Grafted Polyelectrolyte Layer Research Articles

Viscoelectric effect on the chemiosmotic flow in charged soft nanochannels

The charged nanochannel surface and pH-sensitive grafted polyelectrolyte layer (PEL) play a critical role in the design of devices aimed at controlling nanofludic flow. They enable the manipulation of ionic transport by influencing the electric-double (EDL) layers that overlap. Additionally, the viscoelectric effect, amplified by a strong EDL electric field, may enhance the activation energy and viscosity of liquids. Motivated by this, we conducted a numerical investigation using a finite element method-based solver, COMSOL, to examine the effects of the viscoelectric effect on concentration-gradient-driven chemiosmotic flow in a charged soft nanochannel with grafted pH-sensitive polyelectrolyte layer on the inner wall surfaces. It is important to note that the nanochannel is positioned between two reservoirs with different pH values and bulk-ionic concentrations. The PEL is sensitive to protonic association–dissociation due to the presence of carboxylic and amine groups in monomeric units. In our study, we comprehensively demonstrate variations in key variables characterizing the underlying flow. These variations include changing the solute concentration in the left side reservoir within the range of 0.1–5 mol m−3, adjusting the pH of the right-side reservoir (pHR) within the range of 3–10, and varying the viscoelectric coefficient. The viscoelectric effect significantly raises viscosity near the wall due to the stronger EDL electric field generated at the left-side reservoir resulting from the higher solute concentration. On the other hand, viscosity tends to decrease with lower pHR values and remains unaffected by changes at higher pHR values. The average flow velocity shows an increasing–decreasing pattern as the concentration of the right-side reservoir is enhanced. Additionally, the decrease in flow velocity becomes noticeably more pronounced with higher solute concentrations in the right-side reservoir when accounting for the viscoelectric effect. The findings of the present study have practical implications for novel nanofluidic devices, frequently employed in various engineering applications to control flow.

Transient electro-osmotic flow in rotating soft microchannel

Exploiting secondary velocities produced by Coriolis forces in Lab-on-CD systems is key to achieving better transport in pharmaceutical and biomedical applications. We explore the transient behavior of velocities in rotating microchannels aided by a grafted polyelectrolyte layer (a soft layer). We further obtain an analytical solution for governing differential equations of the rotational electro-osmotic flow by the eigenfunction expansion method. We check and benchmark the solution with an in-house finite volume numerical code and also with results in literature for situation after transience has completed. We explore and discuss the effect of channel rotation, electro-osmosis, and polyelectrolyte layer on the oscillatory transient behavior of the flow velocities. We show that the size of the polyelectrolyte layer grafted to the walls aids in better control of the flow velocities and oscillations. We believe that controlled transient oscillatory behavior of velocities can be greatly used in Lab-on-CD based systems to manage the mass and momentum transport.

Ion partitioning effect on the electrostatic interaction between two charged soft surfaces

We theoretically investigate electrostatic properties between two charged surfaces with a grafted polyelectrolyte layer in an aqueous electrolyte solution by using the Poisson-Boltzmann approach accounting for ion partitioning. In order to consider the ion partitioning effect, we focus on changes of electrostatic properties due to the difference in dielectric permittivity of the polyelectrolyte layer and the aqueous electrolyte solution. We find that ion partitioning enhances electrostatic potential in the region between two charged soft surfaces and hence increases the electrostatic interaction between two charged soft surfaces. Ion partitioning effect on osmotic pressure is enhanced not only by an increase in the thickness of polyelectrolyte layer and Debye length but also by a decrease in ion radius.

