CATIONIC POLYELECTROLYTE

CATIONIC POLYELECTROLYTE

The terms Cationic polyelectrolyte, polymer electrolyte, and polymeric electrolyte should not be confused with the term solid polymer electrolyte.
Cationic polyelectrolytes can be either synthetic or natural. Nucleic acids, proteins, teichoic acids, some polypeptides, and some polysaccharides are examples of natural Cationic polyelectrolytes.

CAS No. : 42751-79-1

Synonyms:
Calcium dichloride; Katyonik polielektolit; Katyonik polielektrolit; Katyonik poli elektrolit; Cationic Poly-electrolyte; katiyonik polielektrolit; kationik polyelektrolit; cationic polielectrolite; Cationic Poly electrolyte; Cationic Polyelectrolyte Polyamine; Dimethylamine, polymer with epichlorohydrin and ethylenediamine; 2-(chloromethyl)oxirane; dimethylamine; ethane-1,2-diamine; Anionic & Cationic Polyelectrolyte; Anionic Polyelectrolyte; Cationic-Polyelectrolyte; acrylamide; furan-2,5-dione;2-methylprop-1-ene; Polyelectrolyte 60; Maleic anhydride, isobutylene copolymer; maleic anhydride isobutene; Isobutylene maleic anhydride; PEL; Cationic PEL; CATIONIC PEL; CATIONIC POLYELECTROLYTE; Calcium dichloride; Calcium chloride anhydrous; CaCl2; Calciumchloride; Calcium(II) chloride; Cationic Polyelectrolyte; Calcium chloride pellets; Isocal; Calcium dichloride; Katyonik polielektolit; CP; calcium chloride,anhydrous; CHEBI:3312; Caloride; Liquical; Jarcal; Unichem calchlor; Sure-step; Huppert's reagent; Calcium chloride, ACS reagent, desiccant; Homberg's phosphorus; Calcium chloride, 96%, for analysis, granules; Calcium chloride, 96%, for biochemistry, anhydrous

Cationic Polyelectrolyte

Cationic polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are Cationic polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Cationic polyelectrolyte properties are thus similar to both electrolytes (salts) and polymers (high molecular weight compounds) and are sometimes called polysalts. Like salts, their solutions are electrically conductive. Like polymers, their solutions are often viscous. Charged molecular chains, commonly present in soft matter systems, play a fundamental role in determining structure, stability and the interactions of various molecular assemblies. Theoretical approaches to describing their statistical properties differ profoundly from those of their electrically neutral counterparts, while technological and industrial fields exploit their unique properties. Many biological molecules are Cationic polyelectrolytes. For instance, polypeptides, glycosaminoglycans, and DNA are Cationic polyelectrolytes. Both natural and synthetic Cationic polyelectrolytes are used in a variety of industries.

IUPAC definition of Cationic polyelectrolyte
Cationic polyelectrolyte: Polymer composed of macromolecules in which a substantial portion of the constitutional units contains ionic or ionizable groups, or both.

Notes:
The terms Cationic polyelectrolyte, polymer electrolyte, and polymeric electrolyte should not be confused with the term solid polymer electrolyte.
Cationic polyelectrolytes can be either synthetic or natural. Nucleic acids, proteins, teichoic acids, some polypeptides, and some polysaccharides are examples of natural Cationic polyelectrolytes.

Charge of Cationic polyelectrolyte
Acids are classified as either weak or strong (and bases similarly may be either weak or strong). Similarly, Cationic polyelectrolytes can be divided into "weak" and "strong" types. A "strong" Cationic polyelectrolyte is one that dissociates completely in solution for most reasonable pH values. A "weak" Cationic polyelectrolyte, by contrast, has a dissociation constant (pKa or pKb) in the range of ~2 to ~10, meaning that it will be partially dissociated at intermediate pH. Thus, weak Cationic polyelectrolytes are not fully charged in solution, and moreover their fractional charge can be modified by changing the solution pH, counter-ion concentration, or ionic strength.

