Induced structural changes at aliphatic hydrocarbon-graphite interfaces

被引:8
作者
Gilbert, EP [1 ]
Reynolds, PA [1 ]
White, JW [1 ]
机构
[1] Australian Natl Univ, Res Sch Chem, Canberra, ACT 0200, Australia
关键词
alkane; graphite; structure; adsorption; surface;
D O I
10.1016/S0927-7757(98)00205-2
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
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 less than or equal to n less than or equal to 20 when adsorbed on Papyex. Ordering of the hydrocarbon molecules occurs at the interface, and this surface crystalline structure persists at least 30 degrees 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 C16H34 and C16H34 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. (C) 1998 Elsevier Science B.V.
引用
收藏
页码:81 / 100
页数:20
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