Calculation of the orientational linear and nonlinear correlation factors of polar liquids from the rotational Dean-Kawasaki equation.

A calculation of the Kirkwood and Piekara-Kielich correlation factors of polar liquids is presented using the forced rotational diffusion theory of Cugliandolo et al. [Phys. Rev. E 91, 032139 (2015)]. These correlation factors are obtained as a function of density and temperature. Our results compare reasonably well with the experimental temperature dependence of the linear dielectric constant of some simple polar liquids across a wide temperature range. A comparison of our results for the linear dielectric constant and the Kirkwood correlation factor with relevant numerical simulations of liquid water and methanol is given.

Experimental and simulation studies of unusual current blockade induced by translocation of small oxidized PEG through a single nanopore.

Detection of a single macromolecule based on the use of artificial nanopores is an attractive and promising field of research. In this work, we report a device based on a 5 nm single nanopore with a high length/diameter ratio, tailored by the track etching and atomic layer deposition techniques. The translocation of neutral polyethylene glycol (PEG) and charged polyethylene glycol-carboxylate (PEG-carboxylate) molecules of low molar masses (200 and 600 g mol(-1)) through this nanodevice was studied. It was shown that charged PEG-carboxylate molecules, which permeate through the pore, promote an unusual blockade of ionic current whereas the neutral PEG molecules do not show such behaviour. The molecular dynamics simulation shows that both neutral and charged PEGs permeate through the nanopore close to its inner surface. The main difference between the two macromolecules is the existence of a structured shell of cations around the charged PEG, which is likely to cause the observed unusual current blockade.

Nonlinear susceptibilities of interacting polar molecules in the self-consistent field approximation.

The nonlinear stationary response of an assembly of long range interacting electric dipoles is calculated via Berne's nonlinear rotational diffusion equation [J. Chem. Phys. 62 1154 (1975)]. Analytical formulas are derived, showing that the behavior of ω and 3ω components of the nonlinear external field response spectra in such polar dielectrics strongly deviates from the Coffey-Paranjape formulas [Proc. R. Ir. Acad., Sect. A 78, 17 (1978)] as the long range dipole-dipole interactions increase, while the linear response remains qualitatively unaffected. By qualitatively comparing with recent experimental measurements on glycerol, it is further demonstrated that nonlinear dielectric response experiments provide a powerful tool for the characterization of short range intermolecular interactions in polar liquids.

Determination of the functioning parameters in asymmetrical flow field-flow fractionation with an exponential channel.

The flow conditions in normal mode asymmetric flow field-flow fractionation are determined to approach the high retention limit with the requirement d≪l≪w, where d is the particle diameter, l the characteristic length of the sample exponential distribution and w the channel height. The optimal entrance velocity is determined from the solute characteristics, the channel geometry (exponential to rectangular) and the membrane properties, according to a model providing the velocity fields all over the cell length. In addition, a method is proposed for in situ determination of the channel height.

Reply to Comments on the separation efficiency of asymmetrical Flow Field-Flow fractionation in channels of constant channel and crossflow velocities leading to constant separation efficiency.

The membrane or cross-flow velocity in asymmetric Flow Field-Flow fractionation is not constant in usual geometries. Previously theoretical models however were developed with the hypothesis of a constant membrane velocity. The argument about peak broadening in the comments of K.-G. Wahlund (J. Chromatogr. A 1218 (2011) 6848) is based on this kind of model. Such an assumption was not included in the recently proposed model used to determine the conditions of constant velocities. The model goes beyond this approximation and anyway can provide the two velocity fields in various geometries.

Adsorption on montmorillonite prevents oligomerization of Bt Cry1Aa toxin.

The adsorption of the insecticidal Cry1Aa protein from Bacillus thuringiensis (Bt-toxin) on a model clay surface was studied to understand the structural changes of the protein induced by the clay surface. We studied the adsorption of the monomeric and soluble oligomeric forms of the Cry1Aa toxin as a function of pH and ionic strength conditions on montmorillonite, which is an electronegative phyllosilicate. Cry1Aa secondary structure was determined from the amide I' FTIR absorption profiles. Accessibility to the solvent was determined by NH/ND exchange to characterize conformational flexibility of the different states of the Cry1Aa protein. The size distribution of Cry1Aa solutions was obtained by dynamic light scattering (DLS). From combined DLS and FTIR measurements, we conclude that montmorillonite traps the Cry1Aa toxin in its monomeric state, preventing the oligomerization of the protein. The oligomeric forms were adsorbed onto the clay without significant structural changes.

