Effect of surface grafted polymers on the adsorption of different model proteins

Adsorption of a model protein to a surface with end-grafted polymers was studied by Monte Carlo simulations. In the model the effect on protein adsorption in the presence of end-grafted polymers was evaluated by calculating the change in free energy between an end-grafted surface and a surface without polymers. The change in free energy was calculated using statistical mechanical perturbation theory. Apart from ordinary athermal polymer-polymer and protein-polymer interactions we also study a broad selection of systems by varying the interaction between proteins and polymers and effective polymer-solvent interactions. The interactions between the molecules span an interval from -0.5 to +0.5 kT. Consequently, general features of protein adsorption to end-grafted surfaces is investigated by systematically changing properties like hydrophilicity/hydrophobicity of the polymer, protein and surface as well as grafting density, degree of polymerization and protein size. Increasing grafting density as well as degree of polymerization decreases the adsorption of protein except in systems with attractive polymer-protein interactions, where adsorption increases with increasing chain length and higher grafting density. At a critical polymer-protein interaction neither chain length nor grafting density affects the free energy of adsorption. Hydrophilic polymers were found to prevent adsorption better than hydrophobic polymers. Very small particles with radii comparable to the size of a polymer segment were, however, better excluded from the surface when using hydrophobic than hydrophilic polymers. For systems with attractive polymer-protein interaction not only the volume of the protein was shown to be of importance but also the size of the exposed surface.     

Adsorption of the protein bovine serum albumin in a planar poly(acrylic acid) brush layer as measured by optical reflectometry

The adsorption of bovine serum albumin (BSA) in a planar poly(acrylic acid) (PAA) brush layer has been studied by fixed-angle optical reflectometry. The influence of polymer length, grafting density, and salt concentration is studied as a function of pH. The results are compared with predictions of an analytical polyelectrolyte brush model, which incorporates charge regulation and excluded volume interactions. A maximum in adsorption is found near the point of zero charge (pzc) of the protein. At the maximum, BSA accumulates in a PAA brush to at least 30 vol %. Substantial adsorption continues above the pzc, that is, in the pH range where a net negatively charged protein adsorbs into a negatively charged brush layer, up to a critical pH value. This critical pH value decreases with increasing ionic strength. The adsorbed amount increases strongly with both increasing PAA chain length and increasing grafting density. Experimental data compare well with the analytical model without having to include a nonhomogeneous charge distribution on the protein surface. Instead, charge regulation, which implies that the protein adjusts its charge due to the negative electrostatic potential in the brush, plays an important role in the interpretation of the adsorbed amounts. Together with nonelectrostatic interactions, it explains the significant protein adsorption above the pzc.     

Effects of ionic strength and surface charge on protein adsorption at PEGylated surfaces

PEGylated Nb2O5 surfaces were obtained by the adsorption of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) copolymers, allowing control of the PEG surface density, as well as the surface charge. PEG (MW 2 kDa) surface densities between 0 and 0.5 nm(-2) were obtained by changing the PEG to lysine-mer ratio in the PLL-g-PEG polymer, resulting in net positive, negative and neutral surfaces. Colloid probe atomic force microscopy (AFM) was used to characterize the interfacial forces associated with the different surfaces. The AFM force analysis revealed interplay between electrical double layer and steric interactions, thus providing information on the surface charge and on the PEG layer thickness as a function of copolymer architecture. Adsorption of the model proteins lysozyme, alpha-lactalbumin, and myoglobin onto the various PEGylated surfaces was performed to investigate the effect of protein charge. In addition, adsorption experiments were performed over a range of ionic strengths, to study the role of electrostatic forces between surface charges and proteins acting through the PEG layer. The adsorbed mass of protein, measured by optical waveguide lightmode spectroscopy (OWLS), was shown to depend on a combination of surface charge, protein charge, PEG thickness, and grafting density. At high grafting density and high ionic strength, the steric barrier properties of PEG determine the net interfacial force. At low ionic strength, however, the electrical double layer thickness exceeds the thickness of the PEG layer, and surface charges "shining through" the PEG layer contribute to protein interactions with PLL-g-PEG coated surfaces. The combination of AFM surface force measurements and protein adsorption experiments provides insights into the interfacial forces associated with various PEGylated surfaces and the mechanisms of protein resistance.     

