Volume 6, Issue 8 (September 2009)
Selection of Sequestering Agents Having Affinity for Calcium Ion for Stabilization of the Emulsifiable Concentrate Formulations
Emulsifiable concentrates (ECs) are often used for pesticides, which are highly soluble in a non-polar solvent. When a pesticide is hardly soluble in a normal non-polar solvent, it is difficult to prepare an EC formulation with high pesticide content. In order to overcome this difficulty, polar solvents are used to enhance the solubility of the pesticide. N-methyl-2-pyrrolidone (NMP) is well-known as one of the most preferred polar solvents. However, many agrochemical manufacturers have started research to remove this solvent from EC formulations because of recent regulations by the authorities. However, it is not easy to find alternative solvents that have both good solvency and a good safety profile. The authors selected diethyl oxalate as an alternative candidate for NMP by using the organic conceptual diagram discussed in the previous report. A pesticide in an EC formulation containing diethyl oxalate as a co-solvent was chemically stable and showed good emulsion stability even after accelerated storage. These results suggested that this solvent could be used as an alternative to NMP. However, a decrease in pH and water content were also observed, and the sample became turbid after storage. As this turbidity was confirmed to be derived from the generation of calcium oxalate, the calcium oxalate generation process was studied. As a result, it is speculated that diethyl oxalate was hydrolyzed to oxalic acid under the acidic conditions and calcium ion, derived from an anionic surfactant contained in the EC formulation, reacted with the oxalic acid to form calcium oxalate. In order to inhibit this reaction, several kinds of sequestering agents were tested. Among them, mono-alcohols were the best though they did not completely inhibit the reaction. Though the mechanism is still unclear, mono-alcohols were effective in blocking the binding of calcium ion and oxalic acid. A mono-alcohol is a structured molecule having a hydrophilic head region and a lipophilic tail region like a surfactant. If calcium ions are dissolved in the water in the EC formulations, even though the water content is low, a mono-alcohol may form something like an inverse micelle, which captures the calcium ions inside.