Overcompensation Of Surface Charge Research Articles

Cadmium pollution exacerbated by drought: Insights from the nanoscale interaction at the clay mineral surface

Drought is a global environmental problem, while the effect of drought-induced unsaturation on the fate of heavy metal ions is still poorly understood, particularly the lack of mechanistic information at the molecular level. This study used molecular dynamics simulations to investigate nanoscale interactions at the montmorillonite surface under different moisture conditions. Compared to the saturated condition, drought increased the amounts and strength of Cd2+ ions adsorbed on the montmorillonite (MMT) surface while decreased the diffusivity, which was especially obvious in extreme drought conditions (θv=21%−7%). This is closely related to the compressed electric double layer, overcompensation of surface charge, and increased ion pair interactions, resulting from the confinement of water films under drought stress. Further analysis showed that the decrease of hydration effect was responsible for the exacerbated cadmium pollution. Therefore, this study may break the stereotypes about the interactions between heavy metal ions and soil minerals. The results suggest that water management (e.g., irrigation) may be prioritized before beginning heavy metal remediation.

Formation of Polyelectrolyte Multilayers by Flexible and Semiflexible Chains

Poly(sodium 4-styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDDA) are flexible polyelectrolytes, whereas sulfated chitosan (SC) and cationic guar gum (CGG) are semiflexible polyelectrolytes. By use of a quartz crystal microbalance with dissipation (QCM-D), a zeta potential analyzer (ZPA), and atomic force microscopy (AFM), we have investigated the growth of PSS/PDDA, PSS/CGG, SC/PDDA, and SC/CGG multilayers as a function of NaCl concentration (C(NaCl)). For the same layer number, the changes of frequency (-Δf) and dissipation (ΔD) regarding PSS/PDDA multilayer increase with C(NaCl), whereas -Δf and ΔD for SC/CGG multilayer increase at C(NaCl) < 0.1 M and decrease at C(NaCl) > 0.1 M as C(NaCl) increases. In the cases of PSS/CGG and SC/PDDA multilayer, for the same layer number, -Δf and ΔD increase with C(NaCl) in the range of C(NaCl) < 0.5 M, and they decrease with the increasing C(NaCl) in the case of SC/PDDA multilayer but slightly change for the PSS/CGG multilayer at C(NaCl) > 0.5 M. QCM-D studies indicate that the growth of multilayers as a function of salt concentration is determined by the delicate balance between the weakening of electrostatic repulsion between identically charged groups and the decrease of electrostatic attraction between neighboring layers. ZPA and AFM measurements demonstrate that the extent of surface charge overcompensation and the surface morphology of the multilayers are controlled by the chain conformation.

Atomistic Study of Surface Effects on Polyelectrolyte Adsorption: Case Study of a Poly(styrenesulfonate) Monolayer

Extensive all-atom molecular dynamics simulations have been performed to investigate the effect of surface features on the adsorption of poly(styrenesulfonate) (PSS) oligomers on top of a modified graphite substrate. In particular, we have investigated hydrophilic and hydrophobic model surfaces, accompanied by a variable surface charge density σs in the range 0−0.164 C/m2. Our results demonstrate that short-range interactions originating from the adsorbing substrate play a significant role in the layer structure of the adsorbed PSS, and they alone are already sufficient to induce a stable PSS adsorption layer. The presence of additional hydrophilic hydroxyl groups and charges on the adsorbing surface can further enhance the adsorption of PSS sulfonate groups at lower σs, whereas for the case of σs = 0.164 C/m2, the influence of the surface hydroxyl groups becomes negligible compared to that of the surface charges. The adsorbed PSS chains show mostly conformations where the PSS backbones are approximately parallel to the adsorbing surface. In some case, however, also the PSS backbones stand on top of the surface. Both the obtained surface charge overcompensation and the surface coverage are in good agreement with a previous experimental work [Ahrens et al. Macromolecules 2001, 34, 4504−4512]. Between the first PSS adsorption layer and the adsorbing substrate, we always find a water-rich region. The orientation of the water molecules in this region depends crucially on the features of the adsorbing surface. Our simulations suggest that the water-involved hydrogen bondings play a dominant role in determining the orientation of the water molecules. We also observe a decrease of the dielectric constant of water in the region close to the adsorbing surface in all of the investigated systems that is more pronounced for the hydrophilically modified surface and moreover increases with rising surface charge density. We suggest that this effect could lead to an electrostatic stabilization of the monolayer surface.

