The invention relates to a novel ultra-thin hydrophobic and oleophobic layer, formed by self-assembly on a solid substrate surface, of compounds of the general formula A-B in which A represents a group of the formula ##STR00001## in which Z represents C or N.sup.+, X represents C--H or C-L, L being an electron-attracting group selected from F, Cl, Br, I, CF.sub.3, NO.sub.2 and N(CH.sub.3).sub.3.sup.+, Y represents H or CH.sub.3, or Y forms a 5- or 6-atom heterocycle with X, T represents NH, CO, CONH or NH.sub.2.sup.-U.sup.-, U.sup.- being a soluble anion such as e.g. F.sup.-, Cl.sup.-, Br.sup.-, I, OH.sup.-, NO.sub.3.sup.-, HSO.sub.4.sup.-, SO.sub.4.sup.2-, CO.sub.3.sup.2-, HCO.sub.3.sup.- or SCN.sup.-, and B represents an unsubstituted C.sub.1-C.sub.20 linear aliphatic alkyl group,and a method of preparing this layer and its use as an epilame.