Study of the magnetorheological response of aqueous magnetite suspensions stabilized by acrylic acid polymers

In this paper we describe the magnetorheological (MR) behavior of aqueous suspensions consisting of magnetite particles stabilized by poly(acrylic acid) polymers (PAA). A previous work on the colloidal stability of the same systems for different pH values and polymer concentrations demonstrated that the addition of PAA polymers has a very significant effect on the stability. In the present contribution, we study the MR effect of the suspensions stabilized by two different commercial polymers, as a function of pH, magnetic field strength and magnetite volume fraction. All the results are discussed in terms of the interfacial properties of the systems. It is demonstrated that for a given concentration of micrometer particles, the rheological response strongly depends on pH, on the volume fraction of magnetite particles, on the type of polymer added for increasing the stability and on the magnetic field strength. Changing the polymer used provokes clear rheological differences for the same sample conditions (field strength, volume fraction and pH). This is suggested to be due to the hydrophobic/hydrophilic balance of the polymer affecting the magnetic field ability to form magnetic structures by aggregation of the magnetized particles. The results are compared to the predictions of the so-called standard chain model, based on the assumption that the MR effect is the result of the balance between the magnetic interactions (tending to establish some degree of order in the suspension by formation of particle chains in the direction of the field) and hydrodynamic ones (tending to destroy the formed structures by viscous stress on the chains). It is found that the behavior of the yield stress does not agree well with the predictions of the model when the relative proportion of both particle and polymer confers optimum stability to the dispersions. This is likely due to the fact that the presence of the stabilizing polyelectrolyte provokes that the magnetic field is not as effective in structuring the suspension as deduced from the chain model.     

Magnetic Polymer Composite Particles Via In situ Inverse Miniemulsion Polymerization Process

We aimed at preparing magnetic iron oxide particles by the oxidation-precipitation method in order to encapsulate these particles in polymer matrices composed of poly(acrylamide-styrene sulfonic acid sodium salt). Nanocomposites were synthesized by the incorporation of surface treated magnetic nanoparticles in the synthesized polymers via in situ inverse mini-emulsion polymerization process. The study parameter was the ionic monomer content in the synthesized polymers. The structure and the morphology of the magnetic nanogels were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), dynamic light scattering (DLS), thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM). FTIR and XRD showed that pure magnetite was formed and successfully encapsulated in the composite nanoparticles. The polymer encapsulation could reduce the susceptibility to leaching and could protect the magnetite particle surfaces from oxidation. The ionic monomer content had a great effect on the magnetization behavior. Magnetite prepared by the oxidation precipitation method, of 50 nm mean particle size, was embedded successfully into the polymer nanogels with a reasonable magnetic response, as proved by vibrating sample magnetometer measurement. Magnetic nanocomposites were proven to be super-ferromagnetic materials.     

Preparation and controlled self-assembly of Janus magnetic nanoparticles

Janus magnetic nanoparticles (~20 nm) were prepared by grafting either polystyrene sodium sulfonate (PSSNa) or polydimethylamino ethylmethacrylate (PDMAEMA) to the exposed surfaces of negatively charged poly(acrylic acid) (PAA)-coated magnetite nanoparticles adsorbed onto positively charged silica beads. Individually dispersed Janus nanoparticles were obtained by repulsion from the beads on reversal of the silica surface charge when the solution pH was increased. Controlled aggregation of the Janus nanoparticles was observed at low pH values, with the formation of stable clusters of approximately 2-4 times the initial size of the particles. Cluster formation was reversed, and individually dispersed nanoparticles recovered, by restoring the pH to high values. At intermediate pH values, PSSNa Janus nanoparticles showed moderate clustering, while PDMAEMA Janus nanoparticles aggregated uncontrollably due to dipolar interactions. The size of the stable clusters could be controlled by increasing the molecular weight of the grafted polymer, or by decreasing the magnetic nanoparticle surface availability for grafting, both of which yielded larger cluster sizes. The addition of small amounts of PAA-coated magnetic nanoparticles to the Janus nanoparticle suspension resulted in a further increase in the final cluster size. Monte Carlo simulation results compared favorably with experimental observations and showed the formation of small, elongated clusters similar in structure to those observed in cryo-TEM images.     

