How polymeric dispersants work in industrial water? Difference between dispersant and flocculation polymers

Dispersant's job is to improve dispersion stability of suspended solids [SS] in water. Dispersants are low molecular weight anionic polymers with anionic functional group. Anionic functional group gives negative charge and hydrocarbon chain of polymer gives surface to a dispersant molecule.

Generally, SS have slight negative charge which makes them repelling each other and keep them floating. One source of negative charge on SS is their coating with negatively charged organic impurities. Any destabilization of charge on SS like, it could be by the counter ions in water, causes SS to deposit on heat transfer surface and foul the surface.

How dispersants work?

A floating solid particle in water has two opposite forces on it [1] Vander wall inter molecular attraction which is pulling SS particles towards each other [2] electrostatic repulsion , which is repelling SS particles away from each other. The fundamental of dispersion of solids in water is - dispersants increase the negative charge on SS particles and make the electrostatic repulsion of particles > van der Waals attraction, thus, prevent agglomeration of SS particles.    

Mechanism -Dispersants adsorb themselves by inter-molecular attraction on SS surface and increase negative charge. The anchoring of polymer on SS surface enhances the negative charge on SS particle. This increased negative charge on SS surface is so delicately balanced that [SS- dispersant] adduct can counter strong cations like divalent positive ions without being agglomerated by Vander wall forces. The steric effect of bulky functional groups on [dispersant – SS] adduct also shield their charge destabilization and agglomeration.

What is so special about the structure dispersant polymer?

All polymers are not dispersants. A dispersant molecule is specially crafted polymer keeping four points in mind,[1] should have sufficient –ve charge [2] ability of functional group to ionize or deprotonate in water into ions [3] controlled molecular size or molecular weight and [4] right choice of functional group to provide necessary charge density such that repulsive force between the molecules can overcome their mutual attraction by Vander wall forces.

Dispersion performance - Key factors

Choice of functional group and size of polymer chain

Poly acrylic acid is a very common dispersant with limitations. Selection of dispersant depends on type of impurities in water. Iron oxide impurities, for example, with Fe++ ions[ positive charge ] are most common and challenging impurity in industrial water. Their positive charge gives rise to coagulation effect to SS and they settle down. Poly acrylic acid is not very effective for water with iron with its limited negative charge. Broadly, poly acrylic acid does not have sufficient negative charge to counter the coagulation effect by di-valent or tri valent [cations] positive ions like Al+++ or Fe++ ions. The tailoring of negative charge on dispersant to suit impurities is done by choosing right functional group or combination of multiple functional groups .like – COOH/ -SO3H etc. A dispersant can be a homo-polymer with one functional group, it can also be a co-polymer with two functional groups and a ter-polymer with three functional groups. The idea behind using different multiple functional groups is to increase negative charge on dispersant molecule to suit types of impurities in water. Larger the negative charge on dispersant molecule the better is its effectiveness for dispersion. A ter-polymer is more effective > than a co-polymer and a co-polymer is more effective > homo polymer due to multiple functional groups crafted in the molecule to counter the coagulation effect of tough multi-valent positive ions.

Role of chain length of polymer

Molecular weight or the size of polymer chain is another important criteria for dispersant. Molecular mass or chain length of a polymer gives ' surface ' to polymer for inter-molecular attraction. The size of molecule determines the Vander wall forces generated by the polymer. Larger the polymer chain, larger is surface of a polymer and consequently larger is its ability for inter-molecular attraction. Molecular chain size of dispersant is so chosen that Vander wall interactions do not exceed the negative surface charge repulsion by the molecule. MW kept nearly same, about 2000 DA.

 Dosage of dispersant

It is an important control. Low concentration, of dispersants do not cover full SS surface. At a high concentration, free dispersants do not adsorb. On the other hand an increase of Vander wall forces can promote flocculation. It is a very important concept.

Effect of hardness ions in water

Hardness ions decrease dispersion of iron. These positive ions neutralize a part of negative charge of dispersant.

Effect of pH

Preferred pH is 4.5 to 9. This promotes de-protonation or ionization of functional groups into anions.

How and why polymers used in flocculation different from dispersant? 

Dispersant does dispersion of suspended particles while flocculant does collection of suspended solid particles in water.

Very briefly, the polymers used in dispersant are short chain polymers with anionic functional group. The idea behind having short chain polymer is to make sure that the electrostatic repulsion between negatively charged suspended particles should not be offset by inter-molecular attraction between polymer chains. Larger the size of hydrocarbon chain of polymer, larger is the inter-molecular attraction so they have an appropriately sized hydrocarbon chain.

Polymers used in flocculation have long chain with either cationic or anionic functional group. The rationale is that long polymer chain produces large inter-molecular attraction [ Vander wall forces ]. In flocculation , the polymer when dissolved in water, uncoils into a long rope with huge surface area which pulls the neutral coagulated particles by inter-molecular forces and knot them into the rope and make them to agglomerate as big floc. .