Cartilage extracellular matrix polymers: hierarchical structure, osmotic properties, and function

Proteoglycans are hierarchically organized structures that play an important role in the hydration and the compression resistance of cartilage matrix. In this study, the static and dynamic properties relevant to the biomechanical function of cartilage are determined at different levels of the hierarchical structure, using complementary osmotic pressure, neutron scattering (SANS) and light scattering (DLS) measurements. In cartilage proteoglycans (PGs), two levels of bottlebrush structures can be distinguished: the aggrecan monomer, which consists of a core protein to which are tethered charged glycosaminoglycan (GAG) chains, and complexes formed of the aggrecan monomers attached around a linear hyaluronic acid backbone. The principal component of GAG, chondroitin sulfate (CS), is used as a baseline in this comparison. The osmotic modulus, measured as a function of the proteoglycan concentration, follows the order CS < aggrecan < aggrecan-HA complex. This order underlines the benefit of the increasing complexity at each level of the molecular architecture. The hierarchical bottlebrush configuration, which prevents interpenetration among the bristles of the aggrecan monomers, enhances both the mechanical properties and the osmotic resistance. The osmotic pressure of the collagen solution is notably smaller than in the proteoglycan systems. This is consistent with its known primary role to provide tensile strength to the cartilage and to confine the aggrecan-HA complexes, as opposed to load bearing. The collective diffusion coefficient D governs the rate of recovery of biological tissue after compressive load. In CS solutions the diffusion process is fast, D ≈ 3 × 10-6 cm2 s-1 at concentrations comparable with that of the GAG chains inside the aggrecan molecule. In CS solutions D is a weakly decreasing function of calcium ion concentration, while in aggrecan and its complexes with HA, the relaxation rate is insensitive to the presence of calcium.

Complex Formation in the Sulfobetaine-Containing Entirely Ionic Block Copolymer/Ionic Homopolymer System and Their Temperature Responsivity

The behavior of micelle formation in the sulfobetaine-containing entirely ionic block copolymer/ionic homopolymer system and its functional expression (temperature responsivity) were investigated. Poly(sulfopropyl dimethylammonium propylacrylamide) was used as the sulfobetaine, poly[3-(methacrylamido)propyl trimethylammonium chloride] was used as the cationic polymer, and poly(p-styrenesulfonic acid sodium salt) was used as the anionic polymer. The changes in transition temperature with the concentration and the behavior of micelle formation in the block-/cationic homopolymer and block-/anionic homopolymer system were compared and examined by transmittance, dynamic light scattering, atomic force microscopy, and ¹H nuclear magnetic resonance. Only block-/cationic homopolymer systems with a core-shell (polyion complex-sulfobetaine) structure showed temperature responsivity of upper critical solution temperature type, and the responsiveness was dependent on the concentration. On the other hand, the block-/anionic homopolymer system had a core-shell structure at a concentration of 0.05 wt %, but temperature responsiveness was not observed at this concentration. At higher concentrations, electrostatic attraction caused the anionic homopolymer and block copolymer to interact as a whole, resulting in a loss of responsiveness. When the ionic homopolymer had a higher degree of polymerization than the sulfobetaine, it could not form a core-shell structure by interacting with the sulfobetaine and ionic polymer moieties of the block copolymer, thus resulting in the loss of responsiveness. The block-/ionic homopolymer system prepared by the reforming method through dialysis formed uniform and small micelles but lost responsiveness due to morphological stability and electrostatic interaction between the block copolymer and ionic homopolymer.

Complex Formation of Sulfobetaine Surfactant and Ionic Polymers and Their Stimuli Responsivity

