Ethoxylated amine surfactants as model additives for engine friction reduction

Abstract

The exact mechanism by which organic friction modifiers (OFMs) adsorb onto metallic surfaces and reduce friction remains debated. To optimise the use of these compounds, a precise understanding of their mechanism is essential. This knowledge will drive the development of next-generation additives, which could significantly extend engine lifespans and reduce fuel emissions. Hindered tertiary amine surfactants are promising candidates as OFMs. In this thesis, the self-assembly and adsorption behaviour of an industrially relevant OFM, 2,2’-(Octadecylazanediyl)diethanol (E1812), is investigated both in bulk dodecane and at the hematite/dodecane interface, to shed light on its friction-reducing mechanism. Chapter 3 examines the self-assembly of E1812 in dodecane through pendant drop tensiometry and small angle neutron scattering (SANS). The surfactant was found to form spherical aggregates with a radius of ∼ 11 Å at 25 °C, and its behaviour was influenced by solvent changes and dopant addition. As explored by SANS in Chapter 4, addition of 2.5−20 : 1 molar ratios of acetic acid (AcOH) to E1812 solutions caused the formation of worm-like micelles (WLMs) that were both concentration- and time-dependent. These were hypothesised to lack a well-defined global energy minimum, due to the hydrogen-bonding interaction of E1812 headgroups, AcOH, and native dissolved water, as well as the formation of trialkylammonium acetate salt. Chapter 5 investigates the adsorption of E1812 at the hematite/dodecane interface under static conditions, by neutron reflectometry (NR). The surfactant exhibited multilayer adsorption, as described by Freundlich isotherms, with the onset occurring above a concentration of 2.5 mM. E1812 formed strongly-bound protective films with a thickness of ∼ 20 Å , effectively screening the interface from water and competing with oleic acid (OA) for surface adsorption. As discussed in Chapter 6, the addition of AcOH largely induced off-specular scattering in NR studies, suggesting surface-correlated roughness likely linked to WLM formation in the bulk. It is hypothesised that E1812-AcOH mixtures adsorb as double-layer structures, with E1812 strongly bound to the surface and the laterally correlated, AcOH-containing species weakly adsorbed. To investigate conditions relevant to engine operation, Chapter 7 explores the adsorption of E1812 at the hematite/dodecane interface under shear, by NR. E1812 films remained stable under applied shear rates of 7.9×10³ s⁻¹, presenting a marginally increased thickness of ∼ 24 Å . The surfactant effectively shielded the interface from water adsorption under the same dynamic conditions and, under shear of 6.6×10³ s⁻¹, its co-adsorption with OA indicated that ΔHads,OA ≤ ΔHads,E1812 at the interface under study. Additionally, E1812 protected the interface from direct AcOH adsorption at a shear rate of 7.9×10³ s⁻¹, where off-specular scattering was removed. As discussed in Chapter 8, mini-traction machine (MTM) testing showed that E1812 enhanced the boundary lubrication of pure dodecane, particularly when mixed with AcOH, highlighting the role of WLMs in improving friction performance.