Structural aspects of surfactant selection for the design of vegetable oil semi-synthetic metalworking fluids.

This paper presents a set of surfactant-selection guidelines that can be used to design bio-based semi-synthetic metalworking fluid (MWF) microemulsions as a renewable alternative to conventional petroleum formulations. Ten surfactant classes (six anionic and four nonionic) with different head and tail structures and three vegetable base oils (canola oil, soybean oil, and a fatty acid trimethylolpropane ester) were investigated as representatives of oil and surfactant options currently under consideration in the MWF industry. All combinations of these surfactants and oils were formulated at the full range of oil to surfactant ratios and surfactant concentrations. The stability of each formulation was evaluated based on visual transparency, light transmittance, and droplet diameter. The experimental results yield the following guidelines that produce stable bio-based MWF microemulsions with minimum necessary concentrations of surfactants: (1) a combination of two surfactants, one nonionic and one water soluble co-surfactant (either nonionic or anionic) is preferred over a single surfactant; (2) the nonionic surfactant should have a carbon tail length greater than or equal to the nominal carbon chain length of the fatty acids in the oil as well as a head group that is not excessively small or large (e.g., 10-20 ethylene oxide groups for a polysorbitan ester, ethoxylated alcohol, or ethoxylated glyceryl ester); (3) the difference in tail lengths between the surfactant and the co-surfactant should be less than 6 to maximize the feasible range of oil to surfactant ratios yielding stable emulsions. These guidelines are consistent with general results of micelle solubilization theory and evidence is provided to suggest that common semi-synthetic MWF systems can be thought of as swollen micelle systems.