a) a zwitterionic monomer of the formula I YBX???I wherein B is a bond or a straight or branched alkylene, alkylene-oxa-alkylene or alkylene-oligooxa-alkylene group, any of which optionally include one or more fluorine substituents; X is an organic group having a zwitterionic moiety; and Y is an ethylenically unsaturated polymerisable group; b) a cationic monomer of the formula II Y1B1Q1???II wher