The physical properties of Cationic polyelectrolyte solutions are usually strongly affected by this degree of charging. Since the Cationic polyelectrolyte dissociation releases counter-ions, this necessarily affects the solution's ionic strength, and therefore the Debye length. This in turn affects other properties, such as electrical conductivity.

When solutions of two oppositely charged polymers (that is, a solution of polycation and one of polyanion) are mixed, a bulk complex (precipitate) is usually formed. This occurs because the oppositely-charged polymers attract one another and bind together.

Conformation of Cationic polyelectrolyte
The conformation of any polymer is affected by a number of factors: notably the polymer architecture and the solvent affinity. In the case of Cationic polyelectrolytes, charge also has an effect. Whereas an uncharged linear polymer chain is usually found in a random conformation in solution (closely approximating a self-avoiding three-dimensional random walk), the charges on a linear Cationic polyelectrolyte chain will repel each other via double layer forces, which causes the chain to adopt a more expanded, rigid-rod-like conformation. If the solution contains a great deal of added salt, the charges will be screened and consequently the Cationic polyelectrolyte chain will collapse to a more conventional conformation (essentially identical to a neutral chain in good solvent).

Polymer conformation of course affects many bulk properties (such as viscosity, turbidity, etc.). Although the statistical conformation of Cationic polyelectrolytes can be captured using variants of conventional polymer theory, it is in general quite computationally intensive to properly model Cationic polyelectrolyte chains, owing to the long-range nature of the electrostatic interaction. Techniques such as static light scattering can be used to study Cationic polyelectrolyte conformation and conformational changes.

Polyampholytes
Cationic polyelectrolytes that bear both cationic and anionic repeat groups are called polyampholytes. The competition between the acid-base equilibria of these groups leads to additional complications in their physical behavior. These polymers usually only dissolve when there is sufficient added salt, which screens the interactions between oppositely charged segments. In case of amphoteric macroporous hydrogels action of concentrated salt solution does not lead to dissolution of polyampholyte material due to covalent cross-linking of macromolecules. Synthetic 3-D macroporous hydrogels shows the excellent ability to adsorb heavy-metal ions in a wide range of pH from extremely diluted aqueous solutions, which can be later used as an adsorbent for purification of salty water. All proteins are polyampholytes, as some amino acids tend to be acidic, while others are basic.

IUPAC definition
Ampholytic polymer: Cationic polyelectrolyte composed of macromolecules containing both cationic and anionic groups, or corresponding ionizable group.
Note:
An ampholytic polymer in which ionic groups of opposite sign are incorporated into the same pendant groups is called, depending on the structure of the pendant groups, a zwitterionic polymer, polymeric inner salt, or polybetaine.

Applications of Cationic polyelectrolyte
Cationic polyelectrolytes have many applications, mostly related to modifying flow and stability properties of aqueous solutions and gels. For instance, they can be used to destabilize a colloidal suspension and to initiate flocculation (precipitation). They can also be used to impart a surface charge to neutral particles, enabling them to be dispersed in aqueous solution. They are thus often used as thickeners, emulsifiers, conditioners, clarifying agents, and even drag reducers. They are used in water treatment and for oil recovery. Many soaps, shampoos, and cosmetics incorporate Cationic polyelectrolytes. Furthermore, they are added to many foods and to concrete mixtures (superplasticizer). Some of the Cationic polyelectrolytes that appear on food labels are pectin, carrageenan, alginates, and carboxymethyl cellulose. All but the last are of natural origin. Finally, they are used in a variety of materials, including cement.

Because some of them are water-soluble, they are also investigated for biochemical and medical applications. There is currently much research in using biocompatible Cationic polyelectrolytes for implant coatings, for controlled drug release, and other applications. Thus, recently, the biocompatible and biodegradable macroporous material composed of Cationic polyelectrolyte complex was described, where the material exhibited excellent proliferation of mammalian cells and muscle like soft actuators.

