Ionic liquid surface coatings

A water-repellent material comprises a substrate and an ionic liquid coating of formula [(R'O)3Si-Y-X+-R3] [Z]-, wherein X is nitrogen, sulphur, or phosphorus; Y is a divalent aromatic or C1-10 aliphatic linker; [Z]- is an anion; R' is C1-12 alkyl; and R is independently C1-18 alkyl optionally substituted by halogen and/or C1-16 alkyl or where the R groups combine with [X]+ to form a heterocycle in which one R group may be a bond. The ionic liquid is at least partially crosslinked and chemically bonded to the substrate. Preferably, R' is methyl or ethyl; R is independently selected from methyl or fluorinated C2-14 alkyl; Y is C2-6 alkylene, dialkanylether, or functionalised polystyrene; and [Z]- is halide, phosphate, sulphate, sulphonate, borate, carboxylate, amide, or imide. The substrate may be a construction material or medical device having surface hydroxyl groups. In another aspect, a dispersion for forming a water-repellent coating comprises a liquid medium and the ionic liquid.