A particularly preferred compound of formula (I) is that corresponding to n =0, namely, benzene-1,4-di(3-methylidenecamphor-10-sulfonic) acid. ##STR3## in which R2 is a hydrogen atom, a halogen atom, an alkyl radical having about 1 to 4 carbon atoms or an --SO3 H radical; and R3 and R4 are each a hydrogen atom or an --SO3 H radical, at least one of the radicals R2, R3 or R4 being an --SO3 H radica