Mechanistic Studies of the Effect of Nitrogen-Based Detergents on Gum Adsorption from Gasoline Fuels

Deposition of carbonaceous materials on internal surfaces of spark ignition engines is a long-standing problem that can seriously impact engine performance. For gasoline fuel, deposit control additives (‘detergents’) comprising nitrogen-based detergents are commonly used to inhibit deposition and minimise its effects. However, the molecular basis of their detergency is still poorly understood. The most commonly quoted mechanistic hypothesis assumes that gasoline detergents inhibit deposit formation via surface passivation. i.e. by forming an adsorbed 2D thin film phase that protects the metal surface. The present work constitutes a first step towards establishing a systematic protocol for analysing the molecular mechanisms leading to the macroscopically observable detergency effect. This was done by synthesising a model fuel system that contained carbonaceous material (‘gum’) that can deposit on steel surfaces. First, a method for the synthesis of gum by a radical oxidation of mono- and diolefin components with air was developed. This established a reproducible method for oxidising gasoline surrogate to create gum. FTIR spectroscopy along with other analytical characterisation techniques were employed to validate the formulation and oxidation methods, the resulting oxidation product, and their deposits on steel surfaces. Then, the influence of a model detergent, 1-octadecylamine (ODA), on the oxidised fuel was studied. In solution, it interacts strongly with polar groups of the gum, likely solubilising it and thereby shifting any interfacial adsorption equilibrium towards the solution state. Gum deposition and detergency were then examined by using 316 stainless steel substrates. The ODA concentration was found to not only influence the adsorption equilibrium, but also the morphology of deposits on the surfaces, minimising the surface area covered by them. A systematic experimental design was developed to investigate the solid-liquid interactions at the stainless steel/solution interface in more detail. X-ray photoelectron spectroscopy was extensively used as the main surface sensitive technique to gain insight into the molecular basis of adsorption and detergency in this system. The results revealed that the detergency process was related mainly to solubilisation of gum in the solution phase, rather than surface passivation by formation of an adsorbed ODA layer. Adsorbed ODA was only evident when unrealistically high ODA concentrations were used.