Permeate channel geometry to get constant separation efficiency in asymmetrical Flow Field-Flow fractionation cell with exponential breadth variation.

We consider an asymmetric Flow Field-Flow fractionation cell with a specific permeate channel geometry associated with an exponential variation of the channel breadth b=b(0)exp(-sz). The hydraulic characteristic length of the membrane is constant and linked to other characteristics of the cell. We determine the dependence of permeate channel height with distance to entrance to get both mean sample channel and membrane velocities constant, hence a constant efficiency of the separation process over the whole length of the cell.

Design of an asymmetrical flow field-flow fractionation cell with both mean channel and membrane velocities constant.

We consider an asymmetric flow-field-flow fractionation (FFF) cell with an exponential variation of the channel breadth and a hydraulic conductivity gradient membrane to get a constant efficiency of the separation process over the whole length of the channel. After presenting the model for a membrane of constant hydraulic conductivity, we determine the dependence of conductivity with distance to entrance to get both mean channel and membrane velocities constant.

Nonlinear response of permanent dipoles in a uniaxial potential to alternating fields.

It is shown how the existing theory of the dynamic Kerr effect and nonlinear dielectric relaxation based on the noninertial Brownian rotation of noninteracting rigid dipolar particles may be generalized to take into account interparticle interactions using the Maier-Saupe mean field potential. The results (available in simple closed form) suggest that the frequency dependent nonlinear response provides a method of measuring the Kramers escape rate (or in the analogous problem of magnetic relaxation of fine single domain ferromagnetic particles, the superparamagnetic relaxation time).

Extended rotational diffusion and orientational relaxation of symmetric top molecules in a strong dc electric field: second-rank orientational correlation functions.

Second-rank orientational correlation functions (pertaining to Kerr effect relaxation and Raman scattering) are obtained using the extended rotational diffusion (J-diffusion) model of symmetric top polar molecules in a strong constant external field. It is shown that the shape of the molecule noticeably affects all second-rank correlation functions and relaxation times in the rare collision limit. In the opposite limit of frequent collisions, the quantities of interest are shown to be shape independent as a consequence of vanishingly small inertial effects. An interpolation formula for the orientation relaxation times in the intermediate regime between the rare and frequent collision limits is also given.

Formation of weak polyelectrolyte multilayers studied by spin labeling.

Multilayers of alternately adsorbing poly(allylamine) (PAH) and poly(acrylic acid) (PAA) of opposite charges on silica have been studied by the spin labeling technique, as a function of pH. The two polyelectrolytes have been labeled independently by a nitroxide free radical. Its electron paramagnetic resonance spectrum is mainly sensitive to the local Brownian motion and shows lines typical of two different environments, namely, loops protruding in solution with a fast motion and trains adsorbed on the solid with a hindered motion. These two parts have been evaluated for each of the polymer layers separately, and the thickness of the coatings has been described more precisely by characterizing the four contributions existing, for example, for a bilayer. Complexation is demonstrated by the loss of loops and tails belonging to the first polyelectrolyte. The overall picture emerging from the data is explained in terms of compensation of charges and entropy of confinement.

Interpolation formula between very low and intermediate-to-high damping Kramers escape rates for single-domain ferromagnetic particles.

It is shown that the Mel'nikov-Meshkov formalism for bridging the very low damping (VLD) and intermediate-to-high damping (IHD) Kramers escape rates as a function of the dissipation parameter for mechanical particles may be extended to the rotational Brownian motion of magnetic dipole moments of single-domain ferromagnetic particles in nonaxially symmetric potentials of the magnetocrystalline anisotropy so that both regimes of damping occur. The procedure is illustrated by considering the particular nonaxially symmetric problem of superparamagnetic particles possessing uniaxial anisotropy subject to an external uniform field applied at an angle to the easy axis of magnetization. Here the Mel'nikov-Meshkov treatment is found to be in good agreement with an exact calculation of the smallest eigenvalue of Brown's Fokker-Planck equation, provided the external field is large enough to ensure significant departure from axial symmetry, so that the VLD and IHD formulas for escape rates of magnetic dipoles for nonaxially symmetric potentials are valid.

