Silicon Nitride Colloidal Probe Measurements: Interparticle Forces and the Role of Surface-Segment Interactions in Poly(acrylic acid) Adsorption from Aqueous Solution

Direct measurements of forces between silicon nitride surfaces in the presence of poly(acrylic acid) (PAA) are presented. The force-distance curves were obtained at pH > pH_iep with an atomic force microscopy (AFM) colloidal-probe technique using a novel spherical silicon nitride probe attached to the AFM cantilever. We found that PAA adsorbs onto the negatively charged silicon nitride surface, which results in an increased repulsive surface potential. The steric contribution to the interparticle repulsion is small and the layer conformation remains flat even at high surface potentials or high ionic strength. The general features of the stabilization of ceramic powders with PAA are discussed; we suggest that PAA adsorbs onto silicon nitride by sequential adsorption of neighboring segments ("zipping"), which results in a flat conformation. In contrast, the long-range steric force found in the ZrO₂/PAA system at pH > pH_iep arises because the stretched equilibrium bulk conformation of the highly charged polymer is preserved via the formation of strong, irreversible surface-segment bonds on adsorption.     

INTERACTION FORCES MEASURED BETWEEN MICA-ADSORBED QUATERNARIZED POLY(2-VINYLPYRIDINE) LAYERS: EFFECTS OF pH AND COMPRESSION

The surface forces apparatus was used to measure directly the interaction forces between mica-adsorbed quaternarized poly(2-vinylpyridine) (QP2VP) layers as a function of solution pH. The interaction is repulsive at large-surface separations and is dominated by a double layer interaction. At shorter range, electrosteric forces dominate until, at small-surface separations, attractive bridging forces lead to intersurface adhesion. The bridging attractive forces are attributed to both intersurface bridging and polyelectrolyte chain entanglement. These are extremely sensitive to the conformation of the adsorbed polyelectrolyte, which changes significantly with solution pH. Surface forces measured on compression of the adsorbed QP2VP layers do not clearly reflect changes in the adsorbed conformation and surface excess of this polyelectrolyte. Rather, the polyelectrolyte conformation is manifest more dramatically on measurement of adhesion, upon retraction/decompression of the surfaces from contact. There is a strong dependence of the adhesion between the polyelectrolyte layers on the compressive load, time in contact, and compression history since the molecular rearrangements required for chain entanglement and intersurface bridging occur on a long time scale, i.e., of the order of minutes. Under a small compressive load, the surfaces are closer together, facilitating segment-surface andsegment-segment interactions across the two interacting layers.     

Interaction forces measured between mica-adsorbed quaternarized poly(2-vinylpyridine) layers: Effects of pH and compression

The surface forces apparatus was used to measure directly the interaction forces between mica-adsorbed quaternarized poly(2-vinylpyridine) (QP2VP) layers as a function of solution pH. The interaction is repulsive at large-surface separations and is dominated by a double layer interaction. At shorter range, electrosteric forces dominate until, at small-surface separations, attractive bridging forces lead to intersurface adhesion. The bridging attractive forces are attributed to both intersurface bridging and polyelectrolyte chain entanglement. These are extremely sensitive to the conformation of the adsorbed polyelectrolyte, which changes significantly with solution pH. Surface forces measured on compression of the adsorbed QP2VP layers do not clearly reflect changes in the adsorbed conformation and surface excess of this polyelectrolyte. Rather, the polyelectrolyte conformation is manifest more dramatically on measurement of adhesion, upon retraction/decompression of the surfaces from contact. There is a strong dependence of the adhesion between the polyelectrolyte layers on the compressive load, time in contact, and compression history since the molecular rearrangements required for chain entanglement and intersurface bridging occur on a long time scale, i.e., of the order of minutes. Under a small compressive load, the surfaces are closer together, facilitating segment-surface andsegment-segment interactions across the two interacting layers.     

Colloidal Processing of Silicon Nitride with Poly(acrylic acid): II, Rheological Properties

Poly(acrylic acid) (PAA) was the focus of an investigation in the role and behavior of polyelectrolyte dispersants in aqueous-based colloidal processing of silicon nitride. The steady-shear flow properties of dense suspensions were examined as a function of pH, particle volume fraction, chain length, and polymer concentration. Suspensions exhibited shear-thinning behavior, which was analyzed using the constitutive Bingham equation for plastic flow. The data showed that polyelectrolyte charge and configuration were important in the observed flow behavior. In acidic media, PAA behaved similar to a neutral polymer in a poor solvent, whereas, in alkaline media, PAA behaved similar to a rigid asymmetric particle. Although electrostatic forces dominated flow properties in alkaline media, where PAA was predominantly in the free-polymer form, at lower pH values, PAA interactions with the particle surface resulted in a complex array of suspension effects, including polymer bridging, electrosteric stabilization, and flocculation.     

