Pyridinium based dicationic ionic liquids as base lubricants or lubricant additives

Highlights

• Synthesis of eight new dicationic ionic liquids derived from pyridinium.

• N-alkylpyridinium cations linked by polyethylene glycol chains.

• Thermal analysis of the pure ionic liquids by DSC and TGA.

• The dicationic ILs with [NTf₂] anion have shown good properties as neat lubricants.

• Decrease in COF with low concentration of [NTf₂] DLIs in mixtures with glycerol.

Abstract

A series of eight new dicationic ionic liquids incorporating polyethylene glycol chains linking two N-alkylpyridinium moieties through rings position 2, were synthesized and investigated for their potential use as lubricants or lubricant additives. Alkylsulfate and bis(trifluoromethanesulfonyl)imide [NTf₂] were selected as anions. Dicationic ionic liquids containing [NTf₂] anion demonstrated good properties as neat lubricants showing low coefficient of friction and wear comparable with the reference lubricant (glycerol). Even more interesting could be their use as friction modifier additives since an important decrease in the coefficient of friction was achieved with a really low concentration of ionic liquid. The thermal analysis of all ionic liquids by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) is also described.

Introduction

Ionic liquids (ILs) have gained increasing importance both in the scientific and industrial engineering community during last decade. Due to their remarkable and interesting properties, such as negligible vapor pressure, non-flammability, high polarity, and high thermal stability [1]. ILs are finding widespread use as engineering fluids or as novel lubricating systems [2], [3]. Moreover, the polar nature of ILs is directly connected with good tribological behavior, since their strong interactions with surfaces lead to the formation of adsorbed films, reducing friction and wear.

The first experiment of ILs in tribology was reported in 2001 with the work of Liu et al. [4]. In that research, imidazolium tetrafluoroborate was tested as IL-based lubricant for various tribo-pairs and, in all experiments, the use of the IL showed significant friction reduction. Since that time, the interest of scientists in tribological properties of ILs has considerably increased [5], [6]. Jones et al. have shown that the presence of polyether chains can significantly improve tribological behavior of liquid lubricants or lubricant additives [7]. Jin et al. have examined some polyethylene glycol functionalized ILs as high temperature lubricants, showing how the presence of fluorine in the IL favorably boosts its anti-wear performance [8]. On the other side, dicationic ILs based on imidazolium and pyrrolidinium cation and traditional anions as ${\mathrm{BF}}_4^{-}$ or ${\mathrm{PF}}_6^{-}$, have shown good thermal stability comparing with their monocationic analogues [9]. It has also been shown that that the acute toxicity of DILs is in many cases below the levels observed for those monocationic and that the use of head groups connected via polyethylene glycol could be identified as structural elements reducing the toxicity [10]. Hence, the combination of the tribologically favorable properties of polyethylene glycol and DILs may lead to a series of novel and advanced lubricating systems.

Previous works on the use of DILs derived from imidazolium [11], [12] and pyrrolidinium [13] for lubrication have shown very promising results. Nevertheless, to our knowledge, no papers were published about using pyridinium DILs as lubricants or lubricant additives. It is known that pyridinium cation can show exceptionally high levels of biodegradation under aerobic conditions and can be classified as “ready biodegradable” [14]. In addition, studies on the acute toxicity of DILs have shown levels below those observed for monocationic ILs [15]. Several recent studies dealing with tribological properties of ionic liquids exposed to extreme conditions [16], [17], as well as with the stability of lubricant mixtures of ionic liquids with conventional base oils [18], has been published. Generated knowledge of ionic liquids in tribology can also be found in several review articles [2], [19].

For all these reasons, we have synthesized and studied the tribological properties of a series of DILs that incorporate polyethylene glycol chains linking two alkylpyridinium moieties. Alkylsulfate [RSO₄] and bis(trifluoromethane) sulfonimide [NTf₂] were selected as anions taking into account the reduction on wear and friction observed when these anions were used in previous works on monocationic ILs [20], [21].