Particle adsorption and deposition: role of electrostatic interactions

This work demonstrates how electrostatic interactions, described in terms of the classical DLVO theory, influence colloid particle deposition phenomena at solid/liquid interfaces. Electrostatic interactions governing particle adsorption in both non-polar and polar media (screened interactions) are discussed. Exact and approximate methods for calculating the interaction energy of spherical and non-spherical (anisotropic) particles are presented, including the Derjaguin method. Phenomenological transport equations governing particle deposition under the linear regime are discussed with the limiting analytical expressions for calculating initial flux. Non-linear adsorption regimes appearing for higher coverage of adsorbed particles are analysed. Various theoretical approaches are exposed, aimed at calculating blocking effects appearing due to the presence of adsorbed particles. The significant role of coupling between bulk transport and surface blocking is demonstrated. Experimental data obtained under well-defined transport conditions, such as diffusion and forced convection (impinging-jet cells), are reviewed. Various experimental techniques for detecting particles at interfaces are discussed, such as reflectometry, ellipsometry, streaming potential, atomic force microscopy, electron and optical microscopy, etc. The influence of ionic strength and flow rate on the initial particle deposition rate (limiting flux) is presented. The essential role of electrostatic interactions in particle deposition on heterogeneous surfaces is demonstrated. Experimental data pertinent to the high-coverage adsorption regime are also presented, especially the dependence of the maximum coverage of particles and proteins on the ionic strength. The influence of lateral electrostatic interactions on the structure of particle monolayers is elucidated, and the links between colloid and molecular systems are pointed out.     

Incremental update of electrostatic interactions in adaptively restrained particle simulations

The computation of long‐range potentials is one of the demanding tasks in Molecular Dynamics. During the last decades, an inventive panoply of methods was developed to reduce the CPU time of this task. In this work, we propose a fast method dedicated to the computation of the electrostatic potential in adaptively restrained systems. We exploit the fact that, in such systems, only some particles are allowed to move at each timestep. We developed an incremental algorithm derived from a multigrid‐based alternative to traditional Fourier‐based methods. Our algorithm was implemented inside LAMMPS, a popular molecular dynamics simulation package. We evaluated the method on different systems. We showed that the new algorithm's computational complexity scales with the number of active particles in the simulated system, and is able to outperform the well‐established Particle Particle Particle Mesh (P3M) for adaptively restrained simulations. © 2018 Wiley Periodicals, Inc.     

Hydrophobic and electrostatic interactions in adsorption of surface-active substances. Tetraalkylammonium salts

A relationship between the standard free energies of adsorption from aqueous solution at the oil/water interface and the radii of organic cations as exemplified by symmetric tetraakylammonium salts has been studied. Hydrophobic effects are shown to be major contributors to the interaction of surfactants with the interface. An adsorption coefficient to quantitate the hydrophobic effects and to specify the changes of standard adsorption energy depending upon the cavity surface area of the detergent hydrocarbon radical in aqueous solution has been proposed. A new formulation of the Traube rule, taking into account the hydrophobic effects concomitant with a transfer of surfactants from the water bulk onto the interface, has also been given.Standard free energies for the adsorption of organic and inorganic ions from aqueous solution at the interface of immiscible liquids have been found. The proposed method is based on an extrapolation of the relationship between the standard adsorption energy of tetraalkylammonium salts and the square of cationic radius to zero ionic radius. The standard free energy of adsorption for an inorganic counter-ion is derived from an intercept on the y-axis cut off by a straight line. The experimental adsorption data on inorganic salts have been used to calculate the standard free energies of adsorption for a variety of ions.A method of estimating the difference in potential at the oil/water interface between the adsorption plane and the aqueous solution has been proposed. The sign of potential provides a clue to the orientation of water molecules at the interface between immiscible liquids.     

Hydrophobic and electrostatic interactions in the adsorption of fibronectin at maleic acid copolymer films

Adsorption and desorption of fibronectin (FN) were investigated at thin films of alternating maleic acid copolymers with octadecene (POMA) and with propene (PPMA). The hydrophobicity and charge density of the polymers were modulated by the choice of the comonomer. In consequence, the dominant forces between the substrate and the protein were specified as hydrophobic interaction for POMA and electrostatic interaction for PPMA. The adsorption kinetics were investigated in situ as variations of the optical thickness, adsorbed mass, and viscoelastic properties (detected by reflectometric interference spectroscopy and quartz crystal microbalance technique, respectively) while alterations of the electrosurface properties were derived from surface conductivity data and isoelectric points (by streaming potential/current measurements using a microslit electrokinetic setup). The results demonstrate that the interfacial mode of adsorbed FN depends on the predominant interactions: large amounts of FN were tightly bound to POMA by hydrophobic interactions. In contrast, FN adsorbed on PPMA was concluded to attain an unfolded structure allowing for the "electrostatic matching" of positively charged residues on FN with the maleic acid groups. This conclusion was supported by the acidic IEP of 3.2 found for FN on PPMA and a significant reduction of the surface conductivity of the FN-covered polymer film, whereas FN on POMA showed an IEP of 4.2 (close to the intrinsic IEP of FN), indicating a stochastic orientation of the adsorbed protein.     

