It is important that residue Y is selected so that the total molecule of the formula I is not charged.It is also possible that X and Y are chemically bonded together, such as in moieties as ???(CH2)rN+(R11)2???(CH2)t???SO3???, with r is as defined above and t is an integer of 1 to 10, preferably 1 to 5, e.g. 2.