Solvation forces in ionic and neutral liquids: A density functional approach

The solvation forces between two planar charged surfaces in ionic solutions, corresponding to charged and neutral hard spheres representing the ions and the solvent, respectively, are studied here using a weighted density functional theory for inhomogeneous Coulomb systems developed by us recently. The hard sphere contributions to the one-particle correlation function are evaluated nonperturbatively using a position-dependent effective density, while the electrical contributions are obtained through a perturbative expansion around this weighted density. The calculated results on the solvation forces between two charged hard walls compare well with available simulation results for ionic systems. For a neutral system, the present results show good agreement with the experimentally observed oscillating forces for two mica surfaces in octamethylcyclotetrasiloxane. The present approach thus provides a direct route to the calculation of interaction energies between colloidal particles.     

Nonadditive interactions and the relative stability of neutral and anionic silver clusters

We discuss the physical nature of nonadditivity in many-particle systems and the methods of calculations of nonadditive contributions to the interaction energy. For neutral clusters, a closed recurrence formula which expresses the energy of m-body interactions through the energies of 2-, 3-, and (m – 1)-body ones is obtained. The general approach for calculation of the nonadditive contribution in the interaction energy of charged systems is developed. The comparative calculation of anionic and neutral silver clusters shows that the geometry of the most stable anionic clusters is established mainly by the additive forces. The stability of neutral silver clusters is determined by the competition of attractive additive forces and repulsive nonadditive ones. © 1995 John Wiley & Sons, Inc.     

Friction and normal interaction forces between irreversibly attached weakly charged polymer brushes

Polyelectrolyte brushes were built on mica by anchoring polystyrene-poly(acrylic acid) (PS-b-PAA) diblock copolymers at a controlled surface density in a polystyrene monolayer covalently attached to OH-activated mica surfaces. Compared to physisorbed polymer brushes, these irreversibly attached charged brushes allow the polymer grafting density to remain constant upon changes in environmental conditions (e.g., pH, salt concentration, compression, and shear). The normal interaction and friction forces as a function of surface separation distance and at different concentrations of added salt (NaCl) were investigated using a surface forces apparatus. The interaction force profiles were completely reversible both on loading and receding and were purely repulsive. For a constant polymer grafting density, the influence of the polyelectrolyte charges and the Debye screening effect on the overall interaction forces was investigated. The experimental interaction force profiles agree very well with scaling models developed for neutral and charged polymer brushes. The variation of the friction force between two PAA brushes in motion with respect to each other as a function of surface separation distance appeared to be similar to that observed with neutral brushes. This similarity suggests that the increase in friction is associated with an increase in mutual interpenetration upon compression as observed with neutral polymers. The effect of the PAA charges and added ions was more significant on the repulsive normal forces than on the friction forces. The reversible characteristics of the normal force profiles and friction measurements confirmed the strong attachment of the PAA brushes to the mica substrate. High friction coefficients (ca 0.3) were measured at relatively high pressures (40 atm) with no surface damage or polymer removal.     

Hydration forces underlie the exclusion of salts and of neutral polar solutes from hydroxypropylcellulose

The distance dependence for the preferential exclusion of several salts and neutral solutes from hydroxypropyl cellulose (HPC) has been measured via the effect of these small molecules on the thermodynamic forces between HPC polymers in ordered arrays. The concentration of salts and neutral solutes decreases exponentially as the spacing between apposing nonpolar HPC surfaces decreases. For all solutes, the spatial decay lengths of this exclusion are remarkably similar to those observed between many macromolecules at close spacings where intermolecular forces have been ascribed to the energetics of water structuring. Exclusion magnitudes depend strongly on the nature and size of the particular salt or solute; for the three potassium salts studied, exclusion follows the anionic Hofmeister series. The change in the number of excess waters associated with HPC polymers is independent of solute concentration suggesting that the dominating interactions are between solutes and the hydrated polymer. These findings further confirm the importance of solvation interactions and reveal an unexpected unity of Hofmeister effects, preferential hydration, and hydration forces.     

