Effects of layer-by-layer coating on activated carbon electrodes for capacitive deionization.

Recently, the need for obtaining, reusing, or purifying water has become a crucial issue. The capacitive deionization (CDI) method, which is based on the electric double layer (EDL) concept, can be applied to ion adsorption from an aqueous solution. This process is carried out by applying a potential difference to highly porous electrodes while pumping salty solution between them, partially removing the ions present in the solution and keeping them in the surface of the electrodes. The use of coated carbon electrodes with one polyelectrolyte layer, turning them into "soft electrodes" (SEs), has been proved to improve the efficiency of the system with respect to its original configuration. In this work, we investigate the effect on the ion adsorption and the efficiency of the process when implementing the coating technique known as layer-by-layer (LbL) on the electrode. This consists in successively coating the electrode surfaces with polyelectrolyte layers, alternating their charge polarity in each step. We tested the effect of the number of layers deposited, as well as the impact of this technique by using different carbons. We found that the second polyelectrolyte layer adheres more than the first layer, serving as a support or seed when it is not dense and uniformly distributed. In contrast, if the first layer is well adhered, a third layer is needed to observe improvements in adsorption and process efficiency. The adsorption of the polymer layers depends in any instance on the porosity of the carbon.

Polymer-induced orientation of nanowires under electric fields.

The controlled orientation of metallic wires inside a polymeric medium can enhance desired properties of the composites, such as the electrical conductivity or the optical transmittance. In this work, we study silver nanowire orientation in semidilute suspensions of DNA and find an intriguing effect: under the application of low-frequency AC electric fields with moderate amplitude, the DNA coils can provoke the orientation of the wires in solution. The phenomenon is entirely induced by the polymer, when it is deformed by the application of an electric field. This effect is explained using computer simulations based on excluded-volume interactions. Moreover, we experimentally show that such a behaviour is not exclusive of silver nanowire-DNA suspensions, but rather occurs for other particle-polymer systems. This phenomenon can be taken advantage of to achieve strong orientation of particles otherwise insensitive to electric fields.

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.

Assembly of Soft Electrodes and Ion Exchange Membranes for Capacitive Deionization.

The responsible use of water, as well as its reuse and purification, has been a major problem for decades now. In this work, we study a method for adsorbing ions from aqueous solutions on charged interfaces using highly porous electrodes. This water purification process is based on the electric double layer concept, using the method known as capacitive deionization (CDI): If we pump salty solutions through the volume comprised between two porous electrodes while applying a potential difference to them, ions present in the solution are partially removed and trapped on the electrode surfaces. It has been well established that the use of carbon electrodes in combination with ion exchange membranes (membrane-CDI) improves the efficiency of the method above that achieved with bare activated carbon. Another approach that has been tested is based on coating the carbon with polyelectrolyte layers, converting them into "soft electrodes" (SEs). Here we investigate the improvement found when combining SEs with membranes, and it is shown that the amount of ions adsorbed and the ratio between ions removed and electrons transported reach superior values, also associated with a faster kinetics of the process. The method is applied to the partial desalination of up to 100 mM NaCl solutions, something hardly achievable with bare or membrane-covered electrodes. A theoretical model is presented for the ion transport in the presence of both the membrane and the polyelectrolyte coating.

Charging Poly(methyl Methacrylate) Latexes in Nonpolar Solvents: Effect of Particle Concentration.

The electrophoresis of a well-established model system of charged colloids in nonpolar solvents has been studied as a function of particle volume fraction at constant surfactant concentration. Dispersions of poly(12-hydroxystearic acid)-stabilized poly(methyl methacrylate) (PMMA) latexes in dodecane were prepared with added Aerosol OT surfactant as the charging agent. The electrophoretic mobility (µ) of the PMMA latexes is found to decrease with particle concentration. The particles are charged by a small molecule charging agent (AOT) at finite concentration, and this makes the origin of this decrease in µ unclear. There are two suggested explanations. The decrease could either be due to the reservoir of available surfactant being exhausted at high particle concentrations or the interactions between the charged particles at high particle number concentrations. Contrast-variation small-angle neutron scattering measurements of PMMA latexes and deuterated AOT-d34 surfactant in latex core contrast-matched solvent were used to study the former, and electrokinetic modeling was used to study the latter. As the same amount of AOT-d34 is found to be incorporated with the latexes at all volume fractions, the solvodynamic and electrical interactions between particles are determined to be the explanation for the decrease in mobility. These measurements show that, for small latexes, there are interactions between the charged particles at all accessible particle volume fractions and that it is necessary to account for this to accurately determine the electrokinetic ζ potential.

