Molecular modeling of intermolecular and intramolecular excluded volume interactions for polymers at interfaces

A hybrid modeling approach is proposed for inhomogeneous polymer solutions. The method is illustrated for the depletion problem with polymer chains up to N=103 segments in semidilute solutions and good solvent conditions. In a three-dimensional volume, a set of freely jointed chains is considered for which the translational degrees of freedom are sampled using a coarse grained Monte Carlo simulation and the conformational degrees of freedom of the chains are computed using a modified self-consistent field theory. As a result, both intramolecular and intermolecular excluded volume effects are accounted for, not only for chains near the surface, but in the bulk as well. Results are consistent with computer simulations and scaling considerations. More specifically, the depletion thickness, which is a measure for the bulk correlation length, scales as d~J-0.75 and converges to the mean field result in the concentrated regim

How the projection domains of NF-L and alpha-internexin determine the conformations of NF-M and NF-H in neurofilaments

Making use of a numerical self-consistent field method and polymer brush concepts, we model the solvated corona of neurofilaments (NF) composed of projection domains (unstructured tails) of constituent proteins. Projections are modeled with amino acid resolution. We focus on the importance of the two shortest ones (alpha-internexin and NF-L) in regulating the conformations of the two longer ones (NF-M and NF-H) in an isolated NF. We take the wild-type NF with no alpha-internexin as the reference, for which the phosphorylation-induced translocation of M- and H-tails has been examined previously. We demonstrate that a subbrush of L-tails creates an electrostatic potential profile with an approximately parabolic shape. An experimentally relevant (2:1) ratio of L- to alpha-projections reduces the charge density of the L subbrush and shifts the translocation transition of the H-tails to slightly higher degrees of phosphorylation. Replacing all L-tails by alpha-projections destroys the substructure of the NF corona and this alters the NF response to the phosphorylation of long tail

Self-consistent field theory for obligatory coassembly

We present a first-order model for obligatory coassembly of block copolymers via an associative driving force in a nonselective solvent, making use of the classical self-consistent field (SCF) theory. The key idea is to use a generic associative driving force to bring two polymer blocks together into the core of the micelle and to employ one block of the copolymer(s) to provide a classical stopping mechanism for micelle formation. The driving force is generated by assuming a negative value for the relevant short-range Flory-Huggins interaction parameter. Hence, the model may be adopted to study micellization via H bonding, acceptor-donor interactions, and electrostatic interactions. Here, we limit ourselves to systems that resemble experimental ones where the mechanism of coassembly is electrostatic attraction leading to charge compensation. The resulting micelles are termed complex coacervate core micelles (CCCMs). We show that the predictions are qualitatively consistent with a wide variety of experimentally observed phenomena, even though the model does not yet account for the charges explicitly. For example, it successfully mimics the effect of salt on CCCMs. In the absence of salt CCCMs are far more stable than in excess salt, where the driving force for self-assembly is screened. The main limitations of the SCF model are related to the occurrence of soluble complexes, i.e., soluble, charged particles that coexist with the CCCM

Exactly solved polymer models with conformational escape transitions of a coil-to-flower type

We analyze exact analytical partition functions for Gaussian chains near surfaces and interfaces. These partition functions contain the possibility of conformational first-order phase transitions. Such transitions occur when chains are tethered in space and exposed to a local perturbing field. Then the chain can partially escape from the field: the chain transforms from the confined coil to an inhomogeneous flower conformation. The flower consists of a strongly stretched stem and a very weakly deformed crown. A generic phase diagram including one binodal and two spinodal lines is found for three related systems. The height of the barrier between stable and metastable states as well as the dynamics of barrier crossings is discussed