Nonpolymeric Coatings of Iron Oxide Colloids for Biological Use as Magnetic Resonance Imaging Contrast Agents

Iron oxide nanoparticles are used in vivo as contrast agents in magnetic resonance imaging. Their widely used polymer coatings are directly involved in their biocompatibility and avoid magnetic aggregation. As these polymer brushes also limit their tissular diffusion due to important hydrodynamic sizes, this work looks to obtain particles coated with thin layers of organic biocompatible molecules. Coating molecules were chosen depending on their fixation site on iron cores; carboxylates, sulfonates, phosphates, and phosphonates, and, among them, analogs of the phosphorylcholine. Two coating procedures (dialysis and exchange resins purification) were evaluated for hydrodynamic size, total iron concentration, electrophoretic mobility, and colloidal stability. Furthermore, a complementary test on stainless steel plates evaluated the contamination by competition of phosphonates as a rough estimation of the biocompatibility of the particles. Coating with bisphosphonates, the more interesting fixation moiety, leads to small (less than 15 nm) and stable objects in a wide range of pH including the neutrality. From stability data, the coating density was evaluated at around 1.6 molecules per nm(2). Including a quaternary ammonium salt to the coating molecule lowers their electrophoretic mobility. Moreover, this type of coating protects steel plates against contamination without significant desorption. All these properties allow further developments of these nanoparticles for biomedical applications. Copyright 2001 Academic Press.     

Effect of surface modification on magnetization of iron oxide nanoparticle colloids

Magnetic iron oxide nanoparticles have numerous applications in the biomedical field, some more mature, such as contrast agents in magnetic resonance imaging (MRI), and some emerging, such as heating agents in hyperthermia for cancer therapy. In all of these applications, the magnetic particles are coated with surfactants and polymers to enhance biocompatibility, prevent agglomeration, and add functionality. However, the coatings may interact with the surface atoms of the magnetic core and form a magnetically disordered layer, reducing the total amount of the magnetic phase, which is the key parameter in many applications. In the current study, amine and carboxyl functionalized and bare iron oxide nanoparticles, all suspended in water, were purchased and characterized. The presence of the coatings in commercial samples was verified with X-ray photoelectron spectroscopy (XPS). The class of iron oxide (magnetite) was verified via Raman spectroscopy and X-ray diffraction. In addition to these, in-house prepared iron oxide nanoparticles coated with oleic acid and suspended in heptane and hexane were also investigated. The saturation magnetization obtained from vibrating sample magnetometry (VSM) measurements was used to determine the effective concentration of magnetic phase in all samples. The Tiron chelation test was then utilized to check the real concentration of the iron oxide in the suspension. The difference between the concentration results from VSM and the Tiron test confirmed the reduction of magnetic phase of magnetic core in the presence of coatings and different suspension media. For the biocompatible coatings, the largest reduction was experienced by amine particles, where the ratio of the effective weight of magnetic phase reported to the real weight was 0.5. Carboxyl-coated samples experienced smaller reduction with a ratio of 0.64. Uncoated sample also exhibits a reduction with a ratio of 0.6. Oleic acid covered samples show a solvent-depended reduction with a ratio of 0.5 in heptane and 0.4 in hexane. The corresponding effective thickness of the nonmagnetic layer between magnetic core and surface coating was calculated by fitting experimentally measured magnetization to the modified Langevin equation.     

Enzymatic Polymerization of Natural Anacardic Acid and Antibiofouling Effects of Polyanacardic Acid Coatings

Anacardic acid, separated from cashew nut shell liquid, is well known for its strong antibiotic and antioxidant activities. Recent findings indicate that phenolic compounds from plant sources have an effect on Gram-negative bacteria biofilm formation. In this work, a polyphenolic coating was prepared from anacardic acid using enzymatic synthesis and tested for its effects on biofilm formation of both Gram-negative and Gram-positive bacteria. Natural anacardic acid was enzymatically polymerized using soybean peroxidase. Hydrogen peroxide and phenothiazine-10-propionic acid were used as an oxidizing agent and redox mediator, respectively. Nuclear magnetic resonance and Fourier transform infrared (FTIR) analyses showed the formation of oxyphenylene and phenylene units through the phenol rings. No linkage through the alkyl chain was observed, which proved a high chemo-selectivity of the enzyme. Aqueous solvents turned out to play an important role in the polymer production yield and molecular weight. With 2-propanol, the highest production yield (61%) of polymer (molecular weight = 3,900) was observed, and with methanol, higher-molecular-weight polymers (5,000) were produced with lower production yields (43%). The resulting polyanacardic acid was cross-linked on a solid surface to form a permanent natural polymer coating. The FTIR analysis indicates that the cross-linking between the polymers took place through the unsaturated alkyl side chains. The polyanacardic acid coating was then tested for its antibiofouling effect against Gram-negative and Gram-positive bacteria and compared with the antibiofouling effects of polycardanol coatings reported in the literature. The polyanacardic acid coating showed more reduction in biofilm formation on its surface than polycardanol coatings in the case of Gram-positive bacteria, while in the case of Gram-negative bacteria, it showed a similar reduction in biofilm formation as polycardanol.     

