## 3 Results and Discussion  
### Physicochemical Characterization of PAW-N and PAW-P  
The physicochemical parameters and RONS concentrations of PAW-N and PAW-P were assessed to elucidate the distinct chemical environments produced by the two plasma activation methods, and their potential implications for hair treatment applications. Parameters such as pH, conductivity, oxidation-reduction potential (ORP), total dissolved solids (TDS), salinity, and concentrations of hydrogen peroxide (H2O2), nitrate (NO3-), and nitrite (NO2-) were measured, using deionized (DI) water as a baseline, as summarized in Table 2.  
The most striking difference observed was the significant decrease in pH due to plasma activation. While DI water exhibited a neutral pH (7.70), both PAW-N and PAW-P became highly acidic, registering pH values of 2.15 and 2.54, respectively (Table 2). PAW-N demonstrated greater acidity, likely due to the elevated concentration of nitrate ions (NO3-), which are strongly acidic species. This behavior is consistent with previous findings by [PERSON] et al., who observed that the acidification of PAW is driven by the generation of reactive nitrogen species. These include nitric acid (HNO3) and nitrous acid (HNO2), which are formed from nitrate (NO3-) and nitrite (NO2-) under low pH conditions. Such acid generation is especially pronounced when PAW is produced by air-operated DBD systems rich in nitrogen oxides [19; 20]. Conversely, the significantly higher concentration of H\({}_{2}\)O\({}_{2}\) in PAW-P aligns with the findings of [PERSON] et al. They reported that pin-to-liquid discharges generate more reactive oxygen species (ROS), particularly hydroxyl radicals (\(\bullet\)OH), due to intense localized electric fields and filamentary microdischarges at the plasma-liquid interface [18]. These \(\bullet\)OH radicals recombine to form H\({}_{2}\)O\({}_{2}\). In this configuration, the relatively lower presence of reactive nitrogen species reduces the quenching of \(\bullet\)OH, thereby enhancing peroxide accumulation. Conductivity values provided further insight into the differing ionic environments of the two PAW types. DI water naturally showed minimal conductivity (4.223 \(\mu\)S/cm), whereas PAW-N presented significantly higher conductivity (6244 \(\mu\)S/cm), indicative of substantial ionic generation, predominantly nitrate ions from the coaxial DBD plasma system. PAW-P, generated via the pin-to-water reactor, exhibited a lower conductivity (1926 \(\mu\)S/cm), reflecting its predominantly neutral reactive species composition, including hydrogen peroxide. ORP also differed markedly between the PAW variants. DI water had a slightly reducing ORP (\(-\)48.9 mV), whereas PAW-N and PAW-P displayed significantly elevated ORP values of 274.0 mV and 257.4 mV, respectively (Table 2). These elevated ORP values highlight the strong oxidative environments created by plasma-generated reactive species. The slightly higher ORP of PAW-N aligns well with its nitrate-rich composition, whereas the ORP of PAW-P, although lower, remains strongly oxidative due to hydrogen peroxide. Such oxidative environments play a crucial role in breaking down organic residues and modifying the surface chemistry of hair fibers. TDS values indicated substantial differences in dissolved ionic content. DI water had minimal TDS (1.704 mg/L), consistent with its purity. PAW-N exhibited the highest TDS (2492 mg/L), correlating strongly with its nitrate-dominated ionic environment. Conversely, PAW-P showed a moderate TDS increase (768.7 mg/L), reflective of its lower ionic concentration. Salinity measurements echoed the trends observed in conductivity and TDS. DI water had negligible salinity (0.013), whereas PAW-N demonstrated significantly elevated salinity (3.699), indicative of higher ionic strength and thus greater potential for enhanced oxidative interactions with hair fibers. PAW-P presented with a much lower salinity (0.775), suggesting a comparatively milder chemical environment, which could be preferable for sensitive hair types.