Electrokinetic phenomena of soft particles

Leveraging microfluidic dielectrophoresis to distinguish compositional variations of lipopolysaccharide in E. coli

Lipopolysaccharide (LPS) is the unique feature that composes the outer leaflet of the Gram-negative bacterial cell envelope. Variations in LPS structures affect a number of physiological processes, including outer membrane permeability, antimicrobial resistance, recognition by the host immune system, biofilm formation, and interbacterial competition. Rapid characterization of LPS properties is crucial for studying the relationship between these LPS structural changes and bacterial physiology. However, current assessments of LPS structures require LPS extraction and purification followed by cumbersome proteomic analysis. This paper demonstrates one of the first high-throughput and non-invasive strategies to directly distinguish Escherichia coli with different LPS structures. Using a combination of three-dimensional insulator-based dielectrophoresis (3DiDEP) and cell tracking in a linear electrokinetics assay, we elucidate the effect of structural changes in E. coli LPS oligosaccharides on electrokinetic mobility and polarizability. We show that our platform is sufficiently sensitive to detect LPS structural variations at the molecular level. To correlate electrokinetic properties of LPS with the outer membrane permeability, we further examined effects of LPS structural variations on bacterial susceptibility to colistin, an antibiotic known to disrupt the outer membrane by targeting LPS. Our results suggest that microfluidic electrokinetic platforms employing 3DiDEP can be a useful tool for isolating and selecting bacteria based on their LPS glycoforms. Future iterations of these platforms could be leveraged for rapid profiling of pathogens based on their surface LPS structural identity.

Effects of surface charge and environmental factors on the electrostatic interaction of fiber with virus-like particle: A case of coronavirus

We propose a theoretical model to elucidate intermolecular electrostatic interactions between a virus and a substrate. Our model treats the virus as a homogeneous particle having surface charge and the polymer fiber of the respirator as a charged plane. Electric potentials surrounding the virus and fiber are influenced by the surface charge distribution of the virus. We use Poisson-Boltzmann equations to calculate electric potentials. Then, Derjaguin's approximation and a linear superposition of the potential function are extended to determine the electrostatic force. In this work, we apply this model for coronavirus or SARS-CoV-2 case and numerical results quantitatively agree with prior simulation. We find that the influence of fiber's potential on the surface charge of the virus is important and is considered in interaction calculations to obtain better accuracy. The electrostatic interaction significantly decays with increasing separation distance, and this curve becomes steeper when adding more salt. Although the interaction force increases with heating, one can observe the repulsive-attractive transition when the environment is acidic.

Soft synthetic microgels as mimics of mycoplasma

Artificial model colloids are of special interest in the development of advanced sterile filters, as they are able to efficiently separate pleomorphic, highly deformable and infectious bacteria such as mycoplasma, which, until now, has been considered rather challenging and laborious. This study presents a full range of different soft to super soft synthetic polymeric microgels, including two types with similar hydrodynamic mean diameter, i.e., 180 nm, and zeta potential, i.e., -25 ± 10 mV, but different deformability, synthesized by inverse miniemulsion terpolymerization of acrylamide, sodium acrylate and N,N'-methylenebisacrylamide. These microgels were characterized by means of dynamic, electrophoretic and static light scattering techniques. In addition, the deformability of the colloids was investigated by filter cake compressibility studies during ultrafiltration in dead-end mode, analogously to a study of real mycoplasma, i.e., Acholeplasma laidlawii, to allow for a direct comparison. The results indicate that the variation of the synthesis parameters, i.e., crosslinker content, polymeric solid content and content of sodium acrylate, has a significant impact on the swelling behavior of the microgels in aqueous solution as well as on their deformability under filtration conditions. A higher density of chemical crosslinking points results in less swollen and more rigid microgels. Furthermore, these parameters determine electrokinetic properties of the more or less permeable colloids. Overall, it is shown that these soft synthetic microgels can be obtained with tailor-made properties, covering the size of smallest species of and otherwise similar to real mycoplasma. This is a relevant first step towards the future use of synthetic microgels as mimics for mycoplasma.

