to about 10 ethylene oxide units and from 0 to about 1 glycerol unit, Y denotes an N-, P- or S-containing group, R6 denotes an alkyl or monohydroxyalkyl group with 1 to 3 carbon atoms, x equals 1 when Y is a sulfur atom and x equals 2 when Y is a nitrogen atom or a phosphorus atom, R7 denotes an alkylene or hydroxyalkylene group with 1 to 4 carbon atoms and Z[???]denotes a carboxylate, sulfate, p