Short-range Steric Repulsion Research Articles

Finite ion size and ion permittivity effects on gel electrophoresis of a soft particle

Electrophoresis of a soft particle embedded in a hydrogel medium is studied by considering the hydrated ions as charged dielectric spheres of finite radius. The Nernst-Planck equation for the transport of ions is modified to take into account the ion steric repulsion and the ion-solvent dielectric interactions (MNP-model). The effective polarizability of the hydrated ions lowers the dielectric permittivity of the ionic solution and consequently, the permittivity of the medium varies with local ionic concentration. This spatial variation of the permittivity creates a Born energy difference in ions leading to a Born force experienced by the ions. In addition, ions experience a dielectrophoretic force due to the polarization under the imposed external electric field. The ion steric repulsion are modeled by the Carnahan-Starling equation of state. The convection of ions are governed by the Brinkman extended Navier-Stokes equations. The modified Nernst-Planck equation coupled with the Brinkman Navier-Stokes equations and the equation for electric field are solved numerically through a control volume approach. The ion steric interactions and ion-solvent interactions create a saturation of mobile ions in the polyelectrolyte layer (PEL) of the soft particle, which leads to an attenuation of the counterion condensation of the PEL. This short range steric repulsion and ion-solvent interactions have significance for higher bulk ionic concentration as well as higher range of charge density of the soft particle. These interactions are elucidated by comparing with the mobility obtained by the standard model (PNP-model). The Dukhin number and the effective charge density of the PEL are determined to illustrate the mobile ion saturation. The discrepancy between the present modified model from the standard model magnifies for the case of salts of multivalent counterions.

Adsorption and interaction forces of commercial Poly(naphthalene sulfonate) (PNS) and Poly(carboxylate ether) (PCE) polyelectrolytes with negatively charged surfaces in monovalent and divalent electrolytes

Adsorption of polyelectrolytes on colloidal particles alters the interaction forces and rheological properties of colloidal suspensions. Poly(naphthalene sulfonate) (PNS) and poly(carboxylate ether) (PCE) polyelectrolytes are commonly added to improve the flowability of hydrated cement paste in low-water conditions. The interactions of commercial PNS and PCE admixtures with negatively charged mica in aqueous electrolytes were directly studied with a surface forces apparatus in this work. In 0.1 M K2SO4, robust adsorption of PNS induced a strong steric repulsion. A slightly thicker adsorption layer formed in low ionic strength 5 mM Ca(NO3)2 exhibiting long-range electrostatic repulsion and short-range steric repulsion. Increasing the Ca(NO3)2 concentration to 0.1 M doubled the adsorption layer thickness and strongly screened any electrostatic repulsion. Contact of the adsorbed PNS layers caused weak physical cohesion under both low and high calcium concentrations presumably due to Ca2+ ion bridging of the opposing PNS layers. These findings indicate that (i) divalent Ca2+ possesses a much stronger ability than monovalent K+ to assemble PNS molecules resulting in significantly thicker adsorption and (ii) the interaction force between PNS adsorption layers in high ionic strength conditions is dominated by steric repulsion. In comparison, PCE did not appreciably adsorb from 0.1 M Ca(NO3)2. This finding challenges the well-accepted divalent ion bridging adsorption mechanism of PCE onto negatively charged surfaces and warrants further study.

Various salts employed as precipitant in combination with polyethylene glycol in protein/detergent particle association

Salt/polyethylene glycol (PEG) mixtures are employed as precipitants for biological macromolecules. The dependence of precipitation curves (PCs) on salt species was investigated for integral membrane protein/detergent particles. By relating this dependence to properties of ions dissociated from added salts, the following roles and effects of various ions were clarified. In the presence of ions whose interaction with water is stronger than water–water interaction, the coordination of solvent molecules is rearranged so as to strengthen short-range steric repulsion and hydrophobic attraction. Ions whose interaction with water is weaker than water–water interaction can be a hindrance to hydrophobic–hydrophobic contact. Moreover, strong electric fields of divalent cations can cause an attractive effect between electronegative or polar groups of neighboring particles. The variations of particle–particle and particle–PEG interactions depending on the state of particles and surrounding solvents were correlative. Due to this, the relationship between the horizontal positions of PC and the species of salts added could be formulated as a binary linear function of cationic and anionic species composing the salts.

