I. The Basics – Solvents and Surfactants
Have you ever looked at the ingredients of your household cleaner and wondered, “What on earth are these chemicals? Why are they necessary to clean my home?” If so, you are like many others. In this bite-sized blog series, our Ph.D. scientists will describe the components of a typical household cleaner and clarify some of the jargon used in the field.
A water phase (which is polar) and an “oil” phase (which is non-polar or less polar) do not remain mixed together without separating out – just like mixing olive oil and water together. Some cleaners only have a water-based phase, while others have both “oil” and water phases. Some specialty cleaners only have an “oil” phase.
In a cleaner that has both a water phase and an “oil” phase, the ingredients comprise one of the two phases mentioned above or are additives to help accomplish one of the following tasks: blend the phases together, improve cleaning power of the solution, or otherwise modify the formula to improve certain physical characteristics. With this in mind, most cleaning ingredients can be classified using the following categories: Solvents, surfactants, builders, fragrances, preservatives, pH adjusters and product modifiers.1 Although an ingredient may be selected for a formula to act in one of these categories, these materials can serve multiple roles. This week, our focus will be on understanding solvents and surfactants.
These ingredients comprise the two phases (oil and water) of the cleaner. The main purpose of a solvent in a formula is to dissolve unwanted “soils“. A soil in the cleaning industry is defined as any unwanted material found on a surface, and typical household soils include: grease/oils, proteins, carbohydrates, fatty acids and solid particulates.1 When dissolving a soil, solvent molecules orient themselves around soil molecules and surround or encapsulate them. This action causes the soil molecule to be pulled into solution, removing it from the surface. Solvents make up the largest portion of the cleaning formula and are selected so that they can dissolve a wide array of different soils.
Certain soils, however, are only soluble in certain solvents. The solubility (or ability for a soil to dissolve) in a solvent is determined by the soil’s polarity. Soils like grease, oils and fatty acids are considered to be fairly non-polar and require a non-polar or low polarity solvent to dissolve them. For example, we have all seen that olive oil is not soluble in water and requires a different solvent choice to help bring it into solution. Other soils are more polar and can be dissolved by a polar solvent like water or glycol. To refer to the saying “like dissolves like”, polar solvents are used to dissolve polar soils, and non polar solvents are used to dissolve non polar soils. In order to accommodate both polarity soils more than one solvent is selected to help dissolve these soils. Typically, the most polar solvent chosen for household cleaning products is water. Organic* (or carbon-based) solvents are often chosen as the non-polar or less polar solvents. If water and oil were chosen as the polar and non polar solvents for the cleaning formula, they would not mix well and would separate into two layers over time. As most cleaning products are homogeneous mixtures (uniform mixture), other additives known as hydro-tropes are added to help blend the non-polar and polar solvents and keep the mixture uniform. This could be a third solvent (that mixes well with both the non-polar and polar solvents) or a surfactant.
*In this context, organic is using the definition of covalently-bonded carbon compounds (i.e. chemical definition) and not the USDA certification discussed in our previous blog post.
These compounds are responsible for giving a formula a giant boost in cleaning power. They are responsible for shouldering the bulk of the cleaning responsibility.1 These ingredients are used to accomplish the following tasks:
- Help spread out a cleaning solution over a surface by reducing its surface tension. This process is called wetting. It helps with the cleaning process by bringing the solvent and soil closer together so that a solvent can help dissolve a soil more easily.1
- Help suspend material (like particulate soils). This is so that they do not redeposit onto a surface during the cleaning process. You don’t want to go through the trouble of finally finding a cleaning solution that can remove a pesky soil… only to have it redeposit back on to the surface after the cleaner initially picks it up!
- Help emulsify or blend the oil and water phases together so that the mixture does not readily separate.
- Facilitate soil removal. While touched upon briefly, this will be described more during a future post.
- Adsorb onto a surface to amend the surface’s properties. An example of this is a hair conditioner. Surfactants are used to adsorb on to the surface of the hair to make it softer.
- Stabilize foam for a cleaning product. Often the presence of foam or suds gives the perception that a cleaning product is working well. Although presence of foam does not necessarily equate to good cleaning, many consumers expect suds or foam, so formulators will often enhance a product’s ability to produce foam by adding certain types of surfactants.
- Act as a biocide. In addition to helping to boost cleaning power or enhancing the cleaning solution’s affinity for a surface, some positively charged surfactants (in particular some quaternary amines) can help against microorganisms.
As this is a very comprehensive, area we will go into more depth about the different types of surfactants in a future post
1. Lange, K. Robert. Detergents and Cleaners a Handbook for Formulators, Hanser Publishers: New York 1994. 43-50, 165-168.