The effect of surfactant addition towards dispersion and antioxidant activity of tert-butylhydroquinone in biodiesel

Biodiesel is a renewable fuel as an alternative to fossil fuel. Currently, biodiesel is commercially used as a mixture of solar diesel. Biodiesel has many advantages. However, it has an instability issue towards oxidation which could lead to the low shelf life and quality degradation. Antioxidant additives such as tert-butylhydroquinone (TBHQ) is commonly used to prevent the oxidation. However, as an antioxidant additive, TBHQ has a low dispersion in biodiesel, thus it limits its antioxidant activity. Therefore, TBHQ is incorporated into biodiesel which was previously mixed with surfactant in order to increase its dispersion to increase its performance. In this research, three types of surfactants were used namely Sorbitan Monooleate, Glycerol Monostearate, and Polyglyceryl-4-isostearate to increase the solubility of TBHQ and its antioxidant activity. The functional group changes due to oxidation reaction product was confirmed by using FTIR. The solubility of the biodiesel mixtures was tested using UV-Vis Spectrophotometer. Result shows that the mixture of 100 ppm Polyglyceryl-4- isostearate with 2000 ppm TBHQ performed the most stable dispersion in biodiesel over one week observation followed by Glycerol Monostearate and Sorbitan Monooleate. The biodiesel quality in respect to acid number and iodine value were analyzed to evaluate the antioxidant performance of the additive-surfactant mixtures. The best improvement of oxidative stability of biodiesel over four-week storage time was performed by addition of 100 ppm Polyglyceryl-4-isostearate compared with other surfactants. These findings provide a potential use of surfactant, especially Polyglyceryl-4-isostearate, as a solubility enhancer as well as, more importantly, helping the antioxidant activity of phenolic additives in biodiesel.