# Anti-Fouling Properties of Phosphonium Ionic Liquid Coatings in the Marine Environment
## 1 Introduction  
Marine biofouling is defined as the undesirable accumulation of microbes, algae, and other marine animals on a submerged substrate [1]. When a non-toxic material is submerged in seawater, it will inevitably experience fouling to some extent. Thus, biofouling affects stationary underwater structures, ships, and other marine vessels [2]. Marine biofouling causes economic damage through the translocation of pests and invasive species, faster material degradation, reduced asset lifetime, and increases marine vessels' fuel consumption by up to 40-50% [3; 4; 5]. The primary method to prevent (or reduce) biofouling is to apply anti-fouling coatings to the underwater surfaces of stationary objects and mobile vessels. Anti-fouling solutions are most commonly paints containing a chemical biocide, which can be applied to the outer surface of vessel hulls [1; 6; 7]. Anti-fouling coatings primarily act via the slow release of toxic components that prevent the initial adhesion and colonization stages of biofouling (Figure 1). Tributyl tin (TBT) [7], SLIPS coatings [8; 9; 10], biofilm-based ionic liquid coating [11], and hydrogels are some examples of current anti-fouling materials. These biocides also confer a 'halo' defence, inhibiting biofouling in an area around the coating surface [9]. Seahawk Cukote [13], TotalBoot JD select (ablative) [13], Pettit Odyssey Triton (ablative) [13], Carboline Sea-barrier 1000 [14], and Carboline Sea-barrier 3000 [13] are examples of some commercially available anti-fouling paints. Although the most effective for preventing fouling, the biocides in these coatings have significant off-target environmental impacts. For example, the International Maritime Organization (IMO) banned TBT-based self-polishing co-polymers in 2008 because of the accumulation of organic tin compounds in fish, seabirds, and other marine species [7]. A second class of anti-fouling coatings comprises slippery liquid-infused porous surfaces (SLIPSs), which act by creating a slippery surface that limits the strength of biofouling adhesion. SLIPS materials include SLIPS(r) Foul ProtectTM Marine Addictive, SLIPS(r) DolphinTM Bottom Paint [10], and AkzoNobel Intersleek 1100 SR [15]. SLIP materials are created by infusing microporous membranes with liquid lubricants [16; 17]. Fouling species are detached when vessels are underway due to the high shear forces cleaving the foulants from the hull. While SLIPS coatings are viable for mobile vessels that spend a large proportion of time underway at sea, they are ineffective for recessed areas, such as sea chests, that do not experience high shear flows. Alongside the need for high shear flow to remove attached species, another limitation of a SLIPS is that it only prevents fouling on the surfaces where the SLIPS coating is applied. Unlike chemical biocides, SLIPSare not specifically designed to eliminate or repel attached or approaching microbes. As a result, fouling species can remain viable in the surrounding water and adhere to any surface not protected by SLIPS coating [16; 18; 19]. Consequently, any mechanical damage to the coating surface, e.g., during docking, anchoring, and encountering floating debris, will expose adhesion sites for fouling to occur [9]. Ionic liquids (ILs) and ion gels have yet to be widely explored for marine anti-fouling (Supplementary Section S1) [20; 21; 4]. For context, ionic liquids are salts that melt at or around room temperature (25 \({}^{\circ}\)C) [22; 23]. Of interest to the anti-fouling application, ionic liquids can form ion gels similar to hydrogels but with surface chemistry and toxicity that can be tailored through the chemistry of the ionic liquid. There already exists significant academic and industrial interest in developing ionic liquids for preservative agents, disinfectants, surface protectants, and anti-fouling agents [24; 25; 26; 27; 28]. Ionic liquids with cationic components of imidazolium, pyridinium, ammonia, and phosphonium have been found to have strong antibacterial activity [24; 29; 30; 31].