(a) 0.1 to 50% by weight of a mixture of at least two different compounds selected from glyceryl-ether compounds having the formula (I): R???OCH2CH(OH)CH2OH???(I) wherein R is an alkyl or alkenyl group having 4 to 6 carbon atoms, wherein two of said at least two different compounds (1) are structural isomers of one another, or (2) differ in the number of carbon atoms,