Experimental determination of the thermodynamic parameters affecting the adsorption behaviour and dispersion effectiveness of PCE superplasticizers

For adsorption of three different allylether-based PCE superplasticizers on CaCO₃ surface, the thermodynamic parameters ΔH, ΔS and ΔG were determined experimentally. The GIBBS standard free energy of adsorption ΔG_0ads, the standard enthalpy of adsorption ΔH_0ads and the standard entropy of adsorption ΔS_0ads applying to an unoccupied CaCO₃ surface were obtained via a linear regression of ln K (equilibrium constant) versus 1 / T (VAN'T HOFF plot). Additionally, the thermodynamic parameters characteristic for a CaCO₃ surface loaded already with polymer (isosteric conditions) were determined using a modified CLAUSIUS-CLAPEYRON equation.For all PCE molecules, negative ΔG values were found, indicating that adsorption of these polymers is energetically favourable and a spontaneous process. Adsorption of PCEs possessing short side chains is mainly instigated by electrostatic attraction and a release of enthalpy. Contrary to this, adsorption of PCEs with long side chains occurs because of a huge gain in entropy. The gain in entropy results from the release of counter ions attached to the carboxylate groups of the polymer backbone and of water molecules and ions adsorbed on the CaCO₃ surface. With increased surface loading, however, ΔG_isosteric decreases and adsorption ceases when ΔG becomes 0. The presence of Ca²⁺ ions in the pore solution strongly impacts PCE adsorption, due to complexation of carboxylate groups and a reduced anionic charge amount of the molecule. In the presence of Ca²⁺, adsorption of allylether-based PCEs is almost exclusively driven by a gain in entropy. Consequently, PCEs should produce a strong entropic effect upon adsorption to be effective cement dispersants. Molecular architecture, anionic charge density and molecular weight as well as the type of anchor groups present in a superplasticizer determine whether enthalpy or entropy is the dominant force for superplasticizer adsorption.     

Selection of dispersants for the dispersion of carbon black in organic medium

The dispersion of the carbon black pigment in non-aqueous medium requires the use of dispersant additives. Two carboxylic dispersants of different chemical nature and molar masses have been investigated regarding this purpose. Their adsorption behavior has been evaluated by means of adsorption isotherms. The molecular interactions taking place at the surface have been inferred from IR and ¹³C RMN spectroscopies. The rheological behavior of the dispersions has considerably been improved by the presence of the copolymer dispersant of higher molar mass. It might adsorb at the surface by hydrogen bindings while taking a flat conformation with tails and possibly few loops that contributed to the steric stabilization of particles. The consequences of the deflocculation of the suspension on the colorimetric properties are an enhanced tinting strength and improved color stability.     

Electrostatic and electrosteric stabilization of aqueous slips of 3Y–ZrO2 powder

Yttria-doped zirconia powder (3Y-ZrO₂) was characterized and dispersed in distilled water. The state of dispersion was evaluated in terms of zeta potential, apparent viscosity and the mean particle size of solid phase in the slip. Zeta potential, apparent viscosity and the mean particle size as a function of pH indicated the pH range of electrostatic stabilization. These results showed that electrostatic stabilization of the slip can be accomplished in low acidic and high basic range of pH. Dissolution of yttria from the powder surface in acidic pH was found to be high and fast, risky to the stability of the slip as well as the microstructure of the sintered body. Electrosteric stabilisation by addition of an anionic polyelectrolyte (PMAA-NH₃) shifted the isoelectric point (IEP) to lower pH. The state of dispersion was further investigated by particle size measurements of the solid phase in the slip. The optimum amount of dispersant is discussed in terms of zeta potential and viscosity. The adsorption of polyelectrolyte is considered at pH 4, native pH of suspension and at pH 10, below and above the IEP of powder and correlated with the dissociation rate of polymer and the net surface charge of particles at given pH. Higher adsorption of dissociated polymer to the positively charged surfaces justifies the higher optimum amount of polyelectrolyte at pH 4.     