Multilayers
Cationic polyelectrolytes have been used in the formation of new types of materials known as Cationic polyelectrolyte multilayers (PEMs). These thin films are constructed using a layer-by-layer (LbL) deposition technique. During LbL deposition, a suitable growth substrate (usually charged) is dipped back and forth between dilute baths of positively and negatively charged Cationic polyelectrolyte solutions. During each dip a small amount of Cationic polyelectrolyte is adsorbed and the surface charge is reversed, allowing the gradual and controlled build-up of electrostatically cross-linked films of polycation-polyanion layers. Scientists have demonstrated thickness control of such films down to the single-nanometer scale. LbL films can also be constructed by substituting charged species such as nanoparticles or clay platelets in place of or in addition to one of the Cationic polyelectrolytes. LbL deposition has also been accomplished using hydrogen bonding instead of electrostatics. For more information on multilayer creation please see Cationic polyelectrolyte adsorption.

Formation of 20 layers of PSS-PAH Cationic polyelectrolyte multilayer measured by multi-parametric surface plasmon resonance
An LbL formation of PEM (PSS-PAH (poly(allylamine) hydrochloride)) on a gold substrate can be seen in the Figure. The formation is measured using Multi-Parametric Surface Plasmon Resonance to determine adsorption kinetics, layer thickness and optical density.

The main benefits to PEM coatings are the ability to conformably coat objects (that is, the technique is not limited to coating flat objects), the environmental benefits of using water-based processes, reasonable costs, and the utilization of the particular chemical properties of the film for further modification, such as the synthesis of metal or semiconductor nanoparticles, or porosity phase transitions to create anti-reflective coatings, optical shutters, and superhydrophobic coatings.

Bridging
If Cationic polyelectrolyte chains are added to a system of charged macroions (i.e. an array of DNA molecules), an interesting phenomenon called the Cationic polyelectrolyte bridging might occur. The term bridging interactions is usually applied to the situation where a single Cationic polyelectrolyte chain can adsorb to two (or more) oppositely charged macroions (e.g. DNA molecule) thus establishing molecular bridges and, via its connectivity, mediate attractive interactions between them.

At small macroion separations, the chain is squeezed in between the macroions and electrostatic effects in the system are completely dominated by steric effects – the system is effectively discharged. As we increase the macroion separation, we simultaneously stretch the Cationic polyelectrolyte chain adsorbed to them. The stretching of the chain gives rise to the above-mentioned attractive interactions due to chain's rubber elasticity.

Because of its connectivity the behaviour of the Cationic polyelectrolyte chain bears almost no resemblance to the case of confined unconnected ions.

Polyacid
In polymer terminology, a polyacid is a Cationic polyelectrolyte composed of macromolecules containing acid groups on a substantial fraction of the constitutional units. Most commonly, the acid groups are –COOH, –SO3H, or –PO3H2.

Definition and Usage Areas of Cationic polyelectrolyte:

Cationic polyelectrolyte is added to the sludge line during the pumping of the excess activated sludge taken from the sedimentation pond to filter presses or belt-presses to dewater the sludge.

Cationic polyelectrolyte is widely used in sludge dewatering units of wastewater treatment plants.

In processes where sludge dewatering is performed with a centrifuge decanter, belt press or filter press, the flocculant, which is mixed with the help of a static mixer, is dosed into the pressurized sludge line.

The working principle of the cationic polyelectrolyte product is generally based on ion exchange between the polymer chain in aqueous solution and the electrical charges of the suspended solid particles. The stable structure of solid particles deteriorates, which leads to coagulation or flocculation.

Cationic polyelectrolytes are diluted from 0.05% to 0.1%. The preparation solution is usually prepared at 0.5% by adding the original product to water while mixing. Since the characteristics of the sludge to be dewatered are different, the dosages to be applied are determined as a result of jar test and operation trials in the laboratory.

Usage areas of Cationic polyelectrolyte

Polyelectrolytes used for flocculation in wastewater treatment systems are divided into two main groups as anionic and cationic polyelectrodes. Although they are nanionic polyelectrolytes they are not used much. In general, anionic polyelectrolytes enable the particles in waste water to be combined and precipitated in chemical treatment plants.

