An Investigation into the Applicability of Wideline NMR for the Determination of Oil in Waxes

Abstract

The applicability of solid-state NMR (Nuclear Magnetic Resonance) for determination of oil content in waxes is probed by NMR experimental and data processing approaches. NMR is found to underestimate the value of the oil content in Fischer-Tropsch (FT) waxes due to the ability of the wax to trap small domains of oil within the matrix during crystallisation. No clear relationship could be obtained between T2 relaxation time and oil content, even after applying corrections due to disparities between mass and proton densities. Near linear calibration curves could be obtained by isolating a true oil signal at −32 °C. Model studies using heptadecane as simple oil analogue proved that removing oil/wax interactions and only using the oil as a diluent, near linear calibration curves could be attained. Insight into the mechanism of oil retention in both FT and paraffin waxes is obtained, showing differences due to both molecular and morphological variation. Results showed that, due to underlying molecular interactions, linear calibration curves cannot necessarily be expected even when melting behaviour varies linearly. In applications where oil content plays a crucial role in wax properties, this knowledge enables the industry to make more informed decisions when discrepancies in wax performance arise.