Macroion adsorption: The crucial role of excluded volume and coions

The adsorption of charged colloids (macroions) onto an oppositely charged planar substrate is investigated theoretically. Taking properly into account the finite size of the macroions, unusual behaviors are reported. It is found that the role of the coions (the little salt-ions carrying the same sign of charge as that of the substrate) is crucial to understand the mechanisms involved in the process of macroion adsorption. In particular, the coions can accumulate near the substrate's surface and lead to a counter-intuitive {\it surface charge amplification}.Comment: 11 pages - 4 figures. To appear in JC

Nonlinear screening of charged macromolecules

We present several aspects of the screening of charged macromolecules in an electrolyte. After a review of the basic mean field approach, based on the linear Debye-Huckel theory, we consider the case of highly charged macromolecules, where the linear approximation breaks down and the system is described by full nonlinear Poisson-Boltzmann equation. Some analytical results for this nonlinear equation give some interesting insight on physical phenomena like the charge renormalization and the Manning counterion condensation

Non-linear screening of spherical and cylindrical colloids: the case of 1:2 and 2:1 electrolytes

From a multiple scale analysis, we find an analytic solution of spherical and cylindrical Poisson-Boltzmann theory for both a 1:2 (monovalent co-ions, divalent counter-ions) and a 2:1 (reversed situation) electrolyte. Our approach consists in an expansion in powers of rescaled curvature $1/(\kappa a)$, where $a$ is the colloidal radius and $1/\kappa$ the Debye length of the electrolytic solution. A systematic comparison with the full numerical solution of the problem shows that for cylinders and spheres, our results are accurate as soon as $\kappa a>1$. We also report an unusual overshooting effect where the colloidal effective charge is larger than the bare one.Comment: 9 pages, 11 figure

Discrete aqueous solvent effects and possible attractive forces

We study discrete solvent effects on the interaction of two parallel charged surfaces in ionic aqueous solution. These effects are taken into account by adding a bilinear non-local term to the free energy of Poisson-Boltzmann theory. We study numerically the density profile of ions between the two plates, and the resulting inter-plate pressure. At large plate separations the two plates are decoupled and the ion distribution can be characterized by an effective Poisson-Boltzmann charge that is smaller than the nominal charge. The pressure is thus reduced relative to Poisson-Boltzmann predictions. At plate separations below ~2 nm the pressure is modified considerably, due to the solvent mediated short-range attraction between ions in the the system. For high surface charges this contribution can overcome the mean-field repulsion giving rise to a net attraction between the plates.Comment: 12 figures in 16 files. 19 pages. Submitted to J. Chem. Phys., July 200

Ion condensation on charged patterned surfaces

We study ion condensation onto a patterned surface of alternating charges. The competition between self-energy and ion-surface interactions leads to the formation of ionic crystalline structures at low temperatures. We consider different arrangements of underlying ionic crystals, including single ion adsorption, as well as the formation of dipoles at the interface between charged domains. Molecular dynamic simulation illustrates existence of single and mixed phases. Our results contribute to understanding pattern recognition, and molecular separation and synthesis near patterned surfaces.Comment: 3 figure

Rétention des produits phytosanitaires dans les fossés de connexion parcelle-cours d'eau