表面等离子体子共振技术实时研究蛋白质在修饰表面的静电吸附行为

研究蛋白质在固相表面的静电吸附特性,进而控制蛋白质在修饰表面的静电吸附尤为重要,表面等离子体子共振可以检测金属表面吸附物质厚度和折射率的变化^[1]。这种技术已在研究生物分子相互作用^[2]和考察自组装单层的形成^[3]及蛋白质在固体表面吸附行为^[9-11]等方面得到广泛的应用。对蛋白质在固体表面吸附行为的研究多为考察不同的蛋白质在不同的修饰表面的吸附行为。然而,对蛋白质在修饰表面静电吸附的本质影响因素的研究却少有报道^[4]。本文使用表面等离子体子共振技术实时研究了蛋白质在甲羧基化葡聚糖修饰表面的静电吸附与溶液pH值及离子强度的依赖关系。     

On the theory of polyelectrolyte adsorption : The effect on adsorption behavior of the electrostatic contribution to the adsorption free energy

A theory has been developed for the adsorption of polyelectrolytes on charged interfaces from an aqueous salt solution. This adsorption is determined by the electrical charge density of the polyelectrolyte, the adsorption energy, the salt concentration, the molecular weight, solubility, flexibility, and concentration of polymer. The theory relates these parameters to the properties of the adsorbed polymer layer, i.e., the amount of polymer adsorbed, the fraction of the adsorbent interface covered, the fraction of the segments actually adsorbed on the interface versus the fraction of the segments in the dangling loops, the final surface charge density, and the thickness of the adsorbed layer. As polyelectrolyte adsorption should resemble nonionic polymer adsorption at high ionic strength of the solution or low charge density on the polymer, this work is an extension of the nonionic polymer adsorption theory to polyelectrolyte adsorption. The following effects are taken into account: (a) the conformational change upon adsorption of a coil in solution into a sequence of adsorbed trains interconnected by loops dangling in solution; (b) the interactions of the adsorbed trains with the interface and with each other; (c) the interaction of the dangling loops with the solvent; (d) the change in surface charge density of the adsorbent due to adsorption of charged trains and the accompanying changes in the electrical double layer which contains “small” ions as well as charged loops; (e) the (induced) dipole interaction of the adsorbed trains with the charged adsorbent interface. The theory is worked out for low potentials (Debye—Hückel approximation); in Appendix B an outline of a more complete treatment is given. The predicted adsorption isotherms have the experimentally observed high-affinity character. A relation between the adsorption energy, the surface charge density on the adsorbent, the degree of dissociation of the polymer, and the salt concentration predicts the conditions under which no adsorption will occur. For adsorbent and polymer carrying the same type of charge (both positive or both negative) the adsorption is predicted to decrease with increased charge density on polymer or adsorbent and to increase with salt concentration. If adsorbent and polymer carry different type charges, the adsorption as a function of the degree of dissociation, α, goes through a maximum at a relatively low value of α and, depending on the adsorption energy, an increase in the salt concentration can then increase or decrease the adsorption. At finite polymer concentration in solution the number of adsorbed segments and the fraction of the interface covered practically do not change with an increase in polymer concentration, whereas the total number of polymer molecules adsorbed increases slightly, as does the average fraction of segments in loops. The experimental results for polyelectrolyte adsorption have been reviewed in general and, as far as data are available, the predictions of the theory seem to follow the experimentally observed trends quite closely, except for the thickness of the adsorbed layer. This thickness is systematically overestimated by the theory and two reasons for this are given. The theoretical model implies a not too low ionic strength of the solution. Extrapolation of results to solutions of very low ionic strength is not warranted.     