Cationic polyelectrolytes are used to float flocs to be created in biological water treatment plants or to increase efficiency during dewatering of waste sludge from all treatment plants. Basically, there are various types of polyelectrolytes used in these principles. It is absolutely essential that the jar tests required for waste water systems are carried out by experts and the most appropriate use of the polyelectrolyte suitable for the system is selected. As a result, the treatment system can be operated healthily and efficiently.

Unlike its anionic form, cationic polyelectrolyte is generally used in excessively activated sludge of biological treatment plants. Cationic polyelectrolyte is added to the sludge line during the pumping of the excess activated sludge taken from the sedimentation pond to filter presses or belt-presses to dewater the sludge.

Cationic Polyelectrolyte is a linear polymeric compound, because it has a variety of lively groups, affinity, adsorption and many substances forming hydrogen bonds. Mainly flocculation of negatively charged colloidal, turbidity, bleaching, adsorption, glue and other functions, for dyeing, paper, food, construction, metallurgy, mineral processing, coal, oil, and aquatic product processing and fermentation industries of organic colloids with higher levels of wastewater treatment, especially for urban sewage, sewage sludge, paper mill sludge and industrial sludge dewatering process.

KEY FEATURES AND BENEFITS of Cationic polyelectrolyte:
Water soluble, and can also dissolve completely in cold water.
Add a small amount of anionic polyelectrolyte products, you can receive a lot of flocculation effect.
while using the products and inorganic anion polyelectrolyte flocculant (polymerized ferric sulfate, polyaluminum chloride, iron salts, etc), you can display a greater effect.

Typical Properties of Cationic polyelectrolyte

Appearance Free flowing white powder
Bulk Density g/l @ 25°C 0.75-0.85
Concentration for dilution(g/l) 2.0-3.0

Advantages of Cationic polyelectrolyte

Improved settling rate in clarifier
Improved efficiency of the clarifier
Reduced retention time
Works irrespective of ph
Decreased Mud volumes
Instant colour change
Compressed filter cakes
Effluent colour reduction

Cationic polyelectrolytes are polymers possessing many ionizable groups. The combination of polymeric and electrolyte behaviour gives them a number of useful properties, as indicated in Table 1, but also poses problems of characterization. This chapter provides an introduction to the behaviour of Cationic polyelectrolytes in solution, discusses the difficulties which this behaviour engenders in the determination of molecular weights and considers means of overcoming these difficulties.

Cationic polyelectrolytes are polymers with ionizable repeating groups, such as polyanions and polycations. These groups can dissociate in polar solvents such as water, leaving charges on polymer chains and releasing counterions into the solution (Bhattarai et al., 2010; Schatz et al., 2004; Wu and Delair, 2015). Cationic polyelectrolyte complexes (PECs) offer the possibility of combining physicochemical properties of at least two Cationic polyelectrolytes (Schatz et al., 2004). The PECs are formed by strong electrostatic interactions between oppositely charged Cationic polyelectrolytes, leading to interpolymer ionic condensation and the simultaneous release of counterions (Wu and Delair, 2015; Luo and Wang, 2014). Other interactions between two ionic groups to form PEC structures include hydrogen bonding, hydrophobic interactions, van der Waals’ forces, or dipole–dipole charge transfer.

