C. Jeppesen

Depletion forces between two spheres in a rod solution

We study the depletion interaction between spherical particles of radius R immersed in a dilute solution of rigid rods of length L. The computed interaction potential is, within numerical accuracy, exact for any value of L/R. In particular we find that for L ~ R, the depth of the depletion well is smaller than the prediction of the Derjaguin approximation. Our results bring new light into the discussion on the lack of phase separation in colloidal mixtures of spheres and rods.

Local entropic effects of polymers grafted to soft interfaces

Abstract:In this paper, we study the equilibrium properties of polymer chains end-tethered to a fluid membrane. The loss of conformational entropy of the polymer results in an inhomogeneous pressure field that we calculate for Gaussian chains. We estimate the effects of excluded volume through a relation between pressure and concentration. Under the polymer pressure, a soft surface will deform. We calculate the deformation profile for a fluid membrane and show that close to the grafting point, this profile assumes a cone-like shape, independently of the boundary conditions. Interactions between different polymers are also mediated by the membrane deformation. This pair-additive potential is attractive for chains grafted on the same side of the membrane and repulsive otherwise.

THE COMPUTER AS A LABORATORY FOR THE PHYSICAL CHEMISTRY OF MEMBRANES

A mini-review is given of some recent advances in the use of computer-simulation approaches to the study of physico-chemical properties of lipid bilayers and biological membranes. The simulations are based on microscopic molecular interaction models as well as random-surface models of fluid membranes. Particular emphasis is put on those properties that are controlled by the many-particle character of the lamellar membrane, i.e. correlations and fluctuations in density, composition and large-scale conformational structure. It is discussed how dynamic membrane heterogeneity arises and how it is affected by various molecular species interacting with membranes, such as cholesterol, drugs, insecticides, as well as polypeptides and integral membrane proteins. The influence of bending rigidity and osmotic-pressure gradients on large-scale membrane conformation and topology is described.

Rods near curved surfaces and in curved boxes

We consider an ideal gas of infinitely rigid rods near a perfectly repulsive wall, and show that the interfacial tension of a surface with rods on one side is lower when the surface bends towards the rods. Surprisingly we find that rods on both sides of surfaces also lower the energy when the surface bends. We compute the partition functions of rods confined to spherical and cylindrical open shells, and conclude that spherical shells repel rods, whereas cylindrical shells (for thickness of the shell on the order of the rod-length) attract them. The role of flexibility is investigated by considering chains composed of two rigid segments.

Monte Carlo study of the inflation-deflation transition in a fluid membrane

We study the conformation and scaling properties of a self-avoiding fluid membrane, subject to an osmotic pressure p, by means of Monte Carlo simulations. Using finite size scaling methods in combination with a histogram reweighting techniques we find that the surface undergoes an abrupt conformational transition at a critical pressure p * , from low pressure deflated configurations with a branched polymer characteristics to a high pressure inflated phase, in agreement with previous findings [1, 2]. The transition pressure p * scales with the system size as p * ∞N -α , with α=0.69±0.01. Below p * the enclosed volume scales as V∞N, in accordance with the self-avoiding branched polymer structure, and for p→p * our data are consistent with the finite size scaling form V∞N β+ , where β + =1.43±0.04. Also the finite size scaling behavior of the radii of gyration and the compressibility moduli are obtained. Some of the observed exponents and the mechanism behind the conformational collapse are interpreted in terms of a Flory theory

The bridging conformations of double-end anchored polymer-surfactants destabilize a hydrogel of lipid membranes

Double-end-anchored poly-ethylene-glycol-surfactants (DEA-PEG-surfactants) induce the gelation of lyotropic lamellar Lα phases stabilized by undulation forces. The physical hydrogel (Lα,g) derives its viscoelasticity from the proliferation of defects at a mesoscopic level. The DEA-PEG-surfactants assume both looping and bridging conformations. The existence of novel bridging conformations is indicated by the coexistence of two lamellar phases and the limited swelling of the Lα and Lα,g phases. Modeling of the polymer decorated membranes demonstrates the existence of bridging and yields a rapidly decreasing density of bridging conformations with increasing interlayer spacing.

Grafted polymers are miniaturized pressure tools

Abstract Development of techniques to characterize and manipulate matter at the mesoscopic level (1–103 nm) has been the cornerstone of recent technological and scientific progress in biology, physics and chemistry. Many of the techniques rely on the possibility of applying well defined forces at the relevant lengths scales: revealing the structure of colloidal interactions with a surface force apparatus (SFA) (Israelachvili J., Intermolecular and Surface Forces, Academic Press, London, 1992); imaging biologic and other soft materials with an atomic force microscope (AFM) (Binnig G., Quate C.F., Gerber C., Phys. Rev. Lett. 56 (1986) 930); manipulating DNA, vesicles or emulsions with optical tweezers (Ashkin A., Dziedzic J., Bjorkholm J.M., Chu S., Opt. Lett. 11 (1986) 288)… We argue here that grafted polymers rightfully belong to the family of mesoscopic force tools, and predict the forces that these tools are expected to exert in a number of experimental situations.

Effect of vacancies and surfactants on the dynamics of ordering processes in multi-component systems

The dynamics of far-from-equilibrium ordering processes in multi-component systems, e.g. crystalline solids or fluid mixtures, that are being quenched in temperature is studied by computer-simulation techniques involving Monte Carlo (MC) as well as molecular dynamics (MD) methods. The systems are modeled by using simple two-dimensional statistical mechanical models such as multi-state Potts lattice models and models of particle systems interacting via Lennard-Jones-like potentials. The ordering dynamics is investigated under the conditions of both conservation and non-conservation of the order parameter as well as with and without the presence of additional ‘foreign’ components, such as vacancies and surfactants, that couple to the interfaces which develop during the ordering process. The present paper reviews recent progress in this field with an emphasis on a possible universal description of ordering dynamics.