Ion-specific thermodynamic properties of colloids and proteins

Franz Hofmeister established in 1888 that different salt solutions with the same ionic charges have different efficiencies in precipitating proteins from whole egg white. We will discuss how this can be understood from the modified Poisson–Boltzmann equation that accounts for ion specificity via the ion-surface non-electrostatic potential of mean force (NE-PMF) from molecular dynamics simulations. Using this approach, it is at least in principle possible to capture the important physics of the system due to the inclusion of ion-surface van der Waals forces, short range hydration, image potential and different solvent-mediated forces. The method has been proved to be efficient and suitable for describing phenomena where the water structure close to the interface plays an essential role. As an illustrative example, we demonstrate why the double layer force between two gold electrodes coated with hydrophobic self-assembled monolayers in different electrolytes can be highly ion specific. Important thermodynamic properties related to protein aggregation, essential in biotechnology and pharmaceutical industries, can be obtained from the method shown here.     

Assessment of surfactant adsorption in oil-based magnetic colloids

We describe in this paper different and complementary experimental methods for assessing the adsorption of surfactants on metal particles in oil-based suspensions. Two different kinds of particles are dispersed in mineral oil: iron microparticles and CoNi nanoparticles. The adsorption of oleic acid in the Fe/oil interface in diluted suspensions can be determined by obtaining the adsorption isotherm. In addition, we present a method based on the time evolution of the optical absorbance of suspensions, from which the existence of adsorption can be inferred. For concentrated suspensions, the used of optical methods is not recommended, since they are affected by a significant inaccuracy. We present here a useful alternative based on electromagnetic induction phenomena. The results obtained allow a more comprehensive knowledge of the aggregation process in concentrated suspensions. With the same purpose, a third group of experiments, based on rheological techniques, is carried out in Fe/oil and CoNi/oil concentrated suspensions. In these series of experiments, the effect of three surfactants (oleic acid, aluminum stearate and lecithin) is tested by measuring either the viscosity, or the magnetic field-induced yield stress of the suspensions. The combination of these series of experiments gives us valuable information about the most appropriate surfactant/carrier combination capable of imparting a high stability and a strong magnetorheological response in magnetic colloids.     

Amphiphilic crescent-moon-shaped microparticles formed by selective adsorption of colloids

We use a microfluidic device to prepare monodisperse amphiphilic particles in the shape of a crescent-moon and use these particles to stabilize oil droplets in water. The microfluidic device is comprised of a tapered capillary in a theta (θ) shape that injects two oil phases into water in a single receiving capillary. One oil is a fluorocarbon, while the second is a photocurable monomer, which partially wets the first oil drop; silica colloids in the monomer migrate and adsorb to the interface with water but do not protrude into the oil interface. Upon UV-induced polymerization, solid particles with the shape of a crescent moon are formed; removal of fluorocarbon oil yields amphiphilic particles due to the selective adsorption of silica colloids. The resultant amphiphilic microparticles can be used to stabilize oil drops in a mixture of water and ethanol; if they are packed to sufficient surface density on the interface of the oil drop, they become immobilized, preventing direct contact between neighboring drops, thereby providing the stability.     

Polymer adsorption and its effect on the stability of hydrophobic colloids

Summary As a first step in a study on the interaction between polymers and hydrophobic colloids we investigated in detail the adsorption of polyvinyl alcohol (PVA) on aqueous silver iodide sols. The adsorption is irreversible. Adsorption isotherms are of the highaffinity type. The amount adsorbed increases with molecular weight and with the fraction of acetate groups in the PVA chain. The effective thickness of the adsorbed layer was determined viscosimetrically and independently checked by an electrophoretic method. Double layer studies enabled the determination of the occupancy of the first layer on the surface by polymer segments. It was found that even at maximal coverage with polymer this layer is still about 30% void. The combination of these data enabled the assessment of the polymer segment distribution. It was found that with not too low coverages the distribution isHoeve- like. The distribution, thus obtained reflects itself in the flocculation of AgI sols by PVA.Presented at the 25th Colloid-Meeting in Munich, October 13–15, 1971     

Formation of polysulfone colloids for adsorption of natural organic foulants

An ever-present problem in the use of commercial membranes for treatment of drinking water is fouling of the membranes by natural organic matter (NOM). This work describes a new approach to elimination or minimization of membrane fouling by NOM. When a 2% solution of polysulfone in NMP and propionic acid is slowly injected into water, approximately 50 nm polysulfone particles are spontaneously formed, and these hydrophobic particles quickly coagulate into approximately 12-microm diameter aggregates; the formed material has a surface area of approximately 100 m(2)/g and an equivalent "pore" size of 25 nm. When 50 mg/L of the new material is equilibrated with a local drinking water supply, virtually all adsorptive fouling of a 20-kDa molecular weight cutoff ultrafiltration membrane is eliminated. Interestingly, although only a very small percentage of the NOM is removed by adsorption on the polysulfone aggregates, it appears that exactly this small NOM component is responsible for nearly all of the membrane fouling. This paper describes the fabrication and characterization of the new polysulfone adsorbent and offers an hypothesis for the formation of the product via spontaneous emulsification and spinodal decomposition.     