Improvement in adhesive‐free adhesion by the use of electrostatic interactions between polymer chains grafted onto polyethylene plates

The tensile shear adhesive‐free adhesion properties induced by electrostatic interactions between poly(acrylic acid) (PAA) and poly[2‐(dimethylamino)ethyl methacrylate] (PDMAEMA) chains grafted onto polyethylene (PE) with low‐density (LDPE) or high‐density (HDPE) plates were studied. PAA‐ or PDMAEMA‐grafted PE plates were immersed in a HCl or NaOH solution or water for 24 h, and their electrostatic properties were changed before they were overlapped with each other and heat‐pressed. The breaking of the substrate between the two plates with water‐swollen grafted layers was observed in the low range of grafted amounts in comparison with immersion in the acidic and basic solutions. The ability of the two plates with grafted polymer chains swollen in water to strongly bond with each other was a result of electrostatic interactions formed by positively charged PDMAEMA and negatively charged PAA chains. The breaking of the substrate in the case of adhesive‐free adhesion between quaternized PDMAEMA‐grafted and PAA‐grafted PE plates immersed in the basic solution occurred with lower grafted amounts of PAA. This came from the strong attractive force between dissociated anionic PAA chains and quaternized cationic PDMAEMA chains in the basic solution. In addition, the adhesive‐free adhesion strength of HDPE plates with the same grafted polymer chains encountered the breaking of the substrate with lower grafted amounts than that of LDPE plates. It was concluded that the grafting of polymer chains onto HDPE plates with high crystallinity was considerably restricted to the outer surface regions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2632–2638, 2006     

Role of Interfacial Interactions in the Deposition of Colloidal Clay Particles in Porous Media

Colloidal clay particle transport under saturated conditions is believed to be controlled by its interactions with the surrounding environment. The dominating forces among these interactions are electrostatic forces that are determined by colloidal clay particle and porous medium surface charge density and Lifshitz–van der Waals forces that are determined by colloidal clay particle and porous medium surface thermodynamic properties. Electrostatic forces are greatly affected by solution chemistry in terms of solution ionic strength and pH. In this research, electrostatic and Lifshitz–van der Waals forces of natural colloidal clay particles with a model porous medium of silica sand were quantified at different ionic strength and pH conditions. At the same time, colloidal clay particle transport in the model medium of silica sand was conducted in a laboratory column. The maximum electrostatic forces, F ^EL (max), which occurred when the separation distance between colloidal clay particles and the porous medium was in the range of the sum of the double layer thicknesses of the colloidal clay particles and the porous medium, was found to be the determinant factor for colloidal clay particle deposition in the porous medium. Colloidal clay particle desorption in the porous media was related to the net effect of attractive Lifshitz–van der Waals forces and repulsive electrostatic forces, evaluated at the equilibrium distance where physical contact between the colloidal clay particle and silica sand actually occurred (i.e., affix force). Higher colloidal clay particle desorption was found to coincide with smaller affix force values.     

Effects of Ionic Environments on Bovine Serum Albumin Fouling in a Cross‐Flow Ultrafiltration System

The influence of electrostatic interactions on membrane fouling during the separation of bovine serum albumin (BSA) from solution was studied in a cross‐flow ultrafiltration system. Experiments were carried out at different pH values between 3.78 and 7.46; and for different ionic strengths between 0.001 M and 0.1 M. The changes in permeate flux, cake layer resistance, zeta potentials of BSA and polyether sulfone (PES) membranes, and electrostatic interaction energies, were evaluated. At all of the ionic conditions studied, PES membranes are negatively charged. However, BSA molecules are either negatively or positively charged depending on the ionic environment. Whereas the cake layer resistance decreased with increasing pH and ionic strength, the permeate fluxes increased. The calculated electrostatic energy was a minimum at the isoelectric point (IEP) of BSA. However, at this point, the cake resistances corresponding to fouling at each ionic strength, were not minimized. Below the IEP of BSA, the electrostatic forces were attractive, while above the IEP, repulsive electrostatic forces were dominant.     