Electrostatic and electrosteric stabilization of aqueous suspensions of barite nanoparticles

Electrostatic and electrosteric stabilization of aqueous suspensions of barite nanoparticles were investigated. The state of dispersion was evaluated in terms of zeta potential, apparent viscosity and the mean particle size of solid phase in the solution. Zeta potential, apparent viscosity and the mean particle size as a function of pH were performed in absence of dispersant. The result showed that electrostatic stabilization of the aqueous suspension of barite nanoparticles can be accomplished in low acidic and high basic range of pH. In presence of sodium polyacrylate (PAA-Na) dispersant, the isoelectric point (IEP) of the barite nanoparticles was shifted to lower pH and the negative zeta potential was increased in a large range of pH above the (IEP). The optimum amount of PAA-Na dispersant is discussed in the light of zeta potential and viscosity. It is found that the adsorption of PAA is correlated to the net surface charge of the barite nanoparticles and the fraction of dissociated polymer at pH 4, 8.5 and 10. At pH 4, the state of dispersion was achieved at higher amount of electrolyte due to the low fraction of negatively charged dissociated polymer and the positively charge particles. At basic pH, the fraction of dissociated polymer was high and the surface charge of particle was highly negative, therefore, the lowest viscosity was obtained at a small amount of PAA. In addition, the optimum amount of polymer decreased with the increase in pH of the suspension.     

Distinct dispersion stability of various TiO2 nanopowders using ammonium polyacrylate as dispersant

The dispersion stabilities of three titania (TiO₂) nanopowders with different particle sizes and surface chemistries in aqueous suspensions containing a common water-based dispersant, ammonium polyacrylate (PAA-NH₄), have been investigated and compared. According to adsorption isotherm and Fourier transform infrared spectroscopy analyses, the adsorption conformations of PAA-NH₄ are distinct for the different TiO₂ nanopowders. In addition, PAA-NH₄ exhibited the greatest adsorption affinity to the larger, hydrophilic TiO₂ nanopowder and the least affinity to smaller, hydrophobic nanopowder. From sedimentation and rheological results, the dispersion stability of the larger, hydrophilic TiO₂ nanopowder was demonstrated to be the greatest. Based on thermodynamic and kinetic calculations for the stabilization energies, the larger, hydrophilic TiO₂ nanopowder was also shown to be the best-stabilized powder, although it settles faster than the smaller, hydrophilic TiO₂ nanopowder; this is due to the greater affection of sedimentation flux on the larger nanopowder. In contrast, the hydrophobic TiO₂ nanopowder formed a gel-like structure in the aqueous suspension when the solid content was greater than 10 wt%, which is attributed to polymer bridging between PAA-NH₄-adsorbed TiO₂ nanoparticles.     

Novel Comb-Like Copolymer Dispersant for Polypropylene/CaCO3 Composites and Its Influence on Dispersion,Crystallization, Mechanical,and Thermal Properties

A novel comb-like copolymer with carboxyl group as an anchoring group and polycaprolactone as a solvent chain was first used as the dispersant of CaCO₃ particles in polypropylene (PP). The dispersion of CaCO₃ particles in PP matrix was significantly improved in the presence of comb-like copolymer dispersant because of the strong repulsive force caused by steric hindrance effect. The influences of the coating amount of comb-like copolymer dispersant on crystallization behaviors, mechanical properties, and thermal stabilities were systematically investigated. The crystallization temperature, crystallinity, and crystallization rate of PP/CaCO₃ composites prepared with monolayer-coated CaCO₃ were all improved, where the monolayer comb-like copolymer coating remained as a rigid layer and provided a noticeable nucleating effect. The PP/CaCO₃ composites coated with monolayer SP comb-like copolymer also had the best mechanical properties, including tensile strength, Young’s modulus, flexural modulus, and impact strength because of the good dispersion of CaCO₃ particles in PP matrix. The thermal stability of PP/CaCO₃ composites were measured by thermogravimetric analysis. The results showed that SP comb-like copolymer dispersant treated CaCO₃ filled composites had excellent thermal stability than untreated and neat PP, especially for the composite prepared with monolayer-coated CaCO₃.     