Weak Anion Binding to Poly(N-isopropylacrylamide) Detected by Electrophoretic NMR

Ion specific effects are ubiquitous in solutions and govern a large number of colloidal phenomena. To date, a substantial and sustained effort has been directed at understanding the underlying molecular interactions. As a new approach, we address this issue by sensitive 1H NMR methods that measure the electrophoretic mobility and the self-diffusion coefficient of poly(N-isopropylacrylamide) (PNIPAM) chains in bulk aqueous solution in the presence of salts with the anion component varied from kosmotropes to chaotropes along the Hofmeister series. The accuracy of the applied electrophoretic NMR experiments is exceptionally high, on the order of 10-10 m2/(V s), corresponding to roughly 10-4 elementary charges per monomer effectively associated with the neutral polymer. We find that chaotropic anions associate to PNIPAM with an apparent Langmuir-type saturation behavior. The polymer chains remain extended upon ion association, and momentum transfer from anion to polymer is only partial which indicates weak attractive short-range forces between anion and polymer and, thereby and in contrast to some other ion-polymer systems, the lack of well-defined binding sites.

PEDOT:PSS nano-particles in aqueous media: A comparative experimental and molecular dynamics study of particle size, morphology and z-potential

PEDOT

PSS is the most widely used conducting polymer in organic and printed electronics.

PEDOT

PSS films have been extensively studied to understand the morphology, ionic and electronic conductivity of the polymer. However, the polymer dispersion, which is used to cast or spin coat the films, is not well characterized and not well understood theoretically. Here, we study in detail the particle morphology, size, charge density and zeta potential (z-potential) by coarse-grained MD simulations and dynamic light scattering (DLS) measurements, for different pH levels and ionic strengths. The PEDOT:PSS particles were found to be 12 nm-19 nm in diameter and had a z-potential of -30 mV to -50 mV when pH was changed from 1.7 to 9, at an added NaCl concentration of 1 mM, as measured by DLS. These values changed significantly with changing pH and ionic strength of the solution. The charge density of PEDOT:PSS particles was also found to be dependent on pH and ionic strength. Besides, the distribution of different ions (PSS^-, PEDOT⁺, Na⁺, Cl^-) present in the solution is simulated to understand the particle morphology and molecular origin of z-potential in PEDOT:PSS dispersion. The trend in change of particle size, charge density and z- potential with changing pH and ionic strength are in good agreement between the simulations and experiments. Our results show that the molecular model developed in this work represents very well the PEDOT:PSS nano-particles in aqueous dispersion. With this study, we hope to provide new insight and an in-depth understanding of the morphology and z-potential evolution in PEDOT:PSS dispersion.

AC Electrokinetics of Salt-Free Multilayered Polymer-Grafted Particles

Interest in the electrical properties of the interface between soft (or polymer-grafted) nanoparticles and solutions is considerable. Of particular significance is the case of polyelectrolyte-coated particles, mainly taking into account that the layer-by-layer procedure allows the control of the thickness and permeability of the layer, and the overall charge of the coated particle. Like in simpler systems, electrokinetic determinations in AC fields (including dielectric dispersion in the 1 kHz-1 MHz frequency range and dynamic electrophoresis by electroacoustic methods in the 1-18 MHz range) provide a large amount of information about the physics of the interface. Different models have dealt with the electrokinetics of particles coated by a single polymer layer, but studies regarding multi-layered particles are far scarcer. This is even more significant in the case of so-called salt-free systems; ideally, the only charges existing in this case consist of the charge in the layer(s) and the core particle itself, and their corresponding countercharges, with no other ions added. The aims of this paper are as follows: (i) the elaboration of a model for the evaluation of the electrokinetics of multi-grafted polymer particles in the presence of alternating electric fields, in dispersion media where no salts are added; (ii) to carry out an experimental evaluation of the frequency dependence of the dynamic (or AC) electrophoretic mobility and the dielectric permittivity of suspensions of polystyrene latex spherical particles coated with successive layers of cationic, anionic, and neutral polymers; and (iii) finally, to perform a comparison between predictions and experimental results, so that it can be demonstrated that the electrokinetic analysis is a useful tool for the in situ characterization of multilayered particles.