Chitosan has cationic nature due to the protonation of amino groups on the polymer backbone and becomes a cationic Cationic polyelectrolyte upon dissolution in aqueous acetic acid (Luo and Wang, 2014). Mixing cationic chitosan Cationic polyelectrolyte with negatively charged Cationic polyelectrolyte molecules forms spontaneous, entropy-driven PECs, which can be water-soluble or precipitated. Nonstoichiometric ratios of two Cationic polyelectrolytes lead to particle formation. For chitosan PEC particle formation, many investigators have used cation Cationic polyelectrolyte solution (chitosan) in excess of anionic Cationic polyelectrolytes (Schatz et al., 2004). The size of PECs is influenced by the Cationic polyelectrolyte concentration, charge density, mixing ratio, and pH. The charge density of the chitosan Cationic polyelectrolyte depends on the pH of the solution and degree of deacetylation (DDA) of chitosan. With increasing DDA (DDA >50%), positive charge density of the chitosan polymer increases and hence exhibits a large number of cross-linking sites to make PECs (Fan et al., 2012, Delair, 2011). The particle size of chitosan PECs decreases with decreases in DDA of chitosan and its molar mass (Schatz, 2004). Higher concentrations of low-molecular weight chitosan are required to form PECs with sufficient gel rigidity. High-molecular weight chitosan can form more robust PECs with highly cross-linked networks.

Cationic polyelectrolytes (PEL) are polymers that carry charges within their backbone or in side chains. Usually, discrimination is made between weak and strong Cationic polyelectrolytes. Weak Cationic polyelectrolytes are polymers with weakly acidic or basic groups, which are protonated or deprotonated depending on the pH of the surrounding medium, resulting in a pH-dependent charge density. In contrast, the charge density in strong Cationic polyelectrolytes is not influenced by the pH.

Cationic polyelectrolyte brushes exhibit interesting characteristics with respect to both theoretical and practical aspects because their behavior is fundamentally different from that of uncharged polymer brushes. In the case of strong Cationic polyelectrolyte brushes, in which the charge density is independent of the pH, the molecular structure and properties are dominated by electrostatic interactions. Mutual repulsion between charged polymer segments strongly influences the physical properties of the grafted layers. In weak Cationic polyelectrolyte brushes—in which the charge density of the chains depends on their protonation level—the chain conformation depends on the pH of the solution. In particular, the swelling of weak Cationic polyelectrolyte brushes in different solvents was extensively studied due to its importance for responsive polymer systems. Swelling depends on the nature of the solvent system, as well as its pH and the concentration and chemical nature of other ions in the solution. Furthermore, interactions with selected counterions can be used to tune the wettability of surfaces with anchored Cationic polyelectrolyte brushes.

General aspects of Cationic polyelectrolytes and PEM films
Cationic polyelectrolytes are ionizable polymers that change their polymeric conformations upon their environmental changes. They are of two types: strong and weak Cationic polyelectrolytes. Strong Cationic polyelectrolytes are charged over a wide pH range. Hence, it is a difficult task to manipulate the properties of the assembled film unless one takes specific measures to disturb the polymer-polymer interactions by controlling other stimuli such as ionic strength, temperature, and polarity. Unlike strong Cationic polyelectrolytes, weak Cationic polyelectrolytes are charged only in a smaller pH window; hence, their polymeric conformations can be easily modulated upon changing the pH of the external environment. The unique feature of PEM films assembled from weak Cationic polyelectrolytes is that they can be destroyed at extreme pH conditions as the pH-induced charge imbalances in the film overcompensate the attractive polymer-polymer interactions.

Physicochemical properties of Cationic polyelectrolytes
Cationic polyelectrolytes are macromolecules that, when dissolved in a polar solvent like water, have a (large) number of charged groups covalently linked to them. In general, Cationic polyelectrolytes may have various kinds of such groups. Homogeneous Cationic polyelectrolytes have only one kind of charged group, e. g. only carboxylate groups. If both negative (anionic) and positive (cationic) groups occur, we call such a molecule a polyampholyte. These Cationic polyelectrolytes will only be briefly discussed at the end of this chapter. Self-assembled structures, such as linear micelles or linear protein assemblies, also often have many charged groups; these structures may have properties very similar to those of Cationic polyelectrolytes, but we shall not deal with them in this chapter.