Comparison between the binding of chlorpheniramine maleate to poly(sodium 4-styrenesulfonate) and the binding to other polyelectrolytes

The interactions of the antihistaminic drug chlorpheniramine maleate (CPM) with the negatively charged polyelectrolytes poly(sodium 4-styrenesulfonate) (PSS) and poly(acrylic acid) (PAA) are studied by the washing method of the diafiltration technique at conditions simulating those of the small intestine such as pH 7.5 and 0.13 M NaCl. The results are compared with those already reported involving other pharmacologically important polyelectrolytes such as alginic acid (ALG), carboxymethylcellulose (CMC), and κ- and ι-carrageenan (κ- and ι-CAR). As in the case of ALG, CMC, and CAR, interactions of CPM with PAA appear to be electrostatic and are cleaved in the presence of 0.13 M NaCl. On the contrary, apart from electrostatic interactions, additional interactions are found with PSS and residual interactions are kept in the presence of 0.13 M NaCl, a fact that may be attributed to π-π interactions and hydrophobic forces. The effect of the addition of 4 M urea, branched poly(ethyleneimine) (BPEI), and poly(vinylpyrrolidone) (PVP) is also studied. The addition of urea 4 M or 0.001 M BPEI produces a decrease on the amounts of counterions bound to PSS at infinite elution, while the addition of PVP does not produce any change on the diafiltration profiles.     

Exploring forces between individual colloidal particles with the atomic force microscope

Forces between individual colloidal particles can be measured with the atomic force microscope (AFM), and this technique permits the study of interactions between surfaces across aqueous solutions in great detail. The most relevant forces are described by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, and they include electrostatic double-layer and van der Waals forces. In symmetric systems, the electrostatic forces are repulsive and depend strongly on the type and concentration of the salts present, while van der Waals forces are always attractive. In asymmetric systems, the electrostatic force can become attractive as well, even when involving neutral surfaces, while in rare situations van der Waals forces can become repulsive too. The enormous sensitivity of the double layer forces on additives present is illustrated with oppositely charged polyelectrolytes, which may induce attractions or repulsions depending on their concentrations.     

Gradient nanoporous phenolics as substrates for high-flux nanofiltration membranes by layer-by-layer assembly of polyelectrolytes

Thin film composite(TFC) membranes represent a highly promising platform for efficient nanofiltration(NF)processes. However, the improvement in permeance is impeded by the substrates with low permeances. Herein,highly permeable gradient phenolic membranes with tight selectivity are used as substrates to prepare TFC membranes with high permeances by the layer-by-layer assembly method. The negatively charged phenolic substrates are alternately assembled with polycation polyethylenimine(PEI) and polyanion poly(acrylic acid)(PAA)as a result of electrostatic interactions, forming thin and compact PEI/PAA layers tightly attached to the substrate surface. Benefiting from the high permeances and tight surface pores of the gradient nanoporous structures of the substrates, the produced PEI/PAA membranes exhibit a permeance up to 506 L? m~(-2)?h~(-1)?MPa~(-1), which is ～2–10 times higher than that of other membranes with similar rejections. The PEI/PAA membranes are capable of retaining N 96.1% of negatively charged dyes following the mechanism of electrostatic repulsion. We demonstrate that the membranes can also separate positively and neutrally charged dyes from water via other mechanisms.This work opens a new avenue for the design and preparation of high-flux NF membranes, which is also applicable to enhance the permeance of other TFC membranes.     

Experimental and MD simulation study on the physical and mechanical properties of organically modified montmorillonite clay and compatibilized linear low density polyethylene nanocomposites

Understanding the interfacial interactions plays a key role in controlling mechanical and physical properties of polymer/clay nanocomposites (PCNs). In this work, the surface interactions between constituents of experimentally prepared PCNs which are the pristine linear low density polyethylene (PE) chains, PE compatibilizers, montmorillonite clay surface layer, and surfactants were studied quantitatively by employing molecular dynamics simulation technique. The interaction energy between the polymer and the clay was found to be inversely proportional with the volume of the surfactant which decreases the electrostatic interactions between the compatibilizer and the hydrophilic clay surface. However, the van der Waals (vdW) interactions between alkyl tails of surfactants and the PE chains increase with the tail length of the surfactants. The most attractive interaction was between the surfactant's head group and the clay surface. We showed that there existed fine balance between the electrostatic and vdW type forces on the stability and the enhanced properties of the PE–organoclay nanocomposites. Calculated interaction energies were then correlated to the experimentally measured mechanical properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45817.     

