Electrokinetic phenomena at grafted polyelectrolyte layers

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

Electrokinetic and contact angle measurements of grafted carbon fibers

 The grafting method that has successfully been applied to methacrylic acid and liquid crystalline monomers was expanded to prepare amphoteric carbon fiber surfaces using 2-(N,N-dimethylamino)ethyl methacrylate as monomer. The obtained carbon fiber surfaces were characterized by contact angle and ζ-potential measurements. The expected basic behavior was not observed, instead an amphoteric character of the modified carbon fiber surface was found. The fiber surfaces display a basic character in the acidic pH-range, while they are acidic in the alkaline part of the pH-scale. An important influence is derived from the amount of initiator used to graft the monomers onto the fibers. The smaller the initiator concen-tration used during polymerization, the larger the amount of amino functionalities introduced to the carbon fiber surface. The wetting behavior versus water depends on the overall conformation of the immobilized polymer. During immersion into water the polymer acts hydrophobic, while during emersion, a hydrophilic character is observed, probably derived from conformational changes and swelling during the contact angle measure-ments in water. Received: 9 June 1998 Accepted: 13 August 1998     

Surface conductivity reveals counterion condensation within grafted polyelectrolyte layers

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

On the monomer density of grafted polyelectrolyte brushes and their interactions

Most of the modern theories of grafted polyelectrolyte brushes are valid only for moderate stretching of the polyelectrolyte. However, particularly at low ionic strength and high grafting densities, even a moderate charge of the polyelectrolyte can generate a strong stretching. A simple mean field model for strongly stretched grafted polyelectrolyte brushes is suggested, based on an approximate calculation of the partition function of a polyelectrolyte chain. It is shown that the average Boltzmann factor of a possible chain configuration can be approximated by the Boltzmann factor of a configuration with a constant monomer distribution, for which the free energy can be readily obtained. The monomer density in the brush and the interaction between two surfaces with grafted polyelectrolyte brushes could be calculated as a statistical average over all possible configurations. Some simple analytical results are derived, and their accuracy is examined. The dependence of the brush thickness on the electrolyte concentration is investigated, and it is shown that the trapping of a fraction of counterions in the brush influences strongly the thickness of the brush. When two surfaces with grafted polyelectrolyte brushes approach each other more rapidly than the ion diffusion parallel to the surface, the trapping of the counterions between the brushes can affect the interactions by orders of magnitude.     

Effect of polyelectrolytes and polyelectrolyte mixtures on the electrokinetic potential of dispersed particles. 2. Electrokinetic potential of silica particles in electrolyte, polyelectrolyte and polyelectrolyte mixtures solutions

The effect pH, ionic strength (KCl concentration), weakly and medium charged anionic and cationic polyelectrolytes (PEs) as well as their binary mixtures on the electrokinetic potential of silica particles as a function of the polyelectrolyte/mixture dose, its composition, charge density (CD) of the PE, and way of adding the polymers to the suspension has been studied. It has been shown that addition of increasing amount of anionic PEs increases the absolute value of the negative zeta-potential of particles at pH > pH isoelectric point (IEP = 2.5); this increase is stronger the charge density of the polyelectrolyte is higher. Adsorption of cationic polyelectrolytes at these pH values gives a significant decrease in the negative ζ-potential and overcharging the particles; changes in the ζ-potential are more pronounced for PE samples with higher CD. In mixtures of cationic and anionic PE at pH > pHIEP, the ζ-potential of particles is determined by the adsorbed amount of the anionic polymer independently of the CD of PEs, the mixture composition and the sequence of addition of the mixture components. Unexpectedly, the ζ-potential of silica at pH = 2.1, i.e. < pHIEP, turned out to be positive in the presence of both anionic PE and cationic + anionic PE mixtures. This is explained by formation (and adsorption onto positively charged silica surface) of pseudo-cationic PEs from anionic ones due to transfer of protons from the solution to the amino-group of the anionic polymer. Considerations about the role of coulombic and non-coulombic forces in the mechanism of PE adsorption are presented.     