Membrane emulsification and solvent pervaporation processes for the continuous synthesis of functional magnetic and Janus nanobeads

We discuss the integration of membrane emulsification and pervaporation processes for the continuous production of functional materials, such as silica-encapsulated magnetite nanoparticle clusters and asymmetric Janus nanoparticles, by the emulsion droplet solvent evaporation method, which has traditionally been performed in small-scale batch systems. An organic solvent containing primary magnetite nanoparticles (～10 nm) coated with oleic acid was dispersed in a continuous aqueous phase by membrane emulsification, which enabled the consistent production of nanoparticle-laden solvent droplets of well-controlled size with narrow size distributions. The solvent was removed from the emulsion by pervaporation. Prior to complete solvent removal, the nanoparticle packing density within the clusters was a function of the residence time in the pervaporation unit. The final clusters formed, ～100-300 nm in size, exhibited the same superparamagnetic behavior as the primary nanoparticles, and were stable in aqueous media with a zeta potential of -70 mV at neutral pH. A facile method was used to coat the nanoclusters with a silica shell, providing sites for surface functionalization with a range of organic ligands. The nanoparticles and clusters were analyzed by a variety of techniques, including TGA, DLS, TEM, EDS, and SQUID. The effects of various parameters, such as the membrane dimensions and flow rate through the unit, on the mass transport rates were elucidated through a parametric modeling study. The applicability of the methods to the production of polymeric beads and more complex particles was demonstrated; to create Janus structures, organic polymer solutions were dispersed as droplets in continuous aqueous phases, and the solvent was subsequently evaporated. The Janus particles consisted either of polymeric cores with magnetite nanoparticles clustered as islands on their surfaces, or of two phase-separated polymers, each constituting half of any given polymeric particle.     

Physical and Chemical Properties of Magnetite and Magnetite-Polymer Nanoparticles and Their Colloidal Dispersions

The properties of polymer-coated magnetite nanoparticles, which have the potential to be used as effective magnetic resonance contrast agents, have been studied. The magnetite particles were synthesized by using continuous synthesis in an aqueous solution. The polymer-coated magnetite nanoparticles were synthesized by seed precipitation polymerization of methacrylic acid and hydroxyethyl methacrylate in the presence of the magnetite nanoparticles. The particle size was measured by laser light scattering. It was shown that the particle size, variance, magnetic properties, and stability of aqueous magnetite colloidal dispersion strictly depend on the nature of the stabilizing agent. The average hydrodynamic radius of the magnetite particles was found to be 5.7 nm in the stable aqueous colloidal dispersion. An inclusion of the magnetite particle into a hydrophilic polymeric shell increases the stability of the dispersion and decreases the influence of the stabilizing agent on the magnetic and structural properties of the magnetite particles as was shown by X-ray diffraction and M?ssbauer and IR spectroscopy, as well as by vibrating sample magnetometry. The variation in the polymeric shell size and the polymer net density can be useful tools for evaluation of the polymer-coated magnetite particles as effective contrast agents. Copyright 1999 Academic Press.     

Colloidal stability of magnetite/poly(lactic acid) core/shell nanoparticles

In this work, we describe an experimental investigation on the colloidal stability of suspensions of three kinds of particles, including magnetite, poly(lactic acid) (PLA), and composite core/shell colloids formed by a magnetite core surrounded by a PLA shell. The experiments were performed with dilute suspensions, so that recording the optical absorbance with time gives a suitable indication of the aggregation and sedimentation of the suspensions. The method allowed us to distinguish very accurately between the different surface and magnetic forces responsible for the structures acquired by particle aggregates. Thus, the pure PLA suspensions are very sensitive to ionic strength and almost unaffected by pH changes. On the contrary, the stability of magnetite systems is mainly controlled by pH. The effect of vertical magnetic fields on the stability of magnetite and magnetite/PLA suspensions is also investigated. The PLA shell reduces the magnetic responsiveness of magnetite, but it is demonstrated that the mixed particles can also form structures induced by the field, despite their lower magnetization, and they can be considered in magnetically targeted biomedical applications.     