Special properties of Cationic polyelectrolytes, as compared with uncharged polymers, are their generally excellent water solubility, their propensity to swell and bind large amounts of water, and their ability to interact strongly with oppositely charged surfaces and macromolecules. Because of these features, they are widely used as rheology and surface modifiers. These typical Cationic polyelectrolyte properties are intimately related to the strong electrostatic interactions in Cationic polyelectrolyte solutions and, hence, are sensitive to the solution pH and the amount and type of electrolytes present in the solution.

Cationic polyelectrolytes show many applications in fields, such as in water treatment as flocculation agents, in ceramic slurries as dispersant agents, and in concrete mixtures as super-plasticizers. Furthermore, many shampoos, soaps, and cosmetics contain Cationic polyelectrolytes. Certain Cationic polyelectrolytes are also added to food products, for example, as food coatings and release agents. Some examples of Cationic polyelectrolytes are pectin (polygalacturonic acid), alginates (alginic acid), and carboxymethyl cellulose, of which the last one is of natural origin. Cationic polyelectrolytes are water soluble, but when crosslinking is created in Cationic polyelectrolytes they are not dissolved in water. Crosslinked Cationic polyelectrolytes swell in water and work as water absorbers and are known as hydrogels or superabsorbent polymers when slightly crosslinked. Superabsorbers can absorb water up to 500 times their weight and 30–60 times their own volume (Bolto and Gregory, 2007; Dobrynin and Rubinstein, 2005).

Cationic polyelectrolyte membranes
Cationic polyelectrolyte membranes are synthesized on surface of the charged supports via sequential coating of anionic and cationic Cationic polyelectrolytes. This assembly technique named as layer by layer (LbL) is attractive for the preparation of NF and RO membranes, and the obtained dense structure can limit passage of ions through the membranes. In this method, first, the initially charged membrane is soaked in the positive dilute solution of cationic Cationic polyelectrolyte. After that the membrane is removed from the solution and rinsed with water for elimination of the unbound molecules. Then the obtained positively charged membrane is immersed in the negative dilute solution of anionic Cationic polyelectrolyte followed by water rinsing. In each step, a small content of Cationic polyelectrolytes adsorbs on the membrane surface and consequently the previous charge of the membrane reverses. Multiple positive and negative layers onto the membrane surface cause the preparation of Cationic polyelectrolyte multilayer membranes. The number of formed Cationic polyelectrolyte layers has an essential role in water flux and salt rejection of the Cationic polyelectrolyte membranes. The higher number of the layers increases mass transfer resistance so water flux decreases. On the other hand, salt rejection increases with increment of the deposited dense Cationic polyelectrolyte layers. It is worth noting that there are an optimum number of layers that determine the membrane performance. The separation performance, thickness, surface hydrophilicity, and charge of the LbL membranes are affected by type, concentration, pH, and the layer number of the Cationic polyelectrolytes.

