The molecular structure of sodium lignosulfonate contributes to its dispersing and emulsifying properties in the following ways:
- Sodium lignosulfonate consists of a lignosulfonic acid polymer backbone with phenylpropane repeating units. Some of these units have sulfonate groups attached.
- The sulfonate groups (SO3-) make the polymer strongly hydrophilic and water-soluble. This introduces negative charges along the polymer chains.
- In aqueous solutions, the negatively charged sulfonate groups interact with water molecules via ion-dipole interactions. This helps sodium lignosulfonate dissolve easily in water.
- The hydrophilic and charged sulfonate groups allow sodium lignosulfonate polymer chains to interact strongly with polar surfaces like cement particles in concrete.
- This adsorption onto particle surfaces helps disperse the particles by increasing electrostatic repulsion between them due to the negative charges.
- The adsorbed polymers also increase electrosteric repulsion between particles due to their large size, further improving dispersion.
- At oil-water interfaces, the hydrophilic polymer chains orient themselves with sulfonate groups interacting with water and hydrocarbon segments in oil. This enables it to act as an effective emulsifier.
- Thus, the introduction of sulfonate groups and the resulting hydrophilicity/charges are crucial for its dispersing and interfacial properties.
Compared to other dispersants and emulsifiers, the molecular structure of sodium lignosulfonate has some unique aspects:
- It is a renewable and biodegradable polymer derived from lignin, whereas many alternatives are petroleum-based synthetics.
- The backbone contains aromatic phenylpropane units, giving it a more rigid structure than flexible polymeric chain dispersants.
- Sulfonate groups are heavily substituted along the polymer backbone. This results in a very hydrophilic polymer compared to ones with side chain sulfonation only.
- Polydispersity leads to a wider range of chain lengths and structures versus well-defined synthetic polymers. This enhances multi-scale interactions.
- As a natural polymer, it has a more heterogeneous, irregular structure than synthetic polymers which tend to be more uniform.
- Unlike linear polymers, its branched/crosslinked structure from lignin origin allows for better binding and complexation.
- Its molecular weight distribution provides interactions across different size scales from small molecules to nanoparticles.
- Other natural polymers like lignins are less water-soluble without ionization groups like sulfonate. These make it a stronger dispersant.