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 whichA 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, CF.sub.3, NO.sub.2 and N(CH.sub.3).sub.3.sup.+, Y represents H or CH.sub.5, 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, and B represents a C.sub.1-C.sub.20 linear aliphatic alkyl group partially or completely substituted with F,and a method of preparing this layer and its use as a harrier film.