The galacturonic acid derivatives of formula: ##STR1## R1 being a linear or branched alkyl having 2 to 22 carbon atoms, R being: ##STR2## for which the carbon carrying the hydroxyl group is attached to the endocyclic oxygen atom. R2 being hydrogen, R1, an alkali metal atom, an alkaline-earth metal atom or a quaternary ammonium group of formula: ##STR3## in which R3 to R6 is each independently of the other hydrogen, an alkyl having 1 to 6 carbon atoms or a hydroxyalkyl having 1 to 6 carbon atoms, are surface-active agents.