Synthesis of Anionic Surfactants

            The main classes of surfactants are comprised of anionic, cationic, nonionic, and amphoteric surfactants.  Anionic surfactants are the most widely used because they are such excellent detergents.  The most widely used anionic surfactants are SLS and ALS which stand for sodium lauryl sulfate and ammonium lauryl sulfate, respectively.  The major process for making anionic sulfate based surfactants is called sulfation.  Sulfation is the treatment of an alcohol or phenol with a sulfating agent to give an alcohol sulfate.  However, the resulting alcohol sulfate is not stable and must be neutralized with an alkali.  When the alkali is sodium hydroxide, the product is SLS and when the alkali is ammonium hydroxide, the product is ALS.  Sulfation results in the formation of an oxygen-sulfur bond.  Common sulfating agents include sulfur trioxide, sulfuric acid, chlorosulfonic acid, and sulfamic acid.  Another process of producing sulfate-based surfactants is sulfonation, which produces a carbon-sulfur bond. 

          Figure 1.  Sulfonation process

              Figure 2.  Sulfation process

The processes above use oils that are almost always naturally derived such as palm kernel oil, avocado oil, peach oil, etc.  The huge advantage of the surfactants derived from these methods is the availability of these natural and renewable oils.  Tallow acids are derived from animal fats and are made up of oleic acid(40-45%), palmitic acid(25-30%), and stearic acid(15-20%).  Coconut fatty acids are made up of C₁₂ acids(45-50%), C₁₄ acids(16-20%), C₁₆ acids(8-10%), oleic acid(5-6%), and 10-15% of smaller carbon chains [2].  Another advantage is the high degree of biodegradability because of the fact that it is naturally derived.  The major disadvantage of these anionic surfactants is that they are sensitive to divalent and trivalent cations, which produces soap scum.  This is usually combated by adding builders or secondary surfactants, such as amphoteric or nonionic surfactants.