Adhesion of small particles in electric fields

We discuss experiments performed with an atomic force microscope that has been modified so that an electric field can be applied across a dielectric particle attached to a cantilever. The modified microscope is used to measure the attractive (before contact) and adhesion (after contact) forces for micrometer-sized dielectric particles. From these measurements, the relative contributions of the nonelectrostatic forces and the electrostatic forces to adhesion can be quantified. Surface conductivity is observed to change the particle's attraction to the surface. In large electric fields, the electrostatic forces dominate the particle's adhesion.     

Adhesion forces of charged particles

Adhesion forces of toner and polymer particles to aluminum substrates were measured by the centrifugal, detachment field and microelectrode detachment field methods, and factors affecting the adhesion forces are discussed. The adhesion forces of toner particles increased with an increase in either particle size or particle charge. The adhesion force of an irregularly shaped toner was larger than that of a spherical toner. The mean adhesion force of polymer particles to aluminum substrates decreased with an increase in surface roughness of the substrates. The CF₄ plasma treatment of the polymer particles shifted their adhesion force distribution in a smaller direction. It was confirmed that the results by the centrifugal and the detachment field methods were in good agreement with each other. The contribution of van der Waals, electrostatic and water bridging forces to the adhesion forces of toner particles are also discussed.     

Cellulose ethers and yield stress of cement pastes

We study in this paper the physical and chemical phenomena at the origin of the influence of cellulose ethers on flow onset of cement pastes. We first show that cellulose ether adsorption seems to slow down the nucleation of calcium silicates at the surface of the cement grains in the tens of minutes following mixing. We moreover suggest that the measured collapse of the van der Waals attractive interaction network upon the addition of cellulose ethers finds its origin in repulsive steric forces generated by adsorbed cellulose ether molecules. Finally, we measure the formation of a new interaction network in the system. From dimensional inter-particle force analysis, we suggest that this network finds its origin in the bridging of cement grains by adsorbed ether molecules. Flow onset occurs then through a desorption process, the energy of which can be assessed via adsorption isotherm measurements.     

Polyanion/gelatin complexes as pH‐sensitive gels for controlled protein release

Polyanion/gelatin complexes including poly(methacrylic acid) (PMAA)/gelatin, poly(acrylic acid) (PAA)/gelatin, and heparin/gelatin are investigated as pH‐sensitive gels for controlled protein release. Polyanions can interact with gelatin and form amorphous precipitates within a certain pH range, which is affected by the polyanion nature. The entrapment efficiency of model proteins (myoglobin, cytochrome c, and pepsin) into the complexes is rather high (>80%). By using a modified colloid titration that mixes a solution of gelatin and model proteins titrated with polyanion solution, myoglobin and cytochrome c are found to interact with polyanions by electrostatic forces at low pH, while pepsin either interacts with the polyanion when the pH is below its isoelectric point (IEP) or complexes with gelatin at a pH above IEP_pepsin. At pH 7.4 all the complexes dissociate and proteins are rapidly released within a few hours. The complexes are stable and the proteins are retained within a certain pH range, which is related to the polyanion type (e.g., 5.0–2.0 for PMAA, 4.6–1.2 for PAA, and <4.3 for heparin). The three processes of complex formation, dissociation, and protein release have a good correlation. In addition, the protein release transition takes place within a rather narrow pH range (ca. 0.5 units) and the protein nature has little effect on the protein release profile. The high protein entrapment efficiency and good pH sensitivity of the protein release can be mainly attributed to the electrostatic attractive interactions between proteins and polyanion or gelatin. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1416–1425, 2001     

Usefulness of surfactants in reducing particle adhesion and their effectiveness in cleaning silicon wafers

The effectiveness of surfactants (sodium dodecyl sulfate and dodecyl amine hydrochloride) in the cleaning of silicon wafers has been investigated using atomic force microscopy and image analysis. Recognizing that the surface of SC-1-cleaned silicon wafers is essentially SiO₂ atomic force microscopy was used to determine the interaction forces for the silica/silica and silica/alumina systems with and without surfactant present in solution. Experimental force vs. separation distance curves were found to be in good agreement with theoretical predictions based on electrostatic and van der Waals interactions. Interestingly, the pull-off forces obtained from atomic force microscopy measurements were very close to the adhesion forces calculated on the basis of van der Waals interactions at very close separations. The results from image analysis further confirmed the usefulness of surfactants in reducing particle adhesion and their effectiveness in cleaning silicon wafers. Finally, the results from this study suggest that a more complete understanding of particle interaction forces should be of considerable importance to the electronics industry.     