Role of electrostatic interactions in two-dimensional self-assembly of tobacco mosaic viruses on cationic lipid monolayers

We explore two-dimensional self-assembly of tobacco mosaic viruses (TMVs) on a substrate-supported, fluid lipid monolayer by manipulating the electrostatic interactions, with specific focus on the effects of the cationic lipid concentration in the monolayer and the presence of Ca(2+) ions in the surrounding bulk solution. The TMV assemblies were characterized by grazing-incidence X-ray scattering and atomic force microscopy, and the inter-particle interaction quantified through X-ray scattering data analysis. In the absence of Ca(2+) ions, we found that higher charge densities on the lipid monolayer led to poorer in-plane order, which may be attributed to faster adsorption kinetics, due to the surface potential that increases with charge density. At the same time, higher lipid-charge densities also resulted in weaker repulsion between TMVs, due to partial screening of Coulomb repulsion by mobile cationic lipids in the monolayer. The lipid-charge dependence was diminished with increasing concentration of Ca(2+) ions, which also led to tighter packing of TMVs. The results indicate that Ca(2+) ions strengthen the screening of Coulomb repulsion between TMVs and consequently enhance the role of attractive forces. Control experiments involving Na(+) ions suggest that the attractive inter-TMV interaction has contributions from both the van der Waals force and the counter-ion-induced attraction that depends on ion valence.     

Light-regulated electrostatic interactions in colloidal suspensions

The net charge of a colloidal particle was controlled using light and a new photocleavable self-assembled monolayer (SAM). The SAM contained a terminal ammonium group and a centrally located carboxylic acid group that was masked with an ortho-nitrobenzyl functionality. Once exposed to UV light, the 2-nitrobenzyl group was cleaved, therefore transforming the colloidal particle from a net positive (silica-SAM-NH3+) to a net negative (silica-SAM-COO-) charge. By varying the UV exposure time, their zeta potential could be tailored between +26 and -60 mV at neutral pH. To demonstrate a photoinduced gel-to-fluid phase transition, a binary colloidal suspension composed of silica-SAM-NH3+ and negatively charged, rhodamine-labeled silica particles was mixed to form a gel. Exposure to UV light rendered all of the particles negative and therefore converted the system into a colloidal fluid that settles to form a dense sediment.     

Role of electrostatic interactions for the domain shapes of Langmuir monolayers of monoglycerol amphiphiles

The role of electrostatic interaction in the domain morphology of amide, ether, ester, and amine monoglycerol monolayers (abbreviated as ADD, ETD, ESD, and AMD, respectively) with systematic variation in the molecular structure of the headgroup region is investigated. Experimental studies using Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GIXD) show that the characteristic features of the condensed monolayer phase, such as domain morphology, crystallinity, and lattice parameters, are very different for these monoglycerols. Therefore, the intermolecular interactions of the four amphiphilic monoglycerols are investigated in detail. First, the dipole moments of four monoglycerols of similar structure but with different functional groups are calculated by a semiempirical quantum mechanical technique. The dipole moments for monoglycerols follow the sequence AMD < ETD < ESD < ADD for the population of conformers of compounds investigated. The dipolar repulsion energies for the amphiphilic monoglycerols are also calculated for different possible mutual orientations between the dipoles. The calculated dipolar energies also follow the same trend for different possible headgroup orientations. These results can explain the domain shape of the monoglycerols observed experimentally. Second, ab initio calculations on the basis of the HF/6-31G** method are performed for representative monoglycerol headgroup segments. The results show that the intermolecular interaction energy related to dimer formation follows the order ETD < ESD < AMD < ADD segments, similar to that observed in experiment except in the case of the AMD segment. The relative importance of intra- and intermolecular hydrogen bonding in dimers is analyzed. The enhanced role of the intermolecular interaction relative to intramolecular interaction in the case of AMD contributes to the relatively high intermolecular interaction energy for the particular conformation of the dimer of AMD segment as observed from ab initio calculation. The present work shows that the variations in headgroup molecular structure alter drastically the domain shape, and the theoretical calculations conclusively reveal the important role of the electrostatic interactions for the mesoscopic domain architecture.     