Use of Soft Electrodes in Capacitive Deionization of Solutions.

All efforts to obtain, reuse or purify water are extremely significant for society. Recently, researchers have begun to delve in an idea born decades ago: the desalination of water using highly porous electrodes. It is based on a fundamental aspect of electrical double layers, namely, their huge capacitance. The ions of a solution can be partially removed under the application of an electric field when the solution fills the space between porous electrodes, either bare (CDI, or capacitive deionization), coated with ionic exchange membranes (MCDI) or chemically treated (inverted-CDI or i-CDI). One of the challenges of the last years was to explore new materials and arrangements to improve the efficiency of the system. In this work, we propose a new approach inspired in the electrokinetics of soft particles: a layer of polyelectrolyte (cationic on one electrode, anionic on the opposite one) coats the carbon electrodes, converting them in a sort of "soft" electrode pair. We present a theoretical model and a set of experiments showing how soft electrodes can be successfully employed in capacitive deionization.

Competition between excluded-volume and electrostatic interactions for nanogel swelling: effects of the counterion valence and nanogel charge.

In this work the equilibrium distribution of ions around a thermo-responsive charged nanogel particle in an electrolyte aqueous suspension is explored using coarse-grained Monte Carlo computer simulations and the Ornstein-Zernike integral equation theory. We explicitly consider the ionic size in both methods and study the interplay between electrostatic and excluded-volume effects for swollen and shrunken nanogels, monovalent and trivalent counterions, and for two different nanogel charges. We find good quantitative agreement between the ionic density profiles obtained using both methods when the excluded repulsive force exerted by the cross-linked polymer network is taken into account. For the shrunken conformation, the electrostatic repulsion between the charged groups provokes a heterogeneous polymer density profile, leading to a nanogel structure with an internal low density hole surrounded by a dense corona. The results show that the excluded-volume repulsion strongly hinders the ion permeation for shrunken nanogels, where volume exclusion is able to significantly reduce the concentration of counterions in the more dense regions of the nanogel. In general, we demonstrate that the thermosensitive behaviour of nanogels, as well as their internal structure, is strongly influenced by the valence of the counterions and also by the charge of the particles. On the one hand, an increase of the counterion valence moves the swelling transition to lower temperatures, and induces a major structuring of the charged monomers into internal and external layers around the crown for shrunken nanogels. On the other hand, increasing the particle charge shifts the swelling curve to larger values of the effective radius of the nanogel.

Influence of ionic strength and polyelectrolyte concentration on the electrical conductivity of suspensions of soft colloidal polysaccharides.

The electrokinetic properties of carboxymethyldextran, a soft and anionic polysaccharide, were analysed in aqueous NaNO3 solutions through measurements of the electrical conductivity of the suspensions. The results, which furnish new experimental support for the structure of soft polysaccharides in electrolyte solution show that the polyion concentration governs the conductance behavior of the suspension as the ionic strength decreases. This is particularly evident for large polymer concentrations, for which electrical double layer overlap is more likely. In contrast, the electrical conductivity of the suspension at high ionic strength reduces to the contribution of the ions in solution, as screening of the polyion charges is more efficient in such conditions. The applicability of Ohshima's general conductivity expression to these electrical conductivity measurements was examined, and a major discrepancy against the theory was observed. The calculated values of the electrical conductivity deduced on the basis of this theory were found to be lower than the experimental ones. Possible reasons for this discrepancy are discussed and a numerical model, based on the use of a cell approach to account for hydrodynamic and electrical interactions between particles, has shown to be a good description of the experimental electrokinetic data.

Electric Permittivity and Dynamic Mobility of Dilute Suspensions of Platelike Gibbsite Particles.