We investigated the kinds of complexes sulfobetaine surfactant and ionic polymer formed using lauramidopropyl hydroxysultane (LAPHS) as a sulfobetaine surfactant, poly(sodium styrenesulfonate) (PSSNa) as the anionic polymer and poly[3-(methacrylamido)propyl trimethylammonium chloride] (PMAPTAC) as the cationic polymer. The fundamental properties of LAPHS at various salt concentrations were estimated by various measurements, and it was confirmed that the LAPHS micelles alone did not show temperature responsiveness. The presence of large aggregates in addition to LAPHS micelles was confirmed in the aggregates prepared by adding PSSNa to LAPHS at a charge ratio of 1:0.5, 1:1, and 1:2. However, the aggregates could not be formed when the salt concentration was high or when a monomer was added instead of the polymer. This revealed that the cation part of sulfobetaine, which is the shell of LAPHS micelles, and the anion part of PSSNa electrostatically interacted with each other to form a large aggregate. On the other hand, unlike the case of LAPHS micelles alone and the aggregate consisting of LAPHS micelles and PSSNa, the aggregate of LAPHS micelles and PMAPTAC showed an unprecedented phenomenon of "clear → opaque → clear" with increasing concentration in the concentration range above CMC. The change in the transition temperature due to the change of concentration was a factor. Additionally, we confirmed that the transition temperature was lowered when the concentration was higher than CMC or the salt concentration was increased, and the transition temperature was increased when the PMAPTAC with a high degree of polymerization was added. These results suggested that the LAPHS micelles and the ionic polymer form an aggregate, and the temperature responsivity can be expressed by the interaction with the cationic polymer.

Effect of Salt on the Ordinary-Extraordinary Transition in Solutions of Charged Macromolecules

Using dynamic light scattering technique, we address the role of added salt at higher concentrations on the "ordinary-extraordinary" transition in solutions of charged macromolecules. The "ordinary" behavior has previously been associated with a "fast" diffusion coefficient which is independent of salt concentration C_s and polymer concentration C_p if the ratio C_p/ C_s is above a threshold value. The "extraordinary" transition is associated with formation of aggregates, with a "slow" diffusion coefficient, formed from similarly charged macromolecules. By investigating aqueous solutions of sodium poly(styrenesulfonate) and sodium chloride with variations in C_p, C_s, and polymer molecular weight, M_w, we report the emergence of a new diffusive "fast" relaxation mode at higher values of C_p, C_s, and M_w, in addition to the previously known "fast" and "slow" relaxation modes. Furthermore, we find that M_w plays a crucial role on the collective dynamics of polyelectrolyte solutions with salt, instead of just the C_p/ C_s ratio as previously postulated. As M_w is progressively decreased, the salty solution exhibits dynamical transitions from three modes to two modes and then to one mode of relaxation. The emergence of the new fast mode and the dynamical transitions are in marked departure from the general premise of the ordinary-extraordinary transition developed over several decades. In an effort to rationalize our experimental findings we present a theory for the collective dynamics of polyelectrolyte solutions with salt by addressing the coupling between the relaxations of polyelectrolyte chains, counterions from the polymer and added salt, and co-ions from the salt. The predictions are in qualitative agreement with experimental findings. The present combined work of experiments and theory forms the basis for accurately characterizing dynamics of charged macromolecules in salty solutions, which are ubiquitous in biological systems and polyelectrolyte-based technologies.

Effects of Ca2+ Ion Condensation on the Molecular Structure of Polystyrene Sulfonate at Air-Water Interfaces

The structure of poly(sodium 4-styrenesulfonate) (NaPSS) polyelectrolytes at air-water interfaces was investigated with tensiometry, ellipsometry, and vibrational sum-frequency generation (SFG) in the presence of low and high CaCl₂ concentrations. In addition, we have studied the foaming behavior of 20 mM NaPSS solutions to relate the PSS molecular structure at air-water interfaces to foam properties. PSS polyelectrolytes without additional salt exhibited significant surface activity, which can be tuned further by additions of CaCl₂. The hydrophobicity of the backbone due to incomplete sulfonation during synthesis is one origin, whereas the effective charge of the polyelectrolyte chain is shown to play another major role. At low salt concentrations, we propose that the polyelectrolyte is forming a layered structure. The hydrophobic parts are likely to be located directly at the interface in loops, whereas the hydrophilic parts are at low concentrations stretched out into near-interface regions in tails. Increasing the Ca²⁺ concentration leads to ion condensation, a collapse of the tails, and likely to Ca²⁺ intra- and intermolecular bridges between polyelectrolytes at the interface. The increase in both surface excess and foam stability originates from changes in the polyelectrolyte's hydrophobicity due to Ca²⁺ condensation onto the PSS polyanions. Consequently, charge screening at the interface is enhanced and repulsive electrostatic interactions are reduced. Furthermore, SFG spectra of O-H stretching bands reveal a decrease in intensity of the low-frequency branch when c(Ca²⁺) is increased whereas the high-frequency branch of O-H stretching modes persists even for 1 M CaCl₂. This originates from the remaining net charge of the PSS polyanions at the air-water interface that is not fully compensated by condensation of Ca²⁺ ions and leads to electric-field-induced contributions to the SFG spectra of interfacial H₂O. A charge reversal of the PSS net charge at the air-water interface is not observed and is consistent with bulk electrophoretic mobility measurements.