Role of hydrophobic and electrostatic interactions for initial enteric virus retention by MF membranes

Membranes are of increasing interest for the removal of human enteric viruses from wastewater, especially when the goal of treatment is reuse. Limited work has been undertaken on fundamental issues such as aggregation and the role of electrostatic and hydrophobic interactions, as opposed to the sieving of viruses by membranes. One apparently critical factor would be the iso-electric point (pI) of a virus. As an example of a worst-case model virus, the retention of bacteriophage MS-2 was investigated using hydrophobic (GVHP) and hydrophilic (GVWP) 0.22 μm MF membranes at different pH levels and with different salts. High retention levels were measured at the iso-electric point of MS-2, pH 3.9 (5 log retention) and pH 7 (4.3 log retention) in the presence of salts and with a hydrophobic membrane. When retention was compared on a hydrophilic membrane, it was clear that hydrophobic interactions dominated virus retention, and this was improved by salt, presumably causing reduction of the Gouy double-layer when MS-2 was charged (pH 7). This paper shows that knowledge of the adsorption characteristics of viruses and the suspending conditions are important to predict removal of viruses by hydrophobic MF membranes, and discusses some of the practical implications of these important hydrophobic interactions.     

Study on controlling particle growth in seeded emulsion polymerization with regulated coagulation based on the electrostatic interactions

30wt% solid content, anionic seed copolymer latex P(methyl acrylate-co-methyl methacrylate) was prepared by conventional emulsion polymerization, and then the seeded emulsion polymerization was carried out accompanied with the electrostatic coagulation during the reaction in the presence of counter-ion species, such as cationic monomer and initiator. In this article, effects of cationic monomer (dimethyl aminoethyl methacrylate, DM) content, secondary monomer to seed polymer weight ratio, M/P and amount of emulsifier (polyoxyethylene nonylphenylether with 23 units of ethylene oxide, PEO23) were investigated on the effective particle growth and the stability of final latex. With 10wt% DM in monomer, M/P ratio at 2.0 were recommended. An optimal policy for handling the emulsifier content without the nucleation of secondary particles while achieving the controlled coagulative growth was proposed from the observations of polymer yield and particle size during the polymerization.     

The influence of electrostatic forces on protein adsorption

In this paper we investigate the importance of electrostatic double layer forces on the adsorption of human serum albumin by UV-ozone modified polystyrene. Electrostatic forces were measured between oxidized polystyrene surfaces and gold-coated atomic force microscope (AFM) probes in phosphate buffered saline (PBS) solutions. The variation in surface potential with surface oxygen concentration was measured. The observed force characteristics were found to agree with the theory of electrical double layer interaction under the assumption of constant potential. Chemically patterned polystyrene surfaces with adjacent 5 microm x 5 microm polar and non-polar domains have been studied by AFM before and after human serum albumin adsorption. A topographically flat surface is observed before protein adsorption indicating that the patterning process does not physically modify the surface. Friction force imaging clearly reveals the oxidation pattern with the polar domains being characterised by a higher relative friction compared to the non-polar, untreated domains. Far-field force imaging was performed on the patterned surface using the interleave AFM mode to produce two-dimensional plots of the distribution of electrostatic double-layer forces formed when the patterned polystyrene surfaces is immersed in PBS. Imaging of protein layers adsorbed onto the chemically patterned surfaces indicates that the electrostatic double-layer force was a significant driving force in the interaction of protein with the surface.     

How to distinguish energetic surface heterogeneity from electrostatic interactions in the case of hydrogen ion adsorption from solution onto oxides

It was shown that the adsorption of uncharged particles onto energetically heterogeneous surfaces with Gaussian-like distribution of adsorption energy can be described by the same adsorption isotherm as the adsorption of hydrogen ions from solution onto homogeneous oxide surfaces with surface potentials given by a quasi-Nernst formula. It exemplifies that the separation of electrostatic from energetic factors in the case of ion adsorption onto heterogeneous oxide surfaces is very difficult. Nevertheless, application of calorimetric data for proton adsorption makes it possible to estimate both factors.     

Negative adsorption due to electrostatic exclusion of micelles

Interactions of surfactants with solid substrates are important in the controlling of processes such as flotation, coating, flocculation and sedimentation. These interactions usually lead to adsorption on solids, but can also result in an exclusion of the reagents with dire consequences. In this work electrostatic exclusion of negatively charged dodecylbenzene sulfonate micelles from quartz/water, Bio-Sil/water and alumina/water interfaces has been investigated as a function of pH and ionic strength. Measurable negative adsorption of these surfactants from similarly charged solid/liquid interface was observed in the micellar region. In the case of porous samples with large surface area, comparison of pore size with the micelle size is necessary to avoid any erroneous conclusions regarding the role of electrostatic exclusion in a given system. A theoretical model for the electrostatic exclusion of micelles is developed and used to calculate the adsorption of negatively charged dodecylbenzene sulfonate on negatively charged quartz (pH 7), silica (Bio-Sil A, pH 3) and alumina (pH 11) in the micellar concentration region. The micellar exclusion values calculated using the model are in excellent agreement with the experimental results.     