Collective behavior of self-propelled rods with quorum sensing

Active agents-like phoretic particles, bacteria, sperm, and cytoskeletal filaments in motility assays-show a large variety of motility-induced collective behaviors, such as aggregation, clustering, and phase separation. The behavior of dense suspensions of engineered phoretic particles and of bacteria during biofilm formation is determined by two qualitatively different physical mechanisms: (i) volume exclusion (short-range steric repulsion) and (ii) quorum sensing (longer-range reduced propulsion due to alteration of the local chemical environment). To systematically characterize such systems, we study semi-penetrable self-propelled rods in two dimensions, with a propulsion force that decreases with increasing local rod density, by employing Brownian dynamics simulations. Volume exclusion and quorum sensing both lead to phase separation; however, the structure of the systems and the rod dynamics vastly differ. Quorum sensing enhances the polarity of the clusters, induces perpendicularity of rods at the cluster borders, and enhances cluster formation. For systems where the rods essentially become passive at high densities, formation of asters and stripes is observed. Systems of rods with larger aspect ratios show more ordered structures compared to those with smaller aspect ratios, due to their stronger alignment, with almost circular asters for strongly density-dependent propulsion force. With increasing range of the quorum-sensing interaction, the local density decreases, asters become less stable, and polar hedgehog clusters and clusters with domains appear.

Direct measurement of interaction forces between bovine serum albumin and poly(ethylene oxide) in water and electrolyte solutions.

The net interaction between a probe tip coated with bovine serum albumin (BSA) protein and a flat substrate coated with poly(ethylene oxide) (PEO) polymer was measured directly on approach in water and electrolyte solutions using AFM. The approach force curve between the two surfaces was monotonically repulsive in water and in electrolyte solutions. At pH ~5, slightly above the isoelectric point (pI) of BSA, and at large distances, the force was dominated by electrostatic repulsion between the oxygen atoms of the incoming protein with those belonging to the ether groups of PEO. Such repulsive force and range decreased in NaCl. Under physiological conditions, pH 6, BSA is definitely charged and the electrostatic repulsion with ether groups in PEO appears at larger separation distances. Interestingly, at pH 4, below the pI of BSA, the repulsion decreased because of an attractive, although weak, electrostatic force that appeared between the ether groups in PEO and the positively charged amino groups of BSA. However, for all solution conditions, once compression of PEO begun, the net repulsion was always dominated by short-range polymeric steric repulsion and repulsive enthalpy penalties for breaking PEO-water bonds. Results suggest that PEO in mushroom conformation may also be effective in reducing biofouling.

Effects of Surfactants and Polyelectrolytes on the Interaction between a Negatively Charged Surface and a Hydrophobic Polymer Surface.

We have measured and characterized how three classes of surface-active molecules self-assemble at, and modulate the interfacial forces between, a negatively charged mica surface and a hydrophobic end-grafted polydimethylsiloxane (PDMS) polymer surface in solution. We provide a broad overview of how chemical and structural properties of surfactant molecules result in different self-assembled structures at polymer and mineral surfaces, by studying three characteristic surfactants: (1) an anionic aliphatic surfactant, sodium dodecyl sulfate (SDS), (2) a cationic aliphatic surfactant, myristyltrimethylammonium bromide (MTAB), and (3) a silicone polyelectrolyte with a long-chain PDMS midblock and multiple cationic end groups. Through surface forces apparatus measurements, we show that the separate addition of three surfactants can result in interaction energies ranging from fully attractive to fully repulsive. Specifically, SDS adsorbs at the PDMS surface as a monolayer and modifies the monotonic electrostatic repulsion to a mica surface. MTAB adsorbs at both the PDMS (as a monolayer) and the mica surface (as a monolayer or bilayer), resulting in concentration-dependent interactions, including a long-range electrostatic repulsion, a short-range steric hydration repulsion, and a short-range hydrophobic attraction. The cationic polyelectrolyte adsorbs as a monolayer on the PDMS and causes a long-range electrostatic attraction to mica, which can be modulated to a monotonic repulsion upon further addition of SDS. Therefore, through judicious selection of surfactants, we show how to modify the magnitude and sign of the interaction energy at different separation distances between hydrophobic and hydrophilic surfaces, which govern the static and kinetic stability of colloidal dispersions. Additionally, we demonstrate how the charge density of silicone polyelectrolytes modifies both their self-assembly at polymer interfaces and the robust adhesion of thin PDMS films to target surfaces.