Effects of low-molecular-weight organic ligands and phosphate on DNA adsorption by soil colloids and minerals

Adsorption of DNA on montmorillonite, kaolinite, goethite and soil clays from an Alfisol in the presence of citrate, tartrate and phosphate was studied. A marked decrease in DNA adsorption was observed on montmorillonite and kaolinite with increasing anion concentrations from 0 to 5 mM. However, the amount of DNA adsorbed by montmorillonite and kaolinite was enhanced when ligand concentration was higher than 5 mM. In the system of soil colloids and goethite, with the increase of anion concentrations, a steady decrease was found and the ability of ligands in depressing DNA adsorption followed the sequence: phosphate > citrate > tartrate. Compared to H2O2-treated clays (inorganic clays), a sharp decrease in DNA adsorption was observed on goethite and organo-mineral complexes (organic clays) with increasing ligand concentrations. The results suggest that the influence of anions on DNA adsorption varies with the type and concentration of anion as well as the surface properties of soil components. Introduction of DNA into the system before the addition of ligands had the greatest amount of DNA adsorption on soil colloids and goethite. Organic and inorganic ligands promoted DNA adsorption on montmorillonite and kaolinite when ligands were introduced into the system before the addition of DNA. The results obtained in this study have important implications for the understanding of the persistence and fate of DNA in soil environments especially rhizosphere soil where various organic and inorganic ligands are active.     

Controlling crystallization and its absence: proteins, colloids and patchy models

The ability to control the crystallization behaviour (including its absence) of particles, be they biomolecules such as globular proteins, inorganic colloids, nanoparticles, or metal atoms in an alloy, is of both fundamental and technological importance. Much can be learnt from the exquisite control that biological systems exert over the behaviour of proteins, where protein crystallization and aggregation are generally suppressed, but where in particular instances complex crystalline assemblies can be formed that have a functional purpose. We also explore the insights that can be obtained from computational modelling, focussing on the subtle interplay between the interparticle interactions, the preferred local order and the resulting crystallization kinetics. In particular, we highlight the role played by "frustration", where there is an incompatibility between the preferred local order and the global crystalline order, using examples from atomic glass formers and model anisotropic particles.     

Modeling of oxygen adsorption on silver

Different adsorption forms of oxygen on silver are discussed. Four main types of oxygen forming at different temperatures and oxygen pressures have been distinguished. A kinetic model describing the formation and transformations of the oxygen forms and taking into account the surface amorphization has been proposed. Numerical modeling of stationary concentrations using this model gives evidence for a temperature window ΔT=500–800 K, where a quasimolecular oxygen state (E=530.5 eV, T_des=800–900 K) can exist at high oxygen pressures.     

The role of hydrogen bonding in non-ionic polymer adsorption to cellulose nanocrystals and silica colloids

A piqued interest in nanocellulose has recently arisen due to the growing need to use sustainable and renewable materials in place of those that are derived from petrochemical resources. Although current commercial uses of nanocellulose remain limited, research over the past two decades demonstrates numerous applications including reinforcing agents in polymer and cement composites, coatings, foams, gels, tissue scaffolds, and rheological modifiers, amongst others. Because of the hydrophilic nature of nanocellulose many of the potential uses will likely be in water-based formulations or employ water-based processing methods. Thus understanding the interactions between nanocellulose and water-soluble polymers is critical. Although polyelectrolyte adsorption to cellulose is well understood, adsorption of non-ionic polymers is less clear, with hydrogen bonding often cited as a governing factor. Recent work suggests that in fact hydrogen bonding does not play a significant role in nanocellulose systems, and that non-ionic polymer adsorption is largely entropically driven. Herein we review current literature that investigates non-ionic polymer adsorption to cellulose nanocrystals (CNCs) and draw upon previous papermaking research to better understand the mechanisms involved. Additionally we analyze recent work that compares the adsorption of polyethylene glycol (PEG) to CNCs and fumed silica that provides further insight into this phenomenon. Our findings, along with current literature, suggest that hydrogen bonding does not significantly impact polymer adsorption in aqueous media despite reports to the contrary.     

Structure of adsorption complex: Modeling and experiment

The structures of adsorption complexes formed by SO₂ adsorption on organic layers with various functional groups were calculated using semiempirical methods. A compound possessing high selectivity with respect to SO₂ was chosen on the basis of calculations. The calculated data were confirmed experimentally.     

Phase behavior of colloids and proteins in aqueous suspensions: Theory and computer simulations

The fluid phase behavior of colloidal suspensions with short-range attractive interactions is studied by means of Monte Carlo computer simulations and two theoretical approximations, namely, the discrete perturbation theory and the so-called self-consistent Ornstein-Zernike approximation. The suspensions are modeled as hard-core attractive Yukawa (HCAY) and Asakura-Oosawa (AO) fluids. A detailed comparison of the liquid-vapor phase diagrams obtained through different routes is presented. We confirm Noro-Frenkel's extended law of scaling according to which the properties of a short-ranged fluid at a given temperature and density are independent of the detailed form of the interaction, but just depend on the value of the second virial coefficient. By mapping the HCAY and AO fluids onto an equivalent square-well fluid of appropriate range at the critical point we show that the critical temperature as a function of the effective range is independent of the interaction potential, i.e., all curves fall in a master curve. Our findings are corroborated with recent experimental data for lysozyme proteins.     

