What is "lignosulfonates"

Lignosulfonates, or sulfonated lignin, ( CAS number 8062-15-5) are water-soluble anionic polyelectrolyte polymers: they are byproducts from the production of wood pulp using sulfite pulping.

Most delignification in sulfite pulping involves acidic cleavage of ether bonds, which connect many of the constituents of lignin. The electrophilic carbocations produced during ether cleavage react with bisulfite ions (HSO) to give sulfonates.

R-O-R' + H → R + R'OH R + HSO → R-SOH

The primary site for ether cleavage is the α-carbon (carbon atom attached to the aromatic ring) of the propyl (linear three carbon) side chain. The following structures do not specify the structure since lignin and its derivatives are complex mixtures: the purpose is to give a general idea of the structure of lignosulfonates. The groups R and R can be a wide variety of groups found in the structure of lignin. Sulfonation occurs on the side chains, not on the aromatic ring like in p-toluenesulfonic acid.

Lignosulfonate have very broad ranges of molecular mass (they are very polydisperse). A range of from 1000–140,000 da has been reported for softwood lignosulfonates with lower values reported for hardwoods.