Interaction forces between DPPC bilayers on glass.

The surface force apparatus (SFA) was utilized to obtain force-distance profiles between silica-supported membranes formed by Langmuir-Blodgett deposition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). In the absence of a membrane, a long-range electrostatic repulsion and short-range steric repulsion are measured as a result of the deprotonation of silica in water and the roughness of the silica film. The electrostatic repulsion is partially screened by the lipid membrane, and a van der Waals adhesion comparable to that measured with well-packed DPPC membranes on mica is measured. This finding suggest that electrostatic interactions due to the underlying negatively charged silica are likely present in other systems of glass-supported membranes. In contrast, the charge of an underlying mica substrate is almost completely screened when a lipid membrane is deposited on the mica. The difference in the two systems is attributed to the stronger physisorption of zwitterionic lipids to molecularly smooth mica compared to physisorption to rougher silica.

Influence of Wetting and Dispersing Agents on the Interaction between Talc and Hydrophobic Particles

The interactions between a natural talc surface and a model hydrophobic particle have been investigated in aqueous solutions by employing the atomic force microscopy (AFM) colloidal probe technique. The results demonstrate the presence of long-range attractive forces due to bridging via preadsorbed or induced bubbles/cavities. Due to the natural heterogeneity of talc, and the stochastic nature of the bubble bridging process, the variability in the range and magnitude of the attraction is larger than that for cases when other interactions predominate or than that when only model surfaces are used. Addition of poly(acrylic acid), a common dispersing agent, did not affect the measured forces. Thus, we conclude that poly(acrylic acid) does not adsorb to the basal plane of talc. In sharp contrast, addition of Pluronic PE6400, a nonionic triblock polymer used as wetting agent, resulted in complete removal of the bubble-induced attractive force. Instead, a short-range steric repulsion is the dominating feature. Clearly, Pluronic PE6400 is able to displace air bubbles from the surface and prevent their formation when the particles come into contact. These are suggested to be important features of efficient wetting agents.

Study of the Coalescence of Acoustic Bubbles as a Function of Frequency, Power, and Water-Soluble Additives

The effect that surface-active solutes, such as aliphatic alcohols and sodium dodecyl sulfate (SDS), have on the extent of bubble coalescence in liquids under different sonication conditions has been investigated by measuring the volume change of the solution following a period of sonication. In general, the adsorption of surface-active solutes onto the bubble surface retards bubble coalescence. Within the limitations of the measurement method and the systems studied, bubble coalescence does not appear to be dependent on the applied acoustic power. Also, varying the applied acoustic frequency has a minimal effect on the extent of bubble coalescence in systems where long-range electrostatic repulsion between bubbles, imparted by the adsorbed surface-active solutes, dominates. However, when short-range steric repulsion (or other short-range repulsive forces) is the primary factor in inhibiting bubble coalescence, the dependence on the applied acoustic frequency becomes apparent, with less coalescence inhibition at higher frequencies. It is also concluded that SDS does not reach an equilibrium adsorption level at the bubble/solution interface under the sonication conditions used. On the basis of this conclusion, a method is proposed for estimating nonequilibrium surface excess values for solutes that do not fully equilibrate with the bubble/solution interface during sonication. For the case of SDS in the presence of excess NaCl, the method was further employed to estimate the maximum lifetime of bubbles in a multibubble field. It was concluded that an acoustic bubble in a multibubble field has a finite lifetime, and that this lifetime decreases with increasing applied frequency, ranging from up to 0.35 +/- 0.05 ms for 213 kHz to 0.10 +/- 0.05 ms for 1062 kHz. These estimated lifetimes equate to a bubble in a multibubble field undergoing an upper limit of 50-200 oscillations over its lifetime for applied ultrasound frequencies between 200 kHz and 1 MHz.

