ABSTRACT
Surface functionalization can change the accessibility and affinity of CNTs to adsorbate molecules. The former can be attributed to the change in the aggregation due to increase in the wettability of CNTs surfaces and/or the number of available adsorption sites by either opening (removal of the end tips of CNTs by oxidation) or blocking (with functional groups or water clusters formed around the clusters formed around the functional groups) of the inner cavities. 4.1.1 Adsorption of Aromatic Compounds on CNTs From
Aqueous SolutionCyclic aromatics consist of one or more carbon rings, the planar arrangement of its atoms creates the conditions for filling the in-plane sp2 hybrid orbitals of C and creating a resonant bond shared by all six C atoms. Besides this aromatic bond, there is a set of π orbitals transverse to the molecular plane. Each carbon atom in a CNT has a π electron orbit perpendicular to the CNT surface. Therefore, the aromatic molecules/CNT systems can be viewed as two interacting π systems. Hence, aromatic compounds interacting with CNTs are of particular interest. Noncovalent sidewall CNTs functionalization with aromatic organic molecules has attracted increasing attention. Aromatic compounds have relatively higher sorption affinity to CNTs than nonaromatics. Moreover, aromatic compounds represent an important group of organic contaminants in various environmental matrixes and structural components of large organic molecules in biological system. An increasing number of theoretical (calculation or simulation) and experimental studies have been carried out to understand systematically the interaction between CNT and aromatic compounds. However, researches in this area are still fragmentary and not complete enough for making clear conclusion. By far, the studies indicate that the interactions between CNTs and aromatic compounds can be influenced by many factors (such as property of adsorbate and CNTs, and environmental conditions). 4.1.1.1 Effect of number of benzene ring
Zhao et al. have suggested that the hybridization of π electrons between nanotubes and molecules is the common feature of all aromatic molecules adsorbed on CNTs. The π-π bond has been recognized as a dominating interaction force for the adsorption of aromatic compounds on CNTs. Consequently, sorption affinity of chemicals
containing benzene rings by CNTs increase with increasing number of aromatic rings, e.g., the adsorption affinity of cyclohexanol, phenol, phenylphenol, and naphthol to CNTs follow the order of cyclohexanol < phenol < 2-phenylphenol < 1-naphthol; for nonpolar PAHs, the adsorption coefficient (K) is found to increase with the benzene ring numbers, for instance, naphthalene < phenanthrene < pyrene. 4.1.1.2 Effect of organic functional groups of aromatic
compoundsBenzene derivatives are obtained by substituting one or more hydrogen atoms in the benzene ring with a functional group such as hydroxyl, methyl nitro, or amino. These derivatives have a common π structure from the hexagonal carbon ring, but they have distinct properties, such as permanent dipole moments and different electron affinities from the functional groups. The most widely recognized influence of organic chemical functional groups on organic chemical-CNT interactions is on the electron donor-acceptor (EDA) π-π interaction. Electron-donating substituent on an aromatic ring can strengthen the π-π interaction between organic compounds and CNTs. The graphene surfaces on CNT sidewalls are of high electronic polarizability, and CNTs may act as amphoteric adsorbate π-acceptors to the electron-rich graphene surface area near edged and π-donors to the central regions being relatively electron poor. The substituents on benzene ring provide different inductive and resonance effects. Benzene is a weak charge donor, and the donor strength can be increased with the substitutes of hydroxyl group, methyl group, and amino group on benzene ring. Pan et al. report that the K/KHW values of endocrine disrupting chemical (EDCs) 17a-ethinyl estradiol (EE2) and bisphenol A (BPA) on MWCNT were six order of magnitude higher than PAHs (phenanthrene and pyrene) due to the presence of hydroxyl groups (π-electron donors) on EDCs, although the number of benzene ring of EE2 and BPA was lower than that of PAHs. Both Wang and Chen observed that the Kd of naphthalene (ca. 6000 L/Kg) was about three times lower than 1-naphthol at a similar equilibrium concentration, confirming the importance of –OH substitution on the interaction of aromatics and CNTs. Amino group (–NH2) is a strong electron-donating group (stronger than –OH), the unshared pair of electrons of nitrogen can result in strong electron conjugation with π-electron in the benzene rings, making the benzene rings electron rich. As a result, the adsorption affinity of 1-naphthylamine on CNTs is significantly higher than those of 1-naphthol and naphthalene. Both
chlorine atoms and nitro group exhibit strong electron-withdrawing ability and cause the chloro-or nitro-substituted benzene rings to be electron depleted and hence function as π-electron acceptors, which interact strongly with the π-electron rich sites (π-electron donors) of the graphene surface of CNTs. Therefore, the adsorptions of nitroaromatic and chloroaromatic compounds are much stronger than those of benzene and toluene. Generally, the relative electron-donating trends of the alternative groups decrease with the order of NH2 > OH > OCH3 > Cl > NO2. As a result, –NO2, –Cl, or –CH3 groups on benzene, phenol, or aniline will enhance their adsorption on CNTs and follow the order nitro group > chloride group > methyl group. The number of groups could significantly influence the adsorption, showing that the substitution with more groups has the higher adsorption affinity. For example, adsorption affinity of chlorobenzenes on CNTs increases with the chlorine atom in benzene ring, and follows the order 1,2,4,5-tetrachlorobenzene > 1,2,4-trichlorobenzene > 1,2-dichlorobenzene > chlorobenzene. For chlorophenols, the adsorption ability of 2,4-dichlorophenol is much higher than that of 2-chlorophenol and 4-chlorophenol. The position of groups on phenol and aniline can also influence the adsorption; however, adsorption effects from group