50th Anniversary Perspective: A Perspective on Polyelectrolyte Solutions

From the beginning of life with the information-containing polymers until the present era of a plethora of water-based materials in health care industry and biotechnology, polyelectrolytes are ubiquitous with a broad range of structural and functional properties. The main attribute of polyelectrolyte solutions is that all molecules are strongly correlated both topologically and electrostatically in their neutralizing background of charged ions in highly polarizable solvent. These strong correlations and the necessary use of numerous variables in experiments on polyelectrolytes have presented immense challenges toward fundamental understanding of the various behaviors of charged polymeric systems. This Perspective presents the author's subjective summary of several conceptual advances and the remaining persistent challenges in the contexts of charge and size of polymers, structures in homogeneous solutions, thermodynamic instability and phase transitions, structural evolution with oppositely charged polymers, dynamics in polyelectrolyte solutions, kinetics of phase separation, mobility of charged macromolecules between compartments, and implications to biological systems.

Ordinary-extraordinary transition in dynamics of solutions of charged macromolecules

The occurrence of the ubiquitous and intriguing "ordinary-extraordinary" behavior of dynamics in solutions of charged macromolecules is addressed theoretically by explicitly considering counterions around the macromolecules. The collective and coupled dynamics of macromolecules and their counterion clouds in salt-free conditions are shown to lead to the "ordinary" behavior (also called the "fast" mode) where diffusion coefficients are independent of molar mass and polymer concentration and are comparable to those of isolated metallic ions in aqueous media, in agreement with experimental facts observed repeatedly over the past four decades. The dipoles arising from adsorbed counterions on polymer backbones can form many pairwise physical cross-links, leading to microgel-like aggregates. Balancing the swelling from excluded volume effects and counterion pressure with elasticity of the microgel, we show that there is a threshold value of a combination of polymer concentration and electrolyte concentration for the occurrence of the "extraordinary" phase (also called the "slow" mode) and the predicted properties of diffusion coefficient for this phase are in qualitative agreement with well-known experimental data.

The Role of Correlation and Solvation in Ion Interactions with B-DNA

The ionic atmospheres around nucleic acids play important roles in biological function. Large-scale explicit solvent simulations coupled to experimental assays such as anomalous small-angle x-ray scattering can provide important insights into the structure and energetics of such atmospheres but are time- and resource intensive. In this article, we use classical density functional theory to explore the balance among ion-DNA, ion-water, and ion-ion interactions in ionic atmospheres of RbCl, SrCl2, and CoHexCl3 (cobalt hexamine chloride) around a B-form DNA molecule. The accuracy of the classical density functional theory calculations was assessed by comparison between simulated and experimental anomalous small-angle x-ray scattering curves, demonstrating that an accurate model should take into account ion-ion correlation and ion hydration forces, DNA topology, and the discrete distribution of charges on the DNA backbone. As expected, these calculations revealed significant differences among monovalent, divalent, and trivalent cation distributions around DNA. Approximately half of the DNA-bound Rb(+) ions penetrate into the minor groove of the DNA and half adsorb on the DNA backbone. The fraction of cations in the minor groove decreases for the larger Sr(2+) ions and becomes zero for CoHex(3+) ions, which all adsorb on the DNA backbone. The distribution of CoHex(3+) ions is mainly determined by Coulomb and steric interactions, while ion-correlation forces play a central role in the monovalent Rb(+) distribution and a combination of ion-correlation and hydration forces affect the Sr(2+) distribution around DNA. This does not imply that correlations in CoHex solutions are weaker or stronger than for other ions. Steric inaccessibility of the grooves to large CoHex ions leads to their binding at the DNA surface. In this binding mode, first-order electrostatic interactions (Coulomb) dominate the overall binding energy as evidenced by low sensitivity of ionic distribution to the presence or absence of second-order electrostatic correlation interactions.