Analytical theory of finite-size effects in mechanical desorption

We discuss a unique system that allows exact analytical investigation of first- and second-order transitions with finite-size effects: mechanical desorption of an ideal lattice polymer chain grafted with one end to a solid substrate with a pulling force applied to the other end. We exploit the analogy with a continuum model and use accurate mapping between the parameters in continuum and lattice descriptions, which leads to a fully analytical partition function as a function of chain length, temperature (or adsorption strength), and pulling force. The adsorption-desorption phase diagram, which gives the critical force as a function of temperature, is nonmonotonic and gives rise to re-entrance. We analyze the chain length dependence of several chain properties (bound fraction, chain extension, and heat capacity) for different cross sections of the phase diagram. Close to the transition a single parameter (the product of the chain length N and the deviation from the transition point) describes all thermodynamic properties. We discuss finite-size effects at the second-order transition (adsorption without force) and at the first-order transition (mechanical desorption). The first-order transition has some unusual features: The heat capacity in the transition region increases anomalously with temperature as a power law, metastable states are completely absent, and instead of a bimodal distribution there is a flat region that becomes more pronounced with increasing chain length. The reason for this anomaly is the absence of an excess surface energy for the boundary between adsorbed and stretched coexisting phases (this boundary is one segment only): The two states strongly fluctuate in the transition point. The relation between mechanical desorption and mechanical unzipping of DNA is discusse

Room-Temperature Ionic Liquids: Excluded Volume and Ion Polarizability Effects in the Electrical Double-Layer Structure and Capacitance

We study structures of room-temperature ionic liquids at electrified interfaces and the corresponding electrical double-layer capacities using a self-consistent mean-field theory. Ionic liquids are modeled as segmented dendrimers and the effective dielectric constant is calculated from the local distribution of ions to accommodate the excluded volume and the local dielectric screening effects. The resulting camel-shaped capacitance curve is further analyzed in terms of the thickness of alternating layers and the polarization of ions at electrified interface

Temperature effects in the mechanical desorption of an infinitely long lattice chain: Re-entrant phase diagrams

We consider the mechanical desorption of an infinitely long lattice polymer chain tethered at one end to an adsorbing surface. The external force is applied to the free end of the chain and is normal to the surface. There is a critical value of the desorption force ftr at which the chain desorbs in a first-order phase transition. We present the phase diagram for mechanical desorption with exact analytical solutions for the detachment curve: the dependence of ftr on the adsorption energy (at fixed temperature T) and on T (at fixed ). For most lattice models ftr(T) displays a maximum. This implies that at some given force the chain is adsorbed in a certain temperature window and desorbed outside it: the stretched state is re-entered at low temperature. We also discuss the energy and heat capacity as a function of T; these quantities display a jump at the transition(s). We analyze short-range and long-range excluded-volume effects on the detachment curve ftr(T). For short-range effects (local stiffness), the maximum value of ftr decreases with stiffness, and the force interval where re-entrance occurs become narrower for stiffer chains. For long-range excluded-volume effects we propose a scaling ftr~T1-(Tc-T)/ around the critical temperature Tc, where =0.588 is the Flory exponent and 0.5 the crossover exponent, and we estimated the amplitude. We compare our results for a model where immediate step reversals are forbidden with recent self-avoiding walk simulations. We conclude that re-entrance is the general situation for lattice models. Only for a zigzag lattice model (where both forward and back steps are forbidden) is the coexistence curve ftr(T) monotonic, so that there is no re-entranc

Self-consistent-field calculations of proteinlike incorporations in polyelectrolyte complex micelles

Self-consistent field theory is applied to model the structure and stability of polyelectrolyte complex micelles with incorporated protein (molten globule) molecules in the core. The electrostatic interactions that drive the micelle formation are mimicked by nearest-neighbor interactions using Flory-Huggins X parameters. The strong qualitative comparison with experimental data proves that the Flory-Huggins approach is reasonable. The free energy of insertion of a proteinlike molecule into the micelle is nonmonotonic: there is (i) a small repulsion when the protein is inside the corona; the height of the insertion barrier is determined by the local osmotic pressure and the elastic deformation of the core, (ii) a local minimum occurs when the protein molecule is at the core-corona interface; the depth (a few kBT's) is related to the interfacial tension at the core-corona interface and (iii) a steep repulsion (several kBT) when part of the protein molecule is dragged into the core. Hence, the protein molecules reside preferentially at the core-corona interface and the absorption as well as the release of the protein molecules has annealed rather than quenched characteristics. Upon an increase of the ionic strength it is possible to reach a critical micellization ionic (CMI) strength. With increasing ionic strength the aggregation numbers decrease strongly and only few proteins remain associated with the micelles near the CM