Splay stripe textures in Langmuir monolayers

Ruíz-García, Jaime; Qiu, Xia; Tsao, Mei-Wei; Marshall, Gary T.; Knobler, Charles M.; Overbeck, Gernot A.; Moebius, D.

doi:10.1021/j100129a006

Cited by 65 publications

(47 citation statements)

References 4 publications

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“…1b). These stripes in the tubules are suggested to be analogous to the tilted stripes seen in the Langmuir monolayers (20) but in a cylindrical rather than a planar geometry. To our knowledge, there has been no experimental test of the theoretical predictions because of the difficulty in probing the local direction of the molecular tilt in the tubules.…”

mentioning

confidence: 71%

Liquid-crystal imaging of molecular-tilt ordering in self-assembled lipid tubules

Zhao

Mahajan

Lü

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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Self-assembled cylindrical tubules of chiral phospholipids are interesting supramolecular structures. Understanding the moleculartilt order is a key step in controlling the size and shape of the tubules and designing new functional materials. The current theories based on the chiral interactions, coupled with molecular tilt, have predicted that the tubules could have both uniform and modulated tilt states. Here, we image the molecular-tilt order in the self-assembled tubules of a chiral phospholipid by using liquid crystals as an optical amplification probe. We demonstrate that the organization of the molecular-tilt azimuth in the lipid tubules can induce an azimuthal orientation in the liquid crystals. Both uniform and modulated tilt states of the lipid tubules are observed after liquid-crystal optical amplification.liquid-crystal amplification ͉ molecular ordering ͉ self-assembly ͉ optical amplification ͉ supramolecular structure T he self-assembled cylindrical tubules have attracted considerable attention because of their interesting supramolecular structures and technological applications (1, 2). It has been found that a variety of amphiphilies, including alkylaldonamides (3), diacetylenic phospholipids (4, 5), amino acid-based surfactants (6, 7), and multicomponent mixtures in bile (8, 9), selfassemble into cylindrical tubules in solutions. Recently, advances have been made in controlling the sizes and shapes of tubules by adjusting the chemical compositions and the conditions under which self-assembly occurs (10-16).Theoretical models based on the chiral interactions, coupled with molecular tilt, have been used to explain the formation of the cylindrical tubules (17)(18)(19). In these models, the chiral molecules do not pack parallel to their neighbors but, rather, at a nonzero angle with respect to their neighbors. This chiral packing causes the twisting of a bilayer stripe, which leads to the formation of cylindrical tubules. The model by Selinger et al. (19) suggests that the tubules can have both uniform and modulated tilt states. In the uniform tilt state, the tubule has a constant orientation of the molecule tilt with respect to the equator of the cylinder (Fig. 1a). In the modulated tilt state, the tubule has a periodic modulated structure by helical stripes in the molecular tilted direction winding around the cylinder. The direction of the molecular tilt smoothly varies across the stripes and jumps at the edges of the stripes (Fig. 1b). These stripes in the tubules are suggested to be analogous to the tilted stripes seen in the Langmuir monolayers (20) but in a cylindrical rather than a planar geometry. To our knowledge, there has been no experimental test of the theoretical predictions because of the difficulty in probing the local direction of the molecular tilt in the tubules.In recent years, there has been an increased interest in developing an imaging technique based on liquid-crystal optical amplification for studying the structures and physical properties of organic and biological interfac...

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mentioning

confidence: 71%

Liquid-crystal imaging of molecular-tilt ordering in self-assembled lipid tubules

Zhao

Mahajan

Lü

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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“…Such a progression is evident in the fatty acids as well, where spiral textures are common in tetradecanoic acid and much rarer in hexadecanoic acid. 10 The presence of both two-wall defects and star defects in domains of C18-M demonstrates that the stabilities of the two textures are not very much different; the decrease in the number of stars when the cooling rate is lowered suggests that they are the less stable texture. In some cases the relative stabilities of the textures can be computed from the theory, as has been done, for example, in determining the optimal numbers of arms in a star defect.9 But we do not know the values of the core energies, so it is impossible to determine the relative stabilities of textures with different core defects.…”

Section: Identity Of Phases No Diffraction Studies Of the Esters Havementioning

confidence: 96%

Observation of a Change from Splay to Bend Orientation at a Phase Transition in a Langmuir Monolayer