Microgel-like aggregates of isotactic and atactic poly(methacrylic acid) chains in aqueous alkali chloride solutions as evidenced by light scattering

A comparative light-scattering study of isotactic and atactic poly(methacrylic acid), iPMA and aPMA, respectively, in aqueous solutions with added alkali chlorides, XCl (X = Li, Na, Cs), at 25 °C and XCl concentration of 0.1 mol L(-1), demonstrates that both PMA isomers are strongly associated at low degrees of neutralization, αN (= 0 for aPMA and 0.25 for iPMA), in the presence of all XCls. The shape parameter ρ and the scattering functions suggest that aggregates have the characteristics of microgel particles, with a dense core surrounded by a less dense shell. The extent of aggregation depends on the stereoregular structure of the polymer and on the type of the added cation. Li(+) and Na(+) ions support aggregation better than Cs(+) ions. Besides, iPMA chains are more strongly aggregated than aPMA chains and form particles with a denser core. A model of the aggregation process is suggested for iPMA. At high αN, a slow diffusive process (so-called extraordinary or anomalous mode in diffusion of polyelectrolytes), arising from electrostatic interactions between charged chains, is observed for both PMAs. Results suggest that under the same experimental conditions iPMA is effectively more charged than aPMA. The role of ions in the slow-mode phenomenon is less pronounced than in aggregation.

Self-assembly thermodynamics of pH-responsive amino-acid-based polymers with a nonionic surfactant

The behavior of pH-responsive polymers poly(N-methacryloyl-l-valine) (P1), poly(N-methacryloyl-l-phenylalanine) (P2), and poly(N-methacryloylglycyne-l-leucine) (P3) has been studied in the presence of the nonionic surfactant Brij98. The pure polymers phase-separate in an acidic medium with critical pHtr values of 3.7, 5.5, and 3.4, respectively. The addition of the surfactant prevents phase separation and promotes reorganization of polymer molecules. The nature of the interaction between polymer and surfactant depends on the amino acid structure in the side chain of the polymer. This effect was investigated by dynamic light scattering, isothermal titration calorimetry, electrophoretic measurements, small-angle neutron scattering, and infrared spectroscopy. Thermodynamic analysis revealed an endothermic association reaction in P1/Brij98 mixture, whereas a strong exothermic effect was observed for P2/Brij98 and P3/Brij98. Application of regular solution theory for the analysis of experimental enthalpograms indicated dominant hydrophobic interactions between P1 and Brij98 and specific interactions for the P2/Brij98 system. Electrophoretic and dynamic light scattering measurements support the applicability of the theory to these cases. The specific interactions can be ascribed to hydrogen bonds formed between the carboxylic groups of the polymer and the oligo(ethylene oxide) head groups of the surfactant. Thus, differences in polymer-surfactant interactions between P1 and P2 polymers result in different structures of polymer-surfactant complexes. Specifically, small-angle neutron scattering revealed pearl-necklace complexes and "core-shell" structures for P1/Brij98 and P2/Brij98 systems, respectively. These results may help in the design of new pH-responsive site-specific micellar drug delivery systems or pH-responsive membrane-disrupting agents.

Conformational effect on small angle neutron scattering behavior of interacting polyelectrolyte solutions: a perspective of integral equation theory

We present small angle neutron scattering (SANS) measurements of deuterium oxide (D(2)O) solutions of linear and star sodium poly(styrene sulfonate) (NaPSS) as a function of polyelectrolyte concentration. Emphasis is on understanding the dependence of their SANS coherent scattering cross section I(Q) on the molecular architecture of single polyelectrolyte. The key finding is that for a given concentration, star polyelectrolytes exhibit more pronounced characteristic peaks in I(Q), and the position of the first peak occurs at a smaller Q compared to their linear counterparts. Based on a model of integral equation theory, we first compare the SANS experimental I(Q) of salt-free polyelectrolyte solutions with that predicted theoretically. Having seen their satisfactory qualitative agreement, the dependence of counterion association behavior on polyelectrolyte geometry and concentration is further explored. Our predictions reveal that the ionic environment of polyelectrolyte exhibits a strong dependence on polyelectrolyte geometry at lower polyelectrolyte concentration. However, when both linear and star polyelectrolytes exceed their overlap concentrations, the spatial distribution of counterion is found to be essentially insensitive to polyelectrolyte geometry due to the steric effect.

