A compound of general formula (I) or (II), wherein Rn and Rm independently represent a C3-C27 aliphatic group, R1 to R4, which may be the same or different at each occurrence, represent hydrogen or a C1-C8 alkyl group, X1 and X2, which may be the same or different at each occurrence, represent a linear or branched divalent hydrocarbon radical with 1 to 24 carbon atoms which can be optionally substituted and/or interrupted by one or more heteroatoms or heteroatom containing groups, and R5 and R6, which may be the same or different at each occurrence, represent a group selected from —O−, -Alk-CH(OH)—CH2—SO3— and -Alk-CO2— wherein Alk represents an alkylene group.