Structural analysis of simple base oils under high pressure using synchrotron radiation X-ray diffraction: First trial

The microscopic structures of two base oils at atmospheric pressure and under high pressure were analyzed using synchrotron radiation X-ray diffraction in this work. The experimental apparatuses we used were designed for structural analysis under high pressure: the ‘MAX80’ X-ray diffractometer with a cubic multi-anvil press at the KEK synchrotron facilities and the ‘SMAP2’ one at the SPring-8 synchrotron facilities. The sample oils were n-hexadecane CH3(CH2)14CH3 and dicyclohexyl C6H11C6H11; the former is a chain hydrocarbon, and the latter is a dual-cyclic hydrocarbon. The samples were sealed in a capsule made of Teflon and put under pressure with one of the two apparatuses. Application of synchrotron radiation X-rays to the sample oils at atmospheric pressure produced clear pair distribution functions and under several high-pressure conditions it produced scattering intensity profiles. The results revealed that the oils transitioned from liquid phase to single-crystal phase with increasing pressure. With a constant high pressure, they crystallized over time, making the transition in about an hour.