Mixing in a rotating soft microchannel under electrical double layer effect: A variational calculus approach

In this work, we investigate the mixing dynamics of fluid streams in the rotating narrow-fluidic channel having grafted polyelectrolyte layer on its inner wall surfaces. We invoke the variational calculus approach for solving the coupled nonlinear system of transport equations, which is integrated with the non-homogeneous boundary conditions pertinent to this analysis. We obtain the velocity distribution in the asymptotic limit of geostrophic plug flow and then demonstrate the mixing dynamics from the perspective of qualitative assessment as well as quantitative evaluation. Performing the Poincaré map analysis, we predict the mixing of fluid streams from the qualitative assessment, while for the quantification of underlying mixing, we focus on the entropy of mixing analysis. We show that the grafted polyelectrolyte layer at the channel walls modulates the electrical double layer phenomenon following the involved electrostatics. This phenomenon in the presence of an external electric field strengthens the electroosmotic pumping in the fluidic channel non-trivially. Results show that the effects stemming from a larger thickness of the grafted polyelectrolyte layer, that is, the stronger electroosmotic pumping together with a relatively larger magnitude of friction drag, modulate the rotational force-driven primary as well as the secondary flows in the channel. The correlative–cooperative effects of the grafted polyelectrolyte layer on the rotational electrohydrodynamics lead to the formation of the dumbbell-shaped vortex and results in an enhancement in the underlying mixing.

Rheology modulated high electrochemomechanical energy conversion in soft narrow-fluidic channel

We study the influence of viscoelastic fluid on the enhancement of the electrochemomechanical energy conversion efficiency in the polyelectrolyte layer grafted narrow-fluidic channels. We obtain the velocity distribution using the simplified Phan-Thien-Tanner model and calculate the conversion efficiency in the purview of both with and without Debye-Hückel approximations. The analysis unveils that an increment in the fluid velocity obtained with an increment in the shear-thinning nature of the viscoelastic fluid increases the streaming of the counter-ions, which, in turn, increases the conversion efficiency. Also, it is shown that the increased electroviscous effect due to higher rate of streaming potential being developed results in a reduction in the predicted enhancement of the conversion efficiency. This phenomenon is observed for a window of parameters viz. higher polyelectrolyte layer thickness, higher viscoelastic parameter, and lower Debye-Hückel parameter of the electrolyte. We also report that the relatively higher electrostatic potential stemming from the overlapping electrical double layers and the higher wall potential (greater than 25 mV), lead to non-intuitive variation in the conversion efficiency following the pronounced influence of the electroviscous effect. While targeting an increment in the conversion efficiency using viscoelastic fluid and grafted polyelectrolyte layer, we also examine the influence of slip at the walls of the channel and the spatial variation of electrical conductance between the stern layer conductance and that of the walls of the channel. However, the rheology modulated enhancement in the efficiency is found to be effective in case of variation in the stern layer conductance and interfacial slip at the walls of the channel.

Efficient electroosmotic mixing in a narrow-fluidic channel: the role of a patterned soft layer.

We propose a novel and efficient mixing technique in a soft narrow-fluidic channel under the influence of electrical forcing. We show that a grafted polyelectrolyte layer (PEL) added as a patch to the channel wall modulates the electrical double layer (EDL) so that an applied electric field initiates a local electroosmotic flow (EOF) at the patched section. This EOF develops in the opposite direction to the primary pressure-driven flow. This localized EOF leads to the formation of Lamb vortices at the patched sections through the phenomenon of momentum exchange with the primary stream and promotes the mixing therein. Our study, consistent with the stream-function/vorticity approach, primarily focuses on the numerical analysis of the mixing phenomena. Through a quantitative description, we reveal the effect of different patterns on the underlying mixing phenomena in the convective mixing regime. We also discuss the impact of key parameters on the mixing efficiency, the onset of the recirculation zone, variation in the mixing length, and the shear-driven aggregation kinetics in soft matter systems. Finally, considering the practicability of the present problem, we unveil the values of several design parameters for which the mixing efficiency in the channel reaches the maximum.

Surface Tension Driven Filling in a Soft Microchannel: Role of Streaming Potential

The filling characteristics of a Newtonian electrolyte through a microchannel with grafted polyelectrolyte layer on its inner walls have been investigated. A reduced order model has been deployed f...