Triggering the aqueous interparticle association of γ‒Al2O3 hierarchical assemblies using divalent cations and cellulose nanofibers

Understanding aqueous dispersion, rheological properties and colloidal stabilisation mechanisms of hierarchically assembled ceramic powders is important for progress in the fields of catalysis, separation and/or adsorption. The present study was designed to evaluate the rheological and sedimentation behaviour of highly loaded aqueous suspensions (up to φ_A = 0.126) containing AlN-powder-hydrolysis-derived, micron-sized, mesoporous, gamma alumina (MA) particulates with a high surface area (～180 m²/g) dispersed with sodium polyacrylate (NaPAA). The as-prepared suspensions were prone to sedimentation and segregation. However, when divalent cations (Mg²⁺, Ca²⁺) or cellulose nanofibers were added, the formation of interparticle association networks in the aqueous suspensions containing MA particles was triggered, facilitating their long-term resistance to sedimentation lasting more than 12 weeks.     

Photopolymerization synthesis of polyacrylic acid dispersant with methoxysilicon end groups and its application in a nano‐SiO2 aqueous system

In order to improve the dispersity and stability of the nano‐SiO₂ aqueous system with high solid content, a kind of polyacrylic acid dispersant with methoxysilicon end groups (KH590‐PAA) was synthesized by photopolymerization of acrylic acid (AA) initiated with (3‐mercaptopropyl)trimethoxysilane (KH590). After adding KH590‐PAA into the nano‐SiO₂ aqueous dispersion system (20 wt% solid content), the viscosity and the curing time of the system were measured with a rotational viscometer and the inverted bottle method. Moreover, the dispersion mechanism of KH590‐PAA for the nano‐SiO₂ aqueous system was researched by measuring the adsorption capacity, the particle size and the zeta potential of the nanoparticles with a conductivity meter, dynamic light scattering, SEM and TEM, respectively. The results showed that the methoxysilicon groups in KH590‐PAA could react with hydroxyl groups on the surface of nano‐SiO₂ in the process of stirring, which enhanced the adsorption capacity of the dispersant and then increased the surface charge of the particles. Therefore, electrostatic repulsion and steric hindrance effects between the SiO₂ nanoparticles could be further enhanced by adding the KH590‐PAA dispersant, and then the nano‐SiO₂ aqueous system exhibited better dispersity and stability. Besides, the dispersion properties of SiO₂ nanoparticles in water were closely related to the addition amount and the molecular weight of the KH590‐PAA dispersant. © 2018 Society of Chemical Industry     

Using a Brush Copolymer as Efficient Dispersant for the Preparation of Highly Stabilized Ag Nanoparticles in Aqueous Suspensions

Silver (Ag) nanoparticle has extremely high surface energy and it is difficult to find an efficient dispersant to prevent its agglomeration in suspensions. A new brush copolymer, succinic anhydride modified epoxy-amine poly[(propylene oxide)-co-(ethylene oxide)]-grafted polymer (EPOA), which can efficiently disperse concentrated aqueous suspensions of Ag nanoparticles is revealed. The dispersion efficiency of EPOA for the dispersion of a 60 nm-Ag nanoparticles in aqueous suspension is studied by measuring its sedimentation and rheological behavior, and the results are compared with those of a commercially available dispersant, ammonium poly(acrylic acid) (PAA-NH₄). Interactions between the dispersants and the Ag nanoparticles are characterized by zeta potential and adsorption analyses. Theoretical calculations are conducted to clarify the adsorption and the dominant dispersion stabilization mechanisms of the dispersants. Compared with PAA-NH₄, EPOA obtains a higher stable suspension of Ag nanoparticles with less significant sedimentation over 1 month. The dispersion homogeneity of the suspension remains excellent even at an extremely high solid loading of 30–40 wt%. According to adsorption analysis, it is suggested that both EPOA and PAA-NH₄ adsorb via single-point attachment through the carboxyl group on the Ag surface. Based on theoretical calculations, the Ag nanoparticles are better stabilized by EPOA via an electrosteric dispersion mechanism.     

