Dependence of Structural Forces in Polyelectrolyte Solutions on Charge Density: A Combined AFM/SAXS Study (original) (raw)
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Role of polyelectrolyte charge density in tuning colloidal forces
AIChE Journal, 2004
An atomic force microscope was used to study the effects of polymer charge density on surface interactions between similarly charged silica surfaces. Copolymers of acrylamide and acrylic acid of three different charge densities (f p ϭ 15, 40, and 70%) were used. The dynamic light-scattering technique was used to obtain the characteristic size of these polymers in solution. Flocculation tests were performed to complement force-distance measurement. At 20 mM KCl and pH ϳ 8.0, the low charge density copolymer (f p ϭ 15%) caused a purely repulsive force profile between silica surfaces irrespective of the added polymer concentration, suggesting a strong adsorption of the copolymer on the surface. The medium (f p ϭ 40%) and high (f p ϭ 70%) charge density copolymers, on the other hand, provided an adhesive bridging attraction at low polymer concentrations, but a purely repulsive force at higher polymer concentrations. The range of these repulsive forces, however, was significantly smaller than that measured for the low charge density polymer. The medium and high charge densities exhibit nearly an identical behavior in controlling the intersurface forces, although the forces are different with respect to magnitude. The flocculation tests follow the same trend as that of force-distance data, where a complete, a partial, and no flocculation observed with high, medium, and low charge density polymers, respectively. In all the cases, the range of surface interactions can be correlated with the polymer chain dimension corresponding to the fast diffusion process (individual chains) obtained from dynamic light-scattering measurements. Mean-field models proposed for charged polymers can qualitatively explain both the dependency of bridging interactions on polymer charge density and the dependency of force-distance profiles on added polymer concentrations. Finally, a mean-field model was used quantitatively to account for the measured electrosteric interactions and their dependency on polymer charge density.
Advances in Colloid and Interface Science, 2002
The interactions between two macroscopic surfaces approaching one another underlies many of the phenomena observed in Colloid and Interface science. In Russia this gave rise to the branch of colloid science now referred to as Surface Forces. Important discoveries, such as the molecular organization of solvent molecules at an interface, have been unveiled by surface force measurements. More recently, forces and structures at macromolecular length scales have been uncovered. In particular, oscillatory force profiles have been detected from aqueous solutions containing polyelectrolytes. The force᎐structure relationship can reflect organization in the bulk solution or the internal structure of the adsorbed layer. Using a range of surface force techniques, combined with X-ray and neutron scattering results, we review the main features of these fascinating systems and provide an overview of how they relate to other systems such as micellar solutions, polymer᎐surfactant complexes and simple solvents.
Macromolecules, 2004
We have synthesized a new kind of poly(2-acrylamido-2-methylpropanesulfonic acid) with crown ether and studied comparatively the single molecule force spectroscopy of the polymers with and without crown ether, in terms of desorption and elongation. The smooth desorption process enabled us to calculate the loading rate of the stretching process. For the two polymers, desorption forces were loading rate independent and ionic strength insensitive. Interestingly, the desorption forces of the two polymers were undistinguishable in all conditions. These findings demonstrate (1) the polymer chains adopt a trainlike (flat) conformation at the interface with a high adsorption/desorption rate, (2) the spacer, which separates the charged group from the hydrophobic backbone and combines the two properties together, should account for the retained desorption force at high salt concentration, and (3) the 20% less in linear charge density does not affect the desorption force remarkably since hydrophobic interaction dominates the adhesion force. In deionized water, PAMPS-co-crown is less rigid than PAMPS since the uncharged side groups separate the charged groups, and thus the repulsion between adjacent charged groups is reduced. As the salt concentration increased, the rigidity of the two polymers both decreased, suggesting that the external salt would screen the charges of the polyelectrolytes. The linear charge density and the ionic strength affect only the rigidity of single polyelectrolyte chain but not the adhesion force, which is another result of the "spacer effect". This fundamental finding, which reveals the nonelectrostatic origin of the interfacial interaction of polyelectrolytes, sheds new light on the understanding of polyelectrolytes, especially for those containing spacers.