position are weaker than those from the group types. The group position effects on adsorption cannot be interpreted directly by the group position. Besides hydrophobic interactions and EDA π-π interaction, hydrogen bonds and electrostatic interactions are also important mechanism for the adsorption of organic compounds on CNTs. The contribution of hydrogen bonding to aromatic chemical adsorption on CNTs is generally considered to be negligible by most researchers. However, Yang et al. developed a linear quantitative relationship combining Polanyi theory-based Dubinin-Astakhov (DA) model parameters with solute solvatochromic parameters to evaluate the adsorptive behaviors of nondissociated phenol and aniline substituents on CNTs. They proposed that H-bonding interactions played an important role on the adsorption of phenols and anilines by CNTs, where the solutes might act as hydrogen-bonding donors and the CNTs acted as the hydrogen-bonding acceptors. The π-electron polarizability might also be important for phenol or aniline adsorption, but its effects were weaker than that of hydrogen-bonding donor ability. Chen suggested that Lewis acid-base interaction was likely an extra important mechanism contributing to the stronger adsorption of 1-naphthylamine than 2-naphthol
to CNTs. Liu et al. found that cation organic dye AO showed much higher affinity to MWCNT than the other azo-containing dyes OG and PAN. The authors concluded that electrostatic attraction was likely to play the dominant role in the noncovalent interaction between dyes and MWCNT in aqueous environment. 4.1.1.3 Effect of surface chemical property of CNTs
Surface property of CNTs can affect the adsorption of aromatic-containing compounds. Researchers suggest that the presence of oxygen and hydrogen within the surface groups crucially affect the adsorptive properties of the adsorbent. Oxygen-containing functional groups can act as electron-withdrawing groups and localize the π-electron system of CNTs, lowering the dispersive force with π-electron of adsorbate. Additionally, the introduction of the oxygen-containing function groups is favorable for water molecules adsorption by means of hydrogen bonding, which can hinder the target molecules accessing to the surface of CNTs, and is unfavorable for the adsorption of organic analytes. Finally, the carboxylic, hydroxyl groups on CNTs surface can be ionized at high solution pH, thus make CNTs negatively charged and provide electrostatic repulsion to ionizable organic adsorbate such as phenol and its derivatives. Decreased adsorption capacity of xylene, resorcinol, pentachlorophenol, and aniline had been observed on the surface-functionalized CNTs. Cho et al. reported that a 10% increase in oxygen concentration led to a 71% decrease in maximum sorption capacity of naphthalene. The authors described the relationship between the increasing oxygen concentration on MWCNT and decreasing adsorption capacity of naphthalene using an equation: qad, max = 118-6.6(%O). They deduced that each additional percentage of surface oxides could lead to 5.9% sorption capacity reduction of naphthalene. They further predicted based on this equation that no naphtnalene sorption would be obtained with >18% surface oxygen on MWCNT. However, reverse effect of oxygen-containing groups on aromatic compounds adsorption by CNTs was also reported. In the study by Gotovac et al., the adsorption capacities of phenanthrene and tetracene on SWCNT were found to increase by 5-6 times after SWCNT was treated with nitric acid. They suggested that π-π-interacted phenanthrene or tetracene molecules on the SWCNT surface could be stabilized with the additional interaction with the carboxylic groups. Some researchers propose that the negatively charged
–COO-group is a strong electron donor, and can enhance the π-π EDA interactions of π-electron acceptors. In Chen and Zhu’s study, they observed that increase in solution pH did not affect adsorption of naphthalene but enhanced the adsorption of 1,3-dinitrobenzene and trinitrobenzene by 2-3 times. Lu et al. found that the adsorption capacity of BTEX on CNTs enhanced with the increase in the density of surface carboxylic groups. The authors supposed that the π-π EDA mechanism involving the carboxylic oxygen atom of CNT surface as the electron donor and the aromatic ring of BTEX as the electron acceptor was responsible for the uptake of BTEX by CNTs. 4.1.1.4 Effect of molecular configuration of adsorbate and
Molecular configuration determines the availability of different adsorption sites on CNTs. The effect of molecular configuration on the adsorption affinity of aromatic compounds to CNTs has been discussed by some researchers. The favorable adsorption states of phenanthrene (PNT) and planar tetracene molecules on SWCNT have been attributed to the so-called “bridge positions” (i.e., the PAH molecules aligned along the nanotube axis under an intensive π-π interaction between PAH and SWCNT surface). As the PNT and tetracene molecules are rigid, the longitudinally parallel external surface and interstitial channels of the SWCNT can provide more accessible adsorption sites for PNT or tetracene than the short, entangled MWCNT. Therefore, SWCNT show significantly higher adsorption capacities and site energies for planar compounds than the MWCNT. Whereas, for nonplanar BP and 2PP, the molecular configurations can be adjusted to better pack in the tubular spaces of MWCNT with diameter several times larger than their widths, hence smaller adsorption capacity and site energy difference for BP and 2PP between SWCNT and MWCNT was observed. Liu et al. have observed the structure-dependent interaction between organic dyes and CNTs. They found that molecular morphology was the dominant factor for the attachment of dyes on MWCNT. The molecules with large planar aromatic polynuclear structure (such as AO, AR, AN, and RB) were strongly adsorbed onto CNT sidewalls, because the planar molecules were easy to approach MWCNTs via a face-to-face conformation, which was favorite for π-π interaction between the conjugated aromatic chromophore skeleton and nanotubes. On the contrary, the nonplanar molecules were kept apart from MWCNT due to the spatial restriction, resulting in low π-π interaction with MWCNT.