Structure of a New Type of Transient Network: Entangled Wormlike Micelles Bridged by Telechelic Polymers

The thermodynamics and structural behavior of a new type of transient network are reported. The network is obtained by adding in a solution of entangled surfactant wormlike micelles a telechelic triblock copolymer whose hydrophobic ends anchor into the micelles and whose hydrophilic tails are swollen in the aqueous solvent and reversibly link the entangled cylindrical micelles. For comparison, we have also studied the same surfactant system decorated with an amphiphilic diblock copolymer which corresponds exactly to a triblock telechelic copolymer cut into two identical diblock copolymers. We find that the addition of telechelic polymers induces an effective attraction of entropic origin between the surfactant that may result, in equilibrium, in the coexistence of a dilute phase and a connected network, as predicted by theory. Small-angle neutron scattering experiments show first that the locally cylindrical structure of the micelles is maintained upon copolymer addition over a wide range of copolymer-to-surfactant molar ratio. On the other hand, the addition of telechelic polymer correlates with (i) the emergence of a broad peak in the structure factor at a finite scattering vector, q, due to the short-range steric repulsion induced by the polymer between the cylinders, and (ii) the large rise of the scattered intensity at low q's (for small enough surfactant concentration), which indicates an increase of the osmotic compressibility of the solution due to the effective attractive interaction between cylinders provoked by the telechelic polymers linking them.

Measurement of Interaction Forces between Lignin and Cellulose as a Function of Aqueous Electrolyte Solution Conditions

The interaction between a lignin film and a cellulose sphere has been measured using the colloidal probe force technique as a function of aqueous electrolyte solution conditions. The lignin film was first studied for its roughness and stability using atomic force microscopy imaging and quartz crystal microbalance measurements, respectively. The film was found to be smooth and stable in the pH range of 3.5-9 and in ionic strengths up to and including 0.01 M. This range of ionic strength and pH was hence used to measure the surface force profiles between lignin and cellulose. Under these solution conditions, the measured forces behaved according to DLVO theory. The force-distance curves could be fitted between the limits of constant charge and constant potential, and the surface potential of the lignin films was determined as a function of pH. At a pH greater than 9.5, a short range steric repulsion was observed, indicating that the film was swelling to a large extent but did not dissolve. Thus, lignin films prepared in this manner are suitable for a range of surface force studies.

Monolayer to bilayer transition in a dipolar system

We study the transition from a one-dimensional magnetic dipolar monolayer to a bilayer as it is compressed beyond the close-packed condition. The pressure in a close-packed monolayer is found to be nearly independent of the number of dipoles. In the case of weak dipolar interactions, our experimental results indicate that the bilayer formation is governed by short-range steric and electrostatic repulsion, whereas for strong dipolar interactions the bilayer formation is governed by long-range dipolar repulsion.

Analysis of the action mechanism of polymer dispersant on dense ethanol alumina suspension using colloidal probe AFM

The action mechanism of a polymer dispersant on dense Al 2O 3 ethanol suspension was investigated using a colloidal probe AFM and branched and linear polyethyleneimines (PEIs). To obtain the minimum viscosity and Newtonian flow property of the dense ethanol suspension, the optimum molecular weights of the branched PEIs were determined over the range from 10,000 to 70,000. The linear PEI with Mw 1400 did not reduce the suspension viscosity compared to the branched PEI with the same molecular weight. The amount of adsorbed PEI did not significantly change regardless of the molecular structure and weight of the PEIs. However, the surface interaction between α-Al 2O 3 solids depended on the molecular structure and weight of the PEIs. The branched chain of the PEI adsorbed on the Al 2O 3 surface facilitated the short-range steric repulsion between particles. Based on the results, the increase in steric repulsive force and the disappearance of the adhesion force by the adsorption of the polymer prompted the dispersion of aggregates in suspension and reduced the viscosity of ethanol dense suspension.