In this work we discuss the electrokinetic evaluation of model platelike particles. By model particles we mean homogeneous and controlled size and shape. The electrokinetic analysis in such complex geometries cannot be limited to a single data point as in usual electrophoresis in constant (dc) fields. The information can be made much richer if alternating (ac) fields with a sufficiently wide range of frequencies are used. In this case, two techniques can be applied: one is the determination of the frequency spectrum of the electric permittivity or dielectric constant (low-frequency dielectric dispersion), and the other is the electroacoustics of suspensions and the determination of the frequency dispersion of the electrophoretic mobility (dynamic or ac mobility). In this work, these techniques are used with planar gibbsite (γ-Al(OH)3) particles, modeled as oblate spheroids with a small aspect ratio. As in other laminar minerals, a particular charge distribution, differing between edges and faces, gives rise to very peculiar electrokinetic behavior. It is found that pH 7 approximately separates two distinct field responses: below that pH the dielectric dispersion and dynamic mobility data are consistent with the existence of individual, highly charged platelets, with charge mainly originating on edge surfaces. At pH 4, a low-frequency relaxation is observed, which must originated from larger particles. It is suggested that these are individual ones bridged by negatively charged fiberlike structures, coming from the partial decomposition of gibbsite particles. On the other side of the measured pH spectrum, the overall charge of the particles is low, and this probably produces aggregates with a relatively large average size, with relaxation frequencies on the low side. This is confirmed by dynamic mobility data, showing that a coherent picture of the nanostructure can be reached by combining the two techniques.

Temperature-sensitive nanogels in the presence of salt: explicit coarse-grained simulations.

In this work, coarse-grained simulations of two charged thermo-shrinking nanogels (with degrees of ionization of 0.125 and 0.250) in the presence of 1:1 and 3:1 electrolytes have been explicitly performed through the bead-spring model of polyelectrolyte. In a first set of simulations, salt concentrations for 1:1 and 3:1 electrolytes ranged from 1 to 100 mM and from 0.167 to 16.7 mM, respectively, whereas temperature remained fixed at a value for which hydrophobic forces were negligible in our case (288 K). The sizes of swollen nanogels are smaller when trivalent cations are present, but they do not change significantly in the range of concentrations of 3:1 electrolyte studied here. It should be also stressed that trivalent cations neutralize the nanogel charge more efficiently. According to these results the electrostatic repulsion plays an important role. In a second set of simulations, the temperature varied from 288 to 333 K to study the effect of salt on the thermal response when hydrophobic forces are not negligible. For the nanogels with the lowest degree of ionization, the behavior of the radius with increasing the temperature can be described by a sigmoid function, which shifts towards lower temperatures in the presence of salt. This shift is more clearly observed for trivalent cations, even at low concentrations. For the nanogels with the highest degree of ionization, the effect of additional electrolyte is also noticeable. In this case, hydrophobic forces are not the only responsible for their shrinkage in the presence of trivalent cations. The surface electrostatic potential and the concentration of salt cations inside the nanogel have been computed from simulations and a modified Poisson-Boltzmann (PB) cell model. The thermosensitivity in size have certain influence on the sensitivity of these properties to temperature changes. The rich behavior of the surface electrostatic potential and the uptake of salt cations are successfully predicted by the modified PB cell model proposed (at least qualitatively). Particularly, the model is able to predict how the retention of salt cations depends on their charge and the ionic valence when nanogels shrink.

Excluded volume effects on ionic partitioning in gels and microgels: a simulation study.

In this work the effect of volume exclusion on ionic partitioning in swollen and moderately collapsed gels has been studied through coarse-grained simulations. Our results show that finite size effects yield deviations from the classical theory of Donnan exclusion. At low or moderate reservoir electrolyte concentration these discrepancies become important if one of the ions has diameters of just a few nanometers. When the reservoir electrolyte concentration grows, volume exclusion can lead to a drastic failure of the ideal Donnan exclusion even for conventional hydrated monoatomic ions. In addition, an approximate analytical expression for the partition coefficient of ionic species including the volume exclusion associated with the polymer network and the neutralizing counterions has been proposed and tested. This theoretical approach also provides an expression for the Donnan potential difference that takes such effects into account. Good agreement between theory and simulations is found for slightly and moderately charged gels (both at low and high reservoir electrolyte concentrations). The theory also works acceptably for highly charged gels at high salt concentrations or for electrolytes with large counterions.