Fischer¹,

Tsao²,

Ruíz-García³

et al. 1994

J. Phys. Chem.

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The behavior of Langmuir monolayers of methyl octadecanoate, methyl nonadecanoate, and methyl eicosanoate has been studied by polarized fluorescence microscopy and Brewster angle microscopy in the neighborhood of a transition between two liquid-condensed (LC) phases. As the monolayer is cooled through the transition, the molecular tilt azimuth in six-arm star defects of the LC phase changes by 90° (from a splay to a bend texture), a process that we call "blooming". In other domains the transition is seen as a change from textured distorted hexagons to rectangular domains in which the tilt direction is uniform. The changes in texture and morphology are consistent with a transition from a hexatic phase to a crystalline phase with herringbone order.Phase diagrams of monolayers of long-chain fatty acids and their esters at the air/water interface (Langmuir monolayers) have been studied for more than 50 years. While much of the knowledge about transitions between monolayer phases has been derived from surface pressurearea (?r-A) isotherms, direct visualization of phase transitions is becoming increasingly important. Transitions between the gas (G) and liquid expanded (LE) phases and between the LE phases and liquid condensed (LC)' phases are readily observed by fluorescence micro~copy.~~~ Recently, techniques such as polarized fluorescence microscopy (PFM)4 and Brewster angle microscopy (BAM),5,6 which are sensitive to differences in molecular tilt, have allowed transitions between several condensed monolayer phases to be dete~ted.~J PFM and BAM have also revealed that Langmuir monolayers often self-organize into structures in which the molecular tilt azimuth is ordered over mesoscopic (and even macroscopic) di~tances.~J~ This organization produces an optical "texture" similar to those characteristic of liquid crystals. The textures are sometimes associated with metastable defects? but they may also correspond to equilibrium states.11J2 As in liquid crystals, they reflect the underlying microscopic order in each phase, which, for monolayers of fatty acids, has been determined from diffraction studies.lS15 While no molecular theory of the textures exists, a phenomenological model that incorporates the structural information is capableof describing many of theobserved textures.12J6 Qiu et al.9 examined the textures in LC domains of the C14-C19 methyl esters in coexistence with the isotropic LE phase. At high temperatures, the domains have a uniform texture consistent with a phase in which the molecules are normal to the surface. When the temperature is lowered, the domains undergo a sharp transition to a tilted phase, one in which the molecules are uniformly inclined with respect to the surface normal. The transition is accompanied by the formation of defects. Some domains organize into "star defects" that contain six wedge-shaped regions in which the tilt azimuth is uniform; the azimuthal angle changes sharply by 60" at walls that separate the wedges. Others form defects in which there are three arms that meet at the v...

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“…Although broadened peaks have been observed from 2D hexatic phases of freely suspended thin liquid crystal films, 7,48,50 the evidence that they have been observed for LM is ambiguous. [67][68][69][70] In any event, they would certainly not be observable for liquid LM phases of behenic acid, or similar systems. 11,18,19,28,71,72 …”

Section: S͑q Xymentioning

confidence: 98%

C 60 -propylamine adduct monolayers at the gas/water interface: A Brewster angle microscopy and x-ray scattering study

Fukuto

Penanen

Heilmann

et al. 1997

The Journal of Chemical Physics

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Brewster angle microscopy ͑BAM͒, x-ray specular reflectivity and grazing-incidence x-ray diffraction ͑GID͒ studies of C 60 -propylamine adduct monolayers at the gas/water interface as a function of molecular area are reported. At large molecular areas (AϾϳ150 Å 2 /molecule), BAM images reveal macroscopic heterogeneity in the film, consisting of the coexistence between regions covered with uniform solidlike monolayer and bare water surface. After compression to a limiting molecular area of 150 Å 2 /molecule, the film is observed to be homogeneous, with the uniform monolayer covering the entire available surface. Both the x-ray reflectivity results and the GID patterns are consistent with the formation of a uniform monolayer at Aϳ150 Å 2 /molecule, while the little dependence that the GID patterns have on the molecular area for AϾϳ150 Å 2 /molecule is consistent with the heterogeneity in the film. Upon further compression to higher densities (AϽϳ120 Å 2 /molecule), the x-ray reflectivity results suggest the formation of a partial layer either at the molecule/gas interface or at the molecule/water interface. In this high density regime, the shift in the observed GID pattern with molecular area is much smaller than would be expected if the film were to remain a homogeneous monolayer, also consistent with the formation of an inhomogeneous partial layer. The analysis of the broad GID pattern observed from a uniform monolayer in terms of a model 2D radial distribution function, implies a short range positional correlation, extending to only a few molecular distances. The average nearest neighbor distance (dϳ13 Å͒, extracted from the GID analysis, is consistent with the limiting molecular area (Aϳ150 Å 2 /molecule) assuming local hexagonal packing. These results together with the sharp facets observed in the BAM images demonstrate that the monolayer when uniform is a two-dimensional amorphous solid.

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Splay stripe textures in Langmuir monolayers

Cited by 65 publications

References 4 publications

Liquid-crystal imaging of molecular-tilt ordering in self-assembled lipid tubules

Liquid-crystal imaging of molecular-tilt ordering in self-assembled lipid tubules

Observation of a Change from Splay to Bend Orientation at a Phase Transition in a Langmuir Monolayer

C 60 -propylamine adduct monolayers at the gas/water interface: A Brewster angle microscopy and x-ray scattering study

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