The structure-behaviour relationships of a class of highly hindered monohexadecyl hydrocarbylphosphonates, R^1(R^2O)P(O)OH, where R^1 represents CH~3, C~2H~5, i-C~3H~7, cyclo-C~6H~1~1 or C~6H~5, and R^2 represents C~4H~9CH(C~2H~5)C~2H~4CH(C~6H~1~3) or C~6H~1~3CH(CH~3) CH~2CH(C~6H~1~3), in the extraction of Yb(III) and Lu(III) from chloride medium are investigated. The results show that the extraction and separation abilities for Yb and Lu decrease with increasing the steric hindrance of the substituents in extractant molecules.

Key words: PHOSPHONIC ACID ESTER, EXTRACTION, YTTERBIUM, LUTETIUM, STRUCTURE AND PROPERTY CORRELATION, SUBSTITUENT EFFECT