The interactions between a series of oppositely charged polypeptide pairs are probed using isothermal titration calorimetry (ITC) in combination with turbidity measurements and optical microscopy. Polypeptide complex coacervation is described as a sequence of two distinct binding steps using an empirical extension of a simple ITC binding model. The first step consists of the formation of soluble complexes from oppositely charged polypeptides (ion pairing), which in turn aggregate into insoluble interpolymer complexes in the second step (complex coacervation). Polypeptides have identical backbones and differ only in their charged side groups, making them attractive model systems for this work. The poly(l-ornithine hydrobromide) (PO)/poly(l-glutamic acid sodium salt) (PGlu) system is used to examine the effects of parameters such as the salt concentration, pH, temperature, degree of polymerization, and total polymer concentration on the thermodynamic characteristics of complexation. Complex coacervation in all probed systems is found to be endothermic, essentially an entropy-driven processes. Increasing the screening effect of the salt on the polyelectrolyte charges diminishes their propensity to interact, leading to a decrease in the observed energy change and coacervate quantity. The pH plays an important role in complex formation through its effect on the degree of ionization of the functional groups. Plotting the change in enthalpy with temperature allows the calculation of the heat capacity change (ΔC(p)) for the PO/PGlu interactions. Finally, ITC revealed that complex coacervation is promoted when higher total polymer concentrations or polypeptide chain lengths are used.

The alternate deposition of polyanions and polycations leads to the formation of films called polyelectrolyte multilayer films (PEMs). Two types of growth processes are reported in the literature, leading to films that grow either linearly or exponentially with the number of deposition steps. In this article we try to establish a correlation between the nature of the growth process and the heat of complexation between the polyanions and the polycations constituting the PEM film. Isothermal titration microcalorimetry experiments performed on several polyanion/polycation systems seem to indicate that an endothermic complexation process is characteristic of an exponential film growth, whereas a strongly exothermic process corresponds to a linear growth regime. Finally, weakly exothermic processes seem to be associated with weakly exponentially growing films. These results thus show that exponentially growing processes are mainly driven by entropy. This explains why the exponential growth processes are more sensitive to temperature than the linear growing processes. This temperature sensitivity is shown on the poly-L-glutamic acid/poly(allylamine) system which grows either linearly or exponentially depending on the ionic strength of the polyelectrolyte solutions.

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Polyelectrolyte brushes are essential in many aspects of surface functionality, particularly for colloidal stabilization and lubrication in biological and materials science applications. It has been shown experimentally that the brushes undergo an abrupt shrinkage in the presence of multivalent counter‐ions. This transition is studied here using a phenomenological mean‐field approach with a model that specifically includes bridging of the polyelectrolyte chains by the multiple charges on the multivalent counter‐ions. Using an energy balance represented by the sum of electrostatic, polymeric and entropic mean‐field terms, additional parameterized phenomenological terms are introduced for counter‐ion condensation and for the attractive interaction between adjacent polyelectrolyte chains to account for the bridging effect. The free energy is minimized with respect to the counter‐ion populations and the brush height. In agreement with experimental observations, increasing the concentration of multivalent ions leads to a sharp collapse of the polyelectrolyte brush height. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 284–291

