Wetting Phenomena at the Free Surface of the Isotropic Phase of a Smectic Liquid Crystal

Abstract

We report an ellipsometric study of the free surface of a smectic liquid crystal possessing a smectic-A-to-isotropic phase transition. Approaching the transition from above, an ordered surface layer appears at a discontinuous transition one degree above the bulk transition temperature. With decreasing temperature the thickness of the surface layer increases continuously, indicating a possible logarithmic divergence at the bulk transition. The behavior may be described as complete wetting accompanied by a prewetting transition. [S0031-9007(97)03114-1] PACS numbers: 64.70.Md, 68.45.Gd The free surface of thermotropic liquid crystals exerts a pronounced ordering effect to the molecules close to the surface which becomes apparent even above the transition to the isotropic liquid phase: Beaglehole [1] has reported an ellipsometric study of the free surface of the liquid crystal 5CB [2] above its isotropic-nematic transition. The results showed the presence of a nematic layer interposing the isotropic liquid-vapor interface; the nematic coverage was found to diverge logarithmically at the bulk nematic-isotropic transition temperature thereby indicating complete wetting. Since then, a number of nematic compounds were found to show essentially the same behavior [3,4].Whereas in the nematic phase only a long-range orientational order of the rodlike molecules exists, smectic phases show in addition a quasi-long-range positional order leading to a layered structure. Thus, above a nematicsmectic [5][6][7][8][9] or isotropic-smectic transition [10][11][12][13] usually the formation of smectic layers at the free surface is observed. In particular, above the isotropicsmectic-A ͑Sm-A͒ transition of the compound 12CB [14] a sequence of five successive layering transitions was observed [10,13]; since the number of surface-induced smectic layers was finite, this behavior corresponds to partial wetting of the isotropic liquid-vapor interface by the smectic phase.In this Letter, we report a new wetting behavior above an isotropic-Sm-A phase transition. Compared to the behavior of 12CB there are two main differences: first, the thickness of the ordered surface domain grows continuously with decreasing temperature and possibly diverges at the bulk transition; second, the first appearance of an ordered surface layer on the isotropic liquid is characterized by a sharp, discontinuous transition. The corresponding layer thickness just below this transition is not that of a single smectic layer but about twice as large. Thus, our results may be described by complete wetting accompanied by a prewetting transition.We have studied the 4-hexyloxyphenylester of 4dodecyloxybenzoic acid, labeled in the following as 12.O.6 (cf. Fig. 1), the Sm-A-isotropic transition temperature T AI of the bulk sample is 89.1 ± C. The compound was purified by chromatography and several recrystallizations. The temperature width of the two-phase region at the Sm-A-isotropic transition, observed in a polarization micoscope, was found to be smaller than the resoluti...

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