Control of specific attachment of proteins by adsorption of polymer layers

This study concerns the design of protein-resistant polymer adsorbed layers for the control of surface binding of biospecific recognition entities. Polymer surface layers were prepared using the adsorption of poly(allylamine hydrochloride) (PAH), poly(l-lysine) (PL), and branched and linear polyethyleneimine (PEI) and further modified by the covalent attachment of biotin for specific avidin attachment. The adsorption of PAH, PL, and PEI on silicon substrates was studied as a function of pH and ionic strength using ellipsometry. Average dry layer thicknesses of approximately 10, approximately 5, approximately 9, and approximately 3 A (+/-1 A) were obtained when polymer adsorption occurred from solutions at pH 9.5 that contained 0.5 M NaCl for PAH, PL, branched PEI, and linear PEI, respectively. These polymers showed significant differences in their efficiency to suppress nonspecific avidin adsorption. At low ionic strength, avidin adsorption occurred on all polymer-coated surfaces at basic pH values, despite the same positive electrostatic charge for protein globules and the surface. Though the net electrostatic repulsion between avidin molecules and branched PEI was efficiently screened in a protein solution of pH 7 and 0.15 M NaCl, branched-PEI coatings of high molecular weight were unique in their ability to provide avidin-resistant surfaces as a result of steric hindrance from the branched architecture of adsorbed polymer chains. All polymers studied were effective in suppressing avidin adsorption at pH 3 as a result of protonation of the avidin surface functional groups at this pH. Branched-PEI-coated surfaces were also effective for the suppression of smaller positively charged proteins such as lysozyme and ribonuclease A at pH 7 and 0.15 M NaCl. They were also resistant to the adsorption of negatively charged proteins such as BSA and fibrinogen at pH 7 and 0.75 M NaCl. Furthermore, by using PEI-modified protein-repellent surfaces, selective binding of avidin was achieved to surface-bound silver nanoparticles, which should provide a promising application for the label-free detection of biological species using surface-enhanced Raman scattering (SERS).     

Monte Carlo simulations of antibody adsorption and orientation on charged surfaces

Monte Carlo simulations were performed to study the adsorption and orientation of antibodies on charged surfaces based on both colloidal and all-atom models. The colloidal model antibody consists of 12 connected beads representing the 12 domains of an antibody molecule. The structure of the all-atom antibody model was taken from the protein databank. The effects of the surface charge sign and density, the solution pH and ionic strength on the adsorption and orientation of different colloidal model antibodies with different dipole moments were examined. Simulation results show that both the 12-bead and the all-atom models of the antibody, for which the dipole moment points from the Fc to (Fab)2 fragments, tend to have the desired "end-on" orientation on positively charged surfaces and undesired "head-on" orientation on negatively charged surfaces at high surface charge density and low solution ionic strength where electrostatic interactions dominate. At low surface charge density and high solution ionic strength where van der Waals interactions dominate, 12-bead model antibodies tend to have "lying-flat" orientation on surfaces. The orientation of adsorbed antibodies results from the compromise between electrostatic and van der Waals interactions. The dipole moment of an antibody is an important factor for antibody orientation on charged surfaces when electrostatic interactions dominate. This charge-driven protein orientation hypothesis was verified by our simulations results in this work. It was further confirmed by surface plasmon resonance biosensor and time-of-flight secondary ion mass spectrometry experiments reported elsewhere.     

Poisson-Boltzmann theory of the charge-induced adsorption of semi-flexible polyelectrolytes

