A Molecular Thermodynamic Model of Complexation in Mixtures of Oppositely Charged Polyelectrolytes with Explicit Account of Charge Association/Dissociation

Into an extended Voorn–Overbeek (EVO) free energy model of polyelectrolyte (PE) complexation and phase behavior, we incorporate three classes of short-ranged electrostatic effects, namely counterion association–dissociation, cross-chain ion pairing (IP), and charge regulation by treating each as a reversible chemical reaction leading to a corresponding law of mass action in a self-consistent fashion. The importance of each reaction is controlled by a corresponding chemistry-dependent standard free energy input parameter. Our model also accounts for Born (or ion solvation) energy using a linear mixing rule for the effective dielectric constant. In monophasic systems, the proposed model can qualitatively explain the observed shifts in acidity and basicity observed in potentiometric titration of weak PEs in the presence of salt and oppositely charged PEs in accordance with Le Châtelier’s principle. We demonstrate how a competition between counterion condensation (CC) and IP alone can explain the complex coacervation of strongly charged PEs as well as the existence of a critical salt concentration. Binodal diagrams predicted in our model are also affected by long-ranged electrostatics and are most sensitive to IP strength both for weak and strong PEs. The extent of IP increases in the dense phase at the expense of reduced CC upon coacervation consistent with counter release view of complex coacervation. We compare binodal diagrams predicted by our model against experimental data for both weakly and strongly dissociating polyions pairs and find a plausible parameter set that leads to an acceptable and partial agreement with experiments in the two cases, respectively

Assessing the Efficacy of Poly(<i>N</i>‑isopropylacrylamide) for Drug Delivery Applications Using Molecular Dynamics Simulations

All-atom molecular dynamic simulations (AA-MD) are performed for aqueous solutions of hydrophobic drug molecules (phenytoin) with model polymer excipients, namely, (1) N-isopropylacrylamide, (pNIPAAm), (2) pNIPAAm-co-acrylamide (Am), and (3) pNIPAAm-co-dimethylacrylamide (DMA). After validating the force field parameters using the well-known lower critical solution behavior of pNIPAAm, we simulate the polymer–drug complex in water and its behavior at temperatures below (295 K) and above the LCST (310 K). Using radial distribution functions, we find that there is an optimum comonomer molar fraction of around 20–30% DMA at which interaction with phenytoin drug molecules is strongest, consistent with recent experimental findings. The results provide evidence that molecular simulations are able to provide guidance in the optimization of novel polymer excipients for drug release

One-Step Preparation of Highly Monodisperse Micron-Size Particles in Organic Solvents

In this communication, we report the first simple and fast one-step method for synthesizing highly monodisperse micron-size PMMA particles in organic media through dispersion polymerization in the presence of PHSA (a polyhydroxyl-stearic-acid graft PMMA copolymer) as a stabilizer. There are two significant advantages of our method over earlier methods. First, by optimizing the composition of a solvent mixture of hexane and dodecane, we were able to increase the concentration of monomer up to 50−56% and obtain unusually large (up to 10 μm in diameter) PMMA particles. Second, by strictly controlling the nucleation time, we were able to make PMMA particles with a low polydispersity of around 1%, much lower than has ever before been achieved for such large particles. We also report an unusual apparent metastable state in the nucleation stage

Molecular Dynamics Simulations of Threadlike Cetyltrimethylammonium Chloride Micelles: Effects of Sodium Chloride and Sodium Salicylate Salts

We use atomistic molecular dynamics simulations to probe the effects of added sodium chloride (NaCl) and sodium salicylate (NaSal) salts on the spherical-to-threadlike micelle shape transition in aqueous solutions of cetyltrimethylammonium chloride (CTAC) surfactants. Long threadlike micelles are found to be unstable and break into spherical micelles at low concentrations of NaCl, but remain stable for 20 ns above a threshold value of [NaCl] ≈ 3.0 M, which is about 2.5 times larger than the experimental salt concentration at which the transition between spherical and rodlike micelles occurs. The chloride counterions associate weakly on the surface of the CTAC micelles with the degree of counterion dissociation decreasing slightly with increasing [NaCl] on spherical micelles, but dropping significantly on the threadlike micelles at high [NaCl]. This effect indicates that the electrolyte ions drive the micellar shape transition by screening the electrostatic repulsions between the micellar headgroups. The aromatic salicylate counterions, on the other hand, penetrate inside the micelle with their hydrophilic groups staying in the surfactant headgroup region and the hydrophobic groups partially embedded into the hydrophobic core of the micelle. The strong association of the salicylate ions with the surfactant headgroups leads to dense packing of the surfactant molecules, which effectively reduces the surface area per surfactant, and increases intramicellar ordering of the surfactant headgroups, favoring the formation of long threadlike micelles. Simulation predictions of the geometric and electrostatic properties of the spherical and threadlike micelles are in good agreement with experiments