pH-responsive Cationic polyelectrolyte shell
Cationic polyelectrolyte complex holds a great deal of promise for the formation of inhibitor containers sensitive to the pH. Since there are various possibilities to change the permeability of Cationic polyelectrolyte multilayers, the use of Cationic polyelectrolyte complexes can control the interior of containers. Owing to the presence of hydroxyl groups on the surface of most inorganic NPs, the majority of these particles are negatively charged at the surface; thus, oppositely charged layers of Cationic polyelectrolyte can be alternatingly deposited on the material through electrostatic interaction to prevent undesirable leakage of inhibitor. The release of additives with corrosion-inhibiting function, similar to the layer-by-layer Cationic polyelectrolyte corrosion-protective coatings, is controlled by varying the pH level, which changes the layer-by-layer Cationic polyelectrolyte permeability. In noncross-linked linear Cationic polyelectrolytes, the Cationic polyelectrolyte complexes, due to displaying electrostatic nature, are highly sensitive to the ionic strength and pH. If two types of strong Cationic polyelectrolytes constitute a Cationic polyelectrolyte complex, the obtained complex displays stability in a wide range of pH values and is able to be opened by raising the ionic strength of the solution and release the confined material. Conversely, if weak Cationic polyelectrolytes constitute the Cationic polyelectrolyte complex, the obtained complex can be damaged and destroyed by shifting the local pH to acidic for weak polyanions and to alkine for weak polycations. The Cationic polyelectrolyte complex consisting weak and strong Cationic polyelectrolytes displays sensitivity to the shift in the pH in only one direction, meaning that weak polyacid together with strong polybase can be used only for the release of inhibitors in acidic media and weak polyacid together with strong polybase for the release of inhibitors only in alkine media, whereas the Cationic polyelectrolyte complex composed of two weak Cationic polyelectrolytes constitutes a container shell, which displays sensitivity to the shift in the pH in both regions. Consequently, the Cationic polyelectrolyte shell of corrosion inhibitor carriers is able to prevent leakage of the corrosion inhibitor at nearly neutral pH and achieve smart release properties when corrosion commences with an alkine and acidic shift in the pH. The fabrication of inhibitor nanoreservoirs with sensitivity to either anodic or cathodic process or to both processes is possible by varying the Cationic polyelectrolyte shell material. Skorb and coworkers deposited Cationic polyelectrolyte shell using layer-by-layer method on the mesoporous silica NPs surface loaded with [2-(benzothiazol-2-ylsulfanyl)-succinic acid]. These NPs were doped in the sol–gel coating. The permeability of the shell increased in response to the alkaline and acidic region at the corroded surface, leading to releasing inhibitors. In addition, the zirconia–silica-based hybrid coating containing these NPs exhibited improved long-term protection against corrosion elements. Shi et al. fabricated submicrometer containers with the use of mesoporous silica particles and layer-by-layer method. The submicrometer containers constructed by this method exhibited higher corrosion inhibitor loading efficiency. The pH-triggered release of corrosion inhibitor as well as barrier effects of the matrix increased the corrosion protection performance.

Electrosteric System Rheology
Cationic polyelectrolytes are widely used as dispersants for high solids loaded colloids (>50 vol%). They combine principles of EDL and steric stabilization, or electrosteric stabilization, and they depend on pH and ionic strength, as Naito et al. discuss. At low solids loading (~20 vol%), viscosity is relatively low, and it is affected very little by pH changes. As solids loading increases, however, pH affects viscosity significantly. The amount of added Cationic polyelectrolyte also has a profound effect on colloidal rheology. It should be optimized to just saturate the surface. Additional Cationic polyelectrolytes result in excess amounts of polymer in the system, and excess polymer can cause depletion flocculation in high solids loaded systems.

Conformation of adsorbed Cationic polyelectrolyte also plays an important role in rheological behavior of electrosterically stabilized colloids, and, in turn, Cationic polyelectrolyte conformation depends on the system’s pH. A detailed study of adsorption behavior on Al2O3 shows that Cationic polyelectrolyte adsorption on particles increases as pH decreases. Typically, a 10-fold increase of adsorbed amount is observed from the uncharged to the charged state. When pH increases or decreases beyond zero charge, the fraction of the Cationic polyelectrolyte dissociated moves toward 1. Hence, charges in the Cationic polyelectrolyte reCationic polyelectrolyte each other and the molecule stretches. At this moment, two models exist: the charged Cationic polyelectrolyte adsorbs flat on the surface or the Cationic polyelectrolyte adsorbs in a tail-like brush structure.

Conformation shape of the adsorbed Cationic polyelectrolyte highly influences dispersion quality. Which types of structures – flat, pancake-like, or brush-like – are achieved depends on adsorption conditions and the materials involved. For pancake-like adsorption, the polymer only contributes short-range repulsive force, and EDL forces of the charged Cationic polyelectrolyte mainly contribute to stabilization via long-range interactions. For brush-style structures, the repulsion is much stronger, and true electrosteric contributions are present. Cationic polyelectrolytes can also be used as dispersants when they are uncharged, i.e. at their PZC. However, they will favor coil-like conformations. Hence, much higher molecular weights will be needed to achieve thicker layers of adsorbed polymer coils, and steric forces predominantly contribute to stabilization.