C24 aliphatic acid ester of pentaerythritol, of an oligomer of pentaerythritol, of an alkylene oxide derivative of pentaerythritol having 1 to 10 alkylene oxide moieties, or of an alkylene oxide derivative of an oligomer of pentaerythritol having 1 to 10 alkylene oxide moieties, or a mixture of any two or more thereof, characterised in that it further comprises from 0.2 to 10% of an anionic ...