Equilibrium and Nonequilibrium Features in the Morphology and Structure of Physisorbed Polyelectrolyte Layers (original) (raw)
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Journal of Colloid and Interface Science, 1998
In this investigation surface force, X-ray photoelectron spectroscopy and ellipsometry techniques have been used to study the adsorption of a low-charge-density cationic polyelectrolyte on negatively charged surfaces. It is shown that the low cationicity of this polyelectrolyte induces an adsorption behavior which is limited by steric factors rather than by the substrate surface charge or potential. It is also established that an increase in ionic strength of the solution results in desorption of the polyelectrolyte accompanied by an increase in layer thickness. This phenomenon is typical of a screening-reduced adsorption regime where electrostatic interactions predominate in the adsorption process. An increase in layer thickness most often occurs as a result of an increased adsorbed amount. Here, however, the increase in layer thickness occurs despite a reduction in the adsorbed amount. This can be understood as resulting from a reduced polyelectrolyte-surface affinity and a swelling of the adsorbed layer. Finally, it is demonstrated that the employed techniques complement each other and reveal new information on the interaction forces and conformation of polyelectrolytes at the solid-liquid interface.
Polyelectrolyte adsorption at the solid/liquid interface
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1998
The forces between negatively charged surfaces in the presence of an adsorbing cationic copolymer of acrylamide and 2(methacryloyloxy)ethyltrimethylammonium chloride have been investigated using an atomic force microscope. The results were compared with measurements from adsorption isotherm, electrophoretic mobility, stability, and light scattering experiments. The adsorbed amount of polyelectrolyte and adsorbed layer conformation at the solid/liquid interface were found to be strongly dependent on the polymer concentration from which initial adsorption takes place. At low polyelectrolyte concentrations unstable silica suspensions were observed from stability tests; light scattering experiments indicate a large aggregate size under equivalent conditions. The adsorbed amount was also seen to be low, well less than monolayer coverage, and force measurements indicated that the polymer was adsorbed in a flat conformation. At high polyelectrolyte concentrations, an increase in the adsorbed amount was observed which resulted in a higher surface coverage, a higher mobility and a stable suspension. Direct force measurements indicated the presence of an electrosteric barrier.
Adsorption of Polyelectrolytes at Oppositely Charged Surfaces
Macromolecules, 2001
We develop a scaling theory of adsorption of necklace-like hydrophobic polyelectrolytes at an oppositely charged hydrophilic and hydrophobic surfaces. At low surface charge densities we predict a two-dimensional adsorbed layer with thickness determined by the balance between electrostatic attraction to the charged surface and chain entropy. At high surface charge densities we expect a 3-dimensional layer with polymer density profile determined either by the balance between two-body monomer-monomer attraction or by electrostatic attraction to the surface and three-body monomermonomer repulsion. These different stabilizing mechanisms result in the nonmonotonic dependence of the layer thickness on the surface charge density. For adsorption of polyelectrolyte chains from salt solutions the screening of the electrostatic repulsion between adsorbed polyelectrolyte chains results in large overcompensation of the surface charge for two-dimensional adsorbed layers. The polymer surface coverage for this regime increases with increasing salt concentration. The opposite trend is predicted for 3-D adsorbed layers where the polyelectrolyte surface excess decreases with increasing salt concentration. † Permanent address:
Effect of short-range interactions on polyelectrolyte adsorption at charged surfaces
2003
We have studied the effect of short-range interactions on polyelectrolyte adsorption at oppositely and similarly charged surfaces. The properties of the adsorbed layer, such as polymer surface coverage, layer thickness, and surface overcharging (for adsorption at oppositely charged surfaces), are calculated as a function of the surface charge density, the strength of the short-range interactions, and the ionic strength of the solution. The properties of dilute and semidilute two-dimensional adsorbed layers are calculated in the framework of the strongly correlated Wigner liquid model. In these regimes, the surface overcharging by adsorbed polyelectrolyte chains increases as a function of the square root of the salt concentration. At higher surface charge densities, when adsorbed polyelectrolytes form a three-dimensional adsorbed layer, we use the self-consistent meanfield theory to calculate the layer properties. Here, the polymer surface coverage shows nonmonotonic dependence on the salt concentration; it initially increases as the salt concentration increases at low ionic strengths and then decreases as the ionic strength becomes higher than some critical value. The decrease of the surface coverage at a higher salt concentration is due to additional screening of the surface charge by salt ions. We show that the adsorption of polyelectrolytes at similarly charged surfaces can only occur within the range of surface charge densities where the short-range interactions dominate the electrostatic repulsion between adsorbed chains and the charged surface. In these regimes, the salt dependence of the polymer surface coverage and layer thickness is similar to that for polyelectrolyte adsorption at oppositely charged surfaces that are dominated by short-range interactions. 2. Adsorption of a Polyelectrolyte Chain at a Neutral Surface Consider a flexible polyelectrolyte chain with a degree of polymerization N, a fraction of charged monomers f, and a bond † Part of the special issue "International Symposium on Polyelectrolytes".
Adsorption of Polyelectrolytes at the Oppositely Charged Surface
1999
We develop a scaling theory of polyelectrolyte adsorption at an oppositely charged surface. At low surface charge densities, the thickness of the adsorbed layer is determined by the balance between electrostatic attraction to the charged surface and chain entropy. At high surface charge densities, it is determined by the balance between electrostatic attraction and short-range monomer-monomer repulsion. These different stabilizing mechanisms result in the nonmonotonic dependence of the layer thickness on the surface charge density.