Explanation of Ionic Sequences in Various Phenomena. IV. General Method for Determination of the Type of Bonding in Polymers; Bonds in Ovalbumin,Ribonuclease, Collagen,Hemoglobin, and Starch Determined

Abstract

The salting-out or -in of hydrocarbons, uncharged nitrogen bases, and acids containing polarized hydroxyl groups give different cationic sequences. Also various ionic groups have different solubility sequences. Such neutral or electrostatically charged groups are attached to polymers and are involved as forces which hold together aggregates or helicies in inter- or intramolecular interactions. In aqueous solutions, the addition of various salts can destroy or produce these inter- or intramolecular interactions. Consequently, by applying the ionic solubility sequences, the observed salting-in and -out sequences of low molecular weight molecules, and other solubility criteria to the destruction or formation of such polymer interactions, it is possible to determine the major force or forces which are involved in holding the aggregate or helix together. Such forces involve hydrophobic bonds, hydrogen bonds between neutral bases, hydrogen bonds involving polarized hydroxyl groups, ionic bonds, and/or repulsive forces between electrostatically charged groups. The method is applied to the salting-out of ovalbumin, the destruction of the α-helix of ribonuclease, the collagen-gelatin transformation, the dissociation of hemoglobin, and the retrogradation and solubilization of starch. These five examples involve all the secondary forces considered. The method should find wide applicability because it can be applied to any method of physical measurement involving ionic solutions.     

On the stability of the polymer brushes formed by adsorption of ionomer complexes on hydrophilic and hydrophobic surfaces

We have studied the effect of normal forces and shear forces on the stability and functionality of a polymer brush layer formed upon adsorption of polymeric micelles on hydrophilic and hydrophobic surfaces. The micelles consist of oppositely charged polyelectrolyte blocks (poly(acrylic acid) and poly(N-methyl 2-vinyl pyridinium iodide), and a neutral block (poly(vinyl alcohol)) or neutral grafts (poly(ethylene oxide)). The strength of the attachment of the micellar layers to various substrates was evaluated with Atomic Force Microscopy. Flow cell experiments allowed for the evaluation of long-term stability of coatings in lateral flow. Fixed angle optical reflectometry was used to quantify protein (BSA) adsorption on the micellar layers after their exposure to flow. The results show that adsorbed micellar layers are relatively weakly attached to hydrophobic surfaces and much stronger to hydrophilic surfaces, which has a significant impact on their stability. Adsorbed layers maintain their ability to suppress protein adsorption on hydrophilic surfaces but not on hydrophobic surfaces. Due to the relatively weak attachment to hydrophobic surfaces the structure of adsorbed layers may easily be disrupted by lateral forces, such that the complex coacervate-brush structure no longer exists.     

Polyion Covalency: Exotic Species from the Unexplored World of Electrostatically Shielded Molecular Ion Chemistry

Standard quantum chemical methods have been employed to describe a variety of kinetically stable polyionic molecular species that are trapped in appreciable potential wells by chemical bonding forces, despite powerful electrostatic opposition that challenges conventional chemical detection and characterization. The studied species are covalent or dative analogs of “anti‐electrostatic” hydrogen‐bonded (AEHB) species, all illustrating how short‐range quantum covalency can overcome the powerful “shielding” opposition of long‐range electrostatic forces to form highly charged molecular species, analogous to known neutral or singly ionic counterparts. Computational predictions of representative structural, spectroscopic, and NBO‐based electronic signatures of multiply charged analogs of common neutral species (CH₃CH₃, CO₂, FeCO) are provided to suggest the unique material properties characteristic of this shielded domain of polyionic chemical phenomena.     

Dimeric states of rhodamine B

Aggregation of the coloured neutral and cationic forms of rhodamine β in aqueous solution has been studied. The formation constants and absorption of the dimers were determined. The geometric disposition of monomers in both aggregates and the nature of the association forces are discussed. Trimers are not formed.     

Shear of a planar polyelectrolyte brush

The behavior of a planar polyelectrolyte brush subjected to normal and tangential external forces is considered. A new “polyelectrolyte effect” is predicted: shear of a free polyelectrolyte brush leads to a decrease in brush thickness contrary to the case of a free neutral brush. Such behavior is equivalent to that of a neutral brush subjected to an external normal stretching force. In the case of a polyelectrolyte brush this force is created by the osmotic pressure of mobile counterions neutralizing grafted chain charges. Addition of salt diminishes the polyelectrolyte effect and changes the sign of correlation between brush thickness and tangential deformation.     

Attraction or Repulsion? London Dispersion Forces Control Azobenzene Switches

Large substituents are commonly seen as entirely repulsive through steric hindrance. Such groups have additional attractive effects arising from weak London dispersion forces between the neutral atoms. Steric interactions are recognized to have a strong influence on isomerization processes, such as in azobenzene‐based molecular switches. Textbooks indicate that steric hindrance destabilizes the Z isomers. Herein, we demonstrate that increasing the bulkiness of electronically equal substituents in the meta‐position decreases the thermal reaction rates from the Z to the E isomers. DFT computations revealed that attractive dispersion forces essentially lower the energy of the Z isomers.     