Adhesion between oppositely charged polyelectrolytes

ABSTRACTThe adhesion between a grafted polyelectrolyte layer (brush) and a gel of an oppositely charged polyelectrolyte has been measured as a function of applied pressure, and the interface has been traced using neutron reflectometry. The interface (in aqueous medium at pH 6) between the (polycationic) brush and the (polyanionic) gel has a limited pressure dependence, with a small amount of deformation of the interface at the brush–gel contact. Brushes with a dry thickness of up to 13 nm exhibit weak adhesion (measured using a mechanical force tester) with an adhesive failure when the gel is detached. Thicker brushes result in the gel exhibiting cohesive failure. Reversing the geometry, whereby a polycationic brush is replaced with a polyanion and the polyanionic gel is replaced with a polycation, reveals that the pH dependence of the adhesion is moderately symmetric about pH 6, but that the maximum force required to separate the polycation gel from the polyanion brush over the range of pH is greater than that for the polycation brush and polyanion gel. The polyanion used is poly(methacrylic acid) (PMAA) and polycations of poly[2-(diethylamino)ethyl methacrylate] (PDEAEMA) and poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) were used.

Controlled and tunable design of polymer interface for immobilization of enzymes: does curvature matter?

Control and tuning of surface properties is indispensable for the programmed and rational design of materials. Particularly, polymeric brush-modified colloids can be used as carrier materials for enzyme immobilization. Although it is of prime importance to control the brush architecture, there is still a lack of systematic investigations concerning the impact of grafting density on the properties of the designed interface, as well as on the immobilization of biomolecules. In this work, we investigate the surface properties of polymer brushes with different grafting densities prepared using a "grafting from" approach on flat and on colloidal particle substrates by varying the density of initiator groups. In this way, we control and tune interfacial properties of the carrier material such as swelling, charge, adhesion as well as adsorption of laccase from Trametes versicolor on the grafted polyelectrolyte layer. We show that there is no direct transferability of the results received from planar to curved substrates regarding the swelling behavior in dependence on the grafting density. The maximum of swelling degree of PDMAEMA layers is achieved at 0.34 nm-2 and at 0.1 nm-2 grafting density for planar and curved particle substrates, respectively. The adhesion properties of the polymeric layer on both substrates are also strongly influenced by the grafting density, i.e. a decrease of the grafting density causes a transition from the adhesive to non-adhesive state. As proven by the cryo-TEM and AFM force distance measurements, an immobilization of laccase from Trametes versicolor causes a decrease of the polymer swelling and therefore leads to the changes in the surface morphology, charge and adhesion performance of final polymer-enzyme layer. Moreover, the higher effectiveness and activity of laccase were observed for the intermediate grafting densities which seem to be preferable over the maximum brush densities.

Ionic strength-dependent changes in tentacular ion exchangers with variable ligand density. I. Structural properties

The ligand density critically affects the performance of ion-exchange resins in such measures as the adsorption capacity and transport characteristics. However, for tentacular and other polymer-modified exchangers, the mechanistic basis of the effect of ligand density on performance is not yet fully understood. In this study we map the ionic strength-dependent structural changes in tentacular cation exchangers with variable ligand densities as the basis for subsequent investigation of effects on functional properties. Inverse size-exclusion chromatography (ISEC), scanning electron microscopy (SEM) and small-angle x-ray scattering (SAXS) were used to assess the effect of ionic strength on the pore size and intraparticle architecture of resin variants with different ligand densities. Comparison of ISEC and cryo-SEM results shows a considerable reduction in average pore size with increasing ligand density; these methods also confirm an increase of average pore size at higher ionic strengths. SAXS analysis of ionic strength-dependent conformational changes in the grafted polyelectrolyte layer shows a characteristic ionomer peak at values of the scattering vector q (0.1–0.2Å−1) that depend on the ligand density and the ionic strength of the solution. This peak attribution reflects nanoscale changes in the structure of the grafted polyelectrolyte chains that can in turn be responsible for observed pore-size changes in the resins. Finally, salt breakthrough experiments confirm a stronger Donnan exclusion effect on pore accessibility for small ions in the high ligand density variant.

