# Multifunctional Modifying Systems Based on Ionic Liquids for Epoxy Resin Systems and Composites
## 1 Introduction
Of particular scientific interest is the fact that the positive charge of the cation may be localized on a variety of atoms, frequently encompassing nitrogen, phosphorus, or sulfur, and affording the potential for diverse functional groups within the cation's molecular framework. Additionally, ILs offer the flexibility to incorporate anions spanning the spectrum from organic to inorganic, and the choice of anionic species exerts a profound influence on the nuanced properties exhibited by the entire IL compound [5]. So, a small group of imidazolium ILs is known, which in their structure have organosil-icon groups, mainly based on siloxanes [19; 20]. Recent papers propose the use of this type of compound as electrolytes in lithium-ion batteries [19] and discuss their potential areas of application as surfactants with the ability to specific aggregation in aqueous solutions [21]. In addition, the ability to store hydrogen in this type of ionic liquid was also presented [22], but in most published works, attention is focused on the synthesis and properties of these quaternary salts, and the scope of structures is limited to the basic ones, i.e., imidazolium and ammonium ionic liquids. On the other hand, ionic liquids containing a phosphorus atom in their structure can be divided into two groups: (i) phosphonium ionic liquids (PhILs) in which the positive charge of the cation is located on the phosphorus atom, and (ii) ionic liquids in which the phosphorus atom is located in the alkyl chain or the anion and is devoid of a direct charge. The precursors of PhILs are mostly various types of phosphines. However, the high sensitivity of phosphines to air requires advanced synthetic equipment that provides an appropriate environment for the process. This feature significantly limits the possibilitiesof free design of this type of ILs, and only a small group of phosphines is known that show low sensitivity to air. Nevertheless, the applications of PhILs are diverse and in recent years the areas of their use have been presented as catalysts for the cycloaddition of carbon dioxide to epoxides [23], anticorrosive additives for pigments in epoxy coatings [24] or in the extraction of gold from thiosulfate solutions [25]. Moreover, many applications of this group of compounds in catalysis and carbon dioxide absorption are known [26; 27]. In contrast, ionic liquids containing charge-free phosphorus in their structure are simpler to synthesize and, in most cases, do not require special conditions. This group of compounds has found use primarily in cellulose dissolution processes [28; 29] and electrochemistry [30]. ILs have a number of advantages for use in composite materials and resin systems themselves. One of their most important properties is their ability to initiate the polymerization process of the epoxy resin [5; 31; 32]. ILs can be designed to predict their performance as cure initiators in a selected range of temperatures and intensities. The curing process can be either cationic or anionic, and the amount of ionic liquid required for the curing process is much smaller than for classical amine curing agents. Critical to the success of the process is the temperature reached by the resin system, as the system itself is inactive at room temperature, which also brings the advantages of a very long gelation time (up to 60 days) and the possibility of long-term storage at room temperature of the already finished resin/hardener mixture in the form of ionic liquid. Ionic liquids affect the properties of the final material through the reduction in the crosslinking density of the resin after curing, resulting in an increase in the stress intensity factor and tensile strength without significant loss of other mechanical properties [33]. In addition, ionic liquids have a scientifically proven effect on fibers of various types. Flax fibers functionalized using an ionic liquid--for example, dodecyldimethylammonium nitrate--become resistant to the growth of microorganisms [34]. The antifungal efficacy is derived from a lengthy alkyl chain.