The crystalline structures of carboxylic acid monolayers adsorbed on graphite
The crystalline structures of carboxylic acid monolayers adsorbed on graphite
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DOI：
10.1021/jp054798q
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发表时间：
2006-03-23
影响因子：
3.3
通讯作者：
Inaba, A
中科院分区：
文献类型：
--
作者：
Bickerstaffe, AK;Cheah, NP;Inaba, A
用X射线和中子衍射研究了从C-6到C-14的所有线性羧酸在亚单层覆盖下和从C-8到C-14的多层覆盖下的固体结晶单分子膜的形成，并对其结构进行了表征。X射线和中子突出了单层结构的不同方面，因此它们的组合在结构确定中很重要。对于所有碳原子数为奇数的酸，其晶胞是含有四个分子的平面基团PGG的长方形。具有偶数个碳原子的同源系列的成员具有倾斜的单胞，每个单胞具有两个分子和平面基团p2。晶体结构的这种奇偶变化为单层熔点和混合行为中观察到的奇偶变化提供了解释。在所有情况下，分子都排列在强氢键二聚体中，其延伸轴平行于表面，碳骨架的平面基本上平行于石墨表面。单层晶体结构的晶胞尺寸与大块晶体的某些紧密堆积平面相似，但分子排列不同。在单层上增加覆盖率有1-3%的压缩。
X-ray and neutron diffraction have been used to investigate the formation of solid crystalline monolayers of all of the linear carboxylic acids from C-6 to C-14 at submonolayer coverage and from C-8 to C-14 at multilayer coverages, and to characterize their structures. X-rays and neutrons highlight different aspects of the monolayer structures, and their combination is therefore important in structural determination. For all of the acids with an odd number of carbon atoms, the unit cell is rectangular of plane group pgg containing four molecules. The members of the homologous series with an even number of carbon atoms have an oblique unit cell with two molecules per unit cell and plane group p2. This odd-even variation in crystal structure provides an explanation for the odd-even variation observed in monolayer melting points and mixing behavior. In all cases, the molecules are arranged in strongly hydrogen-bonded dimers with their extended axes parallel to the surface and the plane of the carbon skeleton essentially parallel to the graphite surface. The monolayer crystal structures have unit cell dimensions similar to certain close-packed planes of the bulk crystals, but the molecular arrangements are different. There is a 1-3% compression on increasing the coverage over a monolayer.