There are provided according to the invention compounds of formula (I) or salts or solvate thereof, wherein:n and N independently represent an integer 2 to 12;m1 and m2 independently represent an integer 1 to 15; and R1 and R2 independently represent -(CO)xC1-9 alkyl or -(CO)xC1-9 fluoroalkyl, which fluoroalkyl moiety contains at least 1 fluorine atom and not more than 3 consecutive perfluorocarbon atoms wherein x represents 0 or 1. The invention includes uses of said compounds and processes for their preparation.