Pour lutter contre les pollutions diffuses en milieu rural, de nombreux programmes d'action se mettent en place. Le développement de recherches sur les connexions parcelle - cours d'eau devrait permettre de mieux comprendre le transfert et la dissipation des polluants dans ce milieu. En particulier, les fossés, structures relativement fréquentes dans les territoires cultivés, peuvent, a priori, avoir une fonction de court-circuit et donc faciliter le transfert des produits phytosanitaires, ou au contraire constituer des éléments de pondération de la pollution. Afin d'éclaircir ce point, une première série d'expérimentations a été menée par le Cemagref (Institut français de recherche pour l'ingénierie de l'agriculture et de l'environnement) dans des fossés de drainage agricole. Une solution aqueuse contenant trois herbicides aux caractéristiques physico-chimiques différentes (isoproturon, diuron et diflufénicanil), et un traceur (chlorures) a été injectée pendant quelques minutes dans quatre fossés. Des échantillons d'eau ont été prélevés à pas de temps fins à deux emplacements en aval du point d'injection. Après dosage par chromatographie au laboratoire, les résultats indiquent une diminution du flux et de la concentration maximale du pic de polluants comparativement à un traceur. En outre, la variation observée est corrélée aux propriétés physico-chimiques des produits, en particulier au coefficient de partage Koc. L'étude présentée montre que la surface de contact (liée à la nature du substrat) et le temps de contact (dépendant essentiellement des conditions d'écoulement) entre les polluants et le substrat sont les paramètres qui influent majoritairement sur la dissipation des produits phytosanitaires.The use of pesticide may lead to the contamination of surface and groundwaters. Agricultural nonpoint source pollution originates from land areas which intermittently contribute to the compound transfer to water. Several studies report on the occurrence of pesticides in surface water resources, with concentrations over the limit set by the 80/778 EEC directive for drinking water (0.1 µg/L for each substance and 0.5 µg/L for all pesticides). Numerous herbicides of different chemical families are detected in surface waters, especially triazines and ureas. Their concentrations vary with time and space partly in relation with application patterns and pluviometry. Maximum concentrations are linked to runoff, originating from agricultural fields and primarily occur right after the application periods.Many methods and levels of actions can be used to reduce water pollution. First, better agricultural practices can be set up, such as choosing the best dose and application period, controlling toxic substance impacts, combining with non-chemical practices. However, pesticide losses from fields can't be totally cancelled because of the complexity of the involved parameters (agricultural practices, climatic conditions, soil physical, chemical and biological properties …). In fact drift during application, runoff or drainage systems may still occur and have an effect on water quality. It may be then pertinent to evaluate to which extent the non treated areas between the fields and the surface water bodies can dissipate pesticide concentrations before they reach them. Pesticides leaving a plot in surface runoff may pass through various landscape components before reaching rivers ; including another field, a ditch, a small brook, a vegetative buffer zone. Besides, pesticide leaving the plot through drainage straightly moves agricultural ditches or streams. However, the contribution of each of these elements in pesticide dissipation is not well known, except for buffer zones (grassed or forest strips) (PATTY (1997). Cemagref (a French research institute), CEH Wallingford and ITCF (Institut Technique des Cereales et des Fourrages) attempt to extend their study to the other elements as agricultural ditches.This paper deals with the role of farm ditches and small streams in the transport and retention of pesticides from fields to the main river network. Their presence seems to play a significant role in the transfer of nonpoint source pollution (especially in the West of France). Indeed they can either accelerate pesticide transport or reduce it, according to their characteristics (length, flow, bottom sediment or soil characteristics, plants and organic matter contents, etc.). Since 1998, Cemagref has been investigating the retention of pesticides by several natural ditches with varied flows and substratum. A water solution containing three herbicides with different physico chemical properties (diflufenican, diuron and isoproturon) and potassium chloride, a tracer, is introduced with a pump in each ditch for about five minutes with a constant concentration. Water samples are collected in the ditches every two or five minutes at two distances from the injection point. The samples are stored in amber polyethylene terephtalate bottles and frozen. Laboratory analysis is performed by liquid-liquid extraction with dichloromethane and then liquid or gas chromatography depending on the compounds.The analysis of the water samples highlights a reduction of the maximum concentration and of the accumulative mass of each pesticide with distance compared to the tracer. Indeed, even if all the chloride ions used as tracers are not recovered at each sampling point (due to infiltration or lateral losses), we notice more significant losses for all the studied herbicides. The reduction can reach 70 % of the applied mass for diflufenican compared to the tracer. The retention of pesticides is also linked to their own physical and chemical properties. Thus, diflufenican, which has the highest sorption coefficient value, Koc, is also the most retained pesticide, whereas the total injected mass of isoproturon is recovered in most cases. Diuron has an intermediate behaviour.In brief, this field experiment proves that the surface and time of contact between pollutants and substratum are likely to play a major role in pesticide retention. An estimated adsorption capacity of each ditch has been assessed, which is based on laboratory sorption experiments on different natural substratum. Despite the few data, a relationship between diflufenican retention in ditches and the estimated adsorption capacity of each ditch has been underscored.This study also highlighted major limits of field experiments. For example, accurate flow measurements are really difficult to carry out with simple methods for low values. The conventional techniques can't be used with small water height or in ditch where the bottom is filled with plants or grass. Chloride ion was chosen in this study because it is easy to analyze, but the results showed an initial presence of chloride ion in the natural ditch water which incites to replace it by another tracer such as bromide with is not found in the environment in future field experiments.For all these reasons, some pilot experiments with a physical model (an artificial ditch of 8 m long and 0.4 m wide) are now designed. This equipment allows to adjust and control hydrodynamic parameters such as water flow, water height, and the nature and structure of the substratum. Then, it is possible to quantify both the role played by the substratum, mainly the organic matter content, and the role of the contact time. These parameters could be then taken into account in order to optimize further experiments on adsorption. The primary tests without substratum already give references for hydrodynamic measurements, as the stability of the water flow and the homogeneity of the initial solution concentration

