As property maintenance company, we get calls all the time to pressure wash parking lots and parking garages. Sometimes, the customer wants all the oil stains in each parking stall to be removed, or there is an oil or grease spill on the property that they want cleaned up. We are often asked, does pressure washing remove oil stains?
Why Water Cleans so Effectively
Water molecules are polar, they have a positive charge on one end of the molecule and a negative charge on the other end of the molecule. This allows water to dissolve other polar substances – the negative charged of the substance will be attracted to the positive side of the water molecule or vice versa causing the substance to dissolve and be suspended and distributed throughout the water. Water dissolves more substances than any other liquid.
Water Does not Dissolve Oil
While water is great at cleaning, it will not clean oil. Oil is non-polar by definition. This means it will not dissolve in water and the common saying that “oil does not mix with water” holds true for pressure washing. Even at high pressure and temperature, oil and water do not mix. Those oil stains deep in the pore spaces of a parking lot that have been developing over years of cars leaking oil, will not be removed by pressure washing.
The Surfactant Exception
There is a way to modify the properties of water to allow it to mix with oil. This is through the addition of a surfactant to the water. A surfactant breaks the surface tension of water. The surfactant molecule is hydrophillic (bonds with water) on one end of the molecule and hydrophobic on the other end (bonds with oil). Soap is a surfactant. While the oil still doesn’t dissolve in water, the surfactant allows an oil molecule (attached to a surfactant molecule) to be suspended in water.
The Effects of Pressure Washing Oil Stains.
Pressure washing with only water, will not lift oil stains from pavement. When using soap, these stains can be lightened somewhat depending on how long the oil has been there. If the oil was spilled within a week or so, the majority of it can be removed, however, the longer it has been there, the more it gets deep into the pore spaces of the concrete and bonds through a process called adhesion. This creates a bond that is usually too strong to be removed with soap and high pressure water. In order to remove an oil stain from pavement a more intensive chemical treatment is necessary.