Synthesis of oxidized ricinoleic acid esters as antimicrobial and emulsifier compounds

Ricinoleic acid is the main fatty acid constituent of castor oil which has antimicrobial activity. Research on the use of ricinoleic acid in ester form is more widely carried out, because it has a higher antimicrobial activity. In this study, commercial ricinoleic acid was oxidized using KMnO₄ before it was used for esterification. The ester synthesis was carried out by reacting oxidized ricinoleic acid with acetic acid, lauric acid, or oleic acid at 60°C for 6 hours using ZnCl₂ as catalyst. The molar ratio oxidized ricinoleic acid to varied carboxylic acids was 3:1. The ester products were then characterized using FTIR and examined as antimicrobial assay against Staphylococcus epidermidis and Propionibacterium acnes. Emulsifier test was performed to observe the stability and emulsion type, using ester products as emulsifier. FTIR spectra showed a typical absorption band of C-O ester which indicates that oxidized ricinoleic acid esters had been successfully formed. The antimicrobial activity assay showed positive results against the growth of Propionibacterium acnes and Staphylococcus epidermidis. Oxidized ricinoleic-lauric ester showed the highest antimicrobial activity against Staphylococcus epidermidis. The emulsifier test showed that the three esters had ability to stabilize emulsion up to 24 hours for water-in-oil (w/o) emulsion type.