Stjepan Marčelja

Repulsion of interfaces due to boundary water

Abstract The exponential repulsion of bilayers in lecithin-water dispersions is explained as arising from modification of water structure near the lecithin-water interface.

Chain ordering in liquid crystals: II. Structure of bilayer membranes

Abstract The ordering of the hydrocarbon chain interior of bilayer membranes has been calculated using the molecular field approximation developed in previous work on liquid crystals. Different statistical averages are evaluated by exact summation over all conformations of a single chain in the field due to neighboring molecules. The internal energy of each conformation, as well as contributions arising from interaction with the molecular field and from a lateral pressure on the chain have been included. The results describe properties of both lipid monolayers and bilayers. For monolayers, the calculated pressure-area relationships are in good agreement with experimental observations. The order parameter for hydrocarbon chains in bilayers (or monolayers) as a function of temperature, lateral pressure and position along the chain, is shown and compared with the available NMR data. Combining the results of calculation and NMR measurements we obtain the value for intrinsic lateral pressure within bilayer membranes, in excellent agreement with direct measurements on surface monolayers. The calculation also gives average length of hydrocarbon chains, thermal expansion coefficient and fraction of bonds in gauche conformations. The effect of cholesterol and proteins within the bilayer is qualitatively described, and the contribution of the bilayer interior to membrane elasticity is determined.

Chain ordering in liquid crystals. I. Even‐odd effect

In the liquid crystalline phases of many organic materials molecular end chains take part in the ordering process. For a single chain, this ordering is calculated by exact summation over all conformations of a chain in the molecular field due to the neighboring molecules. The nematic orientational ordering of a system where molecules consist of rigid parts and chains is then obtained within a self‐consistent molecular field approximation. The results are in very good agreement with the experimental phase diagrams for homologous series of liquid crystals, and for the first time provide the explanation for the well known even‐odd effect in isotropic‐nematic transition temperatures. In agreement with observations, the corresponding transition entropies exhibit both the increase with increasing end‐chain length and the even‐odd variations. The decrease of order along the chain is also calculated and compared with the available data.

Lipid-mediated protein interaction in membranes.

This study describes the effects ensuing from a non-specific interaction between membrane integral proteins and the surrounding lipids. The results are obtained using an appropriate molecular field theory to describe the ordering of membrane lipids. The modification of the lipid structure near a protein molecule, while most pronounced within the annulus of the first neighbour molecules, extends two or three layers beyond the annulus. The ordering of lipids within the annulus has a modified temperature dependence, and becomes a continuous function of temperature for low lipid/protein ratios. The change in order of lipid molecules surrounding a protein leads to an indirect, lipid-mediated interaction between membrane integral proteins. This interaction depends sensitively on the bulk lipid order. Under favourable circumstances, it gives rise to protein aggregation.

Inhomogeneous Coulomb fluids with image interactions between planar surfaces. I

Description of Coulomb particles near a surface is complicated by the effect of the surface on the ion–ion correlations, the electrostatic images (if the surface is a dielectric boundary), and the long range of the correlations in the lateral direction. We formulate the problem of the ion distribution for inhomogeneous Coulomb fluids with an arbitrary core potential and confined between two planar surfaces, with which the particles can interect via any short‐ranged potential. All orders of image interactions are included into an effective position‐dependent pair potential. The problem is solved by mapping the inhomogeneus three‐dimensional system into a homogeneous two‐dimensional one. The system is subdivided into M layers, and then shown to be isomorphic to an M‐component fluid mixture in two dimensions. The mapping becomes exact as the number of layers (‘‘components’’) M→∞, and is accordingly an excellent approximation for finite, but large M. The correlations and other statistical mechanical functions can now be obtained with any conventional closure scheme. This forms the basis for a numerical method to solve the complete integral equations for the one‐ and two‐particle distribution functions, using, e.g., the hypernetted chain (HNC) closure for the inhomogeneous pair correlations as the sole approximation. The associated numerical procedures are briefly described. The method could also be applied to study various problems for solute particles interacting via other pair potentials.