Applications of end-tethered polyelectrolyte "brushes" to modify solid surfaces have been developed and studied for their colloidal stabilization and high lubrication properties. Current efforts have expanded into biological realms and stimuli-responsive materials. Our work explores responsive and reversible aspects of polyelectrolyte brush behavior when polyelectrolyte chains interact with oppositely charged multivalent ions and complexes, which act as counterions. There is a significant void in the polyelectrolyte literature regarding interactions with multivalent species. This paper demonstrates that interactions between solid surfaces bearing negatively charged polyelectrolyte brushes are highly sensitive to the presence of trivalent lanthanum, La 3+ . Lanthanum cations have unique interactions with polyelectrolyte chains, in part due to their small size and hydration radius which results in a high local charge density. Using La 3+ in conjunction with the surface forces apparatus (SFA), adhesion has been observed to reversibly appear and disappear upon the uptake and release, respectively, of these multivalent cations acting as counterions. In media of fixed ionic strength set by monovalent sodium salt, at I 0 = 0.003 M and I 0 = 0.3 M, the sign of the interaction forces between overlapping brushes changes from repulsive to attractive when La 3+ concentrations reach 0.1 mol % of the total ion concentration. These results are also shown to be generally consistent with, but subtlety different from, previous polyelectrolyte brush experiments using trivalent ruthenium hexamine in the role of the multivalent counterion. ■ INTRODUCTIONInvestigations of polyelectrolyte brushes are directly relevant to a wide range of fields including colloidal behavior, 1−5 lubrication, 6−8 drug and therapeutic delivery, biological interactions, 9−11 coatings and adhesives, 12−17 and energy storage. Polyelectrolyte "brushes" consist of charged polymer chains extended outward into solution with one end tethered to an interface. These structures, which are found in nature and in industrial applications, and on hard and soft surfaces, exist when polymer chains are anchored to surfaces at high tethering densities. 5,18,19 The proximity of chains in polyelectrolyte brushes creates electrostatic and steric repulsion among monomer segments, which drive chains to stretch away from the surface into solution, resulting in a brush structure. Adding complexity to this system are charged counterions, which directly interact with the charged monomer segments and play a crucial role in polyelectrolyte chain behavior. Most studies of responsive polyelectrolyte materials 12 have centered on temperature, 16,17 pH, 13−16 or monovalent salt effects. 20,21 Here, however, we further investigate the interactions of polyelectrolyte brushes with multivalent cations, namely, using trivalent lanthanum. In previous work, 20−22 we have looked at polyelectrolyte interactions with a number of cations, complexes, and charged-aggregates, all of which act as opposit...

Composite material nanofilms of controlled thickness constituted by ceramics and polymers find more and more applications to improve the properties and functionalities of material surfaces. In this paper we present a new way to deposit such composite coatings by alternated contacts of a surface with a polyamine solution (either poly(allylamine), poly(ethyleneimine), poly-l-lysine, or poly(diallydimethylammonium)chloride and silicic acid. The experiments are mainly realized by spraying of solutions onto the surface. The polyamines deposited in the first spraying step catalyze silica formation upon further spraying of a silicic acid-containing solution. The film thickness increases linearly with the number of deposition steps, the thickness increment being of the order of a few nanometers per silicic acid/polyamine layer. Infrared spectroscopy in the total attenuated reflection mode reveals spectra that are close to those of pure silica particles. The film morphology is further investigated by means of atomic force microscopy and environmental scanning electron microscopy. This reactive layer-by-layer deposition constitutes a new way to build, in an easy way, nanocomposite coatings with precise control of their thickness.

Polyelectrolyte multilayer (PEM) films have become very popular for surface functionalization and the design of functional architectures such as hollow polyelectrolyte capsules. It is known that properties such as permeability to small ionic solutes are strongly dependent on the buildup regime of the PEM films. This permeability can be modified by tuning the ionization degree of the polycations or polyanions, provided the film is made from weak polyelectrolytes. In most previous investigations, this was achieved by playing on the solution pH either during the film buildup or by a postbuildup pH modification. Herein we investigate the functionalization of poly(allylamine hydrochloride)/poly(glutamic acid) (PAH/PGA) multilayers by ferrocyanide and Prussian Blue (PB). We demonstrate that dynamic exchange processes between the film and polyelectrolyte solutions containing one of the component polyelectrolyte allow one to modify its Donnan potential and, as a consequence, the amount of ferrocyanide anions able to be retained in the PAH/PGA film. This ability of the film to be a tunable reservoir of ferrocyanide anions is then used to produce a composite film containing PB particles obtained by a single precipitation reaction with a solution containing Fe(3+) cations in contact with the film. The presence of PB in the PEM films then provides magnetic as well as electrochemical properties to the whole architecture.