SO H or $0 M groups wherein M is a monovalent alkali such as ammonium or an alkali metal, in amounts corresponding to from 1 to 1.3 alkyl substituents each corresponding to an alkyl radical containing from 9 to 15 carbon atoms, e. g. a tripropylene, a tetrapropylene or a pentapropylene radical, and from 1.8, preferably from 2 to 2.3, sulfonate groups per diphenyl ether nucleus, in order to obtain