Boron nitride films possess good hydrophilic properties and wettability with water, while the nanotube coating made from the same material, on the contrary, exhibit hydrophobic properties. How material which is normally perfectly wetted by the water repellent is? This question is answered by Russian scientists in their work, published in the latest issue of the journal "Russian Nanotechnology."
Development of coatings with high hydrophobic properties is of great practical importance. Such water-repellent coating to help effectively combat corrosion, to create self-cleaning surfaces and prevent icing various parts of machines and structures. In general, the water wettability of the material depends on its surface energy: the smaller the surface energy of the material, the worse it is impregnated with water. Droplets of water on a hydrophobic surface having a low surface energy tend to take the form of a perfect sphere and easily roll off it, while hydrophilic, have a high surface energy that the water spreads uniformly over the area.
However, in recent years there have been studies in which refers to the possibility of creating superhydrophobic coatings on the basis of materials with high surface energy. Composed of the same material and studied by scientists of the Institute of Physical Chemistry and Electrochemistry Frumkin coating based on boron nitride nanotubes. Boron nitride nanotubes are considered to be material, it is extremely attractive to the different areas of nanotechnology, because they have chemical resistance and mechanical strength.
In his experiments, investigators have studied three types of coatings deposited on a silicon substrate. The first type was a flat film of boron nitride, and the other two — deposited by chemical vapor deposition of a nanotube material. The latter two coatings differed only by the size of the nanotube: in one case were short (of the order of 500 nm.), Another long (greater than 10 microns.) Nanotubes.
Normal boron nitride film deposited on a silicon substrate has a high surface energy, as shown by the experiments is well wetted by the water while covering both types of nanotubes shows excellent hydrophobic properties. It is this unusual phenomenon prompted scientists to search for the mechanism of occurrence of hydrophobicity.
The first assumption of the researchers was that for the occurrence of hydrophobic responsible curvature and roughness of the surface coating of nanotubes. As the experiments and studies conducted earlier, the curvature of the surface, of course, affect the hydrophobic properties in favor of the increase, but it does not act, and the only main reason for increasing the hydrophobicity of the surface.
Another cause of the increase of hydrophobic properties of the coating of nanotubes can be adsorbed on the surface of the hydrocarbon contaminants. Hydrocarbons themselves — hydrophobic substance, and when connecting with the surface of nanotubes must reduce its surface energy. Indeed, the study of the chemical composition of the coating showed the presence of these compounds, after removing hydrocarbons from a substrate by heat treatment or plasma surface hydrophilic properties are acquired. Moreover, by maintaining the substrate at a hydrophilic-treated air, it will again be due to the strongly re-adsorption of hydrocarbons.
Clarification of the mechanism of the unique ability of coatings with a boron-nitride nanotubes spontaneously recover the strongly state can be used to create new materials, which can wetting cycles vary by external influences.
The research is supported basic research programs number 7 and number 22 of the Presidium of RAS.
Source of information:
AD Aliyev, LB Boynovich, VL Buhovets, AM Emelianenko AM Gorbunov, AE Gorodetsky, AS Pashinin "superhydrophobic coatings based on boron nitride nanotubes: a superhydrophobic and self-healing mechanism of highly hydrophobic properties." Russian Nanotechnologies, 2011, v. 6, № 9-10.