Ultrasonic dispersion of nano TiC powders aided by Tween 80 addition

Nano TiC powders were dispersed in aqueous media. Effects of ultrasonic treatment and Tween 80 addition on dispersion of TiC powders were investigated. The results showed that ultrasonic treatment had a large effect on the dispersion of nano TiC powders, and 30 min of ultrasonic treatment was necessary for fine dispersion from TEM images and particle size measurement. Tween 80 was selected as the dispersant. Sedimentation test indicated that 0.5 vol.% was the optimum addition level of Tween 80 in TiC suspension. FTIR spectrum proved the adsorption of Tween 80 on the surface of nano TiC powders. XPS analysis revealed the existence of TiO₂ on the TiC powder surface, which led to a hydroxylated surface during dispersion. In the presence of Tween 80 in the solution, zeta potential values became more negative. Both electrostatic stabilization and steric stabilization were deduced to be the main mechanisms for well dispersion of the nano TiC powders in aqueous media.     

Roles of polymeric dispersant charge density on lead zirconate titanate aqueous processing

Poly(acrylic acid) (PAA) and poly(acrylic acid-co-maleic acid) (PACM) were used as dispersants in preparation of lead zirconate titanate (PZT) aqueous suspensions. The effects of dispersant structure on particle stabilization were investigated through properties of the suspensions. Viscosity and sedimentation height measurements showed that addition of the dispersants improved particle stabilization. The dispersant concentrations to obtain the lowest viscosity were 0.4 wt% for PAA and 0.2 wt% for PACM based on powder dried weight basis. Furthermore, effects of pH were studied on the suspensions prepared with 0.2 wt% dispersants. Viscosity and sedimentation behaviors indicated the improvements of particle dispersion and suspension stability with an increasing pH. Particle dispersion revealed by laser light scattering and scanning electron microscopy supported an improvement of particle dispersion at alkaline pHs. Detailed analysis of these data indicated that the PACM exhibited higher dispersant efficiency for PZT aqueous suspension in all conditions. The results were discussed based on the concentrations of anionic –COO^? groups at various pHs and charge density along polymeric backbone of the dispersants.     

Effects of dispersant concentration and pH on properties of lead zirconate titanate aqueous suspension

Lead zirconate titanate (PZT) aqueous suspensions were prepared at 60 wt.% solids loading using a commercial ammonium polyacrylate (APA) dispersant. Effects of the dispersant concentration on rheological behavior, dispersion and stability of PZT aqueous suspensions were investigated by means of zeta potential, viscosity and sedimentation height measurements. The results showed that, under suitable conditions, APA dispersant promoted particle dispersion and stabilization in PZT aqueous suspensions. For 60 wt.% solids loading suspensions, the dispersant concentration yielding the lowest viscosity was 0.5 wt.% based on PZT powder dried weight basis. Effects of pH on particle dispersion in the suspensions prepared with APA were studied by laser light scattering technique and scanning electron microscopy. The results showed an improvement in particle dispersion for the alkaline condition, which led to relatively low viscosity and highly stable suspension. Possible particle stabilization mechanisms at various pHs were discussed based on dissociation of the dispersant in water, polymer conformation and adsorption behavior of the dispersant on the particle surface.     

Poly(methacrylate)‐derived diblock dispersant for TiO2 in aqueous suspensions

In this paper, we propose a newly designed dispersant, ammonium poly(methacrylate)‐block‐poly(2‐phenoxyethyl acrylate) (PMA‐b‐PBEA), and our rheological and zeta potential test results verify its superior dispersion efficiency for aqueous suspensions in comparison to the commercial dispersant ammonium polyacrylate (PAA‐NH₄). The extremely high dispersion efficiency of PMA‐b‐PBEA correlates closely to its diblock structure, which simultaneously exhibits a less polar anchoring head group and a water‐dissociable stabilizing moiety. The unique structure of PMA‐b‐PBEA accounts for its high powder adsorption effectiveness, which is demonstrated in its adsorption capability being double that of PAA‐NH₄.     