Theory and simulations of rigid polyelectrolytes

We present theoretical and numerical studies on stiff, linear polyelectrolytes within the framework of the cell model. We first review analytical results obtained on a mean-field Poisson-Boltzmann level, and then use molecular dynamics simulations to show, under which circumstances these fail quantitatively and qualitatively. For the hexagonally packed nematic phase of the polyelectrolytes we compute the osmotic coefficient as a function of density. In the presence of multivalent counterions it can become negative, leading to effective attractions. We show that this results from a reduced contribution of the virial part to the pressure. We compute the osmotic coefficient and ionic distribution functions from Poisson-Boltzmann theory with and without a recently proposed correlation correction, and also simulation results for the case of poly(para-phenylene) and compare it to recently obtained experimental data on this stiff polyelectrolyte. We also investigate ion-ion correlations in the strong coupling regime, and compare them to predictions of the recently advocated Wigner crystal theories.Comment: 32 pages, 15 figures, proceedings of the ASTATPHYS-MEX-2001, to be published in Mol. Phy

Effects of image charges, interfacial charge discreteness, and surface roughness on the zeta potential of spherical electric double layers

We investigate the effects of image charges, interfacial charge discreteness, and surface roughness on spherical electric double layers in electrolyte solutions with divalent counter-ions in the setting of the primitive model. By using Monte Carlo simulations and the image charge method, the zeta potential profile and the integrated charge distribution function are computed for varying surface charge strengths and salt concentrations. Systematic comparisons were carried out between three distinct models for interfacial charges: 1) SURF1 with uniform surface charges, 2) SURF2 with discrete point charges on the interface, and 3) SURF3 with discrete interfacial charges and finite excluded volume. By comparing the integrated charge distribution function (ICDF) and potential profile, we argue that the potential at the distance of one ion diameter from the macroion surface is a suitable location to define the zeta potential. In SURF2 model, we find that image charge effects strongly enhance charge inversion for monovalent interfacial charges, and strongly suppress charge inversion for multivalent interfacial charges. For SURF3, the image charge effect becomes much smaller. Finally, with image charges in action, we find that excluded volumes (in SURF3) suppress charge inversion for monovalent interfacial charges and enhance charge inversion for multivalent interfacial charges. Overall, our results demonstrate that all these aspects, i.e., image charges, interfacial charge discreteness, their excluding volumes have significant impacts on the zeta potential, and thus the structure of electric double layers.Comment: 11 pages, 10 figures, some errors are change

Ion-Size Effect at the Surface of a Silica Hydrosol

The author used synchrotron x-ray reflectivity to study the ion-size effect for alkali ions (Na$^+$, K$^+$, Rb$^+$, and Cs$^+$), with densities as high as $4 \times 10^{18}- 7 \times 10^{18}$ m$^{-2}$, suspended above the surface of a colloidal solution of silica nanoparticles in the field generated by the surface electric-double layer. According to the data, large alkali ions preferentially accumulate at the sol's surface replacing smaller ions, a finding that qualitatively agrees with the dependence of the Kharkats-Ulstrup single-ion electrostatic free energy on the ion's radius.Comment: 17 pages, 4 figure

Radius of a Photon Beam with Orbital Angular Momentum

We analyze the transverse structure of the Gouy phase shift in light beams carrying orbital angular momentum and show that the Gouy radius $r_G$ characterizing the transverse structure grows as $\sqrt{2p+|\ell|+1}$ with the nodal number $p$ and photon angular momentum number $\ell$. The Gouy radius is shown to be closely related to the root-mean-square radius of the beam, and the divergence of the radius away from the focal plane is determined. Finally, we analyze the rotation of the Poynting vector in the context of the Gouy radius.Comment: 11 page