Relationships between the physical properties and biodegradability and bacteria toxicity of fatty acid-based ionic liquids

Abstract

This work is focused on the correlation between several physical properties (kinematic viscosity, viscosity index, refractive index, density and water solubility) and biodegradability and bacteria toxicity of a family of ionic liquids synthesized from fatty acids as anion precursor (FAILs): methyltrioctylammonium hexanoate [N8881][C6:0], methyltrioctylammonium octanoate [N8881][C8:0], methyltrioctylammonium laurate [N8881][C12:0], methyltrioctylammonium palmitate [N8881][C16:0], methyltrioctylammonium stearate [N8881][C18:0] and methyltrioctylammonium oleate [N8881][C18:1]. To that end, new values of these physical properties, biodegradability and bacteria toxicity were determined, although literature data for the [N8881][C8:0], [N8881][C12:0] and [N8881][C16:0] FAILs were also needed. It was found a good linear relationship (r2>0.90) between the biodegradability index (BOD5/COD) and the logarithm of kinematic viscosity, refractive index and water solubility for the saturated FAILs. Besides, the toxicity on both Vibrio fischeri and Escherichia coli can be successfully predicted using the logarithm of kinematic viscosity and viscosity index. Therefore, kinematic viscosity, which is an essential parameter for a lubricant, is the most promising physical property to estimate both biodegradability and bacteria toxicity of a family of ionic liquids. The double bond in the structure of the unsaturated FAIL ([N8881][C18:1]) is responsible for the worsening of the linear dependence between physical and environmental properties.