(\times\) 40 nm \(\times\) 1-2 \(\upmu\)m which constitute the sepiolite primary nanoparticle structure [9]. In the native form, these nanoparticles appear strongly agglomerated into micrometric bundles, held together by hydrogen bonds. To obtain readily dispersible sepiolite nanoparticles, cation exchange with quaternary ammonium salts is the method of selection [4]. Like most of the other phyllosilicate-based clays, sepiolite belongs to the class of cationic clay, meaning that its surface is negatively charged in neutral water therefore its particle charge must be compensated by a cation. The original exchangeable cation of sepiolite is predominantly Mg\({}^{++}\), but it can be replaced with a more affine one as a quaternary ammonium organic salt (Q\({}^{+}\)X\({}^{-}\)). Unlike 2-D lamellar phyllosilicates, sepiolite cannot be exfoliated into its TOT original structure, because of the edge conjunction; hence, quaternary ammonium exchange results mainly in an external surface modification [10]. Since the primary particle is made of edge-bonded elongated lamellae, about half of the nanoparticle volume consists of regular microporous cavities, normally filled with coordinated water. This microporous structure is  
Figure 1: Nanometric (**left**) and atomic (**right**) structure of layered phyllosilicates from Ref. [7].  
responsible for about 2/3 of the original Sepiolite specific surface area (SSA), and quaternary ammonium salt treatment hinders access to the microporous channels and drastically reduces the measured SSA [10]. The zeolitic water might be not the exclusive guest of sepiolite microporous channels: host-guest interactions were studied for the classical case of the maya blue pigment [11] and some published work suggests that this interaction could be exploited to specifically locate functionalization at the nanoparticles tips [12].  
Exchange of Sepiolite Mg\({}^{++}\) cation with Q\({}^{+}\)X\({}^{-}\) generates a class of organomodified clays (OC) that are well studied in the literature and could be represented by the commercial material Pangel B5 produced by Tolsa. This specific sepiolite grade is characterized by a controlled shape (diameter about 40 nm, length about 1 \(\upmu\)m, aspect ratio about 23) and modified with 25% wt of N,N-didodecyl-N-methyl-benzylammonium. Similar to other OCs, Pangel B5 is compatible with several apolar polymer matrixes [13], and since it does not tend to stack like intercalated 2-D OCs [14] it is easy to disperse. Literature reports applications of organomodified sepiolites in thermoplastics, polyolefin and rubber compositions [15; 16; 17]. Some application is also reported for non-modified sepiolite nanoparticles, particularly for waterborne compositions [18] or with polar matrixes [19].  
Apart from an exchange with organic moieties, the Mg\({}^{++}\) cation on the sepiolite surface can also be leached in acidulated water, transforming the surface silicate anions into silanol moieties. This simple yet powerful approach allows generating a silanol-rich surface, which can be consequently modified by reaction with silane coupling agents [20]. Bare sepiolite is easily dispersible but has a poor capability of reacting with silanes [21] leaving little room for surface modification during melt mixing. The silanization ability is the key for boosting filler-rubber interaction and consequent compound properties in silica-based rubber compounds [22].