Electrolytic conductivity of ionic polymers in a nonpolar solvent

The electrolytic conductivity of two electrolytes as solutions in the nonpolar solvent, n -dodecane, as a function of concentration has been studied. One was a small molecule electrolyte (tetraalkyl cation and a highly fluorinated tetraphenylborate anion), and the other was a macromolecular electrolyte (cation-containing poly(alkyl methacrylate) chain with the same anion). Two series of the macromolecular cation were prepared: one with entirely cation-containing molecules and the other with a small proportion (10%) cation-containing and the rest nonionic. The conductivity data were qualitatively similar for all systems, which formed both single ions and triple ions. The data from the two series of macromolecular electrolytes were particularly informative to understand some recent and counterintuitive electrokinetic data for particles that were stabilized by these polymers. Reducing the proportion of cationic chains in the stabilizer of the particles was found to increase their electrophoretic mobility. In the conductivity data in this study, reducing the proportion of cationic chains in solution was found to increase the magnitude of the single-ion equilibrium constant and suppress the formation of triple ions. These data should support the development of models to understand these electrokinetic results.

Dynamic electrophoretic mobility and electric permittivity of concentrated suspensions of plate-like gibbsite particles

In this paper we present experimental results on the electrokinetic behavior of planar gibbsite particles in concentrated suspensions. The dc electrophoretic mobility measurements are in this case of little significance, as they are scarcely informative. In the present investigation, we show that the dielectric dispersion and dynamic electrophoresis can in contrast provide such information. The complicating factors are of course the non-spherical shape and the finite particle concentration, as no complete theory of these phenomena exists for such systems. We propose to use first of all a model of dynamic electrophoresis of spheroids in which the effect of volume fraction is considered by means of an approximate theory previously obtained for spheres, based on the evaluation of electrical and hydrodynamic interactions between particles. In addition, the role of volume fraction on the high frequency inertial relaxation is also ascertained and used to obtain a volume fraction-independent radius of the gibbsite spheroids. A similar approach is used for the evaluation of dielectric dispersion data. Both the dynamic mobility and dielectric constant dependencies on frequency were obtained for gibbsite suspensions of different volume fractions in 0.5mMKCl. The theoretical treatments elaborated were applied to these data, and a coherent picture of the geometrical and electrical characteristics of the particles was obtained.

Complexing Cations by Poly(ethylene oxide): Binding Site and Binding Mode

The binding of K⁺ and Ba²⁺ cations to short poly(ethylene oxide) (PEO) chains with ca. 4-25 monomeric units in methanol was studied by determining the effective charge of the polymer through a combination of electrophoretic NMR and diffusion NMR experiments. These cations were previously found to bind to long PEO chains in a similar strong manner. In addition, ¹H chemical shift and longitudinal spin relaxation rate changes upon binding were quantified. For both systems, binding was stronger for the short chains than that for the longer chains, which is attributed mainly to interactions between bound ions. For K⁺ ions, the equilibrium binding constant of a cation to a binding site was measured. For both cations, the binding site was estimated to consist of ca. six monomeric units that coordinated with the respective ions. For the systems with barium, a significant fraction of the bound ions are (BaAnion)⁺ ion pairs. This leads to a strong anion effect in the effective charge of the oligomers acquired upon barium ion binding. For K⁺, the coordinating oligomer segment remains rather mobile and individual oligomers exchange rapidly (≪s) between their free and ion-complexing states. In contrast, segmental dynamics slows significantly for the oligomer section that coordinates with the barium species, and for individual oligomers, binding and nonbinding sections do not exchange on the time scale of seconds. Hence, oligomers also exchange slowly (>s) between their free and barium complexing states.

