3.1.1. PIL Colloidal Particles

Since colloidal systems have a close connection with nature and human life, PIL colloids are a new platform to investigate the unique properties of ILs with the rather small dimension and the superior dispersity of colloidal particles. Moreover, they setup a powerful platform for a variety of studies on PIL functions and applications. A colloidal system is defined as a state of subdivision dispersed in a medium with at least one dimension between approximately 1 nm and 1 μm [57]. Recently, nanostructured PILs, and especially colloidal nanoparticles, have received significant interest as functional polymer nanoparticles. This is due to the fact that small particle sizes increase surface effects on the interfacial interaction and mass/energy transport. Moreover, a small particle size and the charged character of PILs improve the colloidal stability in aqueous as well as non-aqueous dispersions. So far, a few synthetic routes have been developed to prepare PIL nanoparticles including suspension polymerization [58], water-in-oil concentrated emulsion polymerization [59], and precipitation polymerization in water without stabilizers using ionic liquid monomers with long alkyl chains has been recently reported [60,61]. Among PIL nanoparticles a significant focus in this field is on the nanostructured imidazolium-type PILs [47,62]. One of the examples of this type of applications is the work by Yang et al. [63], which reported the preparation of the crosslinked poly(1-butyl-3-vinylimidazolium bromide) microspheres with the diameter of about 200 nm synthesized via miniemulsion polymerization for application as metal scavenging and catalysis.

In another work by Zhou et al. [64], a thermosensitive type ionic microgels was obtained via the surfactant-free emulsion copolymerization of 1-vinylimidazole and 4-vinylpyridine with thermosensitive monomer N-isopropylacrylamide. The obtained microgels were spherical in shape with narrow size distribution and exhibited thermosensitive behaviour with unique features of PILs in aqueous solution (Figure 6).

**Figure 6.** Schematic summary of synthesis of thermosensitive ionic microgels via quaternized crosslinking reaction and their properties (Reprinted with permission from [64]. Copyright 2014 American Chemical Society).

One of the most common methods used for the fabrication of polymer nanoparticles with the droplet sizes typically in the range of 20–200 nm is miniemulsion. The use of water as the dispersion medium is one of the main advantages of this system, which makes it environmentally friendly and also allows excellent heat dissipation during the polymerization process [65]. On the other hand, very recently, "click" polyaddition reactions in miniemulsions, specifically thiol-mediated chemistry (i.e., thiol-ene/yne, thiol-Michael); have attracted attention as one of the facile methods for synthesis of polymer nanoparticles, and nanocapsules dispersion in heterophase media with high efficiency [66,67]. For example, for the first time Jasinski and co-workers reported the preparation of poly (thioether ester) latex nanoparticles using miniemulsion thiol-ene photopolymerization. Their synthesis was performed in water, at ambient temperature, and without the use of any organic solvent. The resultant linear poly (thioether ester) particles had an average diameter of 130 nm [68]. In another recent work, sub-100 nm crosslinked polythioether nanoparticles were synthesized via thiol-ene photopolymerization in miniemulsion using high-energy homogenization [65]. Our group recently reported the facile preparation of cross-linked PILs based nanoparticles via thiol-ene photo-polymerization in miniemulsion [69]. In this study, the PIL nanoparticles exhibited improved corrosion inhibition properties to the sol-gel coating due to the interaction between the –C=N– group and electronegative nitrogen in the PIL with the metal surface.
