Influence of specific anions on the orientational ordering of thermotropic liquid crystals at aqueous interfaces.
Influence of specific anions on the orientational ordering of thermotropic liquid crystals at aqueous interfaces.
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特定阴离子对嗜热液体晶体在水界面的方向排序的影响。
DOI：
10.1021/la3024293
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发表时间：
2012-09-04
期刊：
影响因子：
--
通讯作者：
Abbott NL
中科院分区：
文献类型：
--
作者：
Carlton RJ;Ma CD;Gupta JK;Abbott NL
我们报道了特定的阴离子(钠盐)在摩尔浓度下加入到水相中，可以在不溶于水的热致液晶(例如，4‘-戊基-4-氰基联苯的向列相，5CB)中引发快速的取向有序化转变。阴离子分为杂性阴离子，特别是碘化物、高氯酸盐和硫氰酸盐，使5CB在加入阴离子20 S内，在液液界面上经历连续的、依赖于浓度的从平面取向到同向取向(垂直)的转变。相反，在自然界中被归类为相对更具同渗性的阴离子(氟化物、硫酸盐、磷酸盐、醋酸盐、氯化物、硝酸盐、溴化物和氯酸盐)不会干扰没有添加盐的情况下观察到的液晶取向(即平面取向)。5CB在水溶液中的表面压力-面积等温线显示出离子特有效应，其顺序与LC有序转变相似。具体地说，杂化盐稳定了5CB的单分子膜，使其具有更高的表面压力和面密度(在27a2/molec时为12.6mN/m)。对于NaClO4)，因此分子倾斜角(与NaClO4的表面法线成30°)小于粘性盐(在38ä2/molec时为5.0mN/m)。对于氯化钠，对应的倾斜角为53°)。这些结果和本文报道的其他结果表明，阴离子与5CB单分子膜的特定相互作用导致了整体LC有序化转变。通过使用具有氰基的第二个LC(E7；观察到类似于5CB的离子特异性效应)和具有氟取代的芳香族基团的第三个LC(TL205；测量了弱偶极子和无离子特异性效应)，获得了对这些离子特异性相互作用涉及丁腈基团的命题的支持。最后，我们还证实了在微米厚的液晶薄膜中，阴离子诱导的取向转变涉及到液晶易轴的变化。总体而言，这些结果提供了对液晶-水界面离子现象的新见解，并揭示了LC油的长程有序化可以放大这些界面上的离子特定相互作用，使其转变为宏观有序化转变。
We report that specific anions (of sodium salts) added to aqueous phases at molar concentrations can trigger rapid, orientational ordering transitions in water-immiscible, thermotropic liquid crystals (LCs; e.g., nematic phase of 4′-pentyl-4-cyanobiphenyl, 5CB) contacting the aqueous phases. Anions classified as chaotropic, specifically iodide, perchlorate and thiocyanate, cause 5CB to undergo continuous, concentration-dependent transitions from planar to homeotropic (perpendicular) orientations at LC-aqueous interfaces within 20 s of addition of the anions. In contrast, anions classified as relatively more kosmotropic in nature (fluoride, sulfate, phosphate, acetate, chloride, nitrate, bromide, and chlorate) do not perturb the LC orientation from that observed without added salts (i.e., planar orientation). Surface pressure-area isotherms of Langmuir films of 5CB supported on aqueous salt solutions reveal ion-specific effects ranking in a manner similar to the LC ordering transitions. Specifically, chaotropic salts stabilized monolayers of 5CB to higher surface pressures and areal densities (12.6 mN/m at 27 Å2/molec. for NaClO4) and thus smaller molecular tilt angles (30° from the surface normal for NaClO4) than kosmotropic salts (5.0 mN/m at 38 Å2/molec. with a corresponding tilt angle of 53° for NaCl). These results and others reported herein suggest that anion-specific interactions with 5CB monolayers lead to bulk LC ordering transitions. Support for the proposition that these ion-specific interactions involve the nitrile group was obtained by using a second LC with nitrile groups (E7; ion-specific effects similar to 5CB were observed) and a third LC with fluorine-substituted aromatic groups (TL205; weak dipole and no ion-specific effects were measured). Finally, we also establish that anion-induced orientational transitions in micrometer-thick LC films involve a change in the easy axis of the LC. Overall, these results provide new insights into ionic phenomena occurring at LC-aqueous interfaces, and reveal that the long-range ordering of LC oils can amplify ion-specific interactions at these interfaces into macroscopic ordering transitions.
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