Oleophobicity can be thought of as an extension to hydrophobicity; it works using the same principles though it is more difficult to impart. Oleophobicity describes how oils interact with a surface, and tends to imply that a surface rejects oils from it. Oils are not just limited to engine oils, cooking oils and oil for fuel; many food products are oil-based, and dirt that stains textiles often contains oils. As such, modifying the way textiles interacts with oils is very important.
Surface Energy is fundamental to how oils interact with textiles, and modifying this is important to obtaining the best oleophobic properties.
In some regards oils can be thought of as the opposite to water, but despite this, oleophobic surfaces also tend to be hydrophobic. There are a great deal of techniques used to make a surface oleophobic, though most rely on either roughening a low energy material or imparting a rough material with a low energy surface.
Recently, oleophobic and hydrophobic surfaces have gained widespread public interest because of online videos that show seemingly incredible results. For example, NeverWet make a superhydrophobic product that is also oleophobic. Their Youtube video has been watched over 3 million times
Oleophobicity is of great importance to almost all aspects of the textile industry. The applications are everywhere: gloves that shed engine oil, upholstery that doesn’t stain, and soldiers’ uniforms that repel chemical contaminants. In many cases these oleophobic materials are already in use: self-cleaning clothes or sheets sound implausible but they already exist. Any textile that comes into contact with oils, fats, or similar organic substances could benefit from a tuned oleophobicity. In some cases, such as in filters or cleaning products, a fabric may require oleophilicity rather than oleophobicity. As they are opposites, understanding how oleophobicity works enables oleophilicity to be developed also.
Regulations and Legislation
There are no standards, regulations or legislation devoted to oleophobicity. A standard test for the resistance of textiles to hydrocarbons is available, though, and for the absorption of oils (oleophilicity).Also, hydrophobicity tests (such as BS ISO 23232:2009 and ISO 23232:2009 for aqueous stain resistance and BS EN 29865:1993 and ISO 9865:1991 for the Bundesmann rain test) could be applied to oleophobic surfaces, only the liquid would need to be changed to an oil.
Terms and Definitions
Neither ASTM nor the Textile Institute define oleophobicity. However, ‘oleo’ refers to water, and ‘phobic’ means to hate. Therefore, oleophobicity can be thought of as oil-hating. It is the opposite of oleophilicity (oil-loving). One dictionary defines oleophobic as “lacking affinity for oils”.
Oils are not easy to define. IUPAC, the International Union of Pure and Applied Chemistry, uses the terms ‘oil’ and ‘organic liquids’ interchangeably. Organic liquids include anything that are liquid at room temperature and pressure and based on carbon. “Organic liquids” is a very wide definition, but is certainly preferable to the dictionary definition of “a viscous liquid derived from petroleum” which is not correct in this instance. A liquid does not have to be 100% oil to be oily. For example, whiskey is oily, yet is over 50% water.
Oleophobicity is more difficult to impart than hydrophobicity, though it relies on the same principles: to be hydrophobic a solid must have a lower Surface Energy than water, and to be oleophobic a solid must have a lower Surface Energy than oil. Surface Energy (also described in Hydrophobicity) originates from the difference between the bulk interactions in a liquid and the surface molecules’ interactions. The interactions present in water and oil are very different, and this is due to their chemical structures, shown in Figure 1and Figure 2:
Figure 1– generalised structure of oil molecules
Figure 2- water molecules
There is only two types of interaction between oil molecules: van der Waals forces. These are very weak forces that originate from instantaneous dipoles (analogous to electrical charge) that are induced in atoms. Because van der Waals forces are so weak the oil molecules are not strongly bonded to one another, meaning that the difference in energy between interactions at the surface and in the bulk is relatively small, resulting in a low Surface Energy. In the case of water there are three intermolecular interactions: van der Waals forces, permanent dipole forces, and hydrogen bonding. Permanent dipole forces originate from the difference in electronegativity between hydrogen and oxygen and mean that the hydrogen and oxygen molecules on adjacent atoms are quite strongly attracted to one another, in a way analogous to magnets. Hydrogen bonding is the strongest of all intermolecular forces and is largely responsible for many of the properties of water, including its high Surface Tension? and its high boiling point. Hydrogen bonding also derives from the difference in electronegativity between hydrogen and oxygen, and is further influenced by the very small size of the hydrogen atoms. Permanent dipoles and hydrogen bonding are both far stronger interactions than van der Waal forces. Because of the strong interactive forces present in water, the difference between the interactions in its bulk and the interactions at its surface are large, meaning it has a high Surface Energy.
Because oleophobicity is harder to impart than hydrophobicity, it can be said that all oleophobic surfaces are hydrophobic but not all hydrophobic surfaces are oleophobic. Water has a Surface Tension? of approximately 73 mN m-1 (millinewtons per metre) at room temperatures, whereas oils’ surface tensions are approximately 20 mN m-1, depending on the oil. To be oleophobic a surface must have a surface lower than 20 mN m-1.
To impart oleophobicity one must either chemically modify a rough surface using a low Surface Energy material, or roughen a low energy surface. Roughness increases a Contact Angle? already in excess of 90 °, but lowers a Contact Angle? lower than 90 °. Simply put, it can be thought of as an amplifier. Chemicals that deliver sufficiently low surface energies to be oleophobic include:
· Paraffin hydrocarbons
There is a list of surface energies for common polymers available online.
Methods to roughen low energy materials include:
· Plasma etching
· Mechanical stretching
· Microphase separation (including layer-by-layer assembly and self assembly)
· Electrochemical deposition
· Etching and lithography
· Nanowire/nanotube deposition
Not all of these methods will work with all materials, but there are new methods being created all the time.
Challenges and Innovation
Oleophobicity is an old science, but it continues to develop. As with so many technologies, major innovations need to be environmentally benign. As is the case for hydrophobic surfaces, there is currently great reliance on fluorochemical coatings to provide oleophobicity. Fluorochemicals are environmentally damaging but also provide high performance at relatively low cost.
There is a lot of research being carried out into new methods to roughen surfaces, and many of these techniques are yet to be applied to textiles, particularly in a commercial setting. For example, lithography has not been used commercially to produce oleophobic surfaces, nor has nanotube deposition. In the case of nanotube deposition, industrial use may be some way off, but there is great room for academic research in this area.