Temperature effects on energy production by salinity exchange.

In recent years, the capacitance of the interface between charged electrodes and ionic solutions (the electric double layer) has been investigated as a source of clean energy. Charge is placed on the electrodes either by means of ion-exchange membranes or of an external power source. In the latter method, net energy is produced by simple solution exchange in open circuit, due to the associated decrease in the capacitance of the electric double layer. In this work, we consider the change in capacitance associated with temperature variations: the former decreases when temperature is raised, and, hence, a cycle is possible in which some charge is put on the electrode at a certain potential and returned at a higher one. We demonstrate experimentally that it is thus viable to obtain energy from electric double layers if these are successively contacted with water at different temperatures. In addition, we show theoretically and experimentally that temperature and salinity variations can be conveniently combined to maximize the electrode potential increase. The resulting available energy is also estimated.

Effect of Solution Composition on the Energy Production by Capacitive Mixing in Membrane-Electrode Assembly.

In this work, we consider the extent to which the presence of multivalent ions in solution modifies the equilibrium and dynamics of the energy production in a capacitive cell built with ion-exchange membranes in contact with high surface area electrodes. The cell potential in open circuit (OCV) is controlled by the difference between both membrane potentials, simulated as constant volume charge regions. A theoretical model is elaborated for steady state OCV, first in the case of monovalent solutions, as a reference. This is compared to the results in multi-ionic systems, containing divalent cations in concentrations similar to those in real seawater. It is found that the OCV is reduced by about 25% (as compared to the results in pure NaCl solutions) due to the presence of the divalent ions, even in low concentrations. Interestingly, this can be related to the "uphill" transport of such ions against their concentration gradients. On the contrary, their effect on the dynamics of the cell potential is negligible in the case of highly charged membranes. The comparison between model predictions and experimental results shows a very satisfactory agreement, and gives clues for the practical application of these recently introduced energy production methods.

Experimental and theoretical study of the silica particle interactions in the presence of multivalent rod-like ions.

The silica particle interactions in the presence of spermidine were systematically investigated both from experimental and theoretical points of view. The hydrodynamic radii and the corresponding polydispersity indices of the colloidal silica particles were determined by dynamic light scattering (DLS) as a function of spermidine concentration. Whereas the effective size of the silica particles increases with increasing spermidine concentration (pointing to the particle aggregation), the polydispersity index first increases reaches a maximum and then further decreases with the increasing spermidine concentration. From the mobility measurements it was concluded that the increase in spermidine concentration causes less negative values of zeta potential, meaning that the adsorption of spermidine leads to the less negative silica surface. Moreover, Monte Carlo (MC) simulations also confirmed that the addition of spermidine reduces the repulsion between silica particles. The MC concentration profiles of spermidine close to the charged silica particle are in a very good agreement with the results obtained by theory. An important motivation for our study is the effectiveness of multivalent ions to coagulate colloidal suspensions; e.g., the multivalent ions are exploited in the water purification process.

Electrophoresis and dielectric dispersion of spherical polyelectrolyte brushes.

Spherical polyelectrolyte brushes (SPBs) consist of a rigid core on which polyelectrolyte chains are grafted in such a way that in certain conditions (low ionic strength and high charge of the chains) the polymer chains extend radially toward the liquid medium. Because of the hairy-like structure of the polymer brushes, the typical soft-particle approach used for explaining the behavior of polyelectrolyte-coated particles must be modified, using the assumptions that the density of charged segments in the polymer chains decreases with the squared distance to the rigid core surface and that the same happens to the friction between the brushes and the surrounding fluid. Interest in clarifying the electrokinetics of these systems is not just academic. It has recently been found experimentally (Jiménez et al., Soft Matter 2011, 7, 3758-3762) that the response of concentrated suspensions of spherical polyelectrolyte brushes in the presence of alternating electric fields shows a number of unexpected features. Both dielectric and dynamic electrophoretic mobility spectra (respectively, dependences of the electric permittivity and the AC electrophoretic mobility on the frequency of the applied field) showed very special aspects, with giant values of the mobility and an unusually strong dielectric relaxation in the kHz region. In the present paper we give a full account of the electrodynamics of such systems, based on a cell model for describing the hydrodynamic and electrical interactions between the particles. It is found that the low-frequency dynamic mobility of SPBs is much higher than that of rigid particles of comparable size and charge, making any interpretation based on zeta potential estimations of very limited applicability. The very characteristic feature of SPBs in concentrated suspensions, namely, the enhanced alpha relaxation, can be explained by considering an adequate description of the field-induced perturbations in the counterion and co-ion concentrations, well developed both outside and inside the soft layer in the case of brush-coated particles. It can be also pointed out that the dynamic electrophoretic mobility of SPBs increases with the volume fraction of particles, as a consequence of the large thickness of the brush. Predictions are also shown for the effects of friction coefficient and charge of the polyelectrolyte layer. The results compare well with experimental spectra of the dynamic mobility and electric permittivity of moderately concentrated suspensions of SPBs consisting of a 50 nm polystyrene core with grafted poly(styrene sulfonate) chains some 140 nm in length.

