# Anti-Fouling Properties of Phosphonium Ionic Liquid Coatings in the Marine Environment
## 3 Result and Discussion
### Field Testing Results
#### 3.2.1 Best-Performing Anti-Fouling Phosphonium Ion Gels  
Figure 3 directly compares the anti-fouling performance of the best trioctyl and tributyl phosphonium ion gels. Visual observation shows that the P\({}_{888}\) formulations outperform both the P\({}_{888}\) with biocide and the P\({}_{444}\) formulations. Anti-fouling performance was quantified using the fouling rate (FR) standard, as guided by the Naval Ships' Technical Manual Chapter 081 (Figure 4) [43]. The leading ion gel material produced in this work, P\({}_{888}\)[84,10,0], displays only 40% fouling coverage after 43 days submersed, with a maximum FR of 30. By day 63, the surface was 100% covered, but only led to an FR of 30. In contrast, the epoxy controls showed 80% coverage after 43 days and 100% coverage after 63 days with established tunicate, increasing the FR to 100. In contrast, P\({}_{888}\)[65,28,0] and other trioctylphosphonium samples that included biocide exhibited 100% coverage at FR 40 after 92 days. However, trioctylphosphonium samples without biocide (e.g., P\({}_{888}\)[93,0,0], P\({}_{888}\)[84,10,0], and P\({}_{888}\)[48,48,0]) showed a higher fouling rate, FR 100, at day 92. Although these performances are inferior to commercial anti-fouling coatings (which contain copper biocides) [10], this work aims to understand the factors that govern the anti-fouling performance of phosphonium ion gel materials. Discovering that the presence of biocide resulted in higher fouling coverage and FR after equivalent time prompted further investigation. 2.2 Effect of Hydrophobicity and Toxicity (via Alkyl Chain Length and Biocide Inclusion) on Anti-Fouling Performance  
The fouling coverage and FR performance of all ion gels tested in this study are summarized in Figures 4, 7 and 10 and shown in Figures 5, 8 and 9. The superior performance of the P\({}_{888}\) ion gels (Figure 8) vs. the P\({}_{444}\) ion gels (Figure 5) is clearly shown by comparing the FR graphs (Figures 4 and 7). The P\({}_{444}\) formulations received rapid colonization by fouling species, similar to the control sample of bare epoxy coating. Although the P\({}_{444}\) showed a somewhat reduced fouling after 56 days, all P\({}_{444}\) formulations were contaminated with excessive tube worm growth. In contrast, no P\({}_{888}\) formulations were contaminated with matured tube worms even after 63 days (Figure 6), although the FR had reached 100% surface coverage and parts of the coupons were judged as having an FR of 30, which is when cleaning for biosecurity reasons is required. Figure 10: Field test results of trioctyl(vinylbenzyl)phosphonium decusate with copper oxide in water. The samples, labeled P888[4,b,c], were graphed in the order of increasing amounts of free ionic liquid in the formulation. The superior performance of the more hydrophobic P888 materials was expected from the 2003 study conducted by [PERSON] et al. In that work, the authors examined the antibacterial and antifungal effects of imidazolium ionic liquids with alkyll substituent lengths ranging from 6 to 16 [23]. Their results showed that increasing alkoxy chain lengths was more effective for antimicrobial inhibition, up to a limit of 12-14 carbon atoms, which was the most effective carbon chain length against bacteria, fungi, and rods. Similarly, [PERSON] et al., 2022 studied the antibacterial properties of trialkylphosphonium ionic liquid by varying the alkyl chain length [32]. In contrast, it was a surprise to observe that coatings containing the copper(II) oxide biocide (Figure 10) had worse anti-fouling performance than the equivalent plain ion gels (Figure 4). Copper(I) and copper(II) oxides are often added to anti-fouling coating to inhibit the microbial growth that makes up the first stages of conditioning film and bacterial biofilm that initiate the fouling process (Figure 1). In this work, coatings containing P888 ion gels without biocides (Figure 8) showed superior anti-fouling performance compared to P888 ion gels with biocides (Figure 9).