A model is suggested for the structure of an adsorbed layer of a highly charged semi-flexible polyelectrolyte on a weakly charged surface of opposite charge sign. The adsorbed phase is thin, owing to the effective reversal of the charge sign of the surface upon adsorption, and ordered, owing to the high surface density of polyelectrolyte strands caused by the generally strong binding between polyelectrolyte and surface. The Poisson-Boltzmann equation for the electrostatic interaction between the array of adsorbed polyelectrolytes and the charged surface is solved for a cylindrical geometry, both numerically, using a finite element method, and analytically within the weak curvature limit under the assumption of excess monovalent salt. For small separations, repulsive surface polarization and counterion osmotic pressure effects dominate over the electrostatic attraction and the resulting electrostatic interaction curve shows a minimum at nonzero separations on the Angstrom scale. The equilibrium density of the adsorbed phase is obtained by minimizing the total free energy under the condition of equality of chemical potential and osmotic pressure of the polyelectrolyte in solution and in the adsorbed phase. For a wide range of ionic conditions and charge densities of the charged surface, the interstrand separation as predicted by the Poisson-Boltzmann model and the analytical theory closely agree. For low to moderate charge densities of the adsorbing surface, the interstrand spacing decreases as a function of the charge density of the charged surface. Above about 0.1 M excess monovalent salt, it is only weakly dependent on the ionic strength. At high charge densities of the adsorbing surface, the interstrand spacing increases with increasing ionic strength, in line with the experiments by Fang and Yang [J. Phys. Chem. B 101, 441 (1997)].     

Effects of chemical functional groups on the polymer adsorption behavior onto titania pigment particles

The effects of functional groups on polymer adsorption onto titania pigment particles have been investigated as a function of pH and ionic strength using polyacrylic acid and modified polyacrylamides. The polyacrylamides include the homopolymer, an anionic copolymer with hydroxyl and carboxylate group substitution, and a nonionic copolymer with hydroxyl group substitution. Adsorption isotherms and infrared spectroscopy were used to examine the polymer-pigment interactions. The adsorption of the polyacrylic acid and anionic polyacrylamide on titania pigment is greatest when electrostatic repulsion is absent or reduced. At low pH values, below the pigment isoelectric point (IEP), or at high ionic strength, the adsorption density of the anionic polymers on titania pigment is high, while at higher pH values above the pigment IEP, the adsorption density decreases. But the adsorption of nonionic polymers on titania pigment is not influenced by either ionic strength or pH. Acrylamide groups were found to hydrogen bond with the titania pigment surface, independent of pH. With the inclusion of hydroxyl functional groups into the polyacrylamide chain, the polymer adsorption density increased without increased adsorption affinity. Carboxylate functional groups in the anionic polymers strongly interact with the pigment surface, producing the highest adsorption density at low pH values. All polymers exhibit Langmuir adsorption behavior with hydrogen bonding found as the dominant mechanism of adsorption in addition to electrostatic interaction occurring for the anionic polymers.     

Electrophoretic mobility of human erythrocytes in the presence of poly(styrene sulfonate)

The adsorption and depletion of the anionic polymer poly(styrene sulfonate) (PSS) on fresh human red blood cells (RBC) were investigated by measuring RBC electrophoretic mobility as a function of polymer molecular mass (48-2610 kDa), ionic strength (15 and 150 mM NaCl) and polymer concentration (相似文献   

Study on the interaction between anionic and cationic latex particles and Portland cement

The interaction between organic latex polymers and the surface of hydrating cement was investigated by measuring the zeta potential and adsorbed amount of polymer on cement. First, differently charged model latex particles were synthesized in aqueous media by well-known emulsion polymerization technique. The latex polymers were characterized by differential scanning calorimetry (DSC), dynamic light scattering (DLS) and environmental scanning electron microscopy (ESEM). Electrokinetic latex surface properties were investigated by means of streaming potential measurements using a particle charge detector (PCD). It is shown that the anionic latexes adsorb a considerable amount of Ca²⁺ from the cement pore solution. Next, adsorption of the latex polymers on the surface of hydrating cement was confirmed by zeta potential measurements using the electroacoustic method. A water to cement ratio in the cement paste as low as 0.5 was studied, representing actual conditions in mortar and concrete. Additionally, adsorption isotherms were determined in a sedimentation test using the depletion method. For all latex polymers, Langmuir type adsorption isotherms were found. The latex dosages required to achieve saturated adsorption on the cement surface obtained from zeta potential measurements correspond well with those determined in the sedimentation test. Electron microscopy photographs confirm that the charged latex polymers adsorb selectively on surface areas of hydrating cement showing opposite charge. This way, domains of organic latex polymers exist on the cement surface. They provide adhesion between the inorganic cement matrix and the organic polymer film formed later on by particle coalescence as a result of cement hydration and drying.     

