In the first part of this section, the characterizations of functionalized CNTs are described. In the second part, the solubility of functionalized CNTs in various solvents, the corresponding HSPs, and their correlation with grafted functional groups are described.
3.1. Diazonium-Mediated Modification
MWNT samples functionalized by diazonium (CNT–COOH, CNT–CH
2–COOH, CNT–OH, CNT–CH
2OH, CNT–C(O)–CH
3, and CNT–O–CH
3) were characterized using X-ray photoelectron spectroscopy spectra and changes to their HSPs. In the case of characterization using XPS, difficulties were created by the fact that functionalization does not introduce a new element to the modified sample. Therefore, the fit of the C1s peak was used to assess functionalization efficacy. The C1s fit table (
Table 2) was established using previously reported values [
21,
22] and analysis of standard samples such as those containing regraphitized MWNTs. The element composition and components of the C1s peaks for the six samples are summarized in
Table 3 and
Figure 2, respectively.
Figure 2 shows the C1s high-resolution XPS spectra for the six samples and contributions of the various components.
Both the element composition and contributions of various components of the C1s peak were highly reproducible, as demonstrated by small standard deviations.
Table 3 shows the amount of increased oxygen in the six samples after functionalization. In samples where the side chain was separated from the aromatic core by a methylene group, a noticeable nitrogen peak was seen. High-resolution spectra of the N1s peak in those samples (CNT–CH
2–COOH and CNT–CH
2–OH) showed an asymmetric peak at 400.0 eV. Diazonium bridges exhibited a binding energy of 398.5 eV (
Figure 3, standardization using methyl orange), which was close to that of amines (399.5 eV, depending on the side chain). Therefore, the conclusion that the N1s peak originated from a diazonium bond was derived from (i) the asymmetry of the peak, which is characteristic of delocalized electrons, and (ii) the absence of a decrease in intensity after further washing, which removes adsorbed molecules.
Figure 4 shows an increase in the Csp
3 component in all functionalized samples. This increase was consistent with diazonium-mediated functionalization, as this modification transforms sp
2 carbons of the CNT sidewall into Csp
3 carbons [
23].
When assessing functionalization via C1s peak fitting, it was found that MWNT–COOH and MWNT–CH2–COOH samples contained a C(O)–O component (2.6% and 2.8%, respectively). Additionally, the Csp3 component was larger in the CNTs functionalized with –CH2–COOH than in their –C(O)–O counterpart, which was consistent with the presence of an additional methylene group.
For CNTs functionalized with –OH and –CH2–OH, the C–O component (285.8 ± 0.2 eV) increased by 4.4% and 4.5%, respectively, with a larger sp3 component in the case of –CH2–OH functionalization. This confirmed the grafting effectiveness for both side chains.
The MWNT–C(O)–CH3 sample contained larger C=O (1.3%, 286.9 eV) and C(O)–O (1.2%, 289.2 eV) components. This confirmed the presence of a highly oxidized functional group (a ketone) on the MWNT surface. An increase in Csp3 contribution was observed due to the presence of a methyl group on the ketone molecule. CNTs modified with an –O–CH3 side chain displayed high C–O contribution (5.6%, 285.8 eV), which is characteristic of ether.
HSPs are a means to describe interactions between a solute and a solvent. Each compound is described by three parameters: A dispersion parameter, accounting for dispersion forces (δ
d); a polar parameter (δ
p), accounting for dipole–dipole interactions; and a hydrogen-bond parameter (δ
h). A solute and solvent that are characterized by close parameters are compatible and thus soluble in each other. By contrast, compounds with largely different parameters are immiscible. Therefore, it is possible to determine HSPs of an unknown compound by testing its solubility in several solvents with known HSPs. Some solvents appear to be “good solvents.” On a tridimensional plot (using the three parameters as axes), “good solvents” are encompassed by a sphere with a center corresponding to the solubility of the compound with unknown HSPs. HSP theory [
20] was proven to be applicable in the case of CNTs (crude and functionalized), and could therefore be used to characterize functionalized CNTs. Modified CNTs should be characterized by different HSPs from those of the starting material, and each functionalization produces CNTs with different parameters. HSPs were determined for six modified samples (
Table 4), and the corresponding solubility spheres are shown in
Figure 5.
The six modified samples exhibited HSPs (
Figure 6 and
Table 4) that differed from those of nonfunctionalized MWNTs (δ
d = 19.7 MPa
1/2, δ
p = 6.2 MPa
1/2, and δ
h = 4.2 MPa
1/2). COOH- and CH
2–COOH-modified CNTs were characterized by similar HSPs. They showed strong polar parameters and moderate hydrogen bond parameters, which was consistent with the grafting of a carboxyl-like functional group. Comparatively, CNTs modified with MWNT–OH and MWNT–CH
2–OH displayed HSPs with greater hydrogen-bond contribution but lower (at least for MWNT–OH) polar contribution, which was consistent with an alcohol-like functional group. CNTs modified with ketone and ether side chains were characterized by lower polar and hydrogen-bond parameters compared with samples functionalized with alcohol or carboxyl functional groups. As shown in
Table 5, the HSPs of MWNT–C(O)–CH
3 and MWNT–O–CH
3 were consistent with the tabulated values for ketones and ethers. Ketones are characterized by a greater polar parameter than the hydrogen-bond parameter. The ether showed low polar and hydrogen-bond parameters, with δ
p being lower than δ
h [
19]. A comparison between
Table 4 and
Table 5 clearly shows that CNTs modified by MWNT–C(O)–CH
3 and MWNT–O–CH
3 were characterized by HSPs that were very close to those of aromatic molecules with corresponding functions (acetophenone and anisole, respectively).
On the basis of XPS and HSP characterizations, it could be concluded that the six different modifications were successful. Therefore, those samples were used to test the selectivity of silylesterification catalysts.