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

In order to characterize grafted polyelectrolyte layers based on electrokinetic measurements a theory of the surface conductivity Ksigma was developed, starting from the model of thick polyelectrolyte layers with uniform segment distribution and dissociable groups with an unknown pK value. According to this model the inner part of the polyelectrolyte layer adjacent to the substrate is considered to be isopotential while the potential decay occurs in a zone near the solution side of the layer. A simple equation for the Donnan potential psiD as a function of pH, pK, electrolyte concentration C0, and volume charge density rho was obtained. In the derived equation Ksigma is directly related to psiD while the other terms have less influence on the magnitude of Ksigma and can be accounted for in a second approximation using psiD as determined from the measured Ksigma. Evaluation of the suggested model indicates that Ksigma measurements provide an effective method to characterize polyelectrolyte layers by analyzing the dependence of psiD on pH and C0: The magnitude of Ksigma yields information about the surface charge at complete dissociation of the ionizable groups. The dependence of Ksigma on pH and C0 can be used for the determination of the pK value of the dissociating functions and the segment volume fraction of the polyelectrolyte can be estimated using the measured value of rho.     

Effect of surfactants on the fluorescence spectra of water-soluble MEHPPV derivatives having grafted polyelectrolyte chains

Poly(2-methoxy-5-[2′-ethylhexyoxy]-1,4-phenylenevinylene) (MEHPPV) derivatives with polyacrylic acid (PAA) chains grafted onto their backbone were found to be water soluble, and they exhibited a dramatic increase in their fluorescence intensity in the presence of a variety of surfactants, even at concentrations far below their critical micelle concentrations (CMC). This increase was accompanied by a blue-shift in the emission maximum. These observations are rationalized based on the postulate that the backbone conformation of the conjugated polymer is modulated upon interaction of the surfactant molecules with the polyelectrolytic tethers, which in turn results in a significant depletion of intra-chain interchromophore interactions that are known to cause red-shifted emission bands with significantly lower emission yields.     

Conformational behavior of grafted weak polyelectrolyte chains: effects of counterion condensation and nonelectrostatic anion adsorption

Poly[(2-dimethylamino)ethyl methacrylate] (PDEM) is completely charged, partially charged, and uncharged at pH 4, 7, and 10, respectively. We have investigated the salt effects on the conformational change of PDEM chains grafted on a surface at different pH by using quartz crystal microbalance with dissipation (QCM-D) and surface plasmon resonance (SPR). The changes in frequency (Δf) and dissipation (ΔD) in QCM-D measurements demonstrate that the conformational behavior is governed by counterion condensation at pH 4 and 7 but by nonelectrostatic anion adsorption at pH 10. The addition of Na(2)SO(4) induces more collapse of the grafted layer than that of NaClO(3) at pH 4 and 7. However, they have a similar effect at pH 10. The shift of resonance unit (ΔRU) in SPR measurements reflects the changes of layer thickness and layer refractive index. At pH 4, ΔRU decreases with ionic strength in the presence of Na(2)SO(4), indicating the decrease of layer thickness or the chain collapse. However, ΔRU exhibits a minimum as the ionic strength increases in the case of NaClO(3). This is because the effects of the layer thickness and refractive index are dominant in the low and high ionic strength regimes, respectively. At pH 7, ΔRU slightly varies with ionic strength in the case of either Na(2)SO(4) or NaClO(3), indicating that the effects of the layer thickness and refractive index are comparable during the layer collapse. At pH 10, the shift in ΔRU suggests that the nonelectrostatic anion adsorption governs the conformational behavior of the PDEM chains.     

A new approach towards redispersable polyelectrolyte–surfactant complex nanoparticles

Redispersable and weakly cross-linked block copolymer particles with a core-shell structure were prepared by the use of a macroinitiator. Subsequent sulfonation of the polystyrene core and complex formation with a variety of cationic surfactants led to sterically stabilized, redispersable polyelectrolyte-surfactant complex particles with spherical shape and diameters of about 400 nm. Spontaneous microphase separation of the hydrophobic surfactant tails and the hydrophilic entities of the polyelectrolyte and the surfactant headgroups induces mesostructure formation within the particle cores. The characteristic lengths of the mesostructures formed depend mainly on the chain lengths of the surfactants and vary between 2 and 4 nm. For the first time, preformed nanoparticles were used as constrained nanogeometries for polyelectrolyte-surfactant complex formation.     

