Relating Alkyl Chain Length of Additives to Wax Crystallization Inhibition: Toward the Rational Design of Pour Point Depressants

Abstract

Precipitation of paraffin wax (long-chained n-alkanes) from engine oil and fuel distillates at low temperatures is generally prevented using low-dosage polymeric additives, known as pour point depressants (PPDs). The ability of a PPD to inhibit wax crystallization is largely determined by how well its alkyl side chains interact with the waxy components present in the base oil. The current study aims to elucidate the mechanism underpinning this structure–property relationship using docosane (n-C22H46) dissolved in a mixture of dodecane and toluene as a model waxy oil and the homologous series of paraffins as “tailor-made” additives. By determining the wax crystal nucleation kinetics using polythermal turbidimetry measurements, a direct link between the carbon chain length of the additive molecule and its ability to retard wax precipitation was established. Thermodynamic and structural analysis of the wax formation process using differential scanning calorimetry and small/wide-angle X-ray scattering techniques respectively revealed that the additive incorporates into the lamellar wax aggregates formed during the nucleation stage and subsequently impedes its crystallization. This mechanism is consistent with the cocrystallizing behavior of n-alkanes from multiparaffinic solutions to form a solid solution. Only additives with alkyl chains longer than the solute wax retarded nucleation and growth along the needle axis of the wax crystal because of the end-chain protrusion/bending disorder in the resultant lamellar wax crystal structure.