Analytic calculation of phase diagrams for solutions containing colloids or globular proteins

Second-order Barker–Henderson perturbation theory gives phase diagrams for colloid and protein solutions that include stable and metastable fluid–fluid, solid–fluid, and solid–solid phases. The potential of mean force is described by a hard-sphere interacting with a Yukawa potential. Calculations for different ranges of attraction show that, as expected, fluid–fluid coexistence becomes metastable when the potential becomes short-ranged. For a very short-ranged Yukawa potential, the phase diagram shows isostructural solid–solid equilibria with a critical point. To test more simplified models, phase diagrams from second-order Barker–Henderson perturbation theory are compared with those from the random-phase approximation for the fluid phase and the van der Waals theory for the solid phase; this comparison shows significantly different phase diagrams. Moreover, with a potential of mean force with primary and secondary minima, calculations using second-order perturbation theory identify conditions where colloidal and protein solutions can present two fluid–fluid regions, each with a critical point; however, the higher-density fluid–fluid region is likely to be metastable. The analytic calculations described here may be useful for interpretation of experimental phase diagrams and for guiding design of separation processes.     

Modeling gas adsorption and transport in small-pore titanium silicates

Engelhard titanium silicate, ETS-4, is a promising new adsorbent for size-selective separation of mixtures of small gases, a leading industrially important example of which is methane-nitrogen separation. Single component equilibrium and kinetics of oxygen, nitrogen, and methane adsorption in Na-ETS-4 and cation-exchanged Sr-ETS-4, measured in an earlier study over a wide range of temperatures and pressures, are analyzed in this study. The adsorbent crystals were synthesized and pelletized under pressure (without any binder), thus giving rise to a bidispersed pore structure with controlling resistance in the micropores. Change in equilibrium and kinetics of adsorption of the aforementioned gases in Sr-ETS-4 due to pore shrinkage with progressively increasing dehydration temperature has also been investigated. Differential uptakes have been measured at various levels of adsorbate loading, which has allowed the elucidation of the nature of concentration dependence of micropore diffusivity. Both homogeneous and heterogeneous models are examined on the equilibrium data, while a bidispersed pore diffusion model is able to capture the differential uptakes very well. On the basis of chemical potential gradient as the driving force for diffusion, the impact of isotherm models on the concentration dependence of micropore diffusivity is also analyzed. It is shown that pore tailoring at the molecular scale by dehydration can improve the kinetic selectivity of nitrogen over methane in Sr-ETS-4 to a promising level. The models investigated are evaluated to identify essential details necessary to reliably simulate a methane-nitrogen separation process using the promising new Sr-ETS-4 adsorbent.     

Influence of adsorption orientation of methyl orange on silver colloids by Raman and fluorescence spectroscopy: pH effect

This work seeks to characterize the UV–Vis absorption, SRES and fluorescence of methyl orange (MO) on silver colloids at different pH. The gradual changes of SERS signals indicate differing adsorption orientations of dyes on silver surface when pH changed and thus, results in different energy transfer efficiencies, i.e., the fluorescence quenching at 428 nm and synchronous enhancement at 560 nm changed gradually with pH increased by step-up. Both experimental evidence and theoretic analysis indicate that, different molecular structures and differing adsorption orientations of dyes on silver surface were existence when pH changed, and thereat caused the diverse spectral phenomena.     

Peptide-based SAMs that resist the adsorption of proteins

In this paper we present a modular approach for the fabrication of surfaces to characterize protein-protein interactions. The approach is based on azido peptides with an optimized sequence which are then thiol-functionalized using an alkynyl thiol and "click" chemistry. From these peptide thiols we fabricated SAMs on gold to evaluate the protein resistance, using surface plasmon resonance spectroscopy, toward streptavidin, bovin serum albumin (BSA), and fibronectin.     

Modeling of binary adsorption on heterogeneous surfaces characterized by a quasi-gaussian adsorption energy distribution

The integral equation (IE) approach coupled with a quasi-Gaussian adsorption energy distribution is used to model the adsorption of single gases and their binary mixture on a heterogeneous solid surface. The adsorbing surface is assumed to be characterized by two, generally different in width, quasi-Gaussian distribution functions, each of them related to a single component of the mixture. The influence of correlations between the distribution functions associated with different components on the corresponding adsorption isotherms and phase diagrams is discussed. In particular, it is demonstrated that a lack of microscopic correlations between the adsorption energies of the components may lead to the formation of an azeotropic mixture. The predictions of the theory are also compared with the results of the grand canonical Monte Carlo (GCMC) simulations carried out for the system studied.