Proteins and polyelectrolytes: A charged relationship

We review the interaction of charged polymeric systems with proteins. In solutions of low ionic strength there are many examples of proteins attracted to polyelectrolytes even if both systems carry the same overall charge. This attractive interaction is widespread, having been observed for single polyelectrolyte chains as well as for polyelectrolytes grafted to surfaces (polyelectrolyte brushes) and charged polymeric networks. In all cases, adding salt weakens the interaction considerably. We discuss the suggestion that the attractive force at low salinity originates from the asymmetry of interaction between charged polymer segments and charged patches on the surface of the protein globule. This can be explained if the attractive force is mainly due to a counterion release force, i.e., the polyelectrolyte chains become the multivalent counterions for the patches of opposite charge localized on the surface of the proteins. We review a selection of simple models that lead to semi-quantitative estimates of this force as the function of salt concentration.     

Protein PEGylation attenuates adsorption and aggregation on a negatively charged and moderately hydrophobic polymer surface

Covalent grafting of poly(ethylene glycol) chains to proteins ("PEGylation") is emerging as an effective technique to increase the in vivo circulation time and efficacy of protein drugs. PEGylated protein adsorption at a variety of solid/aqueous interfaces is a critical aspect of their manufacture, storage, and delivery. A special category of block copolymer, PEGylated proteins have one or more water-soluble linear polymer (PEG) blocks and a single globular protein block that each exert distinct intermolecular and surface interaction forces. We report the impact of PEGylation on protein adsorption at the interface between aqueous solutions and solid films of poly(lactide-co-glycolide) (PLG), a moderately hydrophobic and negatively charged polymer. Using the model protein lysozyme with controlled degrees of PEGylation, we employ total internal reflection fluorescence techniques to measure adsorption isotherms, adsorption reversibility, and the extent of surface-induced aggregation. Lysozyme PEGylation reduces the extent of protein adsorption and surface-induced aggregation and increases the reversibility of adsorption compared to the unconjugated protein. Results are interpreted in terms of steric forces among grafted PEG chains and their effects on protein-protein interactions and protein orientation on the surface.     

Brownian dynamics simulations of polyelectrolyte adsorption onto charged patterned surfaces

The adsorption of single polyelectrolyte molecules onto surfaces decorated with periodic arrays of charged patches was studied using Brownian dynamics simulations. A free-draining, freely jointed bead-rod chain was used to model the polyelectrolyte, and electrostatic interactions were incorporated using a screened Coulombic potential with the excluded volume accounted for by a hard-sphere potential. The simulations predicted that the polyelectrolyte lies close to the adsorbing surface if the patch length, surface charge density, and screening length are sufficiently large. Chain conformations were found to be very sensitive to patch length, patch spacing, and the nature of the charge on adjacent patches. This is due both to the size of the polymer relative to patch length and spacing and to the structure of the electric field near the surface. In some cases, the component of the radius of gyration parallel to the surface can be made smaller than its free-solution value, which is contrary to what is observed for a uniformly charged surface. Isolated charged patches were also considered, and significant adsorption was observed above a critical surface charge density. The results demonstrate how polyelectrolyte conformations can be controlled by the design of the charged patches and may be useful for applications in which adsorbed polyelectrolyte films play a key role.     