Attractive double-layer interactions between calcium clay particles

Abstract The mechanism responsible for restricted swelling of calcium clays in water and in aqueous salt solutions is examined. Particular attention is given to the montmorillonite, vermiculite, and illite systems. The diffuse double-layer interactions are calculated using an advanced statistical mechanical method, the Anisotropic Hypernetted Chain approximation. In contrast to the predictions of the simple Poisson-Boltzmann theory, which is based on inadequate approximations, for divalent ions the double-layer interaction is strongly attractive at relatively small surface separations, provided the density of surface charge is reasonably large. The attraction is a consequence of the correlations between the ions, giving rise to an electrostatic fluctuation force. When the van der Waals forces between the dielectric media are included, the attraction becomes even larger. The attractive force between the Ca-clay particles gives rise to a stable state, a potential minimum. No specific binding or hydration effects involving the Ca2+ ions are needed to explain the existence of the minimum. However, the position of the potential minimum as derived using the model, based on diffuse double-layer and van der Waals forces only, is influenced by hydration interactions. Some implications of the potential minimum are discussed.

Double layer interactions in mono‐ and divalent electrolytes: A comparison of the anisotropic HNC theory and Monte Carlo simulations

The interaction between charged surfaces in 1:1, 1:2, and 2:2 electrolyte solutions at various concentrations have been calculated using the anisotropic hypernetted chain (HNC) theory and Monte Carlo (MC) simulations. For divalent counterions, the surface interaction has an attractive minimum at short separations. This minimum turns more attractive at increasing electrolyte concentration, while the interaction at somewhat larger separations becomes oscillatory. The agreement between the HNC and the MC results is excellent for the 1:2 and 2:2 electrolyte systems. For 1:1 electrolytes the surface interaction is repulsive except at high concentrations at which it shows a weak attractive minimum. The HNC and the MC results agree quantitatively except in a few angstroms wide region at short separations, where the agreement is only qualitative due to a slight difference in the contributions from the hard core interactions. This is a consequence of the neglect of the short‐range bridge function in the HNC approx...

A theoretical and experimental study of forces between charged mica surfaces in aqueous CaCl2 solutions

The interaction between two mica surfaces immersed in CaCl2 solutions has been directly measured. Accurate theoretical calculations, including the anisotropic hypernetted chain (HNC) theory used in this work, predict that at reasonably high surface charge densities the electrical double layer interactions in the presence of divalent counterions should be attractive at short surface separations (in the range 0.6–2 nm). Under most conditions investigated, the experimental results indicate that this indeed is the case. The attraction is a consequence of the correlation between the ions. In addition to the double layer interaction, in most cases the measured force contains an oscillatory contribution. At low CaCl2 concentrations and small surface separations, Ca2+ ions between the surfaces are exchanged for H3O+ ions, which decreases the oscillatory interaction and the ion‐correlation attraction. At high concentrations the force is dominated by a strong hydration repulsion, which is related to the adoration o...

Interaction of charged surfaces in electrolyte solutions

Abstract We report a restricted primitive model calculation of the double-layer interaction between two uniformly charged surfaces immersed in an electrolyte solution, where the anisotropic hypernetted chain approximation is utilized for the pair correlations. The strong, attractive pressure contribution resulting from ion-ion correlations is very similar to that found earlier for double layers with only counterions present. Consequently, the Poisson-Boltzmann equation, which neglects ion-ion correlations, significantly overestimates the double-layer repulsion in most situations. Some ion concentration profiles are presented, and for large separations between the surfaces the profiles close to each surface agree with grand canonical simulation results for a single wall.

Spatially varying polarization in water. A model for the electric double layer and the hydration force

The formalism derived in the preceding paper is used to model the response of aqueous solutions to a spatially-varying applied electric field. It leads to a generalized theory of the electric double layer which explicitly takes into account the microscopic structure of the solvent. As the solvent polarization is allowed to vary depending on both the macroscopic electric field and the specific interactions at the surface, the result is more freedom in the structure of the electric double layer. We consider generalized expressions for the double-layer free energy, as well as the interaction of two double layers which include specific surface polarization. Such interaction consists of both classical double-layer repulsion and the strong, short-range ‘hydration force’.