Disjoining Pressure Isotherms of Water-in-Bitumen Emulsion Films

In the oil sands industry, undesirable water-in-oil emulsions are often formed during the bitumen recovery process where water is used to liberate bitumen from sand grains. Nearly all of the water is removed except for a small percentage (∼1 to 2%), which remains in the solvent-diluted bitumen as micrometer-sized droplets. Knowledge of the colloidal forces that stabilized these water droplets would help to increase our understanding of how these emulsions are stabilized. In this study, the thin liquid film-pressure balance technique has been used to measure isotherms of disjoining pressure in water/toluene-diluted bitumen/water films at five different toluene–bitumen mass ratios. Even though a broad range of mass ratios was studied, only two isotherms are obtained, indicating a possible change in the molecular orientation of surfactant molecules at the bitumen/water interfaces. At low toluene–bitumen mass ratios, the film stability appears to be due to a strong, short-range steric repulsion created by a surfactant bilayer. Similar isotherms were obtained for water/toluene-diluted asphaltene/water films, indicating that the surface active material at the interface probably originated from the asphaltene fraction of the bitumen. However, unlike the bitumen films, films of toluene-diluted asphaltenes often formed very rigid interfaces similar to the “protective skin” described by other researcher.

Polyelectrolyte Brush Layers Studied by Surface Forces Measurement: Dependence on pH and Salt Concentrations and Scaling

Interactions between opposed brush layers of polyelectrolytes ionized poly(l-glutamic acid) (PLGA) or poly(l-lysine) (PLL) in water were directly investigated using surface forces measurement. The brush layers were prepared by the Langmuir−Blodgett deposition of amphiphiles bearing PLGA (degree of polymerization n = 21, 44, and 48) or PLL (n = 41 and 52) as hydrophilic groups. The density of polyelectrolyte chains was around 0.4 chain/nm2. Surface force profiles, consisting of a long-range electrical double layer repulsion and a short-range steric repulsion, were measured and analyzed with varying pH, salt concentration, and polyelectrolyte chain length. The surface potential obtained from the double layer repulsion indicated that nearly all of the ionized groups in the brush layers were neutralized by counterions. In the case of PLGA, the steric repulsion of the brush layer increased with increasing pH from 9 to 10, reflecting the increase in the ionization degree of the carboxylic groups, and then decre...

Analysis of Electrosteric Interaction of Polymer Dispersant in Dense Alumina Suspensions with Different Counter-Ion Densities Using an Atomic Force Microscope.

The mechanism of anionic polymer dispersant acting on dense alumina suspension behavior was analyzed under different solid-volume fractions (ranging from 40 to 55vol%) and counter-ion densities by comparing microscopic solid/liquid interface interaction to suspension viscosity. The solid/liquid interface interaction and adsorbed structure of polymer dispersant on particles were determined by an atomic force microscope (AFM). The dependency of counter-ion density on the suspension viscosity appeared at a relatively low solidvolume fraction (40vol%). However, at a high solid-volume fraction (50vol%), the suspension viscosity was independent of the counter-ion density. To analyze these phenomena, the force curves between a polished surface of alumina and the tip of AFM were measured in solution with polymer dispersant and different counter-ion densities. The increase of counter-ion density reduced the long-range overlapping repulsive force of an electrical double layer ranging over 10nm in surface distance. A short-range steric repulsion force ranging below about 5nm in surface distance slightly depended on counter-ion density. The estimated mean surface distance between particles in dense suspension decreased from about 8 to 4nm with increasing the solid fraction from 40 to 50vol%. Based on both results, it is concluded that the main mechanism for the aggregation/dispersion of alumina powder in dense suspension changed from electrostatic repulsive force to steric force with increasing the solid-volume fraction in suspension.

Density-Dependent Jump in Compressibility of Polyelectrolyte Brush Layers Revealed by Surface Forces Measurement