Site binding and nonlinearity effects on the extending force in polyelectrolyte solutions
Journal of Electrostatics, 1990
Conformational properties of polyelectrolyte chains strongly depend on the extending forces which are ultimately affected by the strong electrical interaction between the high charge density in the chain and the small mobile ions in solution. The polyelectrolyte solution is investigated through the cylindrical model by letting the permittivity of water depend on the electric field and a mass action law hold for the binding of one species of ions to sites along the chain. First, the electric field and ion distribution are determined by setting up a variational formulation. Then, using an appropriate expression for the free energy of the cell, the extending force is evaluated. Accurate numerical solutions are found by means of a computer simulation. As a result, in cases of common interest, the forces are shown to vary by at least one order of magnitude depending on the ionic environment, the most critical parameter being the binding constant.
Polym. Sci. Ser. A 48, 859–869 (2006) , 2006
A self-consistent integral equation theory in the form of a hybrid Monte Carlo/PRISM computation scheme was used to study a polyelectrolyte solution. The static conformational and structural properties of polyions of different rigidities in a good solvent were studied with explicit allowance for counterions over a wide concentration range. An analysis of the calculated effective potentials and correlation functions confirms the presence of effective attraction between units of the charged polymer in semidilute and concentrated solutions; this attraction leads to the collapse of polyions under certain conditions. It was shown that the cause of effective attraction is the dipole–dipole interaction of ion pairs. For the region of polyelectrolyte transition from the semidilute to the concentrated state of solution, the results qualitatively agree with experimental data and theoretical predictions. Visualized images of conformations in the test range of parameters are given.
Determination of three characteristic regimes of weakly charged polyelectrolytes monolayers
Ultramicroscopy, 2008
We have demonstrated that monolayer films of randomly charged polystyrene sulfonated acid (PSSA) can be produced by the Langmuir technique, and observed the micro-domain structures, produced by the phase separation of electrostatically charged moieties and the hydrophobic moieties. Using atomic force microscopy and Langmuir isotherm, we found three specific regimes for the polyelectrolytes with various degrees of sulfonation (4-35%); very low charged PSSA (4-5%) in the hydrophobic regime, moderately charged PSSA (6-16%) which possessed a well-balanced nature between electrostatic and the hydrophobic interactions, and strongly amphiphilic nature of PSSA (6-16%) in the ionomer regime. Finally, we could categorize PSSA 35% in the polyelectrolyte regime, due to the dominance of the electrostatic interactions over the hydrophobic interactions.
The Journal of Physical Chemistry B, 2010
The interaction forces between individual positively charged amidine functionalized latex particles with adsorbed negatively charged sodium poly(styrene sulfonate) were studied with the colloidal probe technique based on atomic force microscopy (AFM). When the polymer dose is progressively increased, the strength of the repulsive force between the particles decreases as the charge neutralization point is approached, then increases again due to overcharging, and finally reaches a plateau. Surface potentials obtained from fits of the force profiles to Poisson-Boltzmann theory agree well with potentials measured with electrophoresis. Close to the charge neutralization point, attractive forces exceeding van der Waals interactions are found. These attractive forces increase in strength with increasing molecular mass of the polymer and decreasing ionic strength. These attractive interactions are of electrostatic origin and result from lateral patch-charge heterogeneities within the adsorbed polyelectrolyte layer. The measured forces are shown to be in semiquantitative agreement with model calculations based on charge distributions with square lattice symmetry.