Counterion condensation theory of attraction between like charges in the absence of multivalent counterions

There is abundant experimental evidence suggesting the existence of attractive interactions among identically charged polyelectrolytes in ordinary salt solutions. The presence of multivalent counterions is not required. We review the relevant literature in detail and conclude that it merits more attention than it has received. We discuss also some recent observations of a low ionic strength attraction of negatively charged DNA to the region of a negatively charged glass nanoslit where the floor of the nanoslit meets the walls, again in the absence of multivalent ions. On the theoretical side, it has become clear that purely electrostatic interactions require the presence of multivalent counterions if they are to generate like-charge attraction. Any theory of like-charge attraction in the absence of multivalent counterions must therefore contain a non-electrostatic component. We point out that counterion condensation theory, which has predicted like-charge polyelectrolyte attraction in an intermediate range of distances in ordinary 1:1 salt conditions, contains both electrostatic and non-electrostatic elements. The non-electrostatic component of the theory is the modeling constraint that the counterions fall into two explicit populations, condensed and uncondensed. As reviewed in the paper, this physically motivated constraint is supported by strong experimental evidence. We proceed to offer an explanation of the nanoslit observations by showing in an idealized model that the line of intersection of two intersecting planes is a virtual polyelectrolyte. Since we have previously developed a counterion condensation theory of attraction of two like-charged polyelectrolytes, our suggestion is that the DNA is attracted to the virtual polyelectrolytes that may be located in the nanoslit where floor meets walls. We present the detailed calculations needed to document this suggestion: an extension of previous theory to the case of polyelectrolytes with like but not identical charges; the demonstration of counterion condensation on a plane with bare charge density greater than an explicitly exhibited critical value; a calculation of the free energy of the plane; a calculation of the interaction of a line charge polyelectrolyte with a like-charged plane; and the detailed demonstration that the line of intersection of two planes is a virtual polyelectrolyte.

Ions in hyaluronic acid solutions

Hyaluronic acid (HA) is an anionic biopolymer that is almost ubiquitous in biological tissues. An attempt is made to determine the dominant features that account for both its abundance and its multifunctional role, and which set it apart from other types of biopolymers. A combination of osmotic and scattering techniques is employed to quantify its dynamic and static properties in near-physiological solution conditions, where it is exposed both to mono- and divalent counterions. An equation of state is derived for the osmotic pressure Pi in the semidilute concentration region, in terms of two variables, the polymer concentration c and the ionic strength J of the added salt, according to which Pi=1.4x10(3)c(9/4)/J(3/4) kPa, where c and J are expressed in mole. Over the physiological ion concentration range, the effect of the sodium chloride and calcium chloride on the osmotic properties of HA solutions is fully accounted for by their contributions to the ionic strength. The absence of precipitation, even at high CaCl(2) concentrations, distinguishes this molecule from other biopolymers such as DNA. Dynamic light scattering measurements reveal that the collective diffusion coefficient in HA solutions exceeds that in aqueous solutions of typical neutral polymers by a factor of approximately 5. This property ensures rapid adjustment to, and recovery from, stress applied to HA-containing tissue. Small angle x-ray scattering measurements confirm the absence of appreciable structural reorganization over the observed length scale range 10-1000 A, as a result of calcium-sodium ion exchange. The scattered intensity in the transfer momentum range q>0.03 A(-1) varies as 1/q, indicating that the HA chain segments in semidilute solutions are linear over an extended concentration range. The osmotic compression modulus c partial differential Pi/partial differential c, a high value of which is a prerequisite in structural biopolymers, is several times greater than in typical neutral polymer solutions.

