On the relationship between the micellar structure and the diamagnetic anisotropy of amphiphilic nematic mesophases
On the relationship between the micellar structure and the diamagnetic anisotropy of amphiphilic nematic mesophases
复制标题
两亲性向列中间相胶束结构与抗磁各向异性的关系
DOI：
10.1080/00268978100100411
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发表时间：
1981
期刊：
影响因子：
--
通讯作者：
M. C. Holmes
中科院分区：
文献类型：
--
作者：
N. Boden;K. Radley;M. C. Holmes
两亲性双金属中间相的结构和动力学性质是胶体科学家们感兴趣的问题[1]。它们的微观织构[2，3]，在强磁场中的重定向运动n [4-6]，以及它们在各向同性胶束溶液和六方或近晶层状相之间的出现与生电子单元是非球形胶束的假设一致[7]。这些中间相根据它们是否分别具有正或负的抗磁各向异性[AX= X~-X]而被分类为I型或II型[8]。癸基硫酸钠/水/癸醇/硫酸钠体系已被广泛研究[4，8，9]，表现出两种类型的中间相。X射线衍射测量[10，11]表明，胶束在I型相中为圆柱形，在II型相中为圆盘形。在最近的一些文章中[1，12-15]的陈述表明，这一结果已被认为是具有普遍适用性的。全氟辛酸铯/水二元体系形成了一个存在于近晶层状相和各向同性胶束溶液之间的反相：它是抗磁性正的，但由盘形胶束组成[16，17]。这与上述规则直接矛盾。为了理解这两个系统的行为差异，有必要比较聚亚甲基和全氟聚亚甲基链的抗磁性。AX对于前者为负[18]，对于后者为正（从19 F化学位移张量推断[19]）。因此，具有烃或全氟化碳部分的两亲物的优选取向将分别垂直于和平行于磁场，这对应于所述胶束结构中这些链的平均取向。显然，它是组成分子的抗磁性各向异性的标志，其主导中间相的抗磁性，而不是胶束的固有形状抗磁性。这个有趣的结果意味着，应该可以改变中间相抗磁各向异性的符号，
The structural and dynamical properties of amphiphilic nematic mesophases are currently intriguing the colloid scientists [1]. Their microscopic textures [2, 3], the reorientational motion of the nematic director n in strong magnetic fields [4-6], and their occurrence between isotropic micellar solutions and either hexagonal or smectic lamellar phases are consistent with the hypothesis that the nematogenic units are non-spherical micelles [7]. These mesophases have been classified [8] as type I or type II according to whether they possess, respectively, positive or negative diamagnetic anisotropies [AX= X~-X• The system sodium decylsulphate/water/decanol/sodium sulphate, which has been studied extensively [4, 8, 9], exhibits both types of mesophase. X-ray diffraction measurements [10, 11] have shown that the micelles are cylindrical in shape in the type I phase and disk shaped in the type II phase. Statements in a number of recent paPers [1, 12-15] suggest that this result has been taken to be of general applicability.The binary system caesium perfluoro-octanoate/water forms a nematic phase which exists between a smectic lamellar phase and an isotropic micellar solution: it is diamagnetically positive and yet consists of disk shaped micelles [16, 17]. This is a direct contradiction of the above rule. To understand the discrepancy in the behaviour of these two systems, it is necessary to compare the diamagnetic properties of the polymethylene and perfluoropolymethylene chains. AX is negative for the former [18] and positive for the latter (inferred from the 19F chemical shift tensor [19]). Thus, the preferred orientations of amphiphiles with either hydrocarbon or perfluorocarbon moieties will be, respectively, perpendicular and parallel to the magnetic field which corresponds with the averaged orientations of these chains in the micellar structures described. Clearly, it is the sign of the diamagnetic anisotropy of the constituent molecules which dominates the diamagnetic properties of the mesophase and not the inherent shape diamagnetism of the micelle. This interesting result implies that it should be possible to change the sign of the mesophase diamagnetic anisotropy,