High molecular weight kininogen adsorption on hemodialysis membranes: influence of pH and relationship with contact phase activation of blood plasma. influence of pre-treatment with poly(ethyleneimine).

Protein adsorption is an essential parameter for the evaluation of the hemocompatibility of biomedical materials. In effect, protein adsorption often generates unfavorable, complex, biochemical reactions and precedes cell adhesion on artificial interfaces. It is therefore necessary to modify the surface in contact with blood in order to minimize the onset of undesirable, biochemical, cascade reactions. Adsorption of high molecular weight kininogen (HK), which participates in the contact phase activation of the endogenous blood coagulation cascade, was evaluated at 37 degrees C. This paper also presents the results of contact phase activation tests carried out in vitro with 1:20 diluted human plasma flowing through minidialyzers containing hollow fibers of synthetic hemodialysis membranes. These tests showed that contact phase activation is strongly dependent on pH around the physiological value (ranging from 7.35 to 7.80) for negatively-charged membranes. The same pH effect was observed on the AN69 membrane as regards HK adsorption from binary labeled solutions of 125I-HK and 131I-Fibrinogen at concentrations corresponding to 1% diluted plasma. It is suggested that the influence of pH could be related to imidazole pKa values in histidine residues of kininogen D5H domain. The same study was also conducted with hemodialysis membranes pre-treated with poly(ethyleneimine). Both HK adsorption and contact phase activation proved to be greatly reduced, irrespective of the pH value (between 7.0 and 7.8). Hence, positively-charged poly(ethyleneimine) adsorbed on the membrane through strong ionic interactions with sulfonate groups of the surface probably constitutes a repelling, water-swollen layer for the kininogen molecule. In addition, the advantages of the high levels of adsorbance of small molecules on the AN69 membrane leading to blood epuration, due to its high porosity and for some of them to its charge density, should not be lost by such a surface treatment.

Adsorption of low molecular weight proteins to hemodialysis membranes: experimental results and simulations.

Adsorption of alpha-lactalbumin and Cytochrome C on different hollow-fiber hemodialysis modules, whose main membrane constituent was polyacrylonitrile (PAN), polymethylmethacrylate (PMMA) or polysulfone (PS), were compared under different pH and flow rate conditions. These proteins were chosen as models of small scarce proteins like beta2-microglobulin from which the patient's blood should be epurated. Influence of pH suggests the importance of electrostatic interactions on a charged membrane since adsorption greatly decreases at pH 10.9 for Cytochrome C (pI = 10.6) and at pH 7.4 for alpha-lactalbumin (pI = 5). The difference in adsorbance between different membranes is most probably due to their different microstructures. However, the chemical nature of the support plays a non-negligible role since PMMA membranes have smaller initial adsorption rates than PAN membranes. This is correlated with the density of membrane surface charge showing the importance of electrostatic interactions. The influence of the wall shear rate on adsorption kinetics is analyzed through numerical simulations, in terms of transport limitation in the liquid phase and interfacial reaction.

Transient nonlinear dielectric relaxation and dynamic Kerr effect from sudden changes of a strong dc electric field: polar and polarizable molecules.

The nonlinear transient response of polar and polarizable particles (macromolecules) diluted in a nonpolar solvent to a sudden change both in magnitude and in direction of a strong external dc field is considered. By averaging the underlying Langevin equation, the infinite hierarchy of differential-recurrence equations for ensemble averages of the spherical harmonics is derived for an assembly of polar and anisotropically polarizable molecules pertaining to the noninertial rotational Brownian motion. On solving this hierarchy, the relaxation functions and relaxation times appropriate to the transient dynamic Kerr effect and nonlinear dielectric relaxation are calculated. The calculations are accomplished using the matrix continued fraction method, which allows us to express exactly the solution of the infinite hierarchy of differential-recurrence relations for the first- and second-order transient responses of the ensemble averages of the spherical harmonics (relaxation functions). The results are then compared with available experimental data and solutions previously obtained for various particular cases.