Relationship between swelling and the electrohydrodynamic properties of functionalized carboxymethyldextran macromolecules

The electrostatic, hydrodynamic, and swelling properties of a well-defined, functionalized carboxymethyldextran (CMD) polysaccharide are investigated in aqueous NaNO3 solution over a broad ionic strength range. The impact of the polycarboxylate charge and molar mass of the CMD macromolecules on their electrohydrodynamic features is thoroughly examined by combined protolytic titration, dynamic light scattering, and electrokinetic analyses. Electrophoretic mobility data obtained for sufficiently high electrolyte concentrations reveal a typical soft particle behavior. Upon decrease of the ionic strength, mobilities strongly increase in magnitude while significant electrostatic swelling takes place, as reflected in a decrease in the diffusion coefficients. CMD entities undergo conformational transitions from compact random coil at large ionic strengths to swollen coil and possibly a wormlike structure at lower NaNO3 concentrations. The magnitude of the variations in size and mobility with electrolyte concentration strongly depends on the overall charge of the CMD entity as well as on its molar mass. These factors control the stiffness of the constituent polymer chains and thus the degree of macromolecular permeability ("softness"). Using the soft-diffuse interface formalism previously developed for the electrohydrodynamics of charged permeable macromolecules, a quantitative analysis of the electrophoretic mobility data is presented. The measured values of the diffusion coefficient and space charge density Gamma degrees, as evaluated independently from the modeling of potentiometric titration curves, are taken into account in a self-consistent manner. It is found that large CMD entities of low charge densities are the most permeable to flow penetration with a limited heterogeneous electrostatic stiffening of the chains, whereas small CMD entities of larger Gamma degrees significantly expand upon lowering the ionic strength, giving rise to a strong anisotropy for the spatial distribution of polymer chain density.

Micelle–vesicle transition of oleyldimethylamine oxide in water

We studied the effects of the degree of ionization(alpha) and the surfactant concentration (C(d)) on the micelle-vesicle transition in salt-free oleyldimethylamine oxide (OlDMAO) aqueous solutions by the dynamic light scattering (DLS), the hydrogen ion titration, the small angle neutron scattering (SANS), the electrophoretic light scattering (ELS) and viscoelastic measurements. From the study of ionization effects, the micelle-vesicle transition was recognized as a change of aggregate size by the DLS measurement; however, the micelle-vesicle transition was not detected both in the ELS measurement and the hydrogen ion titration, suggesting that the electric properties of the worm-like micelles and the vesicles are very similar despite a large difference of shapes between them. From the results of the SANS, the DLS and the viscosity measurements, it was suggested that a concentration-dependent micelle-vesicle transition took place around C(p)=10 mmolkg-1 for the solutions at alpha=0.5. In the concentration-range 10 mmolkg-1<C(d)<150 mmolkg-1, the micelles and the vesicles coexisted. In the concentration region (C(d) = 10-50 mmolkg-1), the vesicle size increased with the surfactant concentration.

Rheology and dynamic light scattering of silk fibroin solution extracted from the middle division of Bombyx mori silkworm

Dynamic light scattering (DLS) and rheological measurements were performed on aqueous silk fibroin solutions extracted from the middle division of Bombyx mori silkworm over a wide range of polymer concentration C from 0.08 to 27.5 wt %. DLS results obtained in the dilute region of C less than 1 wt % are consistent with a model that an elementary unit is a large protein complex consisting of silk fibroin and P25 with a 6:1 molar ratio. Rheological measurements in the dilute C region reveal that those units (or clusters) with the hydrodynamic radius of about 100 nm form a network extending over the whole sample volume with small pseudoplateau modulus mainly by ionic bonding between COO(-) ions of the fibroin molecules and divalent metallic ions such as Ca(2+) or Mg(2+) ions present in the sample and also that, after a yield stress is reached, steady plastic flow is induced with viscosity much lower than the zero-shear viscosity estimated from creep and creep recovery measurements by 4-6 orders of magnitude. Angular frequency omega dependencies of the storage and the loss shear moduli, G'(omega) and G' '(omega), measured in the linear viscoelastic region, indicate that all solutions possess the pseudoplateau modulus in the low omega region and samples become highly viscoleastic for C greater, similar 4.2 wt %. Above C = 11.2 wt % another plateau appears at the high omega end accompanied by a distinct maximum of G' ' in the intermediate omega region. The relaxation motion with tau = 0.5 s corresponding to the maximum of G' ' is one of characteristic properties of the fibroin solutions in the high C region. Thermorheological behaviors of the solution with C = 27.5 wt % show that the network structure formed in the MM part of the silk gland is susceptible to temperature and a more stable homogeneous network is realized by raising the temperature up to T = 65 degrees C.