Molecular Ordering of Layer-by-Layer Polyelectrolyte Films Studied by Sum-Frequency Vibrational Spectroscopy

The molecular arrangement in organic thin films is crucial for their increasing technological applications. Here, we use vibrational spectroscopy by sum-frequency generation (SFG) to study the ordering of polyelectrolyte layers adsorbed on silica for all steps of layer-by-layer (LbL) self-assembly. In situ measurements during adsorption and rinsing showed that the adsorbed polymer has a disordered conformation and confirmed surface charge overcompensation upon polyelectrolyte adsorption by probing the interfacial electric field. In dry films, the polymer chains acquired a net orientational ordering, which was affected, however, by the adsorption of subsequent layers. Such a detailed characterization may allow the control of LbL film structure and functionality with unprecedented power.

In situInvestigation on Layer-by-Layer Deposition of Polyelectrolytes by Quartz Crystal Microbalance

The effect of salt concentration on layer-by-layer deposition of poly(sodium 4-styrene sulfonate) (PSSS)/poly(vinylbenzyl trimethylammonium chloride) (PVTC) was investigated by use of quartz crystal microbalance with dissipation (QCM-D). The changes in frequency and dissipation demonstrate that the addition of NaCl leads the thickness of PSSS/PVTC multilayer to increase. The deposition of PSSS/PVTC is dominated by surface charge overcompensation level at lower salt concentrations. However, it is mainly determined by the interpenetration of polyelectrolytes at a higher salt concentration, as reflected in the oscillation of dissipation change.

Counterion release from adsorbed highly charged polyelectrolyte: An electrooptical study

The adsorption of sodium poly(4-styrene sulfonate) on oppositely charged β-FeOOH particles is studied by electrooptics. The focus of this paper is on the release of condensed counterions from adsorbed polyelectrolyte upon surface charge overcompensation. The fraction of condensed Na + counterions on the adsorbed polyion surface is estimated according to the theory of Sens and Joanny and it is compared with the fraction of condensed counterions on nonadsorbed polyelectrolyte. The relaxation frequency of the electrooptical effect from the polymer-coated particle is found to depend on the polyelectrolyte molecular weight. This is attributed to polarization of the layer from condensed counterions on the polyion surface, being responsible for creation of the effect from particles covered with highly charged polyelectrolyte. The number of the adsorbed chains is calculated also assuming counterion condensation on the adsorbed polyelectrolyte and semiquantative agreement is found with the result obtained from the condensed counterion polarizability of the polymer-coated particle. Our findings are in line with theoretical predictions that the fraction of condensed counterions remains unchanged due to the adsorption of highly charged polyelectrolyte onto weakly charged substrate.

On the Indirect Polyelectrolyte Titration of Cellulosic Fibers. Conditions for Charge Stoichiometry and Comparison with ESCA

The effect of electrolyte (NaHCO3) concentration on the adsorption of poly-DADMAC (poly-diallyldimethylammonium chloride) onto cellulosic fibers with different charge profiles was investigated. Surface carboxymethylated fibers were obtained by grafting carboxymethyl cellulose (CMC) onto the fiber surface and bulk carboxymethylated fibers were obtained by reacting the fibers with monochloroacetic acid. It was shown that nonionic interactions do not exist between cellulose and poly-DADMAC, rather electrostatic interactions govern the adsorption. Charge stoichiometry prevails under electrolyte-free conditions, whereas surface charge overcompensation occurs at higher electrolyte concentrations. It was shown that charge stoichiometry prevails if the thickness of the electric double layer kappa(-1) was larger than the mean distance between the charges on the fiber surface, as predicted by polyelectrolyte adsorption theories, taking lateral correlation effects into account. In a second set of experiments the ESCA technique served to independently calibrate the polyelectrolyte titrations for determining the surface charge of cellulosic fibers. Various molecular masses of poly-DADMAC were adsorbed to carboxymethylated fibers having different charge profiles. The adsorption of low M(w) poly-DADMAC (7.0 x 10(3)), analyzed by polyelectrolyte titration, was about 10 times higher than that of the high M(w) poly-DADMAC (9.2 x 10(5)). Despite the difference in accessibility of these two polyelectrolytes to the fiber cell wall, ESCA surface analysis showed, as expected, only slight differences between the two polyelectrolytes. This gives strong credibility to the idea that surface charge content of cellulosic fibers can be analyzed by means of adsorption of a high-molecular-mass cationic polymer, i.e., by polyelectrolyte titration.

