Mainly
used as a process intermediate or a vital constituent in other products,
surfactants are responsible for the cleansing properties of some substances
such as soaps and detergents. This property of surfactants can be attributed to
their nature of containing a hydrophilic and a hydrophobic group (Lichtarowicz,
2013). The hydrophobic part is commonly a straight or a simply branched chain
and sometimes an aromatic ring, while the hydrophilic portion varies, which is
the basis of classifications of surfactants (Austin, 1984). One of the
hydrophilic functional groups found in surfactants is the sulfonate group,
under the anionic surfactants classification, and is one of the commonly
manufactured synthetic surfactants (Lichtarowicz, 2013).

Sulfonation

The
basic chemistry of sulfonation is “sulfur trioxide reacting with an organic
molecule forming a sulfur-carbon bond” (Foster, 1997). Undergoing
electrophilic substitution mechanism, the sulfur trioxide (SO3)
replaces the hydrogen bonded to a terminal carbon forming a sulfonate. It is
crucial that the sulfur trioxide substitutes a hydrogen bonded to a carbon and
not an oxygen as it would proceed with sulfation (Porter, 1991), which is still
admissible for surfactants but would already create a large deviation from the
physical properties of sulfonates.

Reaction conditions

Sulfonation
proceeds at a fast pace, releasing high amounts of heat amounting up to around
380 kJ/kg of SO3 produced, which is problematic for industrial scale
manufacturing (Porter, 1991). In order to solve this problem, industrial
manufacturers do not directly introduce sulfur trioxide into the reaction and
instead use “diluting and/or complexing agents … to moderate the rate of
reaction” (Porter, 1991). The common sulfonating agents are chlorsulphonic
acid, oleum or fuming sulfuric acid, and gaseous sulfur trioxide, and they are
specifically used depending on the molecular structure of the raw material, such
as if they are aromatic or not. The viscosity of the feedstock also increases
as they are converted to the products making it hard for heat transfer (Porter,
1991).