Lattice structures and molecular tilts in Langmuir monolayers of saturated fatty acid–alcohol mixtures

Shih, Margaret; Durbin, Mary K; Malik, A.; Zschack, P.; Dutta, Pulak

doi:10.1063/1.468042

Cited by 46 publications

(60 citation statements)

References 6 publications

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“…Most of them were devoted to partially ordered [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and 2D crystalline 18-29 phases of simple amphiphiles-fatty acids, alcohols, and their derivatives. At least eight structurally distinct condensed phases were revealed for nonchiral substances.…”

Section: Introductionmentioning

confidence: 99%

Packing of Hydrocarbon Chains and Symmetry of Condensed Phases in Langmuir Monolayers

1998

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We compare the packing characteristics of alkyl chains in Langmuir monolayers of nonchiral and racemic compounds as determined from available grazing incidence X-ray diffraction data. The analysis demonstrates a gradual change of the projected unit cell dimensions from those of a hexagonal packing of hydrocarbon chains, characteristic of high-temperature monolayer phases, to one of two more dense rectangular packing modes with the projected unit cell dimensions 5.0 × 7.5 Å 2 and 4.4 × 8.7 Å 2 , characteristic of low-temperature phases. The 5.0 × 7.5 Å 2 unit cell incorporates the well-known herringbone arrangement, with an ideally 90°dihedral angle between the planes of carbon backbone chains. The 4.4 × 8.7 Å 2 cell, almost never observed in 3D structures, is characterized by a 40°dihedral angle. We characterize the packing modes by lattice energy calculations. The distribution of the projected unit cell dimensions for the various Langmuir monolayers reveals no discontinuity in the local molecular order between crystalline phases and mesophases. The local symmetry of the molecular packing, as determined from the X-ray data, is compared with the symmetry deduced from the Landau theory of phase transitions. The symmetry of the local order in the mesophases is not maintained on the long-range length scale. We show that two phases of the same local molecular arrangement, a herringbone-ordered two-dimensional crystal and the corresponding one-dimensional mesophase, possess mutually orthogonal directions of glide symmetry.

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Section: Introductionmentioning

confidence: 99%

Packing of Hydrocarbon Chains and Symmetry of Condensed Phases in Langmuir Monolayers

1998

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“…The swiveling transition, which has been recently extensively studied, 12,13 is the transition between the L 2 (L 2h ) phase ͓rectangular lattice with molecules tilted towards ͑NN͒ direction͔ and the phase denoted as L 2 Ј(L 2 *) ͓rectangular lattice, characterized by molecular tilt ordering towards ͑NNN͒ direction͔. In terms of the order parameters introduced here, the transition corresponds to the change: ͗Y 4 ͘⇒Ϫ͗Y 4 ͘, or equivalent ⑀ 2 ⇒Ϫ⑀ 2 , for one domain of the phases and with corresponding relations for the two other domains.…”

Section: The Swiveling Transitionmentioning

confidence: 99%

“…Further, we have departed from the assumptions of previous authors by modeling these phases with only one molecule per unit cell. 12 These calculations were done at a low enough pressure ͑isotropic͒ to ensure a global minimum that exhibited tilt ͑Ϫ45 in our arbitrary pressure scale͒. 6 After locating the potential minimum ͑with respect to lattice vectors a and b and the three orientational angles͒, ⑀ 1 was set equal to 0, and a 0 of the parent hexagonal lattice was found by solving the equation…”

Section: Calculationsmentioning

confidence: 99%

“…Advocating the liquid-crystalline approach, Kaganer et al 4 have recently characterized the distortions with respect to a hexagonal lattice using two concepts: the so-called distortion ellipse 11 and the ''projected unit cell.'' 12 With so many parameters, the theory not only became less transparent but also did not help us to understand and to describe phases observed in the generic phase diagram. In particular, the swiveling transition, recently studied in detail, 12,13 provides an example which can critically test current theoretical descriptions of Langmuir monolayers.…”

Section: Introductionmentioning

confidence: 99%

“…12 With so many parameters, the theory not only became less transparent but also did not help us to understand and to describe phases observed in the generic phase diagram. In particular, the swiveling transition, recently studied in detail, 12,13 provides an example which can critically test current theoretical descriptions of Langmuir monolayers.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Langmuir monolayers as disordered solids: Strain-tilt-backbone coupling and natural order parameters for the swiveling transitions

Luty

Swanson

Eckhardt

1999

The Journal of Chemical Physics

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Multistability of Langmuir monolayers, in particular those composed of fatty acids, is reflected by a very rich and complicated phase diagram. We argue that strain-tilt-backbone coupling determines the behavior described by that diagram. Following the solid state approach, we show that a natural order parameter set is defined by thermal averages of spherical harmonics and strain tensor components. In addition, we show that the backbone order parameter can be conveniently represented by an elastic dipole tensor. Treating Langmuir monolayers as disordered solids, we have derived an orientational entropy contribution to the free energy. The swiveling transition between L 2 (L 2h ) and L 2 Ј(L 2 *) phases is discussed in detail and is conveniently described in terms of the proposed order parameters. We discuss why the change in the distortion direction of the 2D unit cell tracks the change in tilt direction of the molecules, and also why the cell dimensions involved in the swiveling transition are the same in both phases.

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Superlattice configurations in linear chain hydrocarbon binary mixtures — Case ofn-C28H58:n-CxH2x+2 (x = 10,12,14,16)

Shashikanth¹,

Neelima²

2005

J Chem Sci

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Powder XRD data of mixtures of title compounds are interpreted in terms of superlattices (SL). It is suggested that SL configurations (orthorhombic-orthorhombic, orthorhombic-monoclinic, monoclinic-monoclinic) are realizable, because of discrete orientational changes in the alignment of molecules of n-C 28 H 58 hydrocarbon, through an angle mθ, where m = 1, 2, 3 … and angle θ has an average value of 3.3°. Supporting literature evidence on the inclinations are discussed.

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Lattice structures and molecular tilts in Langmuir monolayers of saturated fatty acid–alcohol mixtures

Cited by 46 publications

References 6 publications

Packing of Hydrocarbon Chains and Symmetry of Condensed Phases in Langmuir Monolayers

Packing of Hydrocarbon Chains and Symmetry of Condensed Phases in Langmuir Monolayers

Langmuir monolayers as disordered solids: Strain-tilt-backbone coupling and natural order parameters for the swiveling transitions

Superlattice configurations in linear chain hydrocarbon binary mixtures — Case ofn-C28H58:n-CxH2x+2 (x = 10,12,14,16)

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