Tension-Induced Nematic Phase Separation in Bidisperse Homopolymer Melts

We use an analytical mean-field theory and all-atom molecular dynamics (MD) simulations to predict that external tension, together with the nematic coupling interactions, can drive phase separation of long chains from short ones in bidisperse homopolymer melts. The nematic coupling parameter α for polyethylene (PE) oligomers under applied tension is extracted from the MD simulations and used in the mean-field free energy to predict the phase boundary for bidisperse melts in which the longer chains are stretched by uniaxial tension. The predicted phase diagram is validated by direct MD simulations. We also show that extensional flow, and possibly even shear flow, may lead to nematic phase separation in molten PE oligomers, because the flow can impose a stronger tension on the longer chains than the short ones

Marangoni Effect Reverses Coffee-Ring Depositions

We show here both experimentally and theoretically that the formation of “coffee-ring” deposits observed at the edge of drying water droplets requires not only a pinned contact line (Deegan et al. Nature 1997, 389, 827) but also suppression of Marangoni flow. For simple organic fluids, deposition actually occurs preferentially at the center of the droplet, due to a recirculatory flow driven by surface-tension gradients produced by the latent heat of evaporation. The manipulation of this Marangoni flow in a drying droplet should allow one in principle to control and redirect evaporation-driven deposition and assembly of colloids and other materials

One-Step Preparation of Highly Monodisperse Micron-Size Particles in Organic Solvents

In this communication, we report the first simple and fast one-step method for synthesizing highly monodisperse micron-size PMMA particles in organic media through dispersion polymerization in the presence of PHSA (a polyhydroxyl-stearic-acid graft PMMA copolymer) as a stabilizer. There are two significant advantages of our method over earlier methods. First, by optimizing the composition of a solvent mixture of hexane and dodecane, we were able to increase the concentration of monomer up to 50−56% and obtain unusually large (up to 10 μm in diameter) PMMA particles. Second, by strictly controlling the nucleation time, we were able to make PMMA particles with a low polydispersity of around 1%, much lower than has ever before been achieved for such large particles. We also report an unusual apparent metastable state in the nucleation stage

Identification of Topological Constraints in Entangled Polymer Melts Using the Bond-Fluctuation Model

We propose an algorithm to locate individual entanglements along chains, equilibrated using the bond-fluctuation lattice model. The algorithm identifies entanglements as local deviations of the primitive path from the shortest possible path between beads on a chain that are on lattice sites. For well-entangled chains (number of beads, N ≥ 125), the average number of entanglements enumerated using the proposed method is in excellent agreement with the number of entanglements per chain inferred using the ensemble-averaged primitive path length 〈Lpp〉 and mean-squared end-to-end distance 〈R2〉 of the chains, namely Z = 〈Lpp〉2/〈R2〉. As an application of this method, we show that the elimination of an entanglement releases, approximately, one additional entanglement. This implies a value of α = 1.03 ± 0.02 for the “dilution exponent” relating entanglement density ρent to polymer concentration c via ρent ∝ c1+α and is consistent with the description of entanglements as binary contacts

Atomic Force Microscopic Study of Aggregation of RecA-DNA Nucleoprotein Filaments into Left-Handed Supercoiled Bundles

RecA and its complexes with double-stranded DNA (dsDNA) and single-stranded DNA (ssDNA) are responsible for homologous recombination and DNA repair. In this study, we have observed, by atomic force microscopy (AFM), two-filament left-handed superhelices of RecA-dsDNA filaments that further interwind into four- or six-filament bundles, in addition to previously reported left-handed bundles of three or six filaments. Also revealed are four-filament bundles formed by further interwinding of two intrafilament superhelices of individual filaments. Pitches of superhelices of RecA-DNA filaments are similar to each other regardless the number of component filaments, and those formed on Φx174 RFII dsDNA and pNEB206A dsDNA are measured as 339.3 ± 6.2 nm (690 counts of pitch/2) and 321.6 ± 11.7 nm (101 counts of pitch/2), respectively, consistent with earlier measurements made by electron microscopy with a much smaller sample size. The study of these structures provides insight into the self-interactions of RecA and RecA-like proteins, which are present in all living cells, and into the general phenomenon of bundling, which is relevant to both biological and nonbiological filaments