Tag: lipid layer

Phase Transitions in Dense Lipid Monolayers Grafted to a Surface: Monte Carlo Investigation of a Coarse-Grained Off-Lattice Model

Post author By R Hilfer
Post date April 12, 1996

F. M. Haas, R. Hilfer, K. Binder

The Journal of Physical Chemistry 100 (37), 15290-15300 (1996)
DOI: 10.1021/jp9610980

submitted on
Friday, April 12, 1996

Semiflexible amphiphilic molecules end-grafted at a flat surface are modeled by a bead-spring chain with stiff bond angle potentials. Constant density Monte Carlo simulations are performed varying temperature, density, and chain length of the molecules, whose effective monomers interact with Lennard-Jones potentials. For not too large densities and low temperatures the monolayer is in a quasi-two-dimensional crystalline state, characterized by uniform tilt of the (stretched) chains. Raising the temperature causes a second-order transition into a (still solid) phase with no tilt. For the first time, finite size scaling concepts are applied to a model of a surfactant monolayer, and it is found that the technique in this case again is useful to locate the transition more precisely. For comparison, also a one-dimensional version of the model is studied, and directions for future extensions of this modeling are discussed.

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Critical phenomena Simulations

Continuum Monte Carlo Simulation at Constant Pressure of Stiff Chain Molecules at Surfaces

Post author By R Hilfer
Post date August 31, 1995

F. M. Haas, R. Hilfer

Journal of Chemical Physics 105, 3859 (1996)
https://doi.org/10.1063/1.472206

submitted on
Thursday, August 31, 1995

Continuum Monte-Carlo simulations at constant pressure are performed on short chain molecules at surfaces. The rodlike chains, consisting of seven effective monomers, are attached at one end to a flat twodimensional substrate. It is found that the model exhibits phases similar to the liquid condensed and liquid expanded phases of Langmuir monolayers. The model is investigated here for a wide range of pressures and temperatures using a special form of constant pressure simulation compatible with the symmetry breaking during tilting transitions in the liquid condensed phases. At low pressures the chains undergo a tilting transition exhibiting tilt directions towards nearest and also next nearest neighbours depending on temperature. At elevated temperatures and low pressure the film enters a fluidlike phase similar to the liquid expanded phase observed in experiment.

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Critical phenomena Equilibrium Simulations Statistical Physics

Continuum Monte-Carlo Simulations of Phase Transitions in Rodlike Molecules at Surfaces

Post author By R Hilfer
Post date October 28, 1994

R. Hilfer, F.M. Haas, K. Binder

Il Nuovo Cimento D 16, 1297-1303 (1994)
https://doi.org/10.1007/BF02458816

submitted on
Friday, October 28, 1994

Stiff rod-like chain molecules with harmonic bond length potentials and trigonometric bond angle potentials are used to model Langmuir monolayers at high densities. One end of the rod-like molecules is strongly bound to a flat two-dimensional substrate which represents the air-water interface. A ground-state analysis is performed which suggests phase transitions between phases with and without collective uniform tilt. Large-scale off-lattice Monte Carlo simulations over a wide temperature range show in addition to the tilting transition the presence of a strongly constrained melting transition at high temperatures. The latter transition appears to be related to two-dimensional melting of the head group lattice. These findings show that the model contains both, two- and three-dimensional ergodicity breaking solidification transitions. We discuss our findings with respect to experiment.

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Critical phenomena Equilibrium Simulations Statistical Physics

Layers of Semiflexible Chain Molecules Endgrafted at Interfaces: An Off-Lattice Monte Carlo Simulation

Post author By R Hilfer
Post date July 11, 1994

F.M. Haas, R. Hilfer, K. Binder

Journal of Chemical Physics 102, 2960-2969 (1995)
https://doi.org/10.1063/1.468604

submitted on
Monday, July 11, 1994

A coarse‐grained model for surfactant chain molecules at interfaces in the high density regime is studied using an off‐lattice Monte Carlo technique. The surfactant molecules are modeled as chains consisting of a small number (e.g., seven) of effective monomers. For the modeling of lipid monolayers, each effective monomer is thought to represent several CH2 groups of the alkane chain, but applications of the model to other polymers end grafted at solid surfaces also should be possible. The head segments are restricted to move in the adsorption plane, but otherwise do not differ from the effective monomers, which all interact with Lennard‐Jones potentials. Bond angle and bond length potentials take into account chain connectivity and chain stiffness. The advantage of this crude model is that its phase diagram can be studied in detail. Temperature scans show two phase transitions, a tilting transition at low temperatures between a tilted and an untilted phase, and a melting transition at high temperatures where the lattice of head groups loses its crystalline order.

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