Lucht R, Bahr C, Heppke G
Institute of Physical Chemistry, University Marburg, D-35032 Marburg, Germany.
Phys Rev E Stat Phys Plasmas Fluids Relat Interdiscip Topics. 2000 Aug;62(2 Pt A):2324-33. doi: 10.1103/physreve.62.2324.
An ellipsometric study of the wetting behavior at the free surface above the isotropic to nematic or isotropic to smectic-A transition of nine homologous compounds with even alkyl chain lengths n in the range from four to twenty carbon atoms is presented. All compounds show a pretransitional increase of the nematic or smectic surface coverage as the bulk isotropic to liquid-crystal transition is approached from above. The behavior of the nematic compounds (n=4 to 10) can be interpreted, within the framework of a Landau model, as complete wetting. In short nematic homologs the divergence of the nematic coverage is strongly reduced by a decrease of the nematic susceptibility of the isotropic phase. The elastic coefficient L of the Landau model shows a pronounced increase with increasing n, resulting in the occurrence of a discontinuous prewetting transition in the shortest smectic homolog (n=12) that is still describable by the nematic Landau model. In the longer smectic homologs (n=14 to 20), layering steps appear in the pretransitional increase of the coverage. The results indicate probable partial wetting for the longest homolog, whereas for the other smectic compounds the distinction between complete and partial wetting is difficult on the basis of ellipsometry.
本文介绍了对九种具有偶数烷基链长度(n)（范围为4至20个碳原子）的同系物在从各向同性到向列相或从各向同性到近晶A相转变时自由表面润湿行为的椭偏研究。当从上方接近本体各向同性到液晶转变时，所有化合物的向列相或近晶相表面覆盖率都会出现预转变增加。在朗道模型的框架内，向列相化合物（(n = 4)至10）的行为可以解释为完全润湿。在短链向列同系物中，各向同性相的向列磁化率降低会强烈降低向列覆盖率的发散。朗道模型的弹性系数(L)随(n)的增加而显著增加，导致最短近晶同系物（(n = 12)）中出现不连续的预润湿转变，该转变仍可用向列朗道模型描述。在较长的近晶同系物（(n = 14)至20）中，覆盖率的预转变增加中出现分层台阶。结果表明，最长同系物可能存在部分润湿，而对于其他近晶化合物，基于椭偏测量很难区分完全润湿和部分润湿。
