Partners

Abstract:
The interface and particle contributions to the streaming current of flat substrates covered with ordered square or hexagonal monolayers of spherical particles were theoretically evaluated for particle coverage up to close packing. The exact numerical results were approximated using fitting functions that contain exponential and linear terms to account for hydrodynamic screening and charge convection from the particle surfaces exposed to external flow. According to our calculations, the streaming currents for the ordered and random particle arrangements differ within a typical experimental error. Thus, streaming-current measurements, supplemented with our fitting functions, can be conveniently used to evaluate the particle coverage without detailed knowledge of the particle distribution. Our results for equal interface and particle ζ-potentials indicate that roughness can reduce the streaming current by more than 30%, even in the limit of the small size of spherical roughness asperities.

Abstract:
A detailed theoretical and experimental study is presented concerning the streaming current and the derivative effective ζ-potential for a planar surface covered by a monolayer of adsorbed particles. Precise simulation results are obtained for the equilibrium and random-sequential-adsorption (RSA) distributions of monodisperse spherical particles interacting via the excluded-volume potential. The streaming current is calculated in the thin-double-layer regime for all physically accessible particle area fractions. The results are expressed as a linear combination of the interface and particle contributions DI and DP weighted by the interface and particle ζ-potentials ζI and ζP. We find that in the area-fraction regime where both particle distributions exist, the equilibrium and RSA results for the streaming current are nearly indistinguishable. Our numerical data show that DI exponentially decays to zero when the particle area fraction θ is increased, whereas DP exponentially tends to a linear behavior. The results are described (with the accuracy better than 1.5% of the maximal value) by the exponential and linear plus exponential approximations, with only one fitting parameter. The numerical and theoretical predictions are in agreement with experimental data obtained for a wide range of ζ-potentials of the interface and the particles. Results obtained for a rough surface with spherical asperities indicate that the roughness can reduce the effective ζ-potential (as evaluated from the streaming current) by more than 25%; this prediction is also confirmed by experiments.

Abstract:
Hydrodynamic properties of fibrinogen molecules were theoretically calculated. Their shape was approximated by the bead model, considering the presence of flexible side chains of various length and orientation relative to the main body of the molecule. Using the bead model, and the precise many-multipole method of solving the Stokes equations, the mobility coefficients for the fibrinogen molecule were calculated for arbitrary orientations of the arms whose length was varied between 12 and 18 nm. Orientation averaged hydrodynamic radii and intrinsic viscosities were also calculated by considering interactions between the side arms and the core of the fibrinogen molecule. Whereas the hydrodynamic radii changed little with the interaction magnitude, the intrinsic viscosity exhibited considerable variation from 30 to 60 for attractive and repulsive interactions, respectively. These theoretical results were used for the interpretation of experimental data derived from sedimentation and diffusion coefficient measurements as well as dynamic viscosity measurements. Optimum dimensions of the fibrinogen molecule derived in this way were the following: the contour length 84.7 nm, the side arm length 18 nm, and the total volume 470 nm3, which gives 16% hydration (by volume). Our calculations enabled one to distinguish various conformational states of the fibrinogen molecule, especially the expanded conformation, prevailing for pH < 4 and lower ionic strength, characterized by high intrinsic viscosity of 50 and the hydrodynamic radius of 10.6 nm. On the other hand, for the physiological condition, that is, pH = 7.4 and the ionic strength of 0.15 M NaCl, the semi-collapsed conformation dominates. It is characterized by the average angle equal to = 55, intrinsic viscosity of 35, and the hydrodynamic radius of 10 nm. Additionally, the interaction energy between the arms and the body of the molecule was predicted to be 4 kT units, confirming that they are oppositely charged than the central nodule. Results obtained in our work confirm an essential role of the side chains responsible for a highly anisotropic charge distribution in the fibrinogen molecule. These finding can be exploited to explain anomalous adsorption of fibrinogen on various surfaces.