The Journal of Chemical Physics, 2001
Light and neutron scattering measurements on highly charged polyelectrolyte solutions have recently provided firm evidence for the existence of ''domain structures'' containing many chains, even at rather low-polymer concentrations. In the present paper, we systematically investigate the influence of counterion charge valency Z c on the scattering properties of sulfonated polystyrene ͑PSS͒ solutions in water with monovalent and divalent counterions. This study is part of a larger effort to identify essential factors governing polyelectrolyte domain formation and the geometric properties of these transient structures. Neutron scattering measurements indicate that the interchain correlation length d within the domains becomes larger by a factor of 1.5-2 for divalent relative to monovalent counterions. This observation is consistent with the Manning model estimate of the change in effective polymer charge density ⌫* with Z c and with previous observations linking d ͓from the peak position in the scattering intensity ͓I(q)͔ with the bare polymer charge density, ⌫. Light scattering measurements of the radius of gyration R g,d of the domains indicate that their size becomes smaller for divalent counterions and with a reduction of ⌫. We observe that the ''fractal'' dimension of the domains measured from the low-angle scaling of I(q) depends on the chemical structure of the polyelectrolyte. Zero average contrast ͑ZAC͒ neutron scattering measurements show that the radius of gyration R g,c of individual polyelectrolyte chains is also reduced for divalent counterions, suggesting that chain rigidity is strongly influenced by ⌫*. Charge valency effects on relaxation times are investigated by dynamic light scattering. As usual, two diffusive modes are observed in the light intensity autocorrelation function, G(). The ''fast'' mode becomes slower and the ''slow'' mode becomes faster for the divalent counterion (Mg 2ϩ), relative to the monovalent counterion (Na ϩ). Counterion valence has a large influence on the structure and dynamics of highly charged polyelectrolyte solutions through its influence on ⌫*.
Physical Review Letters, 2016
Direct force measurements between negatively charged silica particles in the presence of a like-charged strong polyelectrolyte were carried out with an atomic force microscope. The force profiles can be quantitatively interpreted as a superposition of depletion and double-layer forces. The depletion forces are modeled with a damped oscillatory profile, while the double-layer forces with the mean-field Poisson-Boltzmann theory for a strongly asymmetric electrolyte, whereby an effective valence must be assigned to the polyelectrolyte. This effective valence is substantially smaller than the bare valence due to ion condensation effects. The unusual aspect of the electrical double layer in these systems is the exclusion of the like-charged polyelectrolyte from the vicinity of the surface, leading to a strongly nonexponential diffuse ionic layer that is dominated by counterions and has a well-defined thickness. As the oscillatory depletion force sets in right after this layer, this condition can be used to predict the phase of the oscillatory depletion force.
Journal de Physique, 1990
2014 Nous avons étudié les aspects dynamiques d'un polyélectrolyte en solution aqueuse, en utilisant la technique de la diffusion quasi-élastique de la lumière. Deux conclusions majeures ont été atteintes pour le cas des solutions sans contre-ions extérieurs et le cas où la force ionique devient importante. Pour le cas sans sel, la dynamique du système est caractérisée par deux modes de fluctuations : un mode coopératif et des fluctuations à longue distance. Ceci reflète une conformation étendue des solutes macromoléculaires. Au contraire, l'augmentation de la force ionique est accompagnée par une importante réduction de la conformation des macromolécules. Un seul mode de fluctuations émerge. Le coefficient de diffusion collectif, qui a aussi été mesuré, ne dépend ni de la concentration du soluté, ni de la force ionique quand cette dernière dépasse une valeur critique faible. Nous attribuons ce phénomène à la prédominance d'interactions hydrophobes, étant donné que les effets d'écrantage réduisent la portée des forces de Coulomb entre les polyions, et à cause de la structure d'hydrocarbure du squelette de polymère. Abstract. 2014 Using QELS technique we studied the dynamic aspects of a synthetic flexible polyelectrolyte compound in aqueous solutions. By monitoring the ionic strength from very small to high values, we reached two major conclusions. First, in the absence of added electrolyte(s), the polyions are in a « highly » expanded conformation ; the dynamics are characterized by two fluctuation modes : cooperative and long range fluctuations. The addition of NaCl introduces a « sharp » and a priori unusual change. The solute macromolecules experience a non gradual change in their conformation, which is directly reflected in their dynamics. The mutual diffusion coefficient, deduced from the only existing decay rate for the correlation function, does not reflect any observable variation with polymer or salt concentration (cP and cS respectively), for the values we scanned. We attribute this phenomenon to the rising of hydrophobic interactions, once the shielding of the electrostatic interactions becomes effective at sufficiently high ionic strengths, owing to the hydrocarbon nature of the polyions.