Diffusion interaction and quantitative analysis of zinc dialkyldithiophosphate content in lube base oils in terahertz regime

Abstract

We investigate the diffusion interaction and quantitative analysis of zinc dialkyldithiophosphate (ZDDP) mixed with lube base oil (LBO) at different concentrations using terahertz time-domain spectroscopy (THz-TDS). When the concentration exceeds 6.78%, the characteristic absorption peaks exhibit significantly shift, and the absorption coefficient peak value is nonlinear against concentration. Moreover, the absorption coefficients of mixed samples follow the Beer's law at a concentration below 6.78%. The quantitative analysis enables a strategy for monitoring the formulation of lubricating oil in real time.OCIS codes: 070.4340, 160.4760, 190.4400, 300.6495. doi: 10.3788/COL201109.110010. Zinc dialkyldithiophosphate (ZDDP), as a multifunctional additive and inhibitor in petroleum industry, works mainly as antiwear, antioxidant, and anticorrosion agent [1,2] . ZDDP mixed with lube base oils (LBOs) has been used to study the effect of concentration on lubrication boundary and tribological properties [3] . The concentration of ZDDP in LBOs plays a crucial role in formulation, which is a balance of many different aspects of performance. Lubricating formulation generally results from a molecular diffusion mechanism due to the relative motion of molecules. Molecular diffusion in a steady-state and nonequilibrium system is divided into molecular motion and interaction. Molecular motion, containing the electronic motion, molecular vibration, and molecular rotation, is complicated and multibody. The intermolecular and intramolecular modes of ZDDP and LBO determine the ability of the reactants to approach each other and products to separate from each other. The intermolecular and intramolecular interactions often have their characteristic frequencies in the region lower than 100 cm −1 . Two major strategies can be used to investigate lowfrequency diffusion between ZDDP and LBO. Femtosecond optical Kerr-effect (OKE) spectroscopy has been used over past decades to investigate molecular motion in liquids and liquid mixtures [4] . The second technique is terahertz time-domain spectroscopy (THz-TDS) [5,6] , which is a promising and nondestructive analysis method for petroleum products. A unique attribute of THz-TDS is its ability to sensitively measure the induced molecular dipole moments of many petroleum products. Furthermore, the low photon energy of radiation allows propagation through flammable liquids without any danger of combustion. The past few years have witnessed a remarkable effort in employing THz-TDS for investigating the properties of petroleum products, including gasoline, diesel, lubricating oil, lube base oil, water-oil complexes, and organic solvents [7−9] . The absorption spectra of petroleum products show collective modes in the THz region.In this letter, we described the interaction between ZDDP and LBO using THz-TDS. First, we measured the THz-TDS of polar liquid of ZDDP at different concentrations in LBO from Russia and calculated their absorption coefficients. We then investigated the usefulness of...