Selective protein adsorption on a phase-separated solvent-cast polymer blend

Polymer-based biomedical devices are growing increasingly sophisticated as compositions evolve toward copolymers and blends in order to satisfy complex design criteria. Such polymers afford opportunities for both micro- and macrophase separation at nano- and micro-length scales and raise questions concerning the role of heterogeneous surface morphology on protein adsorption. Adsorbed protein layers play a critical role in mediating the interaction of cells with polymer surfaces, and both understanding and controlling protein adsorption is assuming greater significance in the development of surfaces with enhanced physiological compatibility. Here we study the short-time adsorption of ferritin, a model protein highly resistant to denaturation and easily imaged in the transmission electron microscope (TEM), onto a phase-separated homopolymer blend of polycaprolactone (PCL) and a polycarbonate derived from desaminotyrosyl-tyrosine dodecyl ester (PDTD). At physiological pH, ferritin selectively adsorbs onto the PDTD phase at a surface density approximately three times greater than that on the PCL phase. By decreasing the pH below ferritin's isoelectric point so its average charge becomes positive, the selective adsorption disappears and the surface density of adsorbed ferritin becomes independent of the phase separation. We attribute the selectivity to the electrostatic repulsion between ferritin and hydrolytically charged PCL, both of which will have a net negative charge at physiological pH. To perform these experiments, we solvent-cast ultrathin polymer films onto dissolvable salt substrates, and we characterize the morphology by TEM imaging and quantitative spatially resolved electron energy-loss spectroscopy (EELS). We find that the film morphology depends strongly on such processing-related variables as the solvent evaporation rate and the nature of the surface in contact with the polymer film during casting. The adsorption of ferritin depends on whether the film is phase-separated as well as to which surface of the film the protein solution is exposed, and these findings suggest that seemingly small variations in polymer processing that influence both the bulk and surface morphology can have a profound effect on the short-time protein adsorption.     

Molecular dynamics simulations of polyelectrolyte adsorption

We have performed molecular dynamics simulations of polyelectrolyte adsorption at oppositely charged surfaces from dilute polyelectrolyte solutions. In our simulations, polyelectrolytes were modeled by chains of charged Lennard-Jones particles with explicit counterions. We have studied the effects of the surface charge density, surface charge distribution, solvent quality for the polymer backbone, strength of the short-range interactions between polymers and substrates on the polymer surface coverage, and the thickness of the adsorbed layer. The polymer surface coverage monotonically increases with increasing surface charge density for almost all studied systems except for the system of hydrophilic polyelectrolytes adsorbing at hydrophilic surfaces. In this case the polymer surface coverage saturates at high surface charge densities. This is due to additional monomer-monomer repulsion between adsorbed polymer chains, which becomes important in dense polymeric layers. These interactions also preclude surface overcharging by hydrophilic polyelectrolytes at high surface charge densities. The thickness of the adsorbed layer shows monotonic dependence on the surface charge density for the systems of hydrophobic polyelectrolytes for both hydrophobic and hydrophilic surfaces. Thickness is a decreasing function of the surface charge density in the case of hydrophilic surfaces while it increases with the surface charge density for hydrophobic substrates. Qualitatively different behavior is observed for the thickness of the adsorbed layer of hydrophilic polyelectrolytes at hydrophilic surfaces. In this case, thickness first decreases with increasing surface charge density, then it begins to increase.     

Relationship between interfacial forces measured by colloid-probe atomic force microscopy and protein resistance of poly(ethylene glycol)-grafted poly(L-lysine) adlayers on niobia surfaces

Adsorbed layers of "comb-type" copolymers consisting of PEG chains grafted onto a poly(l-lysine) (PLL) backbone on niobium oxide substrates were studied by colloid-probe AFM in order to characterize the interfacial forces associated with coatings of varying architectures (PEG/PLL ratios and PEG chain lengths) and their relevance to protein resistance. The steric and electrostatic forces measured varied substantially with the architecture of the PLL-g-PEG copolymers. Varying the ionic strength of the buffer solutions enabled discrimination between electrostatic and steric-entropic contributions to the net interfacial force. For high PEG grafting densities the steric component was most prominent, but at low ionic strengths and high grafting densities, a repulsive electrostatic surface force was also observed; its origin was assigned to the niobia charges beneath the copolymer, as insufficient protonated amine groups in the PLL backbone were available for compensation of the oxide surface charges. For lower grafting densities and lower ionic strengths there was a substantial attractive electrostatic contribution arising from interaction of the electrical double layer arising from the protonated amine groups, with that of the silica probe surface (as under low ionic strength conditions, the electrical double layer was thicker than the PEG layer). For these PLL-g-PEG coatings the net interfacial force can thus be a markedly varying superposition of electrostatic and steric-entropic contributions, depending on various factors. The force curves correlate with protein adsorption data, demonstrating the utility of AFM colloid-probe force measurements for quantitative analysis of surface forces and how they determine interfacial interactions with proteins. Such characterization of the net interfacial forces is essential to elucidate the multiple types of interfacial forces relevant to the interactions between PLL-g-PEG coatings and proteins and to advance interpretation of protein adsorption or repellence beyond the oversimplified steric barrier model; in particular, our data demonstrate the importance of an ionic-strength-dependent minimum PEG layer thickness to screen the electrostatic interactions of charged interfaces.     