Monoolein: a magic lipid?

During the last few years, there has been an extraordinary increase in publications describing the manifold applications of monoolein, one of the most important lipids in the fields of drug delivery, emulsion stabilization and protein crystallization. In this perspective we present a comprehensive review of the phase behavior of this 'magic lipid'. An account of various mesophases formed in the presence of water and a collection of formulae for the calculation of their nano-structural parameters are provided. Effects of chemical and biological molecules including lipids, detergents, salts, sugars, proteins and DNA on the classical behavior are also discussed. Physicochemical triggers such as, temperature, pressure and shearing modulate the phase behavior of monoolein self assemblies that are covered in subsequent sections. Finally the growing applications of monoolein in various fields are also reported.

Stabilization of lead sulfide nanoparticles by polyamines in aqueous solutions. A structural study of the dispersions.

Lead sulfide (PbS) nanoparticles have been synthesized in aqueous solutions by a reaction between inorganic lead salts and sodium sulfide and stabilized using the cationic polyelectrolytes branched poly(ethylenimine) (PEI), poly(allylamine hydrochloride) (PAH), and poly(diallyldimethylammonium chloride) (PDDA). The structures of the polyamine-stabilized nanoparticle dispersions were examined in detail using UV-vis spectroscopy, small-angle X-ray scattering (SAXS), static and dynamic electrophoretic mobility measurements, and transmission electron microscopy (TEM). Considerable differences were found between the stabilizing efficiencies of these polyelectrolytes, which cannot be attributed to their charge densities or their persistence lengths. Small monodisperse nanoparticles of PbS with a tight stabilizing shell were consistently found only when PEI was used as a stabilizer even at high pH values, although its charge density is then very low. The excellence of PEI as a stabilizer is mainly due to the extensive branching of the chains and the presence of uncharged secondary and tertiary amine groups, which apparently serve as good anchoring points at the nanoparticle surfaces. None of the polyelectrolytes examined here provide long-term protection of the nanoparticles toward oxidation by air, showing that a need for more complex multipurpose stabilizers exists for aqueous PbS dispersions.

AC electrokinetics of concentrated suspensions of soft particles.

In this work, we show how the cell model traditionally used for the evaluation of the electrokinetic properties of concentrated suspensions can be modified to include the case of soft particles, that is, particles consisting of a rigid core and a polyelectrolyte membrane. The Navier-Stokes and Poisson's equations have been modified to account for the presence of extra friction and a volume-distributed charge in the membrane. In addition to the boundary conditions on the particle and the cell boundary, it is necessary to define conditions on the polymer-electrolyte solution interface. The frequency dependence of the dynamic mobility and electric permittivity of suspensions of soft particles with arbitrary solids concentration is computed. It is shown that the dynamic mobility of these systems is larger than that corresponding to hard particles with the same charge. For the permittivity, the same trends are observed: the R-relaxation amplitude increases upon coating. It is found that friction plays an important role in determining the mobility, while the permittivity is more affected by the concentration of solids. The model also predicts that the charges on the core and in the membrane are very important parameters, although their effects differ on the mobility and the permittivity. While the former depends mainly on the membrane charge, the latter is responsive to both charges at comparable extents.