# Modified Kapok Fibers (_Ceiba pentandra_ ([PERSON]) for Oil Spill Remediation
## 3 Results and Discussion
### Characterization of Natural and Treated Materials
#### 3.2.3 Average Roughness (Ra) and AFM  
The Ra values for the kapok fibers in nature and under the different treatments are presented in Figure 8a; all groups displayed an average surface roughness under 15 nm. In nature, fibers presented an Ra value of 4.757 nm. The ones treated with NaOH (0.1 M), HCl (0.1 M), and at 60 \({}^{\circ}\)C resulted in roughness values of 4.404, 4.470, and 5.092 nm, respectively. These values do not significantly differ (\(p>0.05\)) compared to the Ra value of the fibers in nature, indicating that these conditions do not substantially alter the rugosity of the material. This result, nevertheless, is crucial to prove that the low-temperature drying procedures do not alter the surface functional groups (as seen in the FTIR spectra) or the microstructural morphology. The fibers exposed to harsh treatments, such as the acid or base treatment at 2.0 mol/L and thermal treatment at 120 \({}^{\circ}\)C, exhibited Ra values significantly higher than the natural fibers (\(p<0.0001\)). The highest roughness was observed when the fibers were treated with 2.0 mol/L of NaOH (12.35 nm), followed by 2.0 mol/L of HCl (9.735 nm), and the temperature of 120 \({}^{\circ}\)C (7.267 nm).  
The topographical images acquired by AFM that represent the Ra value of the groups analyzed are presented in Figure 8b-h. These images permit the visualization of the surface roughness of the fibers in nature and after the different treatments. As can be seen, the thermal treatment at 120 \({}^{\circ}\)C increases the surface roughness, corroborating the FTIR and SEM analyses. The 3D image showing the topographical features of the sample treated with 2.0 mol/L of HCl illustrates the presence of fissures along the fiber (Figure 8f). These fissures compromise the fiber capacity to retain diesel, reducing its sorptive power. Contrastingly, the fiber treated with 2.0 mol/L of NaOH suffered structural modifications with a sort of peeling of the waxy layer, which can be better seen in the SEM image. Previous works already attested that roughness could be associated with surface free energy, and the higher its value, the more superhydrophobic the system would be [29; 30].  
Notwithstanding, the present study proves that it is not possible to directly correlate the surface interaction with isolated properties, such as roughness; the sorption behavior relies, in fact, on a group of factors collaborating in synergy. Herein, the enhancement of rugosity lead to improvement in the sorption capacity only in the acid treatment, which leads us to infer that, under the conditions applied, protonation did not cause sufficient alterations in the morphology nor in the functional groups on the surface to hamper the sorptive capacity of the fiber. The loss of organic content in the thermal process as well as the mercerization caused by NaOH, however, built up roughness, but at the cost of exposing the hydrophilic cellulosic nucleus, which ultimately lead to the loss of the diesel oil sorptive capacity of the materials.  
Figure 8: Average roughness (Ra) of natural and post-treatment kapok fibers. (**a**) Average values of roughness (Ra) of all kapok fibers analyzed. (**b–h**) Topographical images obtained using AFM of the fibers: (**b**) in nature, (**c**) treated at 60 \({}^{\circ}\)C, (**d**) treated at 120 \({}^{\circ}\)C, (**e**) treated with HCl 0.1 mol/L, (**f**) treated with HCl 2.0 mol/L, (**g**) treated with NaOH 0.1 mol/L, and (**h**) treated with NaOH 2.0 mol/L.