Binding of Monovalent and Multivalent Metal Cations to Polyethylene Oxide in Methanol Probed by Electrophoretic and Diffusion NMR

Complex formation in methanol between monodisperse polyethylene oxide (PEO) and a large set of cations was studied by measuring the effective charge acquired by PEO upon complexation. Quantitative data were obtained at a low ionic strength of 2 mM (for some salts, also between 0.5 and 6 mM) by a combination of diffusion nuclear magnetic resonance (NMR) and electrophoretic NMR experiments. For strongly complexing cations, the magnitude of the acquired effective charge was on the order of 1 cation per 100 monomer units. For monovalent cations, the relative strength of binding varies as Na⁺ < K⁺ ≈ Rb⁺ ≈ Cs⁺, whereas Li⁺ exhibited no significant binding. All polyvalent cations bind very weakly, except for Ba²⁺ that exhibited strong binding. Anions do not bind, as is shown by the lack of response to the chemical nature of anionic species (perchlorate, iodide, or acetate). Diffusion experiments directly show that the acetate anion with monovalent cations does not associate with PEO. Considering all cations, we find that the observed binding does not follow any Hofmeister order. Instead, binding occurs below a critical surface charge density, which indicates that the degree of complexation is defined by the solvation shell. A large solvation shell prevents the binding of most multivalent ions.

Electrokinetics in polyelectrolyte grafted nanofluidic channels modulated by the ion partitioning effect

The effects of ion partitioning on the electrokinetics in a polyelectrolyte grafted nanochannel, which is the representative of a soft nanochannel, are analyzed. Earlier studies in this regard have considered low polyelectrolyte layer (PEL) grafting density at the rigid nanochannel wall and, hence, an equal permittivity inside and outside the grafted layer. In order to overcome this shortcoming, the concept of Born energy is revisited. The coupled system of the modified Poisson-Boltzmann and Navier-Stokes equation is solved numerically, going beyond the widely employed Debye-Hückel linearization and low PEL densities. The complex interplay between the hydrodynamics and charge distribution, modulated by the ion partitioning effect, along with their consequent effects on the streaming potential and electrokinetic energy conversion efficiency (EKEC) have been systemically investigated. It has been observed that the ion partitioning effect reduces the EKEC in comparison to the case with equal permittivity up to a certain electrical double layer thickness after which it increases the EKEC. For a high concentration of mobile charges within the PEL, the net gain in the maximum EKEC due to the ion partitioning effect is about 10 fold that of the case when the ion partitioning effect is not considered. We delve into the various scaling regimes in the streaming potential and intriguingly point out the exact location of peaks in efficiency. The present study also reveals the possibility of improvement in streaming potential mediated energy conversion by the use of polyelectrolyte materials, which possess substantially lower dielectric permittivity than the bulk electrolyte.

Electrostatic interaction of soft particles

Theories of the electrostatic interaction between two soft particles (i.e., particles covered with an ion-penetrable surface layer of polyelectrolytes) in an electrolyte solution are reviewed. Interactions of soft particles after contact of their surface layers are particularly discussed. Interaction in a salt-free medium and the discrete-charge effect are also treated.