Under-water adhesion of rigid spheres on soft, charged surfaces

Adhesion in a liquid medium is fundamentally important for understanding a myriad of physiological and technological issues such as nanoparticle or bacteria uptake by cells, attachment of viruses on bacterial surfaces, adhesion of a bacteria on a preformed biofilm, biofouling of ships and marine vehicles, and many more. In this paper, we provide a theory to analyze the under-water adhesion of a rigid spherical particle on a soft, charged surface, which is represented as a layer of grafted polyelectrolyte layer (PEL). Our model is based on calculating and minimizing the free energy, appropriately modified to account for the PEL electric double layer (EDL) induced electrostatic energies. The central result of our paper is that the presence of surface charge typically enhances the adhesion, indicated by a larger negative value of the equilibrium free energy and larger value of the equilibrium depth of indentation. Such a behavior can be explained by noting that the lowering of EDL electrostatic energy due to adhesion better balances the increase in elastic energy caused by the adhesion-induced deformation. We anticipate that our theory will provide the hitherto unknown basis of quantifying the effect of surface charge in under-liquid adhesion, which is central to the vast number of phenomena involving charged bio-systems, like cells, bacteria, and viruses.

Electrostatics of soft charged interfaces with pH-dependent charge density: effect of consideration of appropriate hydrogen ion concentration distribution

Explicit consideration of hydrogen ion concentration for describing the electrostatics of grafted polyelectrolyte layers with pH-dependent charge density exhibits the necessity of considering a non-uniform depth dependent monomer distribution.

Versatile Fabrication of Ultralight Magnetic Foams and Application for Oil–Water Separation

Ultralow-density (<10 mg cm(-3)) materials have many important technological applications; however, most of them were fabricated using either expensive materials or complicated procedures. In this study, ultralight magnetic Fe2O3/C, Co/C, and Ni/C foams (with a density <5 mg cm(-3)) were fabricated on the centimeter scale by pyrolyzing commercial polyurethane sponge grafted with polyelectrolyte layers based on the corresponding metal acrylate at 400 °C. The ultralight foams consisted of 3D interconnected hollow tubes that have a diameter of micrometer and nanoscale wall thickness, forming hierarchical structures from macroscopic to nanometer length scales. More interesting was that the wall thickness and morphology of the microtubes could be tuned by controlling the concentrations of acrylic acid and metallic cations. After modification with low-surface-energy polysiloxane, the ultralight foams showed superhydrophobicity and superoleophilicity, which quickly and selectively absorbed a variety of oils from a polluted water surface under magnetic field. The oil absorption capacity reached 100 times of the foams' own weight, exhibiting one of the highest values among existing absorptive counterparts. By controlling the composition and conformation of the grafted polyelectrolyte layers, the present approach is extendable to fabricate a variety of ultralow-density materials desirable for absorptive materials, electrode materials, catalyst supports, etc.

Quartz Crystal Microbalance with Dissipation (QCM-D) studies of the viscoelastic response from a continuously growing grafted polyelectrolyte layer

Poly(acrylic acid) was grown from substrates by photopolymerization, and the grafting process was monitored in situ by Quartz Crystal Microbalance with Dissipation (QCM-D) measurements in a 1:1 v/v mixture of water/ethanol. The polymerization process was monitored into the thick film region, where the change in frequency and dissipation with increasing film mass changes sign as predicted by the Voigt viscoelastic model. Our experimental data are compared with predictions of this model, and satisfactory agreement is found for low overtone numbers. The Voigt model was applied to analyze the measured changes in frequency, Δf, and dissipation, ΔD, in order to extract information on layer thickness, shear elasticity, μ, and shear viscosity, η, of the growing film. The increasing rate of changes in Δf and ΔD observed after about 150s of polymerization was found to correlate with an increasing growth rate of the film thickness. For longer polymerization times a close to linear increase in thickness with time was observed. The sensitivity, defined as the derivatives of Δf and ΔD with respect to thickness, depends on overtone number and is different for the frequency and dissipation signals – facts that should be considered when investigating small changes in thick films used in e.g. sensor applications.

