Characterization of Structural and Dynamical Behavior in Monolayers of Long-Chain Molecules Using Molecular-Dynamics Calculations

Bareman, James P.; Cardini, Gianni; Klein, Michael L.

doi:10.1103/physrevlett.60.2152

Cited by 135 publications

(59 citation statements)

References 36 publications

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“…In this sense the LPE simulation can be viewed as approximating a polymorphic pretransition from a spherical micelle, whose organization is dominated by surface dipole interactions, toward a "cylindrical" micelle, stabilized by a more uniform compact packing ofthe fatty acid chains. Indeed, similar transitions have also been observed in model monolayer simulations (16). Although computer-generated, this transition illustrates how environmental factors such as hydration can affect dynamic organization of phospholipid molecules and result in polymorphism.…”

mentioning

confidence: 63%

Molecular Dynamics Simulation of a Phospholipid Micelle

Wendoloski

Kimatian

Schutt

et al. 1989

Science

104

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The dynamic character of phospholipid aggregates limits conventional structural studies to the determination of average molecular features. In order to develop more detailed descriptions of phospholipid structure for comparison with experiment, the molecular dynamics of a hydrated lysophosphatidylethanolamine (LPE) micelle, incorporating 85 LPE and 1591 water molecules, have been simulated. Comparison of the initial and equilibrated micelles shows substantial differences both in LPE hydrocarbon chain conformation and polar head-group-solvent interactions. Although these changes produce only subtle effects on the averaged structural properties of the system, the alterations in hydrocarbon chain packing and head-group solvation appear to mimic a polymorphic pretransition from a spherical toward a cylindrical micelle structure.

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mentioning

confidence: 63%

Molecular Dynamics Simulation of a Phospholipid Micelle

Wendoloski

Kimatian

Schutt

et al. 1989

Science

104

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“…This is consistent with molecular dynamics simulations that predict a small percentage of gauche conformations in these nearly rigid rod monolayers with the gauche defects concentrated near the -CH 3 end. 15,16 This analysis demonstrates that the triacontanol monolayer at the water-vapor interface is close packed with nearly alltrans and nearly upright molecules (normal to the interface). This is consistent with the understanding that molecules in the condensed phases of alkanol (and also alkanoic acid) Langmuir monolayers are nearly all-trans rigid rods.…”

Section: Monolayers At the Water-vapor Interfacementioning

confidence: 92%

Surfactant and Water Ordering in Triacontanol Monolayers at the Water−Hexane Interface

Tikhonov

Schlossman

2003

J. Phys. Chem. B

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Our view of molecular ordering in Langmuir monolayers at the water-vapor interface influences our understanding of molecular ordering at other interfaces, including liquid-liquid interfaces for which structural information is scarce. We present a comparative study of a monolayer of a long-chain alkanol at the watervapor and water-hexane interfaces using X-ray reflectivity to highlight significant differences between these two interfaces. The molecules in the Langmuir monolayer form an ordered phase of nearly rigid rods. In contrast, at the water-hexane interface, the triacontanol molecules form a condensed phase with progressive disordering of the chain from the -CH 2 OH to the -CH 3 group. Surprisingly, at the water-hexane interface, the density in the headgroup region is 10-15% greater than either bulk water or the ordered headgroup region found at the water-vapor interface. It is conjectured that this higher density is a result of water penetration into the headgroup region of the disordered monolayer.

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“…Although this 3-sigma difference between the densities of the two layers of the tailgroup is a weak effect, it is consistent with molecular dynamics simulations that predict a small percentage of gauche conformations in these nearly rigid rod monolayers with the gauche defects concentrated near the -CH 3 end. 62,63 The electron density profile allows us to calculate N, the number of electrons per area of the interface ͑see Table I͒ by integrating just the monolayer part of the profile over the distance normal to the interface ͑equivalently, N ϭ0.333͚ iϭ1,3 i L i ). Using the area per molecule of 18.7 Å 2 determined by x-ray surface diffraction for a condensed monolayer of C 30 OH molecules at the water-vapor interface 10 yields 252 (ϭ18.7ϫ13.5) electrons per molecule.…”

Section: Oh Alkanol At the Water-vapor Interfacementioning

confidence: 99%

Molecular ordering and phase transitions in alkanol monolayers at the water–hexane interface

Tikhonov

Pingali

Schlossman

2004

The Journal of Chemical Physics

138

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The interface between bulk water and bulk hexane solutions of n-alkanols (H(CH(2))(m)OH, where m=20, 22, 24, or 30) is studied with x-ray reflectivity, x-ray off-specular diffuse scattering, and interfacial tension measurements. The alkanols adsorb to the interface to form a monolayer. The highest density, lowest temperature monolayers contain alkanol molecules with progressive disordering of the chain from the -CH(2)OH to the -CH(3) group. In the terminal half of the chain that includes the -CH(3) group the chain density is similar to that observed in bulk liquid alkanes just above their freezing temperature. The density in the alkanol headgroup region is 10% greater than either bulk water or the ordered headgroup region found in alkanol monolayers at the water-vapor interface. We conjecture that this higher density is a result of water penetration into the headgroup region of the disordered monolayer. A ratio of 1:3 water to alkanol molecules is consistent with our data. We also place an upper limit of one hexane to five or six alkanol molecules mixed into the alkyl chain region of the monolayer. In contrast, H(CH(2))(30)OH at the water-vapor interface forms a close-packed, ordered phase of nearly rigid rods. Interfacial tension measurements as a function of temperature reveal a phase transition at the water-hexane interface with a significant change in interfacial excess entropy. This transition is between a low temperature interface that is nearly fully covered with alkanols to a higher temperature interface with a much lower density of alkanols. The transition for the shorter alkanols appears to be first order whereas the transition for the longer alkanols appears to be weakly first order or second order. The x-ray data are consistent with the presence of monolayer domains at the interface and determine the domain coverage (fraction of interface covered by alkanol domains) as a function of temperature. This temperature dependence is consistent with a theoretical model for a second order phase transition that accounts for the domain stabilization as a balance between line tension and long range dipole forces. Several aspects of our measurements indicate that the presence of domains represents the appearance of a spatially inhomogeneous phase rather than the coexistence of two homogeneous phases.

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Characterization of Structural and Dynamical Behavior in Monolayers of Long-Chain Molecules Using Molecular-Dynamics Calculations

Cited by 135 publications

References 36 publications

Molecular Dynamics Simulation of a Phospholipid Micelle

Molecular Dynamics Simulation of a Phospholipid Micelle

Surfactant and Water Ordering in Triacontanol Monolayers at the Water−Hexane Interface

Molecular ordering and phase transitions in alkanol monolayers at the water–hexane interface

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