Observation of the orbiting backward peak in the differential cross section for Na(2P)-Hg collisions

A typical feature of collisions governed by attractive and repulsive forces is the orbiting phenomenon. Most clearly these collisions are visible as a backward peak in the differential cross section. In this work we report the first observation of this backward peak in the interaction of neutral atoms, specifically for Na(^2P interacting with Hg.     

Direct measurement of interactions between stimulation-responsive drug delivery vehicles and artificial mucin layers by colloid probe atomic force microscopy

A novel thermo- and pH-sensitive nanogel particle, which is a core-shell structured particle with a poly(N-isopropylacrylamide) (p(NIPAAm)) hydrogel core and a poly(ethylene glycol) monomethacrylate grafted poly(methacrylic acid) (p(MMA-g-EG)) shell, is of interest as a vehicle for the controlled release of peptide drugs. The interactions between such nanogel particles and artificial mucin layers during both approach and separation were successfully measured by using colloid probe atomic force microscopy (AFM) under various compression forces, scan velocities, and pH values. While the magnitudes of the compression forces and scan velocities did not affect the interactions during the approach process, the adhesive force during the separation process increased with these parameters. The pH values significantly influenced the interactions between the nanogel particles and a mucin layer. A large steric repulsive force and a long-range adhesive force were measured at neutral pH due to the swollen p(MMA-g-EG) shell. On the other hand, at low pH values, the steric repulsive force disappeared and a short-range adhesive force was detected, which resulted from the collapse of the shell layer. The nanogel particles possessed a pH response that was sufficient to protect the incorporated peptide drug under the harsh acidic conditions in the stomach and to effectively adhere to the mucin layer of the small intestine, where the pH is neutral. The relationships among the nanogel particle-mucin layer interactions, pH conditions, scan velocities, and compression forces were systemically investigated and discussed.     

Scintigraphic Measurement of Liquid Holdup in Foam Fractionation Columns

Alumina-supported copper and copper-manganese oxide catalysts as well as the parent formates were characterized by means of FTIR spectroscopy in situ and nitrogen physisorption. The IR spectroscopic results are discussed on the basis of the deposition scheme proposed recently by Kapteijn et al. (J. Catal. 150, 94 (1994). The infrared bands in the high-frequency region (OH stretches) indicate that the aqueous copper species formed in the solutions are deposited on the basic and neutral surface OH groups, while as our previous studies show the aqueous manganese species are deposited on the basic and acidic OH groups. However, the aqueous manganese species are deposited on the basic and neutral OH groups in the presence of copper ions (pH 4.6-4.7). The deposition of the aqueous metal species on the protonated basic OH groups occurs as a result of the "ion-pairing" process. The driving forces resulting in the deposition of the aqueous metal species on the neutral OH groups are considered to be hydrogen bonds. The deposition of the formate ions is also discussed. The higher uptake of copper as compared to manganese is discussed in terms of the metal ion properties (electronic configuration, the ability to form hydrogen bonds of different strength, metal species-support interaction). The nitrogen physisorption shows that the initial mesoporous character of gamma-Al(2)O(3) structure does not change during impregnation. The r(FHH) values which characterized the adsorbent-adsorbate interaction forces are calculated. The comparative analysis of the pore size distribution curves of the oxide-supported samples and their parent formates proves to be a useful tool to elucidate the metal species-support interaction strength. Copyright 2000 Academic Press.     

表面活性剂中DNA构象变化的研究

以荧光探针法研究了表面活性剂与小牛胸腺DNA的相互作用，结果表明：阳离子表面活性剂主要通过静电引力和疏水方式与DNA作用；阴离子表面活性剂与DNA之间存在静电排斥力，两者之间的相互作用不太明显；而非离子表面活性剂与DNA的相互作用类似于有机溶剂对DNA的影响，即通过溶液的极性、粘度和介电常数来影响DNA的构象，表面活性剂使得DNA构象发生较大的变化，预示了它可能使DNA的生物功能发生较大的变化。     

Polyelectrolyte brushes

The simple scaling theory of weakly-charged polyelectrolyte brush (the layer of polyelectrolyte chains grafted at one end onto an impermeable surface) immersed into a good solvent has been developed.The asymptotic scaling dependences of the free layer thickness on charge density and solvent strength are obtained. The behavior of polyelectrolyte brush subjected to normal and tangential external forces is considered. New “polyelectrolyte effect” is predicted: shear of a free polyelectrolyte brush leads to a decrease in brush thickness in contrast to the case of a free neutral brush. Such behavior is equivalent to that of a neutral brush subjected to external normal stretching force. This force in the case of polyelectrolyte brush is created by the osmotic pressure of mobile counterions neutralizing grafted chain charges.     

