Mainlyused as a process intermediate or a vital constituent in other products,surfactants are responsible for the cleansing properties of some substancessuch as soaps and detergents. This property of surfactants can be attributed totheir nature of containing a hydrophilic and a hydrophobic group (Lichtarowicz,2013). The hydrophobic part is commonly a straight or a simply branched chainand sometimes an aromatic ring, while the hydrophilic portion varies, which isthe basis of classifications of surfactants (Austin, 1984). One of thehydrophilic functional groups found in surfactants is the sulfonate group,under the anionic surfactants classification, and is one of the commonlymanufactured synthetic surfactants (Lichtarowicz, 2013).SulfonationThebasic chemistry of sulfonation is “sulfur trioxide reacting with an organicmolecule forming a sulfur-carbon bond” (Foster, 1997). Undergoingelectrophilic substitution mechanism, the sulfur trioxide (SO3)replaces the hydrogen bonded to a terminal carbon forming a sulfonate. It iscrucial that the sulfur trioxide substitutes a hydrogen bonded to a carbon andnot an oxygen as it would proceed with sulfation (Porter, 1991), which is stilladmissible for surfactants but would already create a large deviation from thephysical properties of sulfonates.Reaction conditionsSulfonationproceeds at a fast pace, releasing high amounts of heat amounting up to around380 kJ/kg of SO3 produced, which is problematic for industrial scalemanufacturing (Porter, 1991).
In order to solve this problem, industrialmanufacturers do not directly introduce sulfur trioxide into the reaction andinstead use “diluting and/or complexing agents … to moderate the rate ofreaction” (Porter, 1991). The common sulfonating agents are chlorsulphonicacid, oleum or fuming sulfuric acid, and gaseous sulfur trioxide, and they arespecifically used depending on the molecular structure of the raw material, suchas if they are aromatic or not. The viscosity of the feedstock also increasesas they are converted to the products making it hard for heat transfer (Porter,1991).