Clusters in strong polyelectrolyte solutions in the condensation theory approach

The interaction free energy of parallel clusters of like-charged rod polyelectrolytes in solution is calculated in the framework of the extended condensation theory. For sufficiently high linear charge density of the polyelectrolyte, clustering takes place. The greater is the number of polyelectrolytes participating to the cluster, the smaller is the equilibrium interpolyelectrolyte distance, and the deeper is the corresponding free energy minimum. It is a counterintuitive organization due to the increasing of the counterion condensed charge and condensation volume, taking place as the polyelectyrolytes approach each other.     

Field theoretical approach to electrochemical deposition

In this work we present an application of the lambdaphi(4) field theoretical model to the adsorption of atoms and molecules on metallic surfaces-the electrochemical deposition. The usual approach to this system consists in the computational simulation using Monte Carlo techniques of an effective lattice-gas Hamiltonian. We construct an effective model towards a comparison between the lattice-gas Hamiltonian and the discrete version of the lambdaphi(4) Hamiltonian, obtaining the relationships between the model parameters and electrochemical quantities. The lambdaphi(4) model is studied in the mean field approximation, and the results are fitted and compared to numerical simulated and experimental data.     

An integrative approach for analyzing metal–polyelectrolyte interactions

An integrative approach based on the combined use of both experiments and modelling is discussed here aimed at investigating metal–polyelectrolyte interactions in solution. Electrochemical techniques are applied because of their potential to measure the actual speciation without disturbing the solution physico–chemical equilibrium. The experimental methodologies are complementary since the ranges of applicability depend on the solution composition itself. To complement and interpret the results of these experimental techniques, a physico–chemical association model, based on the so-called ‘chemical model’ of counterion condensation theory, is used. The model considers that, in addition to the usual electrostatic interactions and entropic effects, territorial affinity and chemical bonding interactions take place between the small counterions in solution and the polyelectrolyte. A number of particular cases of metal/polyelectrolyte systems are discussed aimed at showing that the integrative approach leads to additional information about the solution system which can not be deduced from experimental results solely. Future challenges with respect to the applications in the study of natural aquatic systems are pointed out.     

Probing the permeability of polyelectrolyte multilayer capsules via a molecular beacon approach

Application of polyelectrolyte multilayer (PEM) capsules as vehicles for the controlled delivery of substances, such as drugs, genes, pesticides, cosmetics, and foodstuffs, requires a sound understanding of the permeability of the capsules. We report the results of a detailed investigation into probing capsule permeability via a molecular beacon (MB) approach. This method involves preparing MB-functionalized bimodal mesoporous silica (BMSMB) particles, encapsulating the BMSMB particles within the PEM film to be probed, and then incubating the encapsulated BMSMB particles with DNA target sequences of different lengths. Permeation of the DNA targets through the capsule shell causes the immobilized MBs to open due to hybridization of the DNA targets with the complementary loop region of the MBs, resulting in an increase in the MB fluorescence. The assay conditions (BMSMB particle concentration, MB loading within the BMS particles, DNA target concentration, DNA target size, pH, sodium chloride concentration) where the MB-DNA sensing process is effective were first examined. The permeability of DNA through poly(sodium 4-styrenesulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) multilayer films, with and without a poly(ethyleneimine) (PEI) precursor layer, was then investigated. The permeation of the DNA targets decreases considerably as the thickness of the PEM film encapsulating the BMSMB particles increases. Furthermore, the presence of a PEI precursor layer gives rise to less permeable PSS/PAH multilayers. The diffusion coefficients calculated for the DNA targets through the PEM capsules range from 10-19 to 10-18 m2 s-1. This investigation demonstrates that the MB approach to measuring permeability is an important new tool for the characterization of PEM capsules and is expected to be applicable for probing the permeability of other systems, such as membranes, liposomes, and emulsions.     

A theoretical approach to molecular single-electron transistors

We present theoretical methods and computational strategies of the effects of nanoparticles on linear optical properties of molecules. We present quantum mechanical-molecular mechanics response methods for calculating electromagnetic properties of molecules interacting with nanoparticles and we report strategies for calculating electronic and redox states of molecules sandwiched between gold nanoparticles.     

