# Anti-Fouling Properties of Phosphonium Ionic Liquid Coatings in the Marine Environment
## 4 Conclusions  
This work has provided proof-of-concept results for the application of phosphonium ion gel films to anti-fouling coatings for marine environments. New tributyl(vinylbenzyl) phosphonium docusate and trioctyl(vinylbenzyl)phosphonium docusate monomers were synthesized and used to produce ion gel coatings on epoxy-treated stainless-steel coupons. Ion gels of polymerized trioctyl(vinylbenzyl)phosphonium docusate performed best, with standardized foul ratings only reaching FR30 after 63 days. To emphasize, this foul rating performance was obtained in coastal waters and protected from light and water currents using anti-fouling coatings that did not contain biocide components.  
The combination of strongly hydrophobic cation and anion components provides a possible route to metal-free anti-fouling coatings that would reduce the heavy metal contamination that currently results from anti-fouling coatings containing copper compounds.  
The leading performance of the trioctyl(vinylbenzyl)phosphonium docusate ion gels was attributed to the increased hydrophobicity of the surfaces, with advancing contact angle measurements showing water contact angles >(83-131)\({}^{\circ}\), up to double those of ion gels containing copper(II) oxide (<14-87\({}^{\circ}\)) and tributyl(vinyl benzyl)phosphonium docusate (<25-57\({}^{\circ}\)). It was found that the addition of copper(II) oxide as a biocide and free ionic liquid contributed to much lower hydrophobicities and correlated to significantly worse anti-fouling performance.  
Interestingly, fluorescence assays indicated that marine anti-fouling performance correlated with the inhibition of Gram-positive bacteria. Further experimental work should aim to determine the link between the inhibition of the initial bacterial biofilm and long-term marine anti-fouling performance.  
This work illustrates the promise of phosphonium-containing ion gel materials and provides clear design strategies for the development of phosphonium-containing ion gel materials as anti-fouling coatings for marine environments.  
The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/polym15183677/sl](https://www.mdpi.com/article/10.3390/polym15183677/sl)]([https://www.mdpi.com/article/10.3390/polym15183677/sl](https://www.mdpi.com/article/10.3390/polym15183677/sl)). The 34-page supporting information contains 42 figures, 13 tables, the literature search methodology and results and raw experimental data for the characterization of phosphonium docusate ionic liquids, the polymerization reaction, advancing contact angle measurements, fluorescence assays, disc diffusion assays, and full marine anti-fouling results [47].  
Conceptualization, S.K.B. and M.G.C.; methodology, S.K.B., R.F., M.G.C. and H.R.; and validation, S.K.B. and M.G.C.; formal analysis, S.K.B., R.F. and M.G.C.; investigation, S.K.B., B.J.M. and M.G.C.; writing--original draft preparation, S.K.B. and M.G.C.; writing--review and editing, all authors; supervision, M.G.C. and K.D.S.; project administration, M.G.C.; funding acquisition, K.D.S. and M.G.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the NZ Product Accelerator Master's scholarship.  
Not applicable.  
The data presented in this study are available as part of the Supplementary Materials.  
The authors would like to express their gratitude to [PERSON] from Defence Technology Agency, New Zealand for helping arrange the field testing of the samples, [PERSON] for training SKB in making contact angle measurements, [PERSON] for assisting S.K.B. with processing graphical data, [PERSON] for assisting with NMR and mass spectrometry data acquisition, and [PERSON] for assisting with 3D surface profiling. All authors gratefully acknowledge the New Zealand Product Accelerator for providing a Masters scholarship to [PERSON] and M.G.C. thanks the MacDiarmid Institute for providing funding to support this research.  
The authors declare no conflict of interest.