## 3 Results and Discussion
### Evaluation of Nitrate and Peroxide Depletion in PAWs Using UV-Vis Spectroscopy After Hair Immersion  
The UV-Vis spectra in Figure 6 illustrates the chemical changes in PAW following immersion of hair samples, highlighting differential interactions between PAW-N and PAW-P and the four hair types. Figure 6a,b correspond to PAW-N, with Figure 6b providing a magnified view of the 250-400 nm region to better resolve nitrite-related changes and potential byproduct formation. Figure 6c displays the spectra for PAW-P and its interaction profile with the same hair types. Figure 6: UV–Vis Spectra of PAW-N and PAW-P Before and After Hair Immersion for Different Hair Types. (**a**) UV–Vis spectra of PAW-N before and after immersion of hair samples, showing decreased absorbance near 210–220 nm (nitrate) and 280–300 nm (nitrite), indicating RONS consumption, especially for wavy and coily hair (Samples 1 and 4). (**b**) Magnified view of the 250–400 nm region, highlighting nitrite depletion and byproduct formation, particularly in Sample 3. (**c**) UV–Vis spectra of PAW-P showing peroxide depletion near 200–220 nm, with more pronounced reduction for porous hair types. In Figure 6a, the UV-Vis spectrum of PAW-N prior to hair immersion exhibits a strong absorbance band in the 200-220 nm range, characteristic of NO3\({}^{-}\). After interaction with the hair samples, only slight reductions in absorbance are observed across all spectra, suggesting limited but detectable consumption of reactive species. The changes are subtle, indicating that most of the RONS remain in solution post-interaction, likely due to the short contact time or limited diffusion into the hair cuticle. Although slight variations are present, no substantial difference in spectral attenuation is observed among the different hair types, and thus no definitive correlation with porosity or surface area can be drawn from this specific spectral region alone. The magnified spectral region presented in Figure 6b reveals distinct differences in absorbance between 250 and 400 nm following hair immersion. Sample 3 (straight hair) exhibits the most pronounced increase in absorbance near 300 nm, suggesting the formation of secondary oxidation byproducts, potentially nitro-compounds or aromatic degradation residues. This spectral shift may indicate localized oxidative reactions at the hair-liquid interface, reflecting heterogeneous chemical reactivity even within similar morphological types. Sample 4 (coily/kinky hair) also shows a moderate increase in this region, supporting the hypothesis that porous or chemically heterogeneous hair structures are more susceptible to oxidative interactions with nitrate-rich PAW-N. The overall trend in this zoomed-in region complements the SEM observations (Figure 3), where samples 3 and 4 showed more visible cuticle disruption, aligning with the presence of degradation products in the surrounding medium. Figure 6c displays the UV-Vis spectra of peroxide-rich PAW-P before and after hair immersion, focusing on the spectral region between 190 and 280 nm. The initial PAW-P spectrum exhibits a broad absorption band centered around 200-220 nm, characteristic of hydrogen peroxide and related reactive oxygen species. After contact with hair, all samples show a discernible decrease in absorbance, indicating peroxide consumption during chemical interaction with the hair surface. The most significant absorbance reductions are observed for Samples 1 and 4 (wavy and coily hair types), suggesting greater peroxide uptake, likely due to their higher porosity and larger surface area, which facilitate enhanced diffusion and redox reactions. In contrast, Samples 2 and 3 (straight hair) exhibit less pronounced changes, consistent with reduced surface reactivity and limited oxidative interaction. These findings corroborate the SEM observations (Figure 4), where wavy and coily samples displayed greater morphological alterations, and support the hypothesis that hair type and structure play a critical role in mediating the chemical effects of PAW-P treatment.