Biomimetic hydration lubrication with various polyelectrolyte layers on cross-linked polyethylene orthopedic bearing materials

Natural joints rely on fluid thin-film lubrication by the hydrated polyelectrolyte layer of cartilage. However, current artificial joints with polyethylene (PE) surfaces have considerably less efficient lubrication and thus much greater wear, leading to osteolysis and aseptic loosening. This is considered a common factor limiting prosthetic longevity in total hip arthroplasty (THA). However, such wear could be mitigated by surface modification to mimic the role of cartilage. Here we report the development of nanometer-scale hydrophilic layers with varying charge (nonionic, cationic, anionic, or zwitterionic) on cross-linked PE (CLPE) surfaces, which could fully mimic the hydrophilicity and lubricity of the natural joint surface. We present evidence to support two lubrication mechanisms: the primary mechanism is due to the high level of hydration in the grafted layer, where water molecules act as very efficient lubricants; and the secondary mechanism is repulsion of protein molecules and positively charged inorganic ions by the grafted polyelectrolyte layer. Thus, such nanometer-scaled hydrophilic polymers or polyelectrolyte layers on the CLPE surface of acetabular cup bearings could confer high durability to THA prosthetics.

Self-organization of grafted polyelectrolyte layers via the coupling of chemical equilibrium and physical interactions.

The competition between chemical equilibrium, for example protonation, and physical interactions determines the molecular organization and functionality of biological and synthetic systems. Charge regulation by displacement of acid-base equilibrium induced by changes in the local environment provides a feedback mechanism that controls the balance between electrostatic, van der Waals, steric interactions and molecular organization. Which strategies do responsive systems follow to globally optimize chemical equilibrium and physical interactions? We address this question by theoretically studying model layers of end-grafted polyacids. These layers spontaneously form self-assembled aggregates, presenting domains of controlled local pH and whose morphologies can be manipulated by the composition of the solution in contact with the film. Charge regulation stabilizes micellar domains over a wide range of pH by reducing the local charge in the aggregate at the cost of chemical free energy and gaining in hydrophobic interactions. This balance determines the boundaries between different aggregate morphologies. We show that a qualitatively new form of organization arises from the coupling between physical interactions and protonation equilibrium. This optimization strategy presents itself with polyelectrolytes coexisting in two different and well-defined protonation states. Our results underline the need of considering the coupling between chemical equilibrium and physical interactions due to their highly nonadditive behavior. The predictions provide guidelines for the creation of responsive polymer layers presenting self-organized patterns with functional properties and they give insights for the understanding of competing interactions in highly inhomogeneous and constrained environments such as those relevant in nanotechnology and those responsible for biological cells function.

Surface Conductivity Reveals Counterion Condensation within Grafted Polyelectrolyte Layers

Surface conductivity (SC) has been demonstrated to be a valuable parameter for the characterization of surface-bound polyelectrolyte layers (PLs). The measurement of the SC in dependence of the pH and solution concentration yields information about the Donnan potential, PsiD, the intrinsic charge, the potential of the PL electrolyte interface, Psi0, the pK of the ionizable groups within the PLs, and the concentration of segments, n. We discuss herein that SC measurements may additionally provide information about counterion condensation. The mobility of the counterions within grafted poly(acrylic acid) (PAA) layers was estimated from the density of COOH groups and SC data to be only 14% of that of free ions (Zimmermann, et al. Langmuir 2005, 21, 5108). In view of this large deviation and the limited sterical constraints within the brushes, we conclude that the number of freely moving counterions is decreased due to counterion condensation. This interpretation agrees well with the measurement of the osmotic pressure for PAA solution (Boisvert, et al. Polymer 2002, 43, 141), which can be exclusively attributed to the remaining mobile counterions of the polyelectrolyte.

