Homemade detergents typically contain water softeners (washing soda), bicarb soda (sodium bicarbonate), stain removers (sodium percarbonate) and traditional soap (Sunlight Soap or Lux Flakes).
Without the surfactants found in commercial detergents, soap-based detergents lead to soap scum and inhibit cleaning.
A closer look at these ingredients
Traditional soap is a saponified triglyceride. It is a very mild surfactant and works for spot stain removal. However, traditional soap won’t achieve a good clean in an automatic washing machine because it reacts with minerals in water and forms soap scum.
When soap scum builds up on fabrics it traps bacteria and ammonia. This is the same soap scum that builds up in showers and baths. Soap is not recommended for washing clothes unless you are using a wash board.
Washing soda (sodium carbonate) is a water softener. It does not clean or remove soiling.
Washing soda is used in commercial detergents to bind to minerals in water, and prevent minerals binding to surfactants. Harder water contains more minerals, which why is powders are more suitable for harder water areas
Bicarb or baking soda (sodium bicarbonate) is a pH adjuster. It isn’t a water softener and it doesn’t have any surfactant qualities.
The benefits of surfactants in commercial laundry detergents
Commercial laundry detergents contain surfactants. Surfactants are active cleaning agents, they remove soiling from fabrics and do not lead to soap scum.
Surfactant molecules consist of a hydrophilic head that holds onto water, and a hydrophobic tail that holds onto soiling. The Wash Wise Detergent Ingredients information sheet details how surfactant molecules react in laundry.