There has not been a report of synthesizing oil adsorbent from MWCNTs by means of microemulsification. Fewer than fifty examples exist of CNT based nanocatalyst con-structed using microemulsion technique. Microemulsion modification was used to increase the hydrophobic properties of MWCNTs. Microemulsion is defined as ther-modynamically stable solutions, macroscopically homogeneous due to the dispersion of two immiscible fluids by the aid of surfactant, and it contains at least three compo-nents, specifically a nonpolar phase (often, oil), a polar phase (frequently, water), and a surfactant (Malik et al., 2012) (Figure 25).
Figure 25: Hypothetical phase regions of microemulsion systems (Malik et al., 2012) The µEMWCNTs, which are decorated with the surfactant molecules, have two dis-tinct parts. The hydrophilic polar head likes to come into contact with an aqueous phase
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and the nonpolar hydrophobic tail as it resists interactions with the aqueous phase (Figure 26).
Figure 26: Illustration for µEMWCNTs and the droplet of the microemulsion
Raw and functionalized MWCNTs have been used to study the hydrocarbon adsorp-tion from water. Different surface analytical techniques were used to study the changes in the structure of the MWCNTs after emulsification treatment and to correlate those changes with the adsorption results. The morphological studies revealed that the mi-croemulsification resulted in a decrease in the specific surface area of MWCNTs by 36% and reduction of the pore volume by 9 %. This result proves the successful incor-poration of functional groups into the MWCNTs' pores. However, the average pore size increased from 13.1 nm (MWCNTs) to 18.3 nm (µEMWCNTs). The thermograv-imetric studies showed that during microemulsification treatment fatty acid esters (es-terified miristic acid and lauric acid) were incorporated in 6.8% onto the surface of the MWCNTs, which influences the heat stability of the carbon nanotubes.
The structural analysis was carried out by XRD technique, and the surface investiga-tion was performed by Raman spectroscopy. It can be stated that the MWCNTs have a crystalline structure, and this structure is preserved after the functionalization. The Raman spectroscopic studies confirmed the interaction between the MWCNTs and the functional groups of the fatty acid esters used for the modification.
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It is necessary to examine briefly the mechanisms by which such hydrocarbon are formed, and what interactions they exhibit. Different mechanisms can be considered during organic chemical and MWCNT interactions, such as (i) hydrophobic interac-tions; (ii) π–π bonds, and (iii) electrostatic interactions, (v) hydrogen bonding and mes-opore filling (Das et al., 2018). The sorption mechanism is supposed to be different for different types of organic chemicals (such as polar and nonpolar chemicals). π- π in-teractions are a type of non-covalent interaction that involves π systems. Kar and co-workers have investigated the nature of interactions between aromatic systems and CNTs, their findings so far have demonstrated that π−π stacking configurations are more strongly bound than CH---π analogues (Kar et al., 2008). From this perspective, the controlled noncovalent functionalization of MWCNTs using microemulsion of-fered the possibility of attaching the hydrocarbon chain using van der Waals and π-π stacking forces with only minor perturbations of the electronic network of the tube.
In this context, toluene was chosen as a model hydrocarbon to propose an adsorption mechanism. The adsorption affinity of toluene by µMWCNTs increases by -C=O and aromatic π- π bonds and/or by aromatic –OK+ substitution. The sorption of the toluene on the surface of the µEMWCNTs can be given, as depicted in Figure 27. It should be noted that the mobility of adsorbed toluene on the tube surface can be considered in terms of rotating, tilting, and sliding. Interestingly, the suggested mechanism in this study has found to be in good agreement with the earlier adsorption mechanisms of organic pollutants on MWCNTs suggested by Pourzamani and co-workers (Pourzamani et al., 2015).
Figure 27: The mechanism of the surface functionalization and sorption of toluene over µMWCNTs
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Once again, however, the way in which the adsorption mechanism is proposed remains quite unclear. Due to the possibility of several interactions occurrence at one time, depending on the type of hydrocarbon and the type of functional group on the MWCNTs surface. The principal observation from all experimental studies of hydro-carbon molecules and fragments occupy clearly defined sites on adsorbent surfaces. In essence, the noncovalent interactions of saturated and unsaturated hydrocarbons such as cation−π, π − π, and CH···π with carbon nanomaterials is ubiquitous and vital in explaining several hydrocarbon adsorption mechanisms.
Within this framework, toluene removal efficiency by UV-Vis over µMWCNTs sup-ported the notion that the microemulsification is a suitable method for enhancement of the adsorption capacity of pristine MWCNTs. By using toluene as a model hydrocar-bon, the removal efficiency of µMWCNTs was enhanced to reach up to 90%; these results are in line with the outcome of the TOC measurements. Kinetic studies show that adsorption of toluene obeys a pseudo-second-order model. The adsorption capac-ity of µMWCNTs increased by increasing the temperature.
Undecane used as another hydrocarbon to investigate the potential of µMWCNTs as hydrocarbon adsorbent. Based on the adsorption tests carried out by TOC, a significant increase in undecane adsorption was observed from 32% to 79-83% as a consequence of the surface modification. It is worthy of mentioning that the values obtained for hydrocarbon removal efficiencies by TOC and GC measurements are in harmony with each other. The obtained results confirm that the MWCNTs could earn a significant potential in hydrocarbon depollution control of waters.
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