Polymer-coated ferromagnetic colloids from well-defined macromolecular surfactants and assembly into nanoparticle chains

A novel synthetic route to polymer-coated ferromagnetic colloids of metallic cobalt has been developed. Well-defined end-functional polystyrenes were synthesized using controlled radical polymerization and used as surfactants in the thermolysis of dicobaltoctacarbonyl to afford uniform ferromagnetic nanoparticles. The presence of the polymer shell enabled prolonged colloidal stability of dispersions in a wide range of organic solvents and formed glassy encapsulating coatings around ferromagnetic cores in the solid state. These polymer-coated colloids assembled into robust, micron-sized nanoparticle chains when cast onto supporting surfaces due to dipolar associations of magnetic cores. Hierarchical assemblies were also prepared by blending polystyrene-coated cobalt colloids with larger silica beads.     

Electric conductivity of polymer films filled with magnetic nanoparticles

The conductivity of polymer composites with magnetic nanoparticles (MNP) containing magnetite and other MNP (Ni, Cu–Ni) in the layers and planar cells with Al electrodes is studied. For soluble polymers (polyvinylpyrrolidone and polyvinyl alcohol) containing 1–10 wt % of magnetite MNP, a substantial effect of MNP on surface conductivity is detected over a wide range (from 10^–10 to 10^–3 Ω^–1). It is shown that the addition of magnetite MNP not only results in a considerable change in cell conductivity, but also leads to its partially irreversible variation (by an order of magnitude or more) via minor modifications of the experimental conditions (temperature, electric field). For high-resistance samples with low probabilities of conducting chain formation, temperature current peaks are observed upon moderate heating (up to 350 K). These peaks are similar to the maxima observed upon polymer electret thermodischarges when the charges are captured by the deep centers associated with separate MNP or MNP aggregates. The type and position of the maxima are determined by the characteristics of the polymer matrix. For polyvinylpyrrolidone composites, the maxima are observed some time after heating (the echo effect). With composites based on solventborne polymers (polyalkanesterimides, soluble polyimide) and Ni, Cu–Ni MNP, no change in film conductivity measured electrophotographically is observed, due to the formation of a dielectric coating formed by polymer macromolecules adsorbed on the MNP surface. An explanation based on the possible formation of magnetic aggregates of magnetite MNP and conducting chains is proposed. Magnetic aggregation IPM is proposed as one way of controlling cell conductivity.     

磁性纳米包覆微球的制备和磁性表征

以胶体球形粒子为基体发展起来的纳米包覆(nano鄄coating)技术近来引起人们的极大兴趣,这种纳米包覆技术得到的产物常常被称为核壳粒子(core鄄shellparticles)。这种包覆一般是将纳米颗粒直接吸附在核微球上,或者包覆材料控制沉淀在核微球上[1,2]。这些复合微球常常展现出独特的光、电、力学、化学、催化等性质,因而具有广泛的研究和应用前景[3~7]。近十几年来,用做磁感应成像的超顺磁材料得到了深入的研究[4]。一方面,磁性颗粒的尺寸、电荷和表面成分对其应用有很大影响[8,9],另一方面,材料的磁学性质又主要取决于磁颗粒的大小[10]。Xu和Lindlar制备了含超顺磁颗粒的聚合物胶体颗粒,被用于构建超顺磁性的光子晶体[11,12]。在聚合物微粒上包覆氧化铁颗粒通常采用表面沉淀或表面改性官能团诱导反应包覆的方法。但这些方法不能很好控制复合微粒的均一性和表面平整性;Caruso的层鄄层包覆法(Layer鄄byLayer)虽然实现了磁性颗粒包覆[13],然而这种方法非常繁琐而不利于广泛应用。本文报道了一种新的合成磁性包覆颗粒的方法,即以聚合物微球为基核,通过非均相种子生长法包覆磁性纳米颗粒,并研究了...     