Dispersion behaviour of fine collophane particles in the presence of different dispersants and metal ions

ABSTRACT

Recent, studies have shown that selective flocculation flotation is an effective method for enhancing the flotation of a fine mineral. Fine-grained collophane is difficult to disperse naturally, thereby affecting its subsequent flotation separation. Therefore, it is important to study the dispersion behaviour and mechanism of fine-grained cellophane to choose an appropriate method to improve the dispersion effect of the flotation system. The influences of different types of poly-anionic dispersants, such as (sodium silicate (ZSS); sodium carbonate (ZSC); sodium hexametaphosphate, ZSP) and metallic cations with different valences (Al³⁺, Ca²⁺, Na⁺) on the dispersion behaviour effect of fine-grained collophane were investigated in this work using the sedimentation balance method. The zeta potential measurements and the extended-DLVO (Derain-Landau-Verwey-Overbake) theory were used to explain the distinction of dispersion behaviour. In the presence of dispersants, the dispersion of collophane was in the order of ZSP>ZSS>ZSC. Zeta potential analysis indicated that the absolute value of the surface potential followed the order of ZSP>ZSS>ZSC. That is, the collophane particles in the ZSP system had stronger electrostatic repulsion, resulting in a better dispersion zeta potential effect zeta potential. ZSP was adopted as dispersant to study the effects of metal ions on dispersion. In the ZSP dispersant system, metal ions can suppress the dispersion behaviour of fine-grained collophane. The suppression effect followed the order of Al³⁺>Ca²⁺>Na⁺, because of the greater charge and better neutralization ability of trivalent ions for the same concentration. The results of the extended-DLVO calculations showed that the dispersion stability was attributed to the electrostatic repulsion and hydration repulsion, and hydration repulsion had a greater influence on the stability than electrostatic repulsion. This paper provides reference information for adjusting the dispersion behaviour effect of fine-grained collophane, thereby guiding the actual flotation separation.     

Soap-based detergent formulations: XX. The physical and chemical nature of lime soap dispersions

Blends of soap and surfactants that possess good lime soap dispersing properties were dispersed in hard water. The turbidity of such dispersions varied depending on the type of dispersant used and also on the soap:dispersant ratio. Differences in coarseness of various dispersions could be measured empirically by filtration through a membrane of intermediate (1.2 μm) porosity. For determinations of the chemical composition of the dispersions a somewhat finer membrane (0.8 μm or less) was chosen, which retained most of the dispersed solids. Filter residues and filtrates were analyzed for sodium, calcium, magnesium and lime soap dispersing agents (LSDA). All of the calcium remained on the filter, whereas sodium was found primarily in the filtrate. Magnesium was held completely on the filter only if sufficient soap was present to tie up all Ca⁺⁺ and Mg⁺⁺. Analysis of the organic portion of the residues indicated that the soap:LSDA ratio found was the same as that used in the preparation of the original dispersion. On filtration through a fine membrane (0.05 μm) virtually all dispersed material was retained on the membrane. The filtrate possessed only slight activity in terms of surface tension and detergency, whereas the resuspended solids possessed high surface activity similar to the unfiltered dispersion. This indicates that the dispersed solids are the major source of surface activity. Presented at the AOCS Meeting, September 1975, Cincinnati.     

Preparation of high surface area CaCO3 by reacting CO2 with aqueous suspensions of Ca(OH)2: Effect of the addition of sodium polyacrylate

High surface area CaCO₃ was produced through the reaction between CO₂ and an aqueous suspension of Ca(OH)₂ with the addition of an additive, sodium polyacrylate. The surface area of CaCO₃ prepared was affected markedly by the amount of additive and the solution pH when adding the additive. The CaCO₃ with the highest surface area (87.7 ± 1.3 m²/g) was obtained under the conditions that the initial Ca(OH)₂ concentration was 2.4 wt.%, the amount of sodium polyacrylate added was 0.2 wt.%, and the solution pH at which the additive was added was in the range of 11.4-11.1. The high surface area CaCO₃ also had a high pore volume. The CaCO₃ was highly reactive toward SO₂, and a conversion of 0.95 was achieved when it was sulfated at 950 °C and 4000 ppm SO₂ in air for 1 min. Prior calcination reduced the reactivity of this high surface area CaCO₃.     

