Water hardness or mineral ion content of water has played a big role in bringing you the detergents of today. There is more than one way that detergents can tackle the problem of water hardness. But, first one must understand just what the problem is and how it plays out in the wash.
What is Water Hardness and How does it Affect the Anionic Surfactant?
Surfactant molecules constitute the part of the detergent mixture that does the actual cleaning work. In most laundry detergents the main surfactant used is an anionic surfactant. These surfactants are very effective at drawing oily materials and oil/clay soil into suspension in the wash water. However minerals which may be present in the water, most frequently in the form of positively charged magnesium (+2) ions and positively charged calcium ions (+2), tend to bind themselves to the negatively charged heads of these surfactant molecules. Surfactant molecules which have reacted with the water soluble minerals in this way fall out of solution, as they no longer have the electrically charged head since it was this 'polar' end which was keeping them floating aloft in the aqueous solution before.
Depending upon how hard the wash water is, (the harder it is, the more mineral ions are dissolved in it) varying quantities of surfactant molecules are put out of service as they precipitate with these minerals. Different types of anionic surfactant molecules have different tendencies to react in this way with the mineral ions. Tails and heads of surfactant molecules can vary tremendously, hence the great variety and effectiveness of different surfactants. Some surfactants can remain aloft even in harder water, others, such as those consisting of fatty acid chain found in bar soaps, do not fair as well. Because of this, detergents have come to rely on synthetic surfactants which do not precipitate out as easily under high dissolved mineral conditions in the wash water.
Other ingredients may also be added to the detergent to help to prevent mineral precipitation of the surfactant from occurring.
Other Ways that the Anionic Surfactant may be Affected by the Water Hardness
In an ideal washing situation oils and dirt globules are lifted up from fabric fibers and carried away in a water loving coat of anionic surfactant molecules. Once suspended in this manner, these globules do not re-coalesce onto the fabric strands when water currents bumps them back up against the fabric. This is because the laundry fibers are also coated with negatively charged anionic surfactants just as surfaces of the dirt/oil globule are coated. This means that effectively both surfaces are negatively charged and they will repel one another and the dirt globule will never make meaningful contact with the fabric again.
This is what keeps the dirt emulsified in a washing machine. Dirt globules remains aloft for the duration of the laundry procedure and they are expelled in the dirty water at the end of the wash cycle because of this property of anionic repulsion. However, when a lot of mineral ions are present, as in the case of hard water, the positively charged mineral ions can act as bridges - joining the negatively charged fabric-surfactant layer to the negatively charged dirt globules-surfactant layer. In other words, the dirt which was lifted away from the fabric by the surfactant action will be re-attached by a surfactant- mineral-ion-surfactant bridge.
As noted above, some synthetic surfactants do not tend to bind as readily to the water hardness ions as other more traditional surfactants (especially the common soap fatty acid surfactants). If a detergent relies upon a more easily bound surfactant (as many green detergents do) the water hardness reactions need to be prevented. Either the positively charged water hardness ions need to be removed from the system or they need to be rendered inactive in one way or another. Many detergents include a class of chemicals called hardness sequestrants which are added to for this purpose.
Hardness Sequestrants and Green Laundry Detergents
Hardness sequestrants are chemicals which will preferentially bind with the water soluble ions, and therefor effectively remove them from the wash-water-action, leaving the surfactant molecules free to carry out their dirt-emulsification processes in peace. Naturally, the laundry detergent industry has devised synthetic hardness sequestrants, and puts these chemicals to use. For many years phosphates were added to detergents to act as hardness sequestrants, these are not synthetics, but every one knows to avoid them for environmental reasons. Washing soda is not a synthetic either, and it's environmental impact is much less than the impact that phosphates have. Yet, it works very well as a hardness sequestrant. Green laundry detergents usually contain washing soda for this reason.
Choose a laundry detergent that includes washing soda as its hardness sequestrant. This is a simple mineral additive which will not have long-range environmental impacts, and will not endanger the health of the user.