Ion-exchange voltammetry with nafion/poly(sodium 4-styrenesulfonate) mixed coatings on mercury film electrodes: characterization studies and application to the determination of trace metals

This work aimed to produce improved polymer coatings for the modification of thin mercury film electrodes (TMFEs). The goal is to obtain sensitive, reproducible, mechanically stable and antifouling devices suitable for the determination of trace metal cations in complex media. Therefore, novel mixed coatings of two sulfonated cation-exchange polymers of dissimilar characteristics-Nafion (NA) and poly(sodium 4-styrenesulfonate) (PSS)-were produced by solvent evaporation onto glassy carbon electrodes. The effect of the mass ratio (NA:PSS) on the film morphology was studied by scanning electron microscopy, revealing the formation of biphasic polymer systems, where PSS bead-shaped clusters appeared randomly dispersed into a uniform and compact NA environment. The permselectivity/ion-exchange features of the mixed films onto glassy carbon were evaluated using cathecol, urate, and dopamine. To allow trace metal analysis, thin mercury films were plated through the NA/PSS coatings, being the reproducibility and ion-exchange features of the mixed coatings-TMFE evaluated using lead ions. The best NA/PSS coating was found for the mass ratio of 5.3. Analytical performance of the NA/PSS-TMFE yielded a detection limit of 5.5 nM (3sigma), and the application of this modified electrode to an untreated polluted estuarine water sample produced significant improvements in the quality of the signal compared with that for a bare TMFE.     

Steric repulsion as a way to achieve the required stability for the preparation of ionic liquid-based ferrofluids

With this work we would like to emphasize the necessity of steric repulsion to stabilize novel ionic liquid-based ferrofluids. For this purpose, we prepared a suspension of magnetite nanoparticles coated with a double layer of oleic acid, dispersed in 1-ethyl-3-methylimidazolium ethylsulphate ([EMIM][EtSO(4)]). For comparison, a suspension of bare magnetite nanoparticles in [EMIM][EtSO(4)] was also prepared. The stability of these suspensions was checked by magnetic sedimentation and centrifugation processes. Furthermore, their yield stress was measured as a function of the applied magnetic field, which gave additional information on their stability. The results of these experiments showed that the suspension of bare nanoparticles was rather unstable, whereas the suspension of double layer coated nanoparticles gave rise to a true (stable) ferrofluid.     

Coating of silica sand with aluminosilicate clay

The objective of this work was to coat aluminosilicate clays on an inert silica support, and to characterize the properties and stability of the clay-silica coating. Two polymers, polyacrylamide (PAM) and polyvinyl alcohol (PVA), were used to bind kaolinite, illite, and smectite onto silica grains. The clay-polymer composites were studied by X-ray diffraction, FTIR, and electrophoretic mobility. Clay coatings on silica grains were characterized by mass coverage, scanning electron microscopy, specific surface area, and pH stability. Silica sand was successfully coated with clays by using the two polymers, but with PVA, the clay coating had a greater mass coverage and was more stable against pH variations. Less polymer was needed for the clay coating using PVA as compared to using PAM. Clay-polymer complexes and pure clay minerals had similar cation exchange capacities and electrophoretic mobilities, indicating that overall surface charge of the clays was little affected by the polymers. Some decrease in hydrophilicity was observed for illite and smectite when clays where coated with the polymers. The methodology reported here allows the generation of a clay-based porous matrix, with hydraulic properties that can be varied by adjusting the grain size of the inert silica support.     

Relaxation phenomena of polyethyleneimine molecules within saturated homogeneous and heterogeneous layers adsorbed at a silica/water interface

Surface area exclusion chromatography was used to investigate the adsorption characteristics of the highly branched polyethyleneimine (PEI) molecule and of a related molecule resulting from the grafting of PEI with C12 to C22 alkyl chains. The interfacial relaxation and surface affinity of the two polymers was determined in homogeneous and heterogeneous layers. The presence of hydrophobic moieties within the branched morphology of the grafted PEI molecule was found to modify the adsorption histogram as compared to bare PEI and to lead to greater interfacial stability. The relaxation of the bare and grafted macromolecules proved to be of equal extent but to develop at different rates within homogeneous layers. In heterogeneous layers composed of the two polymers, the slower relaxation of the grafted macromolecules decreased the rate of relaxation of the bare molecules, while the faster relaxation of the bare molecules strongly increased the rate and extent of the relaxation of the grafted macromolecules. The same technique was then used to determine the mode of establishment of the layers. The polymer coating profiles on successive glass fiber filters of the chromatography column were found to depend on the sequence of injection of the two polymers. Simultaneous and sequential adsorption processes were analyzed on the basis of the random sequential adsorption of macromolecules.     

