Abstract: Four p-hydroxybenzoic alkyl esters were synthesized by direct esterification reaction from p-hydroxybenzoic acid (HA) and a series of alkyl alcohols. The antioxidant efficiency of p-hydroxybenzoic acid and its alkyl esters in aqueous methanol solution were determined by the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. The Schaal oven test was used to determine their antioxidant efficiency in rapeseed oil-in-water emulsions with an oil/water volume ratio of 2:8. Results showed that p-hydroxybenzoic octyl esters had the highest antioxidant efficiency in aqueous methanol solution, and its DPPH radical scavenging capacity was 339.40 mmol Trolox/mol. In contrast, p-hydroxybenzoic butyl esters had the maximum antioxidant efficiency in rapeseed oil emulsions. In the case of the addition of p-hydroxybenzoic butyl esters to emulsions with an emulsifier concentration of 1%, the time needed to reach a conjugated diene value (CDV) of 1 and a p-anisidine value (p-AV) of 6 was 27.1 and 19.8 days, respectively. The distribution profiles of p-hydroxybenzoic acid and its alkyl esters were obtained in the interfacial region of the emulsions by using the pseudophase kinetic model to interpret their antioxidant behaviors in the emulsions. Results showed that the antioxidant efficiency of p-hydroxybenzoic acid and its alkyl esters in the emulsions roughly correlated with their concentrations in the interfacial region.

Key words: p-hydroxybenzoic acid; esterification; emulsion; antioxidant efficiency; pseudophase kinetic model