The terms oleophobic is becoming more popular, especially with regard to coatings and surface treatments. But what does this term mean?
“Phobic” comes from the Greek root “phobos,” meaning “fear” and “oleo” refers to oil but as commonly used, refers to most organic liquids. It follows that the literal meaning of oleophobic is “afraid of oil”. The term oleophobic commonly refers to a surface that repels oil and oil-based materials.
When oil and oil-based materials come in contact with an oleophobic surface, individual drops of will bead up and will roll off or slide off the surface with ease. Like hydrophobic surfaces, larger amounts of organic liquids will pool together, the edges pulling back from the oleophobic surface and move as one sheet. This effect can also be described as de-wetting. Where the liquid pulls away the surface seems to remain totally dry.
With the potential to impart properties such as stay-dry, easy-to-clean, oleophobic coatings and treatments are finding their way into more mainstream applications. Anti-smudge and anti-fingerprint coatings are oleophobic and are used widely on touchscreens.
But how can the degree of oleophobicity be measured? As with hydrophobicity, this can be determined by measuring how “spherical” a drop is that is sitting on the oleophobic surface. Whereas in hydrophobicity, water is used as the test fluid, in oleophobicity, other fluids are used, typically short-chain alkanes.
One of the most common standard fluids for measurement is n-hexadecane. As the oleophobicity of the surface increases, the surface tension, although comparatively low, causes the liquid to pull itself away from the surface more and more into a sphere. If a line is drawn tangent to the drop as it contacts the surface, then the angle (θ) between that line and a line parallel with the surface is said to be the Contact Angle (CA).
Surfaces with an n-hexadecane CA of 60-80 are considered very oleophobic. An example of historic importance is polytetrafluoroethylene (Teflon,) with a contact angle of 65o. Polytetrafluoroethylene is well known for its repellent, non-stick and non-friction surface. As with hydrophobicity, oleophobic surfaces do not actually repel a liquid, there is simply no attraction between the surface and the liquid droplet.
If there was an attraction, the surface would be considered oleophillic. Oleophillic means “oil loving.” Oleophillic surfaces attract oils or organic fluids and will allow it to level out or wet out. Most surface are oleophoillic relative to the oily liquid, which because of its low surface tension tends to spread out.
Measuring Contact Angle:
To measure the contact angle, an instrument called a goniometer is used. “Goni” is the Greek prefix for “angle.” This instrument will deposit a drop of liquid onto the surface of the substrate and measure the angle between a line tangent to the drop and a line parallel to the surface.
Newer equipment will deposit the drop automatically and then measure the angle using a video camera and specialized software. More sophisticated equipment and software programs enable the researcher to analyze the surface in different ways, in fact, surface science or the study of surface chemistry and properties is expanding rapidly.
What makes a surface oleophobic?
Since most organic fluids are slightly polar to non-polar, an oleophobic surface must very non-polar. To achieve this condition, a surface must contain non-polar atoms such as silicon, fluorine or chlorine. In many instances, these atoms are formed into polymeric molecules that form a wall of non-polarity at the surface.
It is interesting that some surfaces are hydrophobic but not oleophobic. Some surfaces are both hydrophobic and oleophobic.
Examples of Oleophobicity:
A well-studied example of a surface that is both hydrophobic and oleophobic is polytetrafluoroethylene (PTFE or Teflon). PTFE is made up of long carbon chains where every hydrogen is replaced with a fluorine atom. This chemical structure presents a wall of non-polar fluorine atoms making PTFE highly repellent to both water and oils and thus suitable for many non-stick applications. It is highly inert, making it suitable for many fluid handling, sealing and containment applications.
Another benefit of the fluorine rich structure is the dimensional stability over a wide range of temperatures.
A major limitation of PTFE is that it is a solid substance and so must be cut or formed into various shapes. PTFE can be coated but it requires a multi-step process at high temperatures. This process can be cost prohibitive for many applications.
NanoSlic coatings are both hydrophobic and oleophobic. With a water contact angle in the 105o-107o range and n-hexadecane contact angle in the 65o-70o range, these coatings provide a functional surface similar to PTFE. This high degree of water and oil repellency develops during the curing phase.
As the solvent dries, highly non-polar molecules align themselves and form the coating’s surface. Below this layer, the bulk of the coating polymerizes into an inert silica-based structure. At the substrate surface, the polymer chemically bonds to hydroxyl groups on the substrate.
The advantage of this approach is that a high degree of hydrophobicity, oleophobicity, adhesion, temperature and abrasion resistance can be achieved is a one-step, low cost process. NanoSlic can be applied by methods such as dipping and spraying and 3-dimensional surfaces can easily be coated.
Very thin layers, in the 2-4 micron range provide excellent properties. Therefore, NanoSlic coatings are an excellent choice for the most demanding applications and challenging environments where hydrophobicity, oleophobicity and temperature resistance are required.