Interfacial Tension (IFT)
ASTM D-971 Interfacial Tension of Oil against Water by Ring Method.
IFT is a test used to determine the presence of oxidation by-products, polar contaminants, and other decomposition products. These can be acids, oxidation intermediate compounds, varnishes, other fluids that are in the fluid. These compounds generally lower the IFT value, and that value is a good judge of overall condition of the fluid.
To perform, a platinum ring is pulled from a distilled water layer through a layer of fluid to be tested placed on top. Since fluid are inherently different and the densities of fluids such as oils and esters is less than water, they sit upon the water layer and form a stable interface. The better condition of the fluids form stronger interfaces. The contaminates in the oil work to destabilize the interface. When pulling the platinum ring from the water into the oil, we measure the force need to break this interface. This force is measured in milli Newtons per meter (mN/m) or dynes/centimeter (dynes/cm). For reference a dyne is a measure of force equal to 0.000002247 pounds.
For good clean mineral oil, a value of 40-50 mN/m is obtained, for natural esters a value of 18-20. When values fall below 25 for mineral oil and 10 for natural esters, sludging of the fluid could begin in the fluid.
IFT has a definite relationship with the acid content oil the fluid. As acid content increases within the fluid it should also show a distinct lowering value in the IFT value obtained. IFT is also reduced by the presence of the intermediate compounds created in oxidation before the acids are actually formed. This can be precursor indication to increased acid content. Acid and other fluid quality tests are used to determine if the fluid needs to be reconditioned, reclaimed, or replaced.
