Complexation of cationic 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin (TMPyP) and negatively charged chemically converted graphene (CCG) sheets was performed by simply mixing the diluted aqueous solutions of both components. During this process, a large bathochromic shift of porphyrin Soret band from 421 to 458 nm was observed, which is attributed to the flattening of TMPyP molecules induced by CCG through electrostatic and π−π stacking cooperative interactions. Furthermore, the coordination reaction between TMPyP and Cd2+ ions was greatly accelerated from 20 h to 8 min under ambient conditions by introducing CCG sheets. On the basis of this phenomenon, we used the complex of TMPyP and CCG as an optical probe for rapid and selective detection of Cd2+ ions in aqueous media.

Chemically converted graphene induced molecular flattening of 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin and its application for optical detection of cadmium(II) ions.

Organic compounds can interact with clay minerals by i) adsorption at the external surfaces, ii) adsorption at the external and internal surfaces, iii) by exchange of exchangeable ions at the external surfaces, iv) by exchange of exchangeable ions at the external and internal surfaces, and v) by grafting reactions with silanol and aluminol groups leading to covalent bonds. Kaolin minerals intercalate only are a limited number of compounds whereas the reactions of 2:1 clay minerals, in particular smectites and vermiculites, are very manifold. Special attention is given to the interaction with neutral organic molecules such as alcohols, fatty acids, amines, amino acids, aromatic compounds, macrocyclic compounds, and nuclein bases. The interaction with complexes and dyes also provides the basis of advanced applications of clay minerals. Binding of long chain alkylammonium ions is a fundamental reaction for hydrophobising clay mineral particles as needed in many applications. The interaction of clay minerals with polymers including proteins is not only an actual field of research but also of practical importance. Organo-clay minerals are used as effective adsorbents. As these materials also adsorb solvent molecules together with the adsorptive, the adsorption process must be considered as adsorption from binary solution which, therefore, is also described in this chapter.

Clay mineral-organic interactions

The adsorption of uncharged polymers by clays is largely "entropy-driven." Polymer confor- mation changes from a random coil in solution to an extended form at the surface in which adsorbed polymer segments or trains alternate with loops and tails extending away from the surface. Although the net inter- action energy, e, per segment-surface contact is small (--1 kT unit), the total energy of adsorption is large because the fraction of train segments, p, is commonly between 0.3 and 0.5. The adsorption isotherms are typically of the high-affinity type, and there is an apparent lack of desorption on dilution. Positively charged polymers (polycations) are adsorbed largely through electrostatic interactions between the cationic groups of the polymer and the negatively charged sites at the clay surface. Here e >> 1 kT unit and p > 0.7, leading to an almost complete collapse of the polymer chain onto the surface. Indeed, beyond a given level of adsorption charge reversal can occur in that the clay-polycation system effectively behaves as an anion exchanger. Little adsorption occurs with negatively charged polymers (polyanions) due to initial charge repulsion between the polymer and the clay surface. Acid pH, a high ionic strength, and the presence of polyvalent cations in the system enhance and promote polyanion adsorption. Uncharged polymers and polycations can enter the interlayer space of expanding 2:1 type layer silicates but polyanions generally fail to intercalate. The interactions of clays with biopolymers, such as proteins, nucleic acids, and polysaccharides, can be rationalized in similar terms. When intercalation occurs, the interlayer biopolymer is further stabilized against microbial (enzymatic) degradation giving rise to practical applications of clay-polymer complexes as flocculants and soil conditioners. Polyanions are effective as flocculants because of their large "grappling distance," whereas uncharged polymers are better suited as soil conditioners because they can spread over adjacent clay/soil particle surfaces like a coat of paint.

T. J. Pinnavaia

Papers

Porphyrin intercalation in mica-type silicates

Aromatic radical cation formation on the intracrystal surfaces of transition metal layer lattice silicates

Porphyrin intercalation in mica-type silicates