Dispersion of magnetic susceptibility and the microstructure of magnetic fluid

The microstructure of magnetic fluid produced on the basis of kerosene with oleic acid as a stabilizer is studied experimentally. An analytical procedure based on the known dependence of the time of Brownian relaxation of the magnetic moment of the colloidal particle on its size and the expansion of a low-frequency spectrum of dynamic susceptibility into the series of Debye functions is used. Magnetic susceptibility is measured at frequencies from 10 Hz to 100 kHz and temperatures from 225 to 360 K for colloidal solutions with the volume fraction of magnetite from 0.08 to 0.17. The clusters with uncompensated magnetic moments and sizes varying from 50 to 70 nm that are three-or fourfold larger than the mean diameter of a single colloidal particle are found. It is revealed that characteristic sizes of clusters are virtually independent of temperature and concentration of colloidal particles. The contribution of clusters to the equilibrium susceptibility of magnetic fluid grows exponentially with decreasing temperature, being manyfold larger at low temperatures than that of single particles. The obtained temperature dependence of equilibrium susceptibility is compared with that predicted from current theoretical models.     

Preparation of Surface-Imprinted Polymer Magnetic Nanoparticles with Miniemulsion Polymerization for Recognition of Salicylic Acid

Molecular surface-imprinted polymers nanoparticles encapsulating magnetite modified with oleic acid, for recognition of salicylic acid was prepared by three-step miniemulsion polymerization. The important factors including polymerization process, solvents, miniemulsifying approaches, and co-stabilizer have been investigated to obtain magnetic molecular imprinting polymers (MMIPs) nanoparticles (NPs) with high saturation magnetization (Ms), regular morphology, and good monodispersion. The results showed that the amount of magnetite encapsulated in MMIPs NPs was 43.4 wt% and Ms was 33.584 emu/g. Thus, MMIPs NPs could be separated easily within 2 minutes by an external magnetic field. The transmission electron microscope (TEM) showed MMIPs NPs were of regular sphere with core-shell structure, where magnetite NPs were uniformly encapsulated in homogeneous polymer shells. The average diameter of MMIPs NPs was 98 nm with RSD of 6.6%. Good recognition and high loading of target molecule were achieved by MMIPs NPs in batch rebinding tests.     

Aqueous dispersions of magnetite nanoparticles complexed with copolyether dispersants: experiments and theory

Magnetite (Fe3O4) nanoparticles have been synthesized and complexed with carboxylate-functional block copolymers, and then aqueous dispersions of the complexes were investigated as functions of their chemical and morphological structures. The block copolymer dispersants had either poly(ethylene oxide), poly(ethylene oxide-co-propylene oxide), or poly(ethylene oxide-b-propylene oxide) outer blocks, and all of them had a polyurethane center block that contained pendent carboxylate groups. The complexes were formed through interactions of the carboxylates with the surfaces of the magnetite nanoparticles. The magnetite cores of the magnetite-copolymer complexes were near 10 nm in diameter, and the particles were superparamagnetic. Complexes with mass ratios of polymer to magnetite varying from 50:50 to 85:15 were studied. One of our objectives is to design complexes that form stable dispersions of discrete particles in water, yet that can be actuated (moved together) upon exposure to a uniform magnetic field. DLVO calculations that accounted for magnetic attractive interparticle forces, as well as van der Waals, steric, and electrostatic forces are presented. Compositions were identified wherein a shallow, attractive interparticle potential minimum appears once the magnetic term is applied. This suggests that it may be possible to tune the structures of superparamagnetic nanoparticle shells to allow discrete dispersions without a field, yet weak flocculation could be induced upon exposure to a field.     

Critical factors for high-performance physically adsorbed (dynamic) polymeric wall coatings for capillary electrophoresis of DNA

Physically adsorbed (dynamic) polymeric wall coatings for microchannel electrophoresis have distinct advantages over covalently linked coatings. In order to determine the critical factors that control the formation of dynamic wall coatings, we have created a set of model polymers and copolymers based on N,N-dimethylacrylamide (DMA) and N,N-diethylacrylamide (DEA), and studied their adsorption behavior from aqueous solution as well as their performance for microchannel electrophoresis of DNA. This study is revealing in terms of the polymer properties that help create an "ideal" wall coating. Our measurements indicate that the chemical nature of the coating polymer strongly impacts its electroosmotic flow (EOF) suppression capabilities. Additionally, we find that a critical polymer chain length is required for polymers of this type to perform effectively as microchannel wall coatings. The effective mobilities of double-stranded (dsDNA) fragments within dynamically coated capillaries were determined in order to correlate polymer hydrophobicity with separation performance. Even for dsDNA, which is not expected to be a strongly adsorbing analyte, wall coating hydrophobicity has a deleterious influence on separation performance.