Characterization and control of surfactant-mediated Norovirus interactions

Understanding of the colloidal interactions of Norovirus particles in aqueous medium could provide insights on the origins of the notorious stability and infectivity of these widespread viral agents. We characterized the effects of solution pH and surfactant type and concentration on the aggregation, dispersion, and disassembly of Norovirus virus-like particles (VLPs) using dynamic light scattering, electrophoretic light scattering, and transmission electron microscopy. Owing to net negative surface charge of the VLPs at neutral pH, low concentrations of cationic surfactant tend to aggregate the VLPs, whereas low concentrations of anionic surfactant tend to disperse the particles. Increasing the concentration of these surfactants beyond their critical micelle concentration leads to virus capsid disassembly and breakdown of aggregates. Non-ionic surfactants, however, had little effect on virus interactions and likely stabilized them additionally in suspension. The data were interpreted on the basis of simple models for surfactant binding and re-charging of the virus capsid. We used zeta potential data to characterize virus surface charge and interpret the mechanisms behind these demonstrated surfactant-virus interactions. The fundamental understanding and control of these interactions will aid in practical formulations for virus inactivation and removal from contaminated surfaces.

Ion transport and selectivity in biomimetic nanopores with pH-tunable zwitterionic polyelectrolyte brushes

Inspired by nature, functionalized nanopores with biomimetic structures have attracted growing interests in using them as novel platforms for applications of regulating ion and nanoparticle transport. To improve these emerging applications, we study theoretically for the first time the ion transport and selectivity in short nanopores functionalized with pH tunable, zwitterionic polyelectrolyte (PE) brushes. In addition to background salt ions, the study takes into account the presence of H(+) and OH(-) ions along with the chemistry reactions between functional groups on PE chains and protons. Due to ion concentration polarization, the charge density of PE layers is not homogeneously distributed and depends significantly on the background salt concentration, pH, grafting density of PE chains, and applied voltage bias, thereby resulting in many interesting and unexpected ion transport phenomena in the nanopore. For example, the ion selectivity of the biomimetic nanopore can be regulated from anion-selective (cation-selective) to cation-selective (anion-selective) by diminishing (raising) the solution pH when a sufficiently small grafting density of PE chains, large voltage bias, and low background salt concentration are applied.

Contribution of Adsorbed Protein Films to Nanoscopic Vibrations Exhibited by Bacteria Adhering through Ligand-Receptor Bonds

Bacteria adhering to surfaces exhibit nanoscopic vibrations that depend on the viscoelasticity of the bond. The quantification of the nanoscopic vibrations of bacteria adhering to surfaces provides new opportunities to better understand the properties of the bond through which bacteria adhere and the mechanisms by which they resist detachment. Often, however, bacteria do not adhere to bare surfaces but to adsorbed protein films, on which adhesion involves highly specific ligand-receptor binding next to nonspecific DLVO interaction forces. Here we determine the contribution of adsorbed salivary protein and fibronectin films to vibrations exhibited by adhering streptococci and staphylococci, respectively. The streptococcal strain used has the ability to adhere to adsorbed salivary proteins films through antigen I/II ligand-receptor binding, while the staphylococcal strain used adheres to adsorbed fibronectin films through a proteinaceous ligand-receptor bond. In the absence of ligand-receptor binding, electrostatic interactions had a large impact on vibration amplitudes of adhering bacteria on glass. On an adsorbed salivary protein film, vibration amplitudes of adhering streptococci depended on the film softness as determined by QCM-D and were reduced after film fixation using glutaraldehyde. On a relatively stiff fibronectin film, cross-linking the film in glutaraldehyde hardly reduced its softness, and accordingly fibronectin film softness did not contribute to vibration amplitudes of adhering staphylococci. However, fixation of the staphylococcus-fibronectin bond further decreased vibration amplitudes, while fixation of the streptococcus bond hardly impacted vibration amplitudes. Summarizing, this study shows that both the softness of adsorbed protein films and the properties of the bond between an adhering bacterium and an adsorbed protein film play an important role in bacterial vibration amplitudes. These nanoscopic vibrations reflect the viscoelasticity of the bacterial bond with a substratum and play important roles in bacterial adhesion, detachment and susceptibility to antimicrobials.

