1. Introduction
The continuous advances in diesel engine technology and the more stringent emissions standards have led to higher injection pressure and tomodifications in some fuel properties. Among the latter ones, diesel fuels need to increase their lubricity to protect the fuel injection system and other engine components. A higher boiling point and aromatic, nitrogen, and sulfur contents appear to improve diesel fuel lubricity.1 Several studies have reported that the key agents are the highly polar compounds (especially those containing oxygen and nitrogen) present in diesel fuels which derive in forming a protective layer on the metal surface.2 However, many of these surface-active polar compounds are eliminated during the desulfurization processes causing loss of lubricity.3,4 To meet the wear scar limits established in diesel fuel standards (460 and 520 μm in European and U.S. regulations, respectively (EN 590:2009,5 ASTM D 9756)), a variety of lubricity additives can be used,which have a high affinity to metallic surfaces forming a thin protective metal-metal contact layer. This lubricant film is formed by the adsorption of the polar molecules of the additives on the metal surface, which is negatively charged.4