Overcharging of ellipsoidal particles by oppositely charged polyelectrolytes

Abstract Electric light scattering method is used to study complexation between polyelectrolyte molecules and oppositely charged ellipsoidal particles. Attention is focused on the charge inversion that occurs when polyelectrolytes of different charge density, chain length and flexibility adsorb on oxide particles in water medium. In particular, we are interested in the similarity observed between the electrical properties of adsorbed and nonadsorbed polymers in regime of the surface charge overcompensation. The electro-optical results show that, at plateau adsorption, the particles assume the charge characteristic of the polymer. The extension of the adsorbed layer into a vertical direction, in which case the overcharging is caused by dangling loops and tails, is supposed to explain this observation.

Polyelectrolyte Adsorption onto Insoluble Monolayers at the Air/Water Interface

Poly(styrenesulfonate) (PSS) was adsorbed reversibly from aqueous CsCl or NaCl solutions onto oppositely charged DODAB (diocatadecyldimethylammonium bromide) monolayers at the water surface. By adjusting the DODAB lateral density, the surface charge of the liquid interface is varied. The electron density profile of the surface layer was measured with X-ray reflectivity, using counterion contrast. A flat PSS layer is always found, with a thickness corresponding to one to two chain diameters. At low surface charge, the PSS coverage is constant and large, presumably due to hydrophobic attraction. A surface charge overcompensation of up to 6:1 is observed. The PSS coverage increases both with surface charge and with ionic strength. Intriguingly, at high salt (1 M) and low surface charge, beneath the flat concentrated PSS layer a diffuse extended adsorption layer is found. Its thickness decreases on increase of the surface charge and disappears at ca. 80 A2 DODAB molecular area, causing a minimum in PSS covera...

Mechanisms behind concentration profiles illustrated by charge and concentration distributions around ions in double layers

Abstract The mechanisms behind the behaviour of concentration and charge density profiles in diffuse electric double layers are investigated quantitatively for 1:1 and 2:2 electrolytes. This is done by analysing various contributions to the mean force that acts on each ion. The forces are obtained from the calculated ionic charge and concentration distributions around individual ions at various positions in the double layer. These distributions are presented graphically which allows an immediate visual illustration of the mechanisms in action. Some features studied are charge inversion in double layers for divalent aqueous electrolytes, overcompensation of surface charge due to large amounts of physisorbed counterions, ion size effects in the double layer structure and various mechanisms that cause deviations from the predictions of the Poisson–Boltzmann approximation. A major objective of the paper is to present the results in a visual form and explain aspects of modern double layer theory in a simple manner.

Self-Consistent-Field Modeling of Polyelectrolyte Adsorption on Charge-Regulating Surfaces

An extension of the self-consistent-field (SCF) lattice theory of polyelectrolyte adsorption, which takes into account electrochemical equilibria at an interface (charge regulation), is presented. The extension consists of replacement of the usual boundary condition of constant surface charge or potential by a charge-potential relation derived from the surface electrochemical equilibria. Most parameters of the model were based on characteristics of real systems, with minimal number of fitting parameters. Theoretical calculations of surface charge density, adsorbed amount, and degree of surface charge compensation by polyelectrolytes as a function of pH and electrolyte concentration were carried out. The results were compared with experimental data. The extended SCF model describes well (at a quantitative level) the surface charge density of the substrate both in the absence and in the presence of adsorbed polyelectrolyte. The theory is also successful in prediction of the main features of the surface charge overcompensation upon polyelectrolyte adsorption under different electrolyte conditions (pH, cs). By contrast the adsorbed amount is poorly predicted. We believe that the discrepancies between the experiment and the theory are mostly due to nonequilibrium aspects of the polyelectrolyte adsorption, which are not described by the present equilibrium theory.