Calcification-resistant Nafion/Fe3+ assemblies for implantable biosensors

An electrostatic layer-by-layer deposition technique was employed for the formation of thin films consisting of alternating layers of perfluorinated ionomer (Nafion) and ferric ions. UV-vis spectroscopic and ellipsometric data indicate a stepwise growth that in certain cases is as high as 47 nm per dip cycle. The growth characteristics of these assemblies can be correlated with Nafion's hydrodynamic radius, iron content, as well as the ionic strength and pH of Nafion and the wash solution. When these assemblies were compared to cast Nafion films, they exhibit the following advantages: (i) increased hydrolytic stability, attained without thermal treatment required for pristine Nafion films, and (ii) resistance to calcification, by more than an order of magnitude. These results, along with the ability to control glucose permeability by varying the number of Nafion/Fe3+ layers, could prove vital in prolonging the lifetime of implantable biosensors.

"Ordered" structure in ionic dilute solutions: dendrimers with univalent and bivalent counterions

As an intermediate sample of ionic solutes between colloidal particles (macroions) and simple electrolyte ions, we made small-angle x-ray scattering (SAXS) measurements for aqueous solutions of poly(amido amine) dendrimers of three generations (G4, G7, and G10). The SAXS curves of univalent acid solutions showed a single scattering peak, as observed for synthetic macroions. The peak position was dependent on the dendrimer concentration but independent not only of the acid concentration (degree of protonation) but also of the counterion species. The effective charge density of the dendrimer determined by conductivity measurements was found to be insensitive to the acid concentration and the counterion species. The nearest neighbor interparticle distance 2D(exp) calculated from the peak position of the structure factor of G7 and G10 was obviously smaller, though slightly, than the average interparticle distance 2D(0) calculated from molecular weights and concentrations of dendrimers, implying that acid solutions of dendrimers formed the two-state structures by the attractive force. The ultra-small-angle x-ray scattering curve for the hydrochloric acid solution did not show an upturn, which indicates the existence of large scale structural inhomogeneities such as localized ordered structures, probably due to the weak attraction and hence less clear distinction of the ordered and disordered regions. For sulfuric acid solutions, clear scattering peaks were not observed. The bivalent counterions were more strongly associated with the dendrimer ions than the univalent ones. The resulting low charge number of the dendrimers with the bivalent counterion was confirmed directly by the conductivity measurements. These observations confirm that the counterion-mediated attraction does exist even with the univalent counterions and point out that the frequently advanced claim that the effective potential is essentially repulsive with univalent counterions while attraction appears with bivalent counterions is not necessarily correct. It is noted that the intensity of the counterion-mediated attraction in dendrimer solutions is dictated by both the effective charge density and the effective charge number, in contrast with macroionic solutions or colloidal dispersions in which only the effective charge density appeared to be important.

New evidence of the nonequilibrium nature of the "slow modes" of diffusion in polyelectrolyte solutions

The time-dependent behavior of the dissolution of polyelectrolyte powders in pure water and moderate ionic strength aqueous solvent was monitored by flowing dissolving material through an online filter, and then through a multiangle light scattering unit, a refractometer, and a capillary viscometer. When the polyelectrolytes were dissolved in solutions of moderate ionic strength, their dissolution behavior was similar to that of neutral polymers. When dissolved in pure water, however, there was consistently a small population of aggregates that appeared at the beginning of the dissolution process, which then rapidly diminished. For large pore filtration, the aggregates reached a final low level, and slowly disappeared over the span of many days, whereas for small pore filtration the aggregates disappeared completely over a scale of minutes. The real-time data, together with size exclusion chromatography analysis, shed light on previously unanswered questions concerning the nonequilibrium nature of this small population of polyelectrolyte aggregates in low ionic strength solutions, and its relation to the "extraordinary phase" of diffusion (or "slow modes"). Further evidence is also provided that both angular scattering maxima due to interpolyion correlations and the maximum of reduced viscosity vs polyion concentration ("electroviscous" effect) at low ionic strength are equilibrium properties that are unrelated to these aggregates.