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Isobutyl oleate is an ester, while oleic acid is a carboxylic acid. Since esters are typically less polar than carboxylic acids, you could use liquid-liquid extraction with a suitable solvent. A solvent like hexane or diethyl ether would preferentially dissolve the less polar isobutyl oleate, allowing you to separate it from the more polar oleic acid. You could then evaporate the solvent to obtain pure isobutyl oleate.