Numerical study of the interplay of monomer-surface electrostatic and non-electrostatic interactions in the adsorption of weak polyelectrolytes on oppositely charged surfaces

The adsorption of weak polybase on oppositely charged planar surfaces has been investigated numerically by using the self-consistent field theory (SCFT). Particular attention was paid to the interplay of monomer-surface electrostatic and non-electrostatic interactions in the adsorption behaviors of weak polybase. In this study, the strength of monomer-surface non-electrostatic interactions was set to be no more than the thermal energy k _B T. It was found from the numerical study that in the regime of low surface charge density of the substrate and low pH or high bulk degree of ionization, both the screening-enhanced and screening-reduced salt effects emerge. On the contrary, in the opposite regime, only the screening-reduced salt effect was observed. Moreover, the overall charge neutrality inside the adsorption layer was analyzed. The underlying mechanism governing the adsorption behaviors of weak polybase on oppositely charged surfaces was elucidated.     

Experimental investigation of electrostatic particle-particle interactions in optoelectronic tweezers

This paper reports experimental and theoretical investigation of electrostatic attraction and repulsion of microparticles in an optoelectronic tweezers (OET). When we manipulate dielectric particles suspended in a fluid using OET, the electrostatic interactions of the polarized particles occur, limiting the effective manipulation of microparticles using a light-induced dielectrophoresis. In this study, we first demonstrate the electrostatic particle-particle interactions in the OET device using a liquid crystal display. At the same time, the experimental investigation of the dipole interactions between two spherical particles has been performed using the OET device. On the basis of the point-dipole model, simulation studies on the dipole forces acting on the particles and their trajectories by the forces are also performed. The experimental results show good agreement with the previously reported numerical studies as well as the results of our simulation studies.     

Key role of electrostatic interactions in bacteriorhodopsin proton transfer

The first proton transport step following photon absorption in bacteriorhodopsin is from the 13-cis retinal Schiff base to Asp85. Configurational and energetic determinants of this step are investigated here by performing quantum mechanical/molecular mechanical minimum-energy reaction-path calculations. The results suggest that retinal can pump protons when in the 13-cis, 15-anti conformation but not when 13-cis, 15-syn. Decomposition of the proton transfer energy profiles for various possible pathways reveals a conflict between the effect of the intrinsic proton affinities of the Schiff base and Asp85, which favors the neutral, product state (i.e., with Asp85 protonated), with the mainly electrostatic interaction between the protein environment with the reacting partners, which favors the ion pair reactant state (i.e., with retinal protonated). The rate-limiting proton-transfer barrier depends both on the relative orientations of the proton donor and acceptor groups and on the pathway followed by the proton; depending on these factors, the barrier may arise from breaking and forming of hydrogen bonds involving the Schiff base, Asp85, Asp212, and water w402, and from nonbonded interactions involving protein groups that respond to the charge rearrangements in the Schiff base region.     

Evidence for electrostatic interactions of steroids in thin-layer chromatography

Chromatographia - The retention behaviour of steroids was investigated using normal and reversed-phase high-performance TLG systems. Homologous series of esters and n-alcohols were applied as the...     

Intermolecular interactions in liquid adsorption chromatography

Summary For the investigation of intermolecular interactions in adsorption from solution, which are the basis of selectivity in molecular liquid chromatography (LC), it is convenient to use the LC method itself. Using this method the Henry's constants, K₁, and other thermodynamic adsorption characteristics of hydrocarbons and of a series of polar substances on hydroxylated silica surface were determined from aqueous solutions. On the basis of the adsorption of hydrocarbons from water solutions the structure of the chemically modifying layers formed by different hydrocarbon groups on the silica surface is considered. The role of conformation ability of straight-chain bonded phases is demonstrated. Hydrocarbons are adsorbed on the hydroxylated silica surface more strongly from aqueous solutions than from solutions in saturated hydrocarbons and their retention increases with the increase in the number of carbon atoms in the molecule. The retention in LC is determined by the intermolecular interaction of the solute and solvent molecules with the adsorbent, as well by the contribution of the intermolecular interaction, between the solute and the solvent.The thermodynamic characteristics of adsorption of cymarin from water-ethanol solutions on hydroxylated silica gel and on silica gel surface modified by diphenylsilyl groups is compared. The solubility of silica gel modified by diphenylsilyl groups at different composition of water-ethanol eluent at different temperatures is investigated.Enlarged text of a paper presented at the Sixteenth International Symposium on Advances in Chromatography, Barcelona, Spain, September 28–October 1, 1981.