A theoretical approach to molecular conformational analysis

The application of ab initio molecular orbital theory to the study of molecular conformational analysis is discussed. Examples presented include methyl rotational barriers, internal rotation in 1,2-dihalogenoethanes, $\text{cis-trans}$ isomerism in 1,2-dihalogenoethylenes, rotational barriers in substituted acetones and conformational preferences in substituted hydrazines.     

A simple theoretical approach to bond energies

The linear combination of fragment configurations (LCFC) method is used to study factors which control the relative strengths of bonds. Trends in bond strengths and the relative stability of structural isomers are predicted for a variety of organic molecules. It is argued that complex interactions within large organic molecules can be simplified to the interaction of the two electrons of a single bond. A compilation of experimental data is presented to support the proposed theoretical model.     

Redistribution in combinatorial synthesis. A theoretical approach

In modified versions of the split-mix method, the conventional solid support resin was replaced by labeled macroscopic support units in order to produce individual compounds in multi milligram quantities while the high productivity of the original procedure is preserved. Recently it has also been shown that tagging the units is unnecessary. Omission of the tags is possible if (i) the unlabeled support units are arranged into spatially ordered groups and the relative spatial arrangement of the units is maintained during the chemical reactions (ii) the support units are redistributed between the reaction steps according to a predetermined pattern permitted by the combinatorial redistribution rule and (iii) the sorting process is simulated by a computer that can trace the synthetic history of each support unit. Different kinds of solid support units, formation of spatially ordered groups, sorting devices and basic redistribution patterns (serial, semi-parallel and parallel) are discussed. It is also shown that particularly the semi-parallel and the parallel redistribution assure fast sorting.     

Electron-enhanced vibrational spectroscopy: a theoretical approach.

Enhancement of the Raman scattering and IR absorption activities due to the electron-attachment was investigated for water systems by DFT calculations. DFT calculation of a 6-ring water cluster system that included the diffusive nature of electrons well reproduced the Raman enhancement effects and Raman shifts of the OH stretching modes observed in experiments. Based on the same model and calculations, enhancement of the IR absorption activity was also studied and was found to also be improved. Furthermore, the same calculation revealed that the enhancement can be also expected not only in the OH stretching but also in the lower wavenumber region. The enhancement factors for the various vibrational modes of the OH groups range from 10(2) - 10(5) thanks to the electron addition. Based on the coincidence between the theoretical model and the experimental results for the Raman signals and theoretical prediction for IR absorption, new enhancement techniques based on an electron-attachment in both Raman scattering and IR absorption, denoted as "electron-enhanced vibrational spectroscopy (EEVS)", is proposed, where molecular polarizability itself is modulated by the strong electrostatic field induced by neighboring electrons.     

Arsenic in prebiotic species: a theoretical approach

A recent controversy about the presence of arsenic in biological systems prompted us to investigate the possible replacement of phosphorus by arsenic in prebiotic species small enough to be potentially identified in space. Systematic computational experiments were carried out on simple systems able to form a peptide or analogous bond. Density Functional Theory (DFT) within the B3LYP formalism, MP2 and CCSD(T) methods were used to determine the most stable isomers that can possibly form from the [C,H,O,As] and [C,3H,O,As] sets of atoms. It was found that HAsCO, like HPCO and HNCO was the most stable isomer. With three hydrogen atoms, the peptide-like bond (AsH(2)-CH=O) is not the most stable structure, contrary to NH(2)-CH=O. It is ～9 kcal mol(-1) higher than the most stable structure, CH(2)[double bond, length as m-dash]As-OH. To assess the plausibility of the As to P substitution, a comparative study of the dimethylphosphate (DMP) and dimethylarsenate (DMA) anions was then carried out. It was found that the gauche-gauche arrangement that mimics the helix structure is the most stable one in both model molecules, showing that there is no structural evidence to discard the hypothesis of the possible inclusion of As in place of P in the DNA architecture. The topological analysis of the ELF function showed a weakening by 50% of two As-O covalent bonds in all the DMA conformers. It means that if As replaces P, the structure of the DNA helix could be weakened. Rotational constants and IR frequencies of the low-lying isomers are given to encourage laboratory experiments on these prototype molecules.