The Science of Clean: Cleaning 101

Bite-Sized Blog: What it Means to Clean Green Series

We all have those pesky things around the house that we dread having to clean… dirty dishes covered in old, crusted tomato sauce or peanut butter someone conveniently forgot about or perhaps it is a greasy stove, a dirty bathroom or stains on your favorite shirt.

Rather than wearing yourself out again with the same scrubbing with hot soapy water and a ton of “elbow grease”, take some time to consider the mess itself and what type of cleaning may be best to remove it. Also, check out our next post with tips to help you tackle even the trickiest household mess!

We’ve all seen this at-least once in our lives…

To effectively approach cleaning a stubborn mess, it’s a good idea to first review and understand the main ways dirt or grime (often generically called “ soil”) is removed from a surface and how the methods work. These methods are mechanical, chemical, and detergency.

  • Mechanical: physically removing the dirt or grime from a surface, often by means of abrasion (scrubbing).
Mechanical action is seen through suds! Naturally we are trained to see this behavior to know it WORKS!
  • Chemical: use of a material that reacts with the soil to alter or degrade it so that it is easier to remove or dissolve (e.g. enzymes to breakdown a soil, a base to saponify grease/oil)
Solvents composed of specific proportions of material and high pH can be used historically to remove resins and hard soils. Modern formulations have moved away from this technique.
  • Detergency (surface chemical process): which uses surfactants to assist with removing soil  by the surface chemical process1 that is explained in further detail below.

If it sounds like you’re often doing more than one of the above – you’re right! Much of our everyday cleaning involves combining these methods, such as scrubbing dishes with a brush or sponge (mechanical) and using dish soap and water (detergency)

Detergency in Detail

All detergency processes involve “wetting” to some degree.1 Wetting is the ability of a solution to spread out or become flat over a given surface, rather than beading up. Think of the grade school experiment of adding water droplets on a penny – this beading indicates that it is not wetting (even though it is water!). Wetting of the cleaning solution helps to bring the cleaning solution and soil closer together so that the soil can more easily be removed.1 As we mentioned, plain water tends to form beads on many common household surfaces. To avoid this beading, and improve the efficiency of their cleaning products, formulators select surfactants to assist with reducing the surface tension of water and other solvents so that the resulting solution can more evenly spread across a surface (or “wet”)  to improve cleaning. For more information on surfactants and cleaners, visit our Anatomy of a Cleaner post!

Aside from wetting, detergency processes differ depending on the soil being removed. For solid or particulate soils like dirt, surfactants in a cleaning product adsorb or aggregate onto the surface of the particulate soil and surface needing to be cleaned (substrate). Buildup of the surfactant on both the soil and substrate allow for a buildup of like charge on the substrate and soil, causing repulsion which in turn causes a reduction in soil adhesion to the substrate and allows the soil to be removed.

Figure 2: Example Showcasing particulate soil being dislodged from a substrate by means of like charge build up and repulsion
Green Chem., 2007,9, 203-212

The process is different for solid organic soils (non-particulates like wax and grease). After wetting, this type of soil is “liquified” or brought into the cleaning solution. Liquification is thought to involve converting the soil layer by layer into a more liquid phase or phases which aids in mechanical removal.1 Finally, liquid or oily soils are removed by a process called “roll-up”.  When the soil and substrate are exposed to a water-based cleaning product, the surfactants present in the solution can preferentially interact with the substrate (i.e. the surface being cleaned) instead of the soil that is being removed. As more surfactant interacts with the substrate, less surface area becomes available for the soil-substrate interaction. Once the surface available for the soil is reduced enough, it can be dislodged from the substrate.

Figure 3: Roll up mechanism showing the removal of oily soils from a substrate

As we mentioned earlier, we generally use a combination of cleaning techniques. Often, detergency is used in combination with mechanical or chemical means to remove soils effectively and quickly from substrates. Our earlier example was using a sponge or brush when washing dishes along with soap and water. This method uses detergency to remove soils as well as mechanical means. When you have multiple soils on a surface, it can be helpful to employ more than one mechanism for cleaning to effectively accomplish your task.

Additional Source

  1. Lange, Robert K. Detergents and Cleaners a Handbook for Formulators. New York, 1994

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