# Deposition Efficacy of Natural and Synthetic Antioxidants on Fabrics
## 3 Results and Discussion
### Antioxidant Activity Test in Solution  
Several available methods to test the antioxidant activity of different compounds are reported in the literature [33; 34; 55; 56; 57; 58]. Among them, those based on direct measurement of radical scavenging capability are the most widely used ones. They are less expensive and less time-consuming in comparison to the inhibition procedures in which the inhibition of oxidative damage of a target molecule is measured in the presence of the examined antioxidants [59; 60]. Based on what was found in the literature, the currently most common methods for analysing the antioxidant capability of compounds deposited on fabrics use chromogen compounds of a radical nature are the stable free radicals 2,2-diphenyl-1-picrylhydrazyl (DPPH\({}^{\bullet}\)) and 2,2'-azino-bis (3-ethylbenzothazoline-6-sulphonic acid) (ABTS\({}^{\bullet+}\)) [61; 62; 63; 64; 65; 66]. Both display a remarkable stability in specific assay conditions but also present several differences in their response to the antioxidants [67; 68]. DPPH is a stable and commercially available organic free radical with a peak of absorbance at \(\lambda=515\) nm. It undergoes a decrease in its absorption when reduced by an antioxidant or a free radical species. While ABTS\({}^{\bullet+}\) can be solubilised in aqueous and organic solvents, in which the antioxidant activity can be measured due to the hydrophilic and lipophilic nature of the compounds in samples, on the contrary, DPPH cannot be dissolved in aqueous solvents but only in organic ones, which is an important limitation when interpreting the role of hydrophilic antioxidants. Moreover, as it is reported in literature [68], a possible colour interference of DPPH with the plant material extracts can lead to underestimation of antioxidant activity. Therefore, in order to minimise a potential interference with the pine tannins being studied and thus with the aim to obtain more reliable data, the antioxidant activities of cotton fabrics were investigated using the quick and easy colorimetric procedure based on the ABTS assay, also characterised by measurement simplicity and short experimental time. Preliminarily, we determined the antioxidant activity of two different pine tannins and of BHT, M\(t\)BHPP and octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propanoate in solution by carrying out the ABTS\({}^{\bullet+}\) decolourisation assay; results are reported in Table 3. Due to the solubility issues in water observed for **PTT-AW1** (Figure 2D), the antioxidant activity measurements were repeated using aqueous ethanol (30% (v/v) water) as a solvent for preparing the starting antioxidant solution (Figure 2B). **PST-sW1** as water soluble extract (Figure 2C) gave reliable data in the standard ABTS assay run with water as only solvent. Since **PST-sW1** was equally soluble in the aqueous ethanol necessary for solubilising **PTT-AW1**, it was used for creating data for comparison for estimating the differences in observed antioxidant activity as a function of the test used. Figure 2: Representative starting tannin solutions used in the ABTS\({}^{\bullet+}\) decolourisation assay; aqueous ethanol (70%) sample solutions (**A**,**B**,**E**) and aqueous sample solution (**C**,**D**). Overall, a slightly better antioxidant activity was observed in the case of the use of starting pine tannin preparations in aqueous ethanol instead of water (Table 3, entries 1 and 2) a fact that could be ascribed to an increased solubility in this solvent system of those components responsible for the radical scavenging capability. Therefore, the use of aqueous ethanol for both the samples allowed bringing in solution a greater quantity of components responsible for the antioxidant activity. Noteworthy, after centrifugation of the starting liquid suspensions of tannins (Figure 2D,E) and performing the measurements only with the supernatants, respective AAE values essentially did not change (Table 3, entries 1 and 2).