Keywords:
Bead model of fibrinogen, Charge distribution over fibrinogen, Conformations of fibrinogen molecule, Fibrinogen molecule conformations, Hydrodynamic radius of fibrinogen, Viscosity of fibrinogen solutions

Abstract:
Recent developments in the electrokinetic determination of particle, protein and polyelectrolyte monolayers at solid/electrolyte interfaces, are reviewed. Illustrative theoretical results characterizing particle transport to interfaces are presented, especially analytical formulae for the limiting flux under various deposition regimes and expressions for diffusion coefficients of various particle shapes. Then, blocking effects appearing for higher surface coverage of particles are characterized in terms of the random sequential adsorption model. These theoretical predictions are used for interpretation of experimental results obtained for colloid particles and proteins under convection and diffusion transport conditions. The kinetics of particle deposition and the structure of monolayers are analyzed quantitatively in terms of the generalized random sequential adsorption (RSA) model, considering the coupling of the bulk and surface transport steps. Experimental results are also discussed, showing the dependence of the jamming coverage of monolayers on the ionic strength of particle suspensions. In the next section, theoretical and experimental results pertaining to electrokinetics of particle covered surfaces are presented. Theoretical models are discussed, enabling a quantitative evaluation of the streaming current and the streaming potential as a function of particle coverage and their surface properties (zeta potential). Experimental data related to electrokinetic characteristics of particle monolayers, mostly streaming potential measurements, are presented and interpreted in terms of the above theoretical approaches. These results, obtained for model systems of monodisperse colloid particles are used as reference data for discussion of experiments performed for polyelectrolyte and protein covered surfaces. The utility of the electrokinetic measurements for a precise, in situ determination of particle and protein monolayers at various interfaces is pointed out.

Keywords:
Colloid deposition, Nanoparticle deposition, Particle covered surfaces, Polyelectrolyte deposition, Protein deposition, Streaming potential of covered surfaces

Abstract:
The multiple expansion method was applied for calculating friction tensors and hydrodynamic radii RH of rigid molecules of various shape, composed of ns equal sized, touching spheres. The maximum value of ns studied was 450, which covers most situations met in practice. Calculations were performed for linear chains, half-circles, circles (cyclic molecules) and S-shaped aggregates. It was shown that our results agreed with previous theoretical data obtained for linear chains and cyclic aggregates, for ns < 100. For larger ns, studied exclusively in our work, interpolating analytical expressions were formulated for the hydrodynamic radii RH. These expressions, involving logarithmic function of the aspect ratio parameter (length to width ratio of the macromolecules), are the main finding of our work. Using these expressions, the ratio of the hydrodynamic radius of cyclic-to-linear aggregate qf was calculated, which is a parameter of vital significance. It was determined that qf attained values close to 0.95 for ns ∼450. This suggests that the previous analytical results derived by Tchen [19], in the slender body limit, who predicted that qf → 12/11 = 1.09, are not applicable for ns < 450. Using the RH values, the average translation diffusion coefficients and the sedimentation coefficients for these aggregate shapes were calculated. It was shown that our theoretical results are in good agreement with experimental data obtained for polyelectrolytes and for DNA fragments of various molecular mass. It was concluded that our results can be effectively used to determine the shape of macromolecules, in particular to discriminate between linear and cyclic DNA configurations.

Keywords:
Aggregates of particles of various shapes, Bead model of particle aggregates, Diffusion coefficients of particle aggregates, DNA bead model of, DNA fragment hydrodynamic radii, Hydrodynamic radius of aggregates, Linear chain aggregates, Sedimentation coefficients of aggregates and macromolecules

Abstract:
Streaming potential changes induced by deposition of particles at solid/liquid interfaces are considered theoretically. The solution is obtained in terms of a virial expansion of the streaming potential up to the third order of the surface coverage of particles, assumed to be distributed according to the hard sphere equilibrium distribution function. Theoretical methods, including the idea of cluster expansion, are adopted from statistical physics. In the cluster expansion, two-body and three-body hydrodynamic interactions are evaluated with a high precision using the multipole method. The multipole expansion algorithm is also used to perform numerical simulations of the streaming potential, valid for the entire surface coverage range met in practice. Results of our calculations are in good agreement with the experimental data for spherical latex particles adsorbed on a mica surface.

Keywords:
Streaming current, streaming potential, particle-covered wall, Stokes equations, hydrodynamic interactions, multiple expansion, viral exapnsion