It has been speculated that the catechol (1,2-dihydroxybenzene) functionality of marine mussels is responsible for its strong and versatile adhesion on various wet surfaces. To elucidate features of this adhesion, we performed a periodic density functional theory calculation for catechol adsorption on silica surfaces. We obtained its binding energy and geometry on two representative hydroxylated surfaces of cristobalite, which mimic amorphous silica. Catechol strongly adhered to both surfaces by making three or four hydrogen bonds. Catechol achieved versatility in adhesion via torsion of its hydroxyls. The binding energy of catechol, which amounts to 14 kcal/mol, was larger than that of water, irrespective of the surface. With the inclusion of dispersion interaction, the binding energy of catechol further increased up to 33 kcal/mol, and its preferential adsorption over water became evident. Both the hydroxyls and phenylene ring of catechol contribute to its strong adhesion due to hydrogen bonds and dispersion.
Mussels have a remarkable ability to bond to solid surfaces under water. From a microscopic perspective, the first step of this process is the adsorption of dopa molecules to the solid surface. In fact, it is the catechol part of the dopa molecule that is interacting with the surface. These molecules are able to make reversible bonds to a wide range of materials, even underwater. Previous experimental and theoretical efforts have produced only a limited understanding of the mechanism and quantitative details of the competitive adsorption of catechol and water on hydrophilic silica surfaces. In this work, we uncover the nature of this competitive absorption by atomic scale modeling of water and catechol adsorbed at the geminal (001) silica surface using density functional theory calculations. We find that catechol molecules displace preadsorbed water molecules and bond directly on the silica surface. Using molecular dynamics simulations, we observe this process in detail. We also calculate the interaction force as a function of distance, and observe a maximum of 0.5 nN of attraction. The catechol has a binding energy of 23 kcal/mol onto the silica surface with adsorbed water molecules.
Carbon nanotubes (CNTs) are known as nano-architectured allotropes of carbon, having graphene sheets that are wrapped forming a cylindrical shape. Rolling of graphene sheets in different ways makes CNTs either metals or narrow-band semiconductors. Over the years, researchers have devoted much attention to understanding the intriguing properties CNTs. They exhibit some unusual properties like a high degree of stiffness, a large length-to-diameter ratio, and exceptional resilience, and for this reason, they are used in a variety of applications. These properties can be manipulated by controlling the diameter, chirality, wall nature, and length of CNTs which are in turn, synthesis procedure-dependent. In this review article, various synthesis methods for the production of CNTs are thoroughly elaborated. Several characterization methods are also described in the paper. The applications of CNTs in various technologically important fields are discussed in detail. Finally, future prospects of CNTs are outlined in view of their commercial applications.
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