Induced structural changes at aliphatic hydrocarbon–graphite interfaces

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Physicochemical and engineering aspects, 1998-10, Vol.141 (1), p.81-100
• Main Authors: Gilbert, E.P., Reynolds, P.A., White, J.W.
• Format: Article
• Language: English
• Subjects: Adsorption; Alkane; Graphite; Structure; Surface
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/S0927-7757(98)00205-2

Small-angle X-ray scattering and quasielastic neutron scattering have been used to investigate the structures and phase behaviour of normal alkanes with carbon number n for 10⩽ n⩽20 when adsorbed on Papyex. Ordering of the hydrocarbon molecules occurs at the interface, and this surface crystalline structure persists at least 30°C above the melting temperature of the bulk crystalline phase. The surface-adsorbed structure is a lamellar arrangement different from the bulk phase. Below the bulk melting point, T m, and for alkanes with even n, two distinct diffraction peaks are observed in the small-angle region, one associated with the bulk phase and the other with the surface structure. For alkanes with odd n, only one peak is observed below T m; above T m a significant reduction in peak intensity is found, indicating melting of the bulk, but a peak remains. The peak intensity at T= T m increases with n, indicating increasing surface stabilisation with increasing chain length. The separation between T m and the temperature at which the diffracted intensity is greatest decreases as n increases and the values of the former agree well with values associated with a metastable phase transition observed previously by Schaerer. A comparison of the QENS from bulk C 16H 34 and C 16H 34 adsorbed on Papyex below T m indicates no significant difference in elastic peak broadening. Above T m, the chains are shown to undergo translational motion. The lower bound to the rotational correlation time is estimated to be ca. 3 ns, which is some two orders of magnitude larger than the typical alkane rotator phase.