Syntheses,structures and properties of three neutral bisupporting heteropolyoxometalates

Three neutral polyoxometalates [PVW₁₁O₄₀{M(phen)₂H₂O}₂] · 3H₂O (phen = 1,10′-phenanthroline, M=Co 1, Zn 2, Ni 3) are composed of water and the neutral Keggin-type polyoxometallate covalently linking a pair of transition metal complex fragments on opposite sides. The transition metal lies in the center of a distorted octahedron. Multi hydrogen-bonding interactions between coordinated waters of neutral polyoxometalate molecules and terminal oxygen atoms of Keggin units and between bridging oxygens of Keggin units and lattice water create a two dimensional layer and between the layers there exist van der Waals forces. The framework of the neutral molecule begins to decompose at ca 500°C. Compound 1 exhibits a weak antiferromagnetic interaction in the 2–300 K range.     

The Effect of Surface Charge on Adsorption of a Cationic Polyelectrolyte

The regularities of adsorption of a cationic polyelectrolyte, poly(diallyldimethylammonium chloride), on the surface of fused quartz are studied at different values of solution pH by capillary electrokinetics. It is shown that the polyelectrolyte adsorption on a negatively charged surface depends on the value of the surface charge and increases with its growth. At a low charge value (pH 3.8), the polyelectrolyte adsorption increases the quartz surface charge. The driving forces of the adsorption are both electrostatic interaction and forces of nonelectrostatic nature, probably hydrophobic interactions and a change in entropy due to the displacement of counterions from a double layer. The adsorption of poly(diallyldimethylammonium chloride) on quartz from alkaline and neutral solutions is irreversible, which indicates the key role of the electrostatic interaction. At low values of the surface charge, the nonelectrostatic interactions play the main role, thereby resulting in polyelectrolyte desorption.     

Two-fluid model for the simultaneous flow of colloids and fluids in porous media

To describe the velocities of particles such as ions, protein molecules and colloids dispersed or dissolved in a fluid, it is important to also describe the forces acting on the fluid, including pressure gradients and friction of the fluid with the particles and with the porous media through which the fluid flows. To account for this problem, the use of a two-fluid model is described, familiar in the field of fluid mechanics, extended to include osmotic effects. We show how familiar relationships follow in various situations and give examples of combined fluid/particle transport in neutral and charged membranes driven by a combination of electrostatic, diffusional and pressure forces. The analysis shows how the same modeling framework can be generally used both for multidimensional electrokinetic flow through macroscopic channels and around macroscopic objects, as well as for mean-field modeling of transport through porous media such as gels and membranes.     

PAA/PEO comb polymer effects on rheological properties and inter-particle forces in aqueous silica suspensions

The effects of a poly(acrylic acid) (PAA)-poly(ethylene) (PEO) comb polymer dispersant on the rheological properties and inter-particle forces in aqueous silica suspensions have been studied under varying pH conditions. The comb polymer was found to adsorb more strongly under acidic than basic conditions, indicating that the PAA backbone of the copolymer preferentially adsorbs onto silica surfaces with the PEO "teeth" extending out from the surface into the solution. In the presence of low concentrations of copolymer, the silica suspensions were stable due to electrostatic repulsions between the silica surfaces. At higher copolymer concentrations and under neutral and basic conditions, where the copolymer interacted only weakly with silica, the suspensions showed a transition from a dispersed to weakly flocculated state and attractive forces were measured between silica surfaces. Under acidic conditions, the silica dispersion also destabilized at intermediate copolymer adsorbed density and then was re-stabilized at higher adsorbed coverage. The silica suspensions were stable at high copolymer coverage due to steric repulsions between the particles. The destabilization at intermediate coverage is thought to be due to polymer bridging between particles or possibly depletion forces.     

Dielectrophoresis for manipulation of micro/nano particles in microfluidic systems

Dielectrophoretic (DEP) force is exerted when a neutral particle is polarized in a non-uniform electric field, and depends on the dielectric properties of the particle and the suspending medium. The integration of DEP and microfluidic systems offers numerous applications for the separation, trapping, assembling, transportation, and characterization of micro/nano particles. This article reviews the applications of DEP forces in microfluidic systems. It presents the theory of dielectrophoresis, different configurations, and the applications of such systems for particle manipulation and device fabrication.