Ethanol is a hydrophilic compound, it’s hydroscopic, which means it naturally attracts water. Therefore, when water enters a tank containing an ethanol blended fuel, (either via natural means or through contamination/intrusion), the water will eventually be absorbed by the
ethanol. Once critical mass is reached Phase Separation can occur.
How Phase Separation forms
Water in the tank will continue to be absorbed into the ethanol-blended fuel until it reaches a saturation point. With E-10 fuel at 15°C, this saturation point is at approximately 0.5% water to the volume of fuel. Once the
blended fuel reaches this saturation level, the ethanol-water mixture separates from the fuel and falls to the bottom, since its density is now higher than the fuel with which it is mixed. This separated layer is commonly known as Phase Separation. The phase separation layer typically contains 3-4 parts of ethanol for each part water. An alternate phenomenon can occur with far less water than that needed for phase separation.
As previously discussed, some of the water will drop directly to the bottom of the tank as it enters. This water will pull some ethanol from the fuel as it passes through it. This water-ethanol mixture will then pull additional ethanol from the surrounding fuel over time, increasing in volume and resulting in a mixture that is very similar to phase separation; this layer is referred to as Partial Phase Separation. Partial phase separation can occur with far less than 0.5% water. The resulting layer may contain less ethanol than normal phase separation, making it more dense.
What are the possible effects of Phase Separation?