Liquid Chromatography-Mass Spectrometric Study of Composition and Structure of Alkyl Salicylate as Detergent Additives in Lubricants

Abstract

The high-temperature detergency of alkyl salicylates (ASs) as lubricant detergents is influenced by their composition and molecular structure. Analysis via normal-phase liquid chromatography (LC) combined with mass spectrometry (MS) reveals that alkyl salicylic acid (ASA) primarily consists of alkylphenol (AP), monoalkyl salicylic acid (MASA), dialkyl salicylic acid (DASA), and salicylic acid (SA), with MASA being the dominant component. Using reversed-phase LC and MS identification, MASA is characterized as a mixture of homologues with varying alkyl carbon chain lengths (C14, C16, C18), with C16 exhibiting the highest proportion. Furthermore, isomers sharing identical alkyl carbon numbers are successfully separated under these conditions. Their precise structures and elution order are determined through theoretical inference and quantum chemical calculations, namely, polarity is higher at the 5th position alkyl substitution than at the 3th, and alkyl isomerization results in greater polarity compared to non-isomerized structures. This separation, qualitative, and quantitative analytical method not only guides the industrial production and purification processes of ASA products but also enables the evaluation of performance differences among ASs sourced from different manufacturers. Additionally, it provides theoretical support for developing high-performance detergents based on ASA.