Analyses of the Adsorption Structures of Friction Modifiers by Means of Quantitative Structure-Property Relationship Method and Sum Frequency Generation Spectroscopy

Abstract

Blending an optimum amount of friction modifiers into lubricant is one of the important measures to reduce fuel consumption induced by the frictional loss for automobiles. However, the agents containing metal or phosphorus compounds can cause catalyst poisoning and clogging in the filters of the exhaust gas refining system. Thus, development of metal-free and phosphorus-free agents with a long-term stability under real working condition is highly desirable. In the present work, the friction coefficients on the metal surface in lubricants containing different friction modifiers were investigated by the reciprocating friction test in detail. A statistical method based on the quantitative structure-property relationship (QSPR) analysis has been employed to correlate the chemical nature of additives with their effect on the friction reduction behaviors. An empirical equation relating the molecular structure and its friction reduction efficiency has been proposed as a standard for a good friction modifier. Furthermore, a surface-sensitive vibration spectroscopy, sum frequency generation (SFG) spectroscopy, has been used to analyze the molecular structures of the lubricants adsorbed on the metal surface. The SFG observation suggested that a certain relationship between the adsorption structure and friction reduction effect of these agents.

Friction Modifier Additives

“…These include the surface forces apparatus (SFA) [58][59][60][61], atomic/lateral force microscope (AFM/LFM) [62][63][64][65], quartz crystal microbalance (QCM) [66], ultrathin film interferometry (UTFI) [67], neutron reflectometry [68,69] and various surface-specific vibrational spectroscopies [70][71][72]. Methods were also developed to generate model close-packed monolayers by chemical grafting of self-assembled alkylthiols and alkyl siloxanes on solid surfaces [73][74].…”

“…Beattie and co-workers studied a Langmuir-Blodgett film of zinc arachidate in sliding sapphire/quartz contacts and found that film order was maintained during sliding but some film was transferred from the monolayer to the opposing surface [87]. In a recent study by Koshima et al the friction coefficients of solutions of a range of model OFMs were correlated with the SFG spectral characteristics of their selfassembled films on iron, suggesting that OFMs that give the most ordered surface films also show the lowest friction [72].…”

In Situ Study of Model Organic Friction Modifiers Using Liquid Cell AFM: Self-Assembly of Octadecylamine

“…They are widely used as additives in low friction lubricant formulations, including modern, fuel-efficient engine oils. Typical OFM structures are shown in [1]. Although OFMs have been in use for almost a century [2], there is still considerable debate about the nature of the boundary films that they form.…”

Interfacial Structure of Soft Matter Probed by SFG Spectroscopy

“…In the present Personal Account, our recent SFG studies on the molecular structures at interfaces of functional soft matter such as polymers, self‐assembled monolayers (SAMs) and Langmuir–Blodgett (LB) ultrathin layers and lipid bilayers in various environments will be briefly reviewed. A general model dealing with the optical interference effect in thin layers of soft matter, which can dramatically affect the SFG spectral shape and intensity, will be introduced first.…”