Fibrinogen adsorption on Pyrex glass tubes: a continuous kinetic study.

We present an experimental system to record continuously the adsorption kinetics of radiolabeled proteins, following an earlier study (Voegel et al., Colloids Surfaces 10 (1984) 9). We found results in accordance with the Lévêque equation at a wall shear rate of 50 s-1, for adsorption from fibrinogen solutions in Tyrode's buffer on Pyrex glass tubes. Dependence on concentration in the range 4 to 200 micrograms/mL and on distance to the tube entrance were examined. At the highest concentrations, a second slower regime appeared when coverage exceeded about 0.14 micrograms/cm2.

Influence of a preadsorbed terpolymer on human platelet accumulation, fibrinogen adsorption, and ex vivo blood activation in hemodialysis hollow fibers.

Results are presented on kinetics of platelet accumulation in charged polyacrylonitrile (AN69) hollow fibers by continuous data recording under flow conditions (wall shear rate 108-1050 s-1), using suspensions of washed 111In-labeled human platelets in Tyrode's-albumin buffer, containing washed red blood cells (0-40%). Preadsorption of a terpolymer of acrylonitrile, poly(ethyleneoxide) methacrylate and trimethylaminoethyl chloride methacrylate leads to very efficient passivation with respect to platelet accumulation and fibrinogen adsorption. In human ex vivo tests, evaluation of complement peptide C3a, platelet beta-thromboglobulin, leucocyte-polymorphonuclear neutrophile elastase and fibrinopeptide A shows no detectable activation. Furthermore, preadsorption appears to result in simultaneous improvement in hemocompatibility of the blood lines leading to and from the dialysis module. This single pretreatment of dialysis membranes should allow injection of lower doses of anticoagulant to patients submitted to hemodialysis.

In vitro and in vivo studies of the properties of an artificial membrane for pancreatic islet encapsulation.

Encapsulation of pancreatic islets with an artificial membrane has been proposed as a means of immunoprotection after transplantation. Such a membrane should be biocompatible, nondegradable, and should allow the passage of insulin and glucose while preventing that of antibodies and lymphocytes. Thus, we have studied in vitro and in vivo, the characteristics of an acrylonitrile membrane (AN69, HOSPAL, Sweden) for islet encapsulation. The AN69 membrane composed of a fiber network with a porous structure, allowed a satisfactory passage of glucose (75% of the initial amount within one hour) but not of insulin (only 7%). The morphological state of rat islets cultured on membranes under both conditions for 2 weeks was similar to that of islets cultured on dishes; in addition rat fibroblasts retracted after a 3-day culture. Finally, the membrane was unaltered after a 12 month implantation in the peritoneal cavity of rats. When the surface properties of the AN69 membrane were changed by adsorption of a hydrophilic copolymer or by protein coating, the permeability of the membrane was modified. Glucose and insulin diffusion were significantly decreased after protein-coating, whereas glucose diffusion was preserved and that of insulin doubled after adsorption of a copolymer onto the membrane. In addition, after a 12-month implantation in the rat, the membrane surface treated by the copolymer was altered leading to the adhesion of macrophages. In conclusion, the AN69 acrylonitrile membrane may be useful for pancreatic islet encapsulation; its insulin permeability should be increased by a surface treatment aimed at increasing its hydrophilic properties. However the stability of this treatment seems to be an important factor in preserving the biocompatibility of the membrane.

Preadsorption of polymers on glass and silica to reduce fibrinogen adsorption.

Glass and silica beads were precoated with various polymers to obtain steric exclusion chromatography (SEC) supports which are nonadsorbant for hydrophilic macromolecules. The efficiency of this treatment was estimated by subsequent radiolabeled fibrinogen adsorption. The result obtained with a block copolymer was better than with various hydrophilic homopolymers. This ABA type block copolymer, where A is a poly(N-acetylethyleneimine) (PAEI) sequence and B a polyethylene oxide (PEO) sequence was preadsorbed at pH 4.5 and 25 degrees C; the fibrinogen adsorption was reduced to less than 5% of the value observed on untreated solid surfaces. Thus the hemocompatibility of solid supports should be increased by precoating with this block copolymer. Results for nonporous glass beads and porous silica particles were in good correlation.