Neutral and Charged Polymers at Interfaces

Chain-like macromolecules (polymers) show characteristic adsorption properties due to their flexibility and internal degrees of freedom, when attracted to surfaces and interfaces. In this review we discuss concepts and features that are relevant to the adsorption of neutral and charged polymers at equilibrium, including the type of polymer/surface interaction, the solvent quality, the characteristics of the surface, and the polymer structure. We pay special attention to the case of charged polymers (polyelectrolytes) that have a special importance due to their water solubility. We present a summary of recent progress in this rapidly evolving field. Because many experimental studies are performed with rather stiff biopolymers, we discuss in detail the case of semi-flexible polymers in addition to flexible ones. We first review the behavior of neutral and charged chains in solution. Then, the adsorption of a single polymer chain is considered. Next, the adsorption and depletion processes in the many-chain case are reviewed. Profiles, changes in the surface tension and polymer surface excess are presented. Mean-field and corrections due to fluctuations and lateral correlations are discussed. The force of interaction between two adsorbed layers, which is important in understanding colloidal stability, is characterized. The behavior of grafted polymers is also reviewed, both for neutral and charged polymer brushes.Comment: a review: 130 pages, 30 ps figures; final form, added reference

Free Energy and Surface Forces in polymer systems: Monte Carlo Simulation Studies

The thesis is focused on two major subjects: - Calculation of free energy is of great help in the study of any molecular system. The free energy determines phase behaviour of colloids and polymer solutions. It also controls the conformational properties of single macromolecules, e.g. DNA, proteins, copolymers. One of the most robust computer simulation techniques for free energy estimation is an Expanded Ensemble approach which is used throughout all the present work. Starting with a simple lattice polymer model, we develop and test an iterative procedure that makes the method automatic and accurate. Then we apply it to various polymer systems. The ideas behind a few similar approaches for overcoming rough free energy barriers lead to a non-equilibrium simulation method that yields e.g. the Gibbs free energy profile, G(V)=F(V)+pV. The latter is a direct route to phase diagrams which are invaluable when studying a thermodynamic system. This way we investigate the phase behaviour of constrained polymer systems. - Surface force calculation: another application of the expanded ensemble is to equalize the chemical potential of a polymer solution confined to a planar pore while changing its width. Forces acting along and perpendicular to the surfaces are then obtained by usual means of statistical mechanics and from the thermodynamic relations, the surface free energy in particular. A number of different polymer systems were dealt with in this framework

Depletion and structural forces in confined polyelectrolyte solutions

Monte Carlo simulations and density functional calculations have been performed for charged macromolecules confined to planar slits. The force between the confining walls has been evaluated as a function of separation, while keeping the chemical potential of the macromolecules constant. Highly charged spherical particles and flexible polyelectrolyte chains in confinement give rise to depletion and structural oscillatory forces as a function of surface separation. The sign and magnitude of the surface charge of the confining walls have no dramatic effect on the qualitative behavior of the confined liquid. With neutral or oppositely charged surfaces, an accumulation of charged macroions is seen in the slit driven by the repulsive interaction between the macroions, while equally charged surfaces give rise to a pure depletion. The net charge, the range of interaction, and the particle density affect the details of the force curve. For spherical macroions, the period of the oscillations scales approximately as the bulk aggregate concentration, C-bulk(-1/3). Confined polyelectrolyte chains share some of these properties, but they partly display a different behavior. One clear difference is that the polyelectrolyte net charge, that is, the degree of polymerization, has no effect on the osmotic pressure. This is an indication that polyelectrolyte chains pack not as spheres but rather as cylindrical objects. Another difference is that the effective repulsive interaction between polyelectrolyte chains can be more long ranged and oscillatory forces can appear more readily than for a corresponding solution of equally charged spherical macroions

Polyampholyte-induced repulsion between charged surfaces: Monte Carlo simulation studies

The force between two planar charged surfaces in the presence of polyampholytes (PAs) is investigated as a function of the surface separation. The model system contains PA molecules with zero net charge adsorbing onto the charged surfaces from a dilute surrounding solution without salt. We compare the results obtained on three levels of approximation: (i) polyampholytes moving in the mean field due to the counterions, i.e., in the Poisson-Boltzmann field, (ii) PAs in a self-consistent field generated by both counterions and PA monomers, and (iii) with all interactions treated explicitly. Either the amount of PA is kept constant for varying slit widths or chemical equilibrium with a bulk solution is considered. The PA adsorption and the surface force are found to strongly depend on the charge sequence along the chain. That is, polyampholytes with alternating charges do not adsorb, and their effect on the force is similar to that of neutral polymers. For PAs with long blocks oppositely-charged to the surfaces, however, the adsorption is more favorable and the monomer distribution for these blocks resembles that of polyelectrolytes. The counterions are in this case efficiently displaced from the surfaces, which leads to a significant extension of the electric double layer. Thus, our main conclusion is that adsorbing polyampholytes always increase the double layer repulsion between planar charged surfaces, and, the major cause for this phenomenon is the counterion redistribution. Yet, PA chains are capable of establishing "bridges" at short surface separations, but the resulting attraction can only slightly reduce the repulsive net pressure. The simplest approximation (i) is in principle only valid in the limit of zero PA density. At a finite concentration, it is sufficient, though essential, to include a linear correction to the Poisson-Boltzmann solution in order to accurately predict the interfacial force or the amount of PA in the slit at chemical equilibrium

Surface forces in polymer fluids: A comparison between simulations and density functional theory

A polymer density functional theory is evaluated in terms of its ability to predict interactions between large surfaces in a polymer fluid. Comparisons are made with results from simulations in an expanded isotension ensemble. The variation of the net surface-surface interaction with adsorption strength is examined. Cases where the monomers interact via a pure hard sphere potential are investigated, but we have also studied the effect of attractions between the monomers. In all cases, we obtain an almost quantitative agreement between the simulated results and the predictions from the polymer density functional theory. (C) 2004 American Institute of Physics