Influence of Adsorption Conditions on the Structure of Polyelectrolyte Multilayers
Langmuir, 2002
Polyelectrolyte adsorption onto an oppositely charged interface is determined by electrostatic and secondary interactions. Since polyelectrolytes precipitate at elevated temperatures, the secondary interactions are presumably temperature dependent. This idea is tested for poly(allylamine) hydrochloride/ polystyrene sulfonate (PAH/PSS) films adsorbed from aqueous KCl solution (high salt conditions) at temperatures between 5 and 40°C. KCl was chosen because the films were thicker than those obtained from NaCl or CsCl solutions indicating strong specific binding between K and PSS. The film thickness increases continuously with the adsorption temperature; the changes amount to 20-40%, depending on salt conditions. Furthermore, the roughness is increased, up to a factor of 5. The latter is attributed to the decreased percentage of strong electrostatic bonds within the polyelectrolyte multilayer. Another path to increased roughening is using low-weight polymers with a contour length similar to the thickness of a polycation/polyanion pair.
Polyelectrolyte adsorption on charged surfaces: study by electrokinetic measurements
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1998
In order to describe the influence of cationic polyelectrolytes on flocculation of disperse systems the adsorption of poly (diallyldimethylammonium chloride) (PDADMAC) onto silica, mica and acidic polymer latex was investigated. The plateau value of the adsorption isotherms grows with increasing surface charge density of the substrates and electrolyte concentration. The adsorbed layer of the polycation was characterized by zeta potential measurements with KCl solutions of constant ionic strength and varied pH. The zero point of the charge as well as the shape of the zeta potential-pH plot depends on the coverage of the surface with polycations. For fully covered substrates the zero point of the charge as well as the pK A and pK B values calculated by a stochastic search programme are independent of the substrate. Maximum flocculation was observed at about 30% of the plateau value of the adsorption isotherms.
Anomalous Adsorption of Polyelectrolyte Layers
Macromolecules, 2001
We study the adsorption of polyelectrolytes onto oppositely charged surfaces as the surface charge density is varied while keeping the polyelectrolyte charge density fixed. As observed previously from multilayer adsorption studies, 1,2 nonmonotonic adsorption behavior is obtained in the intermediate surface charge density regime, with anomalously thick "supermonolayers" transitioning to molecularly thin layers over a small range of surface charge densities. A simple adsorption model is introduced to explain these findings, in which the surface is characterized by discrete and fully compensated adsorption sites.
Polyelectrolyte adsorption on solid surfaces: theoretical predictions and experimental measurements
Langmuir : the ACS journal of surfaces and colloids, 2013
This work utilizes a combination of theory and experiments to explore the adsorption of two different cationic polyelectrolytes onto oppositely charged silica surfaces at pH 9. Both polymers, poly(diallyldimethylammonium chloride), PDADMAC, and poly(4-vinyl N-methylpyridinium iodide), PVNP, are highly charged and highly soluble in water. Another important aspect is that a silica surface carries a relatively high surface charge density at this pH level. This means that we have specifically chosen to investigate adsorption under conditions where electrostatics can be expected to dominate the interactions. Of specific focus in this work is the response of the adsorption to the addition of simple salt (i.e., a process where electrostatics is gradually screened out). Theoretical predictions from a recently developed correlation-corrected classical density functional theory for polyelectrolytes are evaluated by direct quantitative comparisons with corresponding experimental data, as obtai...
Polyelectrolyte adsorption onto an initially-bare solid surface of opposite electrical charge
We contrast the adsorption, over a wide range of pH and ionic strength, of polyelectrolyte chains with different fractions of charged segments but similar degree of polymerization. The system was a cationic polymer, poly͑1,4 vinyl͒pyridine ͑PVP͒, with 14%, 48%, and 98% quaternized repeat units, adsorbed from aqueous solution ͑D 2 O or H 2 O͒ onto a single silicon oxide substrate at 25°C. Measurements were based on Fourier transform infrared spectroscopy in attenuated total reflection ͑FTIR-ATR͒. In the first phase of this study, we varied the surface charge density by changing pH and showed that attraction of PVP to the surface was electrostatic. The amount adsorbed of charged ͑quaternized͒ PVP segments was nearly the same regardless of the overall fraction of charged segments in the chain. In addition, polymer adsorption appeared to enhance the dissociation of silanol groups on the solid surface. In a second phase of this study, the ionic strength was varied systematically under conditions of high negative surface charge density ͑high pH͒, focusing on 98% quaternized PVP. Strong chemical specificity was found; the polyelectrolyte was insoluble in KI above a low salt concentration, but soluble in NaCl, signifying that the anions, Cl Ϫ and I Ϫ , competed with the negatively-charged surface for association with the polyelectrolyte. At the same time, the cations, Na ϩ and K ϩ , competed with the polyelectrolyte for access to the limited surface area. The mass adsorbed increased strongly with increasing salt concentration and, for polymer in aqueous NaCl, passed through a maximum with subsequent decrease, reflecting a greater abundance of loops and tails at intermediate ionic strength and ultimately complete desorption of the chains when the salt concentration was very high. The maximum in mass adsorbed occurred at very high ionic strength ͑1 molar NaCl͒, indicating competitive adsorption of Na ϩ with charged segments of the polymer. Direct measurements of the infrared dichroism of pyridinium rings of the adsorbed PVP confirmed the presence of a relatively flattened state at low ionic strength and nearly isotropic orientation otherwise. In the third phase of this study, we studied the competitive adsorption to surfaces of high negative charge of Na ϩ and a monomeric analog of the PVP repeat unit, the 1,4-dimethylpyridinium ion (P ϩ ). A tentative quantitative estimate of the effective surface sticking energy of P ϩ relative to Na ϩ ions indicated a decrease from 7k B T in low-ionic-strength buffer solution to 4.5k B T in 0.5 M NaCl. These numbers appear to exceed the weak-adsorption limit in which facile equilibration of the adsorbed layer should be expected.