Electrokinetic phenomena at grafted polyelectrolyte layers

During the last decades the electrokinetic theory of Smoluchowski (Z. Phys. Chem. 92 (1918) 129) was extended to be applicable for soft surfaces (grafted polyelectrolyte layers (PL), biological and artificial membranes, etc.) by either using the Debye approximation or numerical solutions. In the theory of Ohshima (Colloids Surf. A 103 (1995) 249) the nonlinearized Poisson–Boltzmann (PB) equation for thick and uniform PL is solved analytically and a general hydrodynamic equation is derived in an integral form. These advantages in the theory of Ohshima provided a base for the further development of a generalized electrokinetic theory for soft surfaces. In his theory the final equation for the electroosmotic (electrophoretic) velocity is specified for the case of the complete dissociation of ionic sites within PL. Accordingly, the equation may be used only if the difference between p K and pH is very large. However, it turned out that an analytical solution of the nonlinearized PB equation for thick PL is possible for any degree of dissociation. This was achieved using the approximation of excluded coions if the absolute value of the reduced Donnan potential is larger than 2 and due to the simplification in the case of weak dissociation, when the absolute value of the reduced Donnan potential is less than 2. Combining this generalized double layer (DL) theory for PL and the theory of Ohshima enables to obtain an analytical equation for electroosmosis for the general case of any degree of dissociation. This equation creates for the first time a theoretical base for the interpretation of electrokinetic fingerprinting (EF) for the characterization of soft surfaces.

Intrinsic charge and Donnan potentials of grafted polyelectrolyte layers determined by surface conductivity data

In order to characterize grafted polyelectrolyte layers based on electrokinetic measurements a theory of the surface conductivity K σ was developed, starting from the model of thick polyelectrolyte layers with uniform segment distribution and dissociable groups with an unknown p K value. According to this model the inner part of the polyelectrolyte layer adjacent to the substrate is considered to be isopotential while the potential decay occurs in a zone near the solution side of the layer. A simple equation for the Donnan potential Ψ D as a function of pH, p K, electrolyte concentration C 0, and volume charge density ρ was obtained. In the derived equation K σ is directly related to Ψ D , while the other terms have less influence on the magnitude of K σ and can be accounted for in a second approximation using Ψ D as determined from the measured K σ . Evaluation of the suggested model indicates that K σ measurements provide an effective method to characterize polyelectrolyte layers by analyzing the dependence of Ψ D on pH and C 0: The magnitude of K σ yields information about the surface charge at complete dissociation of the ionizable groups. The dependence of K σ on pH and C 0 can be used for the determination of the p K value of the dissociating functions and the segment volume fraction of the polyelectrolyte can be estimated using the measured value of ρ.

Complexation and distribution of counterions in a grafted polyelectrolyte layer

The complexation and the distribution of various cations, bound to a poly(styrene sulfonate) brush, have been investigated using infra-red spectroscopy and neutron reflectivity. Small counterions (like tetremethylammonium) are distributed throughout the brush in such a way that a local electroneutrality is ensured. They also exchange readily with other bulk small cations. On the other hand, model polycations are irreversibly trapped to the brush despite a relative small number of ionic bonds involved in the complexation. These complexed polycations are localized at the outer border of the brush, forming a macromolecular barrier. However, this spatial segregation does not allow the buildup of polyelectrolyte multilayers. Cationic surfactants are associated stoichiometrically with the brush sulfonates but unlike small counterions, this complexation is “irreversible” and induces a restructuring of the polymer interface.