Introduction
Ionic liquids (ILs) are molten salts containing organic or inorganic
cations and anions which have lower boiling point than water (Battez et
al., 2016; Khan et al., 2018; Khemchandani et al., 2014; Matczak et al.,
2018). Their unique features such as high thermal stability, broad
liquid range, low vapour pressure and non-flammability makes ionic
liquids as versatile molecules for lubrication applications (Jimeenez
and Bermudez, 2008; Pensado et al., 2008; Qu et al., 2006; Somers et
al., 2013; Stolte et al., 2012; Yu et al., 2008). ILs offers some
distinctive advantages in reducing friction and wear between surfaces in
direct contact (Ichiro, 2009; Zhou et al., 2009). The ILs composed with
ammonium, phosphonium, pyrrolidinium and imidazolium cations along with
tetrafluoroborate, tosylate, halide and hexafluorophosphate anions were
evaluated as lubricants and their interaction with metal surfaces was
studied (Minami et al., 2010; Qu et al., 2006; Somers et al., 2012;
Tiago et al., 2015; Zhou et al., 2009). These studies proved that the
ILs act as neat lubricants and also as additives to the basestocks. It
is believed that due to their inherent polarity ILs strongly adsorb on
metal surfaces to form tribochemical film, this film enhances antiwear
and antifriction performance (Liu et al., 2006; Mendonca et al., 2012;
Mu et al., 2008; Qu et al., 2012; Ye et al., 2001). However, there is a
limitation for using ILs as neat lubricants due to their higher cost and
multistep reaction procedure involved in their synthesis compared to
traditional base oils (Barnhill et al., 2014; Khan et al., 2018; Zhou et
al., 2014) . However, with a minimum weight percent of ILs in mineral
oil and other synthetic oils performed better than the neat base oils
(Amiril et al., 2017; Ma et al., 2019; Otero et al., 2014). Batteze et
al. detected the film forming capacity of phosphonium cation-based ILs
at low concentrations in mineral oil and also evaluated the effect of IL
concentration on film forming capacity and coefficient of friction under
different test conditions. The choice of changing the combination of
cation and anion is possible in ILs to attain specific properties (Ma et
al., 2019; Mordukhovich et al., 2013; Salih et al., 2011). Recently some
of the ILs gained much interest as lubricant additives such as
dicationic bis(ammonium) and
bis(imidazolium)-di[bis(salicylato)borate] ionic liquids (Gusain et
al., 2014), phosphonium-alkylphosphate (Qu et al., 2015) and
N,N’-dialkylimidazolium (Pejakovic et al., 2016) etc. These ILs when
used as additives to lubricating oil remarkably enhanced its antiwear
and antifriction properties. Zhang et al. (2017), studied the
synergistic effect of polymeric IL additives with conventional additive
ZDDP and found best synergism with 1:1 w/w of ZDDP and boron-containing
polyisobutylene-based IL. Compared to ZDDP ILs are ash less and have
stronger adsorption towards metallic surface, thus they provide better
lubricity performance with low engine deposits (Mendonca et al., 2012;
Oulego et al., 2018).
Currently, most of the ionic liquid based additives used for the
tribological applications were synthesized with anions such as the
halide [Fˉ, Cl‾, Br‾ or I‾], phosphate and sulfate these ILs exhaust
high amount of toxic components to the environment. The ILs consisting
of [PF6]‾ and [BF4]‾ shows
severe corrosion effect due to the formation of hydrofluoric acid by
vigorous reaction with water (Stolte et al., 2012; Swatloski et al.,
2003). Kronberger et al. (2012), investigated the eco toxicity and bio
degradability of lubricant additives with pyrrolidinium and quaternary
ammonium based cations combined with
CH3O4S‾,
CH3O3S‾ and
(CF3SO2)2N‾ as counter
ions. The anion
(CF3SO2)2N‾ being highly
resistant to biodegradation and cannot be considered for applications
with potentially high environmental exposure. Conventional IL additives
containing halide, sulphate and PF6 anions are replaced
with environmentally benign components like fatty acids and amino acids.
Biodegradable and bio compatible behaviour of amino acid and fatty acid
anions are alternatives to the hazardous anions. Liwen et al.
synthesized two green ILs of choline with amino acids glycene and
proline, these two ILs [choline][glycine] and
[choline][L-proline] significantly improved the tribological
properties of base oil (Mu et al., 2015). Minami et al. (2012), stated
that the both tribological and thermo-oxidative stability depend on
hydrophobicity of IL anion, as the hydrophobicity of anionic part
increases results in improved performance. In recent years, the fatty
acid derived ILs has been explored as environmentally benign lubricating
oil additives. Khatri et al. (2018), reported the synthesis of
tetramethylguanidinium ionic liquids with various fatty acid anions for
steel/steel contact in grade I mineral oil. Long alkyl chain constituted
ILs significantly reduced the friction and wear. The present work
describes the synthesis of ricinoleate anion based ILs containing
various cations and evaluation of their tribological performance by
blending in base oil epoxy karanja 2-ethylhexyl esters (EKE) and dioctyl
sebacate (DOS). The formation of tribofilms on surface with base oil and
additive was investigated by SEM and EDX analysis was used to establish
the elemental composition on the worn surfaces.