Retention in treated wastewater affects survival and deposition of Staphylococcus aureus and Escherichia coli in sand columns

The fate and transport of pathogenic bacteria from wastewater treatment facilities in the Earth's subsurface have attracted extensive concern over recent decades, while the impact of treated-wastewater chemistry on bacterial viability and transport behavior remains unclear. The influence of retention time in effluent from a full-scale municipal wastewater treatment plant on the survival and deposition of Staphylococcus aureus and Escherichia coli strains in sand columns was investigated in this paper. In comparison to the bacteria cultivated in nutrient-rich growth media, retention in treated wastewater significantly reduced the viability of all strains. Bacterial surface properties, e.g., zeta potential, hydrophobicity, and surface charges, varied dramatically in treated wastewater, though no universal trend was found for different strains. Retention in treated wastewater effluent resulted in changes in bacterial deposition in sand columns. Longer retention periods in treated wastewater decreased bacterial deposition rates for the strains evaluated and elevated the transport potential in sand columns. We suggest that the wastewater quality should be taken into account in estimating the fate of pathogenic bacteria discharged from wastewater treatment facilities and the risks they pose in the aquatic environment.

Using click chemistry to dial up the modulus of doubly crosslinked microgels through precise control of microgel building block functionalisation

Vinyl functionalisation of poly(2-vinylpyridine-propargylacrylate) microgels via click chemistry gives hydrogels of inter-linked microgels with tuneable modulus values.

Loading and release mechanism of red clover necrotic mosaic virus derived plant viral nanoparticles for drug delivery of doxorubicin

Loading and release mechanisms of Red clover necrotic mosaicvirus (RCNMV) derived plant viral nanoparticle (PVN) are shown for controlled delivery of the anticancer drug, doxorubicin (Dox). Previous studies demonstrate that RCNMV's structure and unique response to divalent cation depletion and re-addition enables Dox infusion to the viral capsid through a pore formation mechanism. However, by controlling the net charge of RCNMV outer surface and accessibility of RCNMV interior cavity, tunable release of PVN is possible via manipulation of the Dox loading capacity and binding locations (external surface-binding or internal capsid-encapsulation) with the RCNMV capsid. Bimodal release kinetics is achieved via a rapid release of surface-Dox followed by a slow release of encapsulated Dox. Moreover, the rate of Dox release and the amount of released Dox increases with an increase in environmental pH or a decrease in concentration of divalent cations. This pH-responsive Dox release from PVN is controlled by Fickian diffusion kinetics where the release rate is dependent on the location of the bound or loaded active molecule. In summary, controllable release of Dox-loaded PVNs is imparted by 1) formulation conditions and 2) driven by the capsid's pH- and ion- responsive functions in a given environment.

Mobility reversal of polyelectrolyte-grafted colloids in monovalent salt solutions

We present molecular dynamics simulations on the electrophoresis of a negative colloid grafted with positive polyelectrolytes. Net-neutral colloids show a varying mobility in monovalent salt. For colloids with negative net charge the mobility is negative at low and positive at high salt concentrations. This mobility reversal is an electrokinetic effect, and thus different from that observed in multivalent salt. Our results agree with numerical calculations based on the Darcy-Brinkman formalism, with which we predict the mobility reversal to also occur for experimentally accessible colloids.

Effect of induced electric field on migration of a charged porous particle

The effect of ambient fluid flow on a charged porous spherical particle suspended in an aqueous medium is analyzed. The porous particle is ion permeable and fluid penetrable. The induced electric field due to the polarization of the particle's electric double layer and counterion condensation leads to a hindrance effect on particle migration by producing an electric force. The influence of this retardation force on the hydrodynamics of the particle is studied through the Nernst-Planck equations, which are coupled with the Stokes-Brinkman equation. The interactions of the double-layer polarization, shielding effect, electroosmosis of unbalanced ions and fluid convection are analyzed. The settling velocity and fluid collection efficiency of the charged aggregate is determined. We have studied the electrokinetics for a wide range of fixed charge density and permeability of the particle with no assumption made on the thickness of the double layer relative to the dimension of the particle.

