M. Wahab

Helical Nanofibers of Self-Assembled Bipolar Phospholipids as Template for Gold Nanoparticles

Bipolar phospholipids (bolalipids) represent an exciting class of amphiphilic molecules as they self-assemble in water to distinct structures of nanoscopic dimensions. Reported here are structural details of helical nanofibers, composed of achiral, symmetrical single-chain bolalipids with phosphocholine headgroups. These nanofibers are used as template for the fixation of gold nanoparticles (AuNPs) without prior functionalization. This realization of a metal array on bolalipid nanofibers is one of the rare examples of one-dimensional AuNP arrangements in solution. The loading and the heat of binding of AuNPs are determined applying transmission electron microscopy and isothermal titration calorimetry.

Interactions between spheroidal colloidal particles.

Using Derjaguins approximation, we have evaluated the interaction energy associated with van der Waals, electrostatic, depletion, and capillary forces between colloidal spheroids. If the interaction range between spheroids is distinctly smaller than the lengths of their principal axes, then simple pair potentials that depend on particle distance and orientation can be derived. Attractive interactions between adjacent spheroids favor their parallel alignment. Parallel spheroids can be arranged into a variety of densely packed configurations. All of these configurations turn out to have the same lattice energy. We discuss the implications of this degeneracy with respect to the stability of photonic crystals consisting of spheroids.

Monte Carlo Study of the Self-Assembly of Achiral Bolaform Amphiphiles into Helical Nanofibers

It is shown by coarse-grained off-lattice Monte Carlo simulations that a geometrically induced frustration of the parallel arrangement of rigid achiral bolaform amphiphiles can cause chirality in self-assembled nanostructures. The amphiphilic molecules are represented as rigid linear chains of 8 equally sized hydrophobic spheres (tail) and a hydrophilic sphere (head) at each end. The hydrophilic and hydrophobic spheres differ in size. A very simple interaction scheme consisting of only hard-core repulsion between all spheres and square-well attraction between hydrophobic spheres is sufficient for self-assembly into helical fibers for molecules with head/tail diameter ratios ranging from 1.3 to 1.8.

Computer Simulations of the Formation of Bile Salt Micelles and Bile Salt/DPPC Mixed Micelles in Aqueous Solutions

Brownian dynamics simulations for a coarse-grained model have been performed to study the formation of micelles from bile salts and mixed micelles with dipalmitoyl-phosphatidylcholine (DPPC) in aqueous solutions. The particular association behavior of bile salts as facial surfactants was shown to be caused by their special molecular architecture with a hydrophilic and a hydrophobic side. The experimentally observed smooth transition into the micellar region with increasing concentration is reproduced. Micelle size distributions have been evaluated at different bile salt concentrations. Typical structures of pure bile salt micelles could be identified. The composition and the structure of mixed micelles have been studied in their dependence on the bile salt/lipid concentration ratio in the aqueous solution. We have found that the bile salt fraction in the mixed micelles increases considerably with increasing bile salt/lipid concentration ratio and decreasing micelle size. The structural and thermodynamic features of micelle formation in the aqueous bile salt solutions with DPPC, which we have studied with the coarse-grained model, are in good qualitative agreement with experimental findings.

Monte Carlo Simulation of Surfactant Adsorption on Hydrophilic Surfaces

Monte Carlo simulations have been carried out to study the adsorption behavior of small flexible amphiphilic molecules on solid surfaces from aqueous solutions. A simple coarse-grained solvent-free off-lattice model, with a square-well pair potential and hard core excluded volume effect, has been used. Adsorption isotherms for weakly and strongly hydrophilic homogeneous surfaces have been determined. The adsorbed layer displays a coexistence region with an upper critical point. Below the critical temperature a densely packed patch coexists with a two-dimensional gas-analogous phase. Above the critical temperature, a percolating network forms at higher surfactant concentrations. Depending on the ratio between the strength of the hydrophobic effect and the adsorption energy, a large variety of associates has been observed. Monolayers, bilayers, admicelles, small clusters, and percolating networks as typical associate structures have been found. In the four-region model, which is extended by the coexistence region, a characteristic adsorbed layer structure for each region can be detected. Intermediate structure types have been produced by variation of the adsorption energy.

Depletion Force between Anisometric Colloidal Particles

A simple mathematical model for the depletion force between two arbitrarily shaped large convex colloidal particles immersed in a suspension of small spherical particles is proposed. Using differential geometry, the interaction potential is expressed in terms of the mean and Gaussian curvature of the particle surfaces. The accuracy of theoretical results is tested by Monte Carlo simulations for parallel and nonparallel circular cylinders. The agreement between theoretical results and simulated data is very good if the density of the depletion agent is not too high.

Vesicle Solubilization by Bile Salts: Comparison of Macroscopic Theory and Simulation

Lipid metabolism is accompanied by the solubilization of lipid bilayer membranes by bile salts. We use Brownian dynamics simulations to study the solubilization of model membranes and vesicles by sodium cholate. The solubilization pathways of small and large vesicles are found to be different. Both results for small and large vesicles can be compared with predictions of a macroscopic theoretical description. The line tension of bilayer edges is an important parameter in the solubilization process. We propose a simple method to determine the line tension by analyzing the shape fluctuations of planar membrane patches. Macroscopic mechanical models provide a reasonable explanation for processes observed when a spherical vesicle consisting of lipids and adsorbed bile salt molecules is transformed into mixed lipid-bile salt micelles.

Monte Carlo simulations of small vesicles under osmotic pressure.

A simple model for amphiphilic molecules is sufficient to predict self-assembly into spherical vesicles. The model is also useful to study the influence of osmotic pressure on the shape of vesicles, which serve as carriers in biological cells. The stability of small vesicles subjected to relatively high osmotic pressures is demonstrated. Fluctuations of small vesicles under osmotic stress are found to be in semiquantitative agreement with a macroscopic description. Small deviations between simulated results for vesicle fluctuations and macroscopic elasticity theory could result from the relative large membrane thickness in comparison to the vesicle radius. The simulations demonstrate that an osmotic pressure exerted by solute molecules outside a spherical vesicle can cause a shape transition, in agreement with results based on the elasticity theory of membranes.

A Monte Carlo study of bilayer formation in a lattice model

A Monte Carlo technique was applied to simulate a mixture of water with three-segment or with bolaform six-segment amphiphilic molecules in a coarse-grained lattice model. The molecular interactions between hydrophobic and hydrophilic segments, and between hydrophobic segments and water are purely repulsive; no additional attractive interactions are included. With a constant water content of 90% in both systems, we obtained the formation of layer structures at low temperatures and a sharp phase transition to high-temperature phases. The heat capacity has a significant peak at the phase transition in both systems. Structural and thermodynamic properties of the layers as a function of temperature were studied. Typical snapshots and statistically averaged density profiles illustrate the bilayer structure.

Simulation of fluid bridges and films

Adsorbed vapour molecules can form dense monolayers or even multilayers on solid surfaces. For sufficiently thick films the mesoscopic concept based on the disjoining pressure can be applied to evaluate the film thickness. The equilibrium thickness of an adsorbed film is evaluated by Monte Carlo simulation for a Lennard-Jones fluid. The results of the simulation are compared with the mesoscopic approach. Predictions of the mesoscopic theory are found to be valid also for film thicknesses exceeding only a few molecular diameters. Curved menisci are found to be well described by the macroscopic Young–Laplace equation, even when the curvature radius of the liquid–vapour interface is rather small.