the groups of formula (I): ##STR1## in which n is zero or an integer from 1 to 30, the groups Z are chosen from alkylene or alkenylene groups having 2 to 30 carbon atoms, Z' is an alkyl or alkenyl-group having 2 to 30 carbon atoms and X and X' independently represent --O- M+, --S- M+ or an alkyl, alkoxy, aryloxy, aminoalkyl or aminoalkoxy group, M+ being an alkali metal cation or NH4 +, it being u