High-performance liquid chromatography of amino acid peptides and proteins

Summary The retention behaviour of seven globular proteins ranging in molecular weight from 12,000 to 69,000 was investigated using Mono-Q anion-exchange resin as the stationary phase and sodium chloride as the displacer salt. In particular the influence of changes in ionic strength and mobile phase pH on the isocratic retention properties was assessed. Several proteins were found to have significant retention when the pH of the mobile phase was below the reported pl values of the proteins. This behaviour results from the non-uniform charge distribution on the protein surface, which allows interaction with the charged stationary phase even though the protein net charge is equal to or greater than zero. The influence of pH and ionic strength on experimentally observed bandwidths was also investigated. The dependence of the effective reduced plate height on solute capacity factor was found to vary significantly with the mobile phase pH, a behaviour consistent with the interplay of complex multisite binding kinetics. These results provide a basis for further detailed investigations into the mechanism of interaction of proteins not only with charged surfaces associated with adsorptive chromatographic media but also with other macromolecules. For Part LXXXII, see ref. [27].     

SANS and SAXS analysis of charged nanoparticle adsorption at oil-water interfaces

A systematic study of the adsorption of charged nanoparticles at dispersed oil-in-water emulsion interfaces is presented. The interaction potentials for negatively charged hexadecane droplets with anionic polystyrene latex particles or cationic gold particles are calculated using DLVO theory. Calculations demonstrate that increased ionic strength decreases the decay length of the electrostatic repulsion leading to enhanced particle adsorption. For the case of anionic PS latex particles, the energy barrier for particle adsorption is also reduced when the surface charge is neutralized through changes in pH. Complementary small-angle scattering experiments show that the highest particle adsorption for PS latex occurs at moderate ionic strength and low pH. For cationic gold particles, simple DLVO calculations also explain scattering results showing that the highest particle adsorption occurs at neutral pH due to the electrostatic attraction between oppositely charged surfaces. This work demonstrates that surface charges of particles and oil droplets are critical parameters to consider when engineering particle-stabilized emulsions.     

Polyelectrolyte–particle complex formation. Polyelectrolyte linear charge density and ionic concentration effects. Monte Carlo simulations

The influence of the linear charge density (LCD) of a polyelectrolyte on its adsorption on an oppositely charged colloidal particle is investigated by Monte Carlo simulations. Adsorption characteristics are studied at different linear charge densities and ionic concentrations and for a given polyelectrolyte/particle size ratio so that particle curvature has full effect. The isolated polyelectrolyte goes through a smooth transition from a collapsed structure to an extended rod-like conformation with increasing the linear charge density in the low ionic concentration regime. In the high ionic concentration regime, the polyelectrolyte is less sensitive to the increase in the linear charge density and adopts a coil conformation. We found that complex formation is promoted by decreasing the ionic concentration and increasing the linear charge density and that large changes in the polymer dimensions are observed at the adsorption-desorption limit. By adjusting the linear charge density and ionic strength, we demonstrate that the adsorption-desorption limit corresponds to a sharp transition from non-adsorbed to adsorbed conformations and that the mean adsorption energy per monomer has to be less than -0.4 kT to achieve adsorption. We calculated that the linear charge density at the adsorption-desorption limit is related to the Debye-Hückel length according to LCD^crit ~3². At small values of the linear charge density and low ionic strength (no adsorption is observed at high ionic strength), a large amount of monomers are present in loops and tails. By increasing LCD, the amount of monomers in trains reaches a maximum value and the polyelectrolyte adopt flat conformation at the surface of the particle.