Streaming potential and electroviscous effects in soft nanochannels: towards designing more efficient nanofluidic electrochemomechanical energy converters

In this paper we provide analytical solutions for the streaming potential and electroviscous effects in soft nanochannels. The analysis is based on the solution of the linearized Poisson-Boltzmann equation, valid for small electrostatic potentials. We identify the important dimensionless parameters that dictate these two effects. Results are provided for a large range of electric double layer (EDL) thickness values, spanning from the case of very thin to very large overlapped EDL thicknesses. We compare the results with those of a rigid nanochannel, having zeta potential equal to the electrostatic potential at the solid-polyelectrolyte interface of the soft nanochannels. For the soft nanochannel, the streaming potential varies very weakly with the EDL thickness and can be substantially larger than that corresponding to the rigid nanochannel. The electroviscous effects for the soft nanochannel, unlike the rigid nanochannel, virtually always exhibit a monotonic decrease with the EDL thickness, and for certain parameter ranges can be several times larger than that for a rigid nanochannel. Most importantly, for the soft nanochannels the electrochemomechanical energy conversion, associated with the generation of streaming potential, is found to be highly efficient, with the efficiency being several times higher than that of a rigid nanochannel.

Effect of finite ion sizes in an electrostatic potential distribution for a charged soft surface in contact with an electrolyte solution

We provide a theory to analyze the impact of finite ion sizes (or steric effect) in electrostatic potential distribution for a charged soft surface in contact with an electrolyte solution. The theory is based on a free energy model that appropriately accounts for the contribution of finite ion sizes as well as the structural characteristics of a soft interface, represented by a combination of a rigid surface and a fixed charge layer (FCL), with the FCL being in contact with an electrolyte solution forming an electric double layer (EDL). This FCL contains a particular kind of ion which is impermeable to the electrolyte solution, and this impermeability is quantified in terms of the corresponding Donnan potential of the "membrane" represented by the FCL-electrolyte interface. We find that consideration of the finite ion size increases the magnitude of this Donnan potential, with the extent of increase being dictated by three length scales, namely, the thickness of the FCL, the thickness of the electrolyte EDL, and the thickness of an equivalent EDL within the FCL. Such regulation of the Donnan potential strongly affects the distribution of the permeable electrolyte ions within the FCL, which in turn will have significant implications in several processes involving "soft" biological membranes.

Electrophoretic behavior of microgel-immobilized polyions

Electrophoretic behavior was studied for N-isopropylacrylamide (NIPA) microgels, into which different amounts of poly(acrylic acid) (PAAc) were physically entrapped. Copolymer microgels of NIPA with acrylic acid (AAc) were also studied as a control. Electrophoretic mobility was measured in 0.1 M NaCl solution at 25 °C as a function of pH, using an electrophoretic light scattering technique. The mobility of the copolymer microgel whose COOH groups are fully ionized agreed with that of PAAc when its ionization degree (α(n)) is close to the mole fraction (f(AAc)) of the AAc unit in the copolymer gel. There was good agreement between the mobility values of the copolymer microgel and the linear NIPA/AAc copolymer when their AAc contents are very close to each other. However, the mobility of the microgel with immobilized PAAc was higher than that of the copolymer microgel, even when there was no difference in the AAc content for both microgels. Moreover, the immobilized PAAc showed a higher mobility than the free PAAc when its α(n) is equal to f(AAc) in the immobilized system. No correlation was observed between the mobility and the hydrodynamic radius. These results were discussed in terms of the free draining model (FDM) for the electrophoresis of polyelectrolytes. It became apparent that the mobility difference depending upon whether (i) the PAAc ions are in the cage of the NIPA network or (ii) the AAc units are copolymerized with the network chain is due to the structural difference of the segments considered in the FDM.