Interactions between apposed brush layers of polyelectrolytes ionized poly(l-glutamic acid) (polymerization degree n = 21, 44, 48) were investigated in water at pH 10 by the direct surface forces measurement. Brush layers were prepared by the Langmuir−Blodgett (LB) deposition of amphiphiles bearing a poly(l-glutamic acid) chain and two octadecyl groups. The obtained surface force and stress profiles consisted of a long-range electrostatic repulsion and a short-range steric repulsion. The distance where the steric repulsion appeared was in good agreement with twice the length of the polyelectrolyte chains in the extended form. The stress profiles of the polyelectrolyte brushes at n = 21, 44, and 48 produced the identical curve when the distance was scaled by the length corresponding to twice the thickness of an undeformed polyelectrolyte layer. Interactions between the layers of a poly(l-glutamic acid) (n = 48) were also studied as a function of the polyelectrolyte chain density in the brush layers. The density was varied by mixing the poly(l-glutamic acid) amphiphile with dioctadecylphosphoric acid. The sudden increase in the short-range repulsion attributed to the steric component was found at the critical density of 0.20 ± 0.07 chain/nm2 with decreasing chain density in the brush layers, indicating the existence of the transition in the interaction mode of polyelectrolytes. The steric repulsion was quantitatively analyzed to provide the elastic compressibility modulus of the polyelectrolyte brushes. The obtained modulus of 0.6 ± 0.1 pN/chain was in the high-density region, and 4.4 ± 0.7 pN/chain was in the low-density region. The transition in the counterion binding to polyelectrolytes may account for the density-dependent change in the interactions of the polyelectrolyte chains.

A Surface Force, Light Scattering, and Osmotic Pressure Study of Semidilute Aqueous Solutions of Ethyl(hydroxyethyl)celluloseLong-Range Attractive Force between Two Polymer-Coated Surfaces

Semidilute (above the overlap concentration, c*) aqueous solutions of ethyl(hydroxyethyl)cellulose (EHEC) as a function of concentration were investigated with the surface force apparatus (SFA), and by static light scattering (SLS) and osmotic pressure measurements. The anomalous excess in scattering intensity observed in SLS experiments at small angles indicates concentration inhomogeneities in the samples, probably as a result of association of the EHEC chains. The light scattering and osmotic pressure results show that EHEC in aqueous soluttion does not behave as expected for a polymer in a good solvent. The results from the SFA experiments show that EHEC adsorbs strongly on mica. These adsorbed layers give rise to a short-range steric repulsion when the two EHEC-coated mica surfaces are brought into close contact. The most remarkable result, however, is the very long-range attractive force observed at larger surface separations. This attractive force features oscillations with a periodicity that decreases when the polymer concentration increases. The periodicity corresponded to the mesh network size determined by SLS. To our knowledge, this type of long-range attraction has not previously been reported. The origin of the force could not be explained in terms of bridging or classical depletion effects. Rather, this long-range attraction might be induced by segregation of different polymer fractions, with different surface activities, present in the EHEC solutions.

Electrokinetic and Direct Force Measurements between Silica and Mica Surfaces in Dilute Electrolyte Solutions

An atomic force microscope has been used to study the forces between a silica sphere in the colloidal size range and silica or mica flat surfaces as a function of distance of separation. At low ionic strength, independent electrokinetic measurements (ζ potentials) of both the spheres (by electrophoresis) and flat surfaces (by streaming potential) under the same conditions show excellent agreement with the diffuse double layer potentials derived from the force data using conventional DLVO theory. At higher ionic strength, the electrokinetically derived potentials were found to deviate from those derived from the fitted atomic force microscopy data, and a short range steric type repulsion was observed between the surfaces, the magnitude of which increased with decreasing pH.

Steric forces between brush layers of poly( l-glutamic acid) and their dependence on secondary structures as determined by FT-IR spectroscopy

Interactions between apposed poly( l-glutamic acid) monolayers in water were investigated by direct surface forces measurement. The average degree of polymerization was 21. The monolayers of anchored poly( l-glutamic acid) were prepared on mica surfaces by the Langmuir-Blodgett deposition of monolayers of an amphiphile containing the polypeptide as the hydrophilic head group. Various secondary structures of poly( l-glutamic acid) were formed in this monolayer at the air-water interface, through intra- and intermolecular hydrogen bonding, depending on the surface pressure and the pH value of the aqueous subphase, and transferred onto mica surfaces. Formation of the secondary structures was characterized by Fourier transform-infrared (FT-IR) spectroscopy prior to the forces measurement. Surface force profiles consisted of a long-range electrostatic and a short-range steric repulsion. Secondary structures of poly( l-glutamic acid), identified by FT-IR with respect to their composition and orientation, were found to determine the short-range steric repulsion. The obtained elastic compressibility moduli of the polypeptide monolayers were 5.6 ± 2.2 MPa for the β 2-structure rich layer, and 0.2 ± 0.1 MPa for the layer of ionized extended chains.