Stabilization of nano-TiO2 aqueous dispersions with poly(ethylene glycol)-b-poly(4-vinyl pyridine) block copolymer and their incorporation in photocatalytic acrylic varnishes

In this work, TiO₂ nanoparticles were dispersed and stabilized in water using a novel type of dispersant based on tailor-made amphiphilic block copolymers of poly(ethylene glycol)-block-poly(4-vinyl pyridine) (mPEG-b-P4VP) prepared by atom transfer radical polymerization (ATRP). The performance of this new block copolymer as dispersant was compared to a polyelectrolyte dispersant commonly used for TiO₂, sodium salt of polyacrylic acid (Na-PAA). The effect of dispersion technique and type and amount of dispersant on deagglomeration and stability of the TiO₂ aqueous suspensions were studied. After incorporation in a standard waterborne acrylic varnish formulation, dry film transparency, photocatalytic activity, and nanoparticle cluster size were also evaluated. The results show that mPEG-b-P4VP copolymer with appropriate block lengths can have a better performance than Na-PAA in terms of aqueous dispersion stabilization and cluster size reduction in the acrylic matrix. This translates into higher film transparency and photocatalytic performance.     

Dispersion behaviour of laser-synthesized nanometric SiC powders in aqueous medium with ammonium polyacrylate

The dispersion behaviour of laser-synthesized nanometric SiC powders in water using ammonium polyacrylate (molecular WEIGHT=10,000) as dispersant was investigated. The influence of oxidation, presoaking time, ammonium polyacrylate (NH₄PA) concentration, and pH on suspension stability and coagulation rate of aggregates was determined. The stabilization mechanism is discussed. Excellent dispersion stability was obtained for oxidized (500 °C) powders containing 2.45 wt.% NH₄PA at pH 9.25 after a lengthy aging treatment.     

Effects of impurities on particle sizing by acoustic attenuation spectroscopy

It is important to have correct information regarding particle size in order to interpret, control, and optimize many industrial processes. Prior to the recent advent of acoustic attenuation spectroscopy, it was difficult to study particle size distribution online and under real process conditions in processes involving concentrated dispersions (suspensions or emulsions). The technique still needs improvement because it is less known how and under which conditions to employ the technique when dispersions involve impurities that could be soluble, insoluble, in the form of additives, and so on. This lack of understanding has almost halted the advancement in applications of the technique to various processes that essentially involve dispersions with impurities. This study investigates aqueous suspensions of CaCO₃ at different concentrations (i.e., 5%, 10% and 20% mass/mass) with added impurities of MgCO₃ (insoluble impurity), NaNO₃ (soluble impurity) and sodium polyacrylate (soluble additive) at varying proportions (5%, 10%, 20% and 30% of the weight of CaCO₃). The study characterizes and compares dispersion with and without impurity in order to demonstrate the possible ways in which addition of an impurity change the original acoustic attenuation spectrum of a dispersion. The study brings the conditions in which acoustic attenuation spectroscopy is capable of explaining that addition of an impurity will not change original particle size of the disperse medium.     

The effect of acids and bases on the dispersion and stabilization of ceramic particles in ethanol

The dispersion and stability of alumina, titania, and silicon carbide powders in ethanolic medium have been investigated. An operational pH-scale, pH*, based on an ethanol-based reference electrode, was used to systematize the suspension properties. The electrokinetic behavior was determined as a function of pH*. The isoelectric points in ethanol — pH¹_iep(SiC)=7.5; pH¹_iep(Al₂O₃)=4.4 and pH¹_iep(TiO₂)=4.2 — were discussed in relation to the dissociation constants of the charge determining reactions at the powder surfaces. We have evaluated the long-term stability of the ethanolic dispersions through settling studies which showed that the primary particle size could be retained for extended times providing that the surface potential and ionic strength were optimized.