Electrochemical studies of the automotive lubricant additive zinc n-dibutyldithiophosphate
Zinc dialkyldithiophosphates (ZDTPs) are widely incorporated in lubricant formulations as anti-oxidant and anti-wear additives. Recent years have seen the emergence of considerable research aimed at defining the mode of action of these compounds. Despite this, the mechanistic action of ZDTPs in their capacity as both anti-wear and anti-oxidant agents remains ill defined. Previous investigations have used a wide variety of techniques, however, electrochemical methods have been poorly exploited. This thesis describes application of electrochemical techniques to the study of zinc ndibutyldithiophosphate (Bun-ZDTP) in an investigation of the value of such methods as a tool for ZDTP analysis. A comprehensive study of Bun-ZDTP redox activity under standard electrochemical conditions has been implemented. Using cyclic voltammetry, Bun-ZDTP has been shown to be oxidised in an irreversible 2 electron transfer process. The diffusion coefficient of this species in DMF has been calculated via rotating disc electrode (RDE) voltammetry. Bun-ZDTP reduction proceeds via nucleation of zinc at the electrode surface. The limiting current of the 2 electron reduction process is lower than previously observed for Bun- ZDTP oxidation. This occurs since zinc deposition is confined to growth centres which effectively limit the electroactive area of the electrode. A more thorough investigation of zinc nucleation at the electrode surface was undertaken using chronoamperometry. Transients plotted in dimensionless form were compared to theoretical models of nuclear growth. The potential of atomic force microscopy as a promising technique for the imaging of ZDTP filming action has been demonstrated. Ex-situ atomic force microscopy was conducted of electrode surfaces following reduction in Bun-ZDTP solution under cyclic and RDE voltammetric conditions. Development of a variable elevated temperature apparatus for the study of Bun-ZDTP under conditions more typical of an engine environment has been described. Application of the system to a wide range of electrochemical problems was demonstrated as an illustration of the advantages of this novel experimental approach. Studies of N, N, N/, N/-tetramethyl-p-phenylenediamine and tris(4-bromophenyl)amine oxidation yielded diffusion co-efficients (D) at elevated temperatures from which activation parameters for diffusion were obtained. Variation of D with temperature was compared to predicted theory according to the Stokes-Einstein and Wilke-Chang relationships. Studies of ortho-bromonitrobenzene and 9-chloroanthracene reduction demonstrated the elevated temperature apparatus to be a viable method for the interrogation of rapid kinetic processes. Both compounds are reduced according to an ECE mechanistic scheme where halide bond cleavage constitutes the chemical step. Since the use of microelectrodes permits "outrunning" of bond cleavage kinetics to give an neff of less than two, rate constants at varying temperatures were evaluated. Arrhenius plots were used to deduce activation parameters of halide bond cleavage in both compounds. Preliminary studies of Bun-ZDTP redox activity under conditions more typical of an engine environment are discussed. Application of the novel microelectrode apparatus to an elevated temperature study of Bun-ZDTP is described. Voltammetric investigations in toluene, a resistive solvent which serves as a convenient model for oil, are discussed.