A new method to measure adhesion and surface forces using a closed-loop accelerometer

A force-balanced MEMS sensor is developed to measure the forces between two surfaces with controllable distance. Its mechanical structure is like a pendulous micromachined accelerometer, and it is designed as a closed-loop system with electrostatic force feedback. The surface force on the sensor probe is balanced by the electrostatic force and the probe operates without displacement. This method avoids the displacement of the conventional cantilever, and the distance between the two surfaces is precisely controlled during the measurement. Experiments on surface force measurements between a probe surface and a ball surface are performed, and the attractive force and adhesion force between the two surfaces are measured under a precise distance controlled by a nanopositioner.     

Surface-initiated ring opening polymerization of N-carboxy anhydride of benzyl-l-glutamate monomers on soft flexible substrates

The potential of pulsed plasma deposited polyallylamine (PAA) adlayer has been successfully demonstrated for fabrication of polypeptide brushes functionalized soft flexible polymeric surfaces. Polymeric substrates functionalized with the plasma deposition PAA adlayer resulted in polymeric surfaces functionalized with amino groups, which are the suitable initiating moieties for ring-opening polymerization (ROP) of N-carboxy anhydride of benzyl-l-glutamate (NCA-BLG) monomer. Poly(γ-benzyl-l-glutamate) (PBLG) brushes were grown on PAA functionalized polypropylene (PP), and polytetrafluoroethylene (PTFE) polymeric substrates. These substrates were intentionally chosen for their inert chemical nature towards most wet chemical surface modification reactions. Surface grafted thin films of poly(γ-benzyl-l-glutamate) PBLG on both the PP and PTFE polymeric substrates yielded high density PBLG brushes. PBLG chain orientation, secondary structure and grafting density were characterized by infra-red spectroscopy. The synthesis of PBLG brushes on a flexible polymeric substrate is unprecedented and technologically important, since PBLG possess good electro-optical activity. Analysis of brush layers by Attenuated Total Reflectance Infra-Red (ATR-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) as well as atomic force microscopy (AFM) fully corroborated the success of the plasma activated soft surface grafting approach.     

Interactions of polyelectrolytes bearing carboxylate and/or sulfonate groups with Cu(II) and Ni(II)

The interactions between the water-soluble polyelectrolytes poly(acrylic acid) (PAA) and poly(vinyl sulfonic acid) (PVS), and Cu(II) and Ni(II) are studied by the liquid-phase polymer-based retention (LPR) technique. Assuming a Ni(II)-PVS interaction of electrostatic nature, the nature of the Ni(II)-PAA interaction is found to be electrostatic, while Cu(II)-PAA interactions imply the formation of coordinative bonds. The charge related formation constants for the systems Ni(II)-PAA, Ni(II)-PVS, and Cu(II)-PVS are found to be 57.57×10², 43.4×10², and 60.5×102 M⁻¹, respectively in a 0.010 M NaNO₃ aqueous solution at pH 5, and 1.4×10² for both systems containing Ni(II) and 1.3×10² M⁻¹ for the system Cu(II)-PVS in a 0.10 M NaNO₃ aqueous solution at pH 5.     

Interactions in the Silicon Carbide–Polyacrylic Acid–Yttrium Ion System

Interactions in the SiC powder–polyacrylic acid (PAA, dispersant)–Y³⁺ ion (sintering additive) system were investigated in the pH range from 2 to 6. The amount of Y³⁺ ions adsorbed on SiC particles increased with an increase of pH because of the electrostatic attraction between the negatively charged SiC surface and Y³⁺ ions. On the other hand, the amount of PAA adsorbed on SiC particles decreased with increasing pH because of the electrostatic repulsion between the negatively charged SiC surface and dissociated PAA. The addition of PAA to the SiC suspension with Y³⁺ ions increased the amount of Y³⁺ ions fixed to SiC particles through the strong interaction between Y³⁺ ions and PAA adsorbed on SiC particles. The above-described interactions in the SiC–PAA–Y³⁺ ions system were closely related to the coagulation of SiC particles and the rheology of SiC suspensions. The coagulation of SiC particles through the adsorbed Y³⁺ ions decreased the specific surface area of SiC powder after calcination in an argon atmosphere. The addition of PAA to the SiC suspensions with Y³⁺ ions kept the SiC particles separate during calcination, i.e., the PAA addition contributed to enhancement of the driving force of sintering (no decrease of specific surface area) and to control of the amount of Y³⁺ ions uniformly fixed to the SiC surface.