Liquid fabric softeners (eg Fluffy) contain cationic surfactants, which are positively charged. They are attracted to the negatively charged fabric, coating the surface, thus causing threads to ‘stand up’ and making the fabric feel softer. The conditioning component of the surfactant adds lubrication.
Liquid fabric softeners can be used infrequently (for example every third wash). A mainstream detergent and hot wash are needed to ensure that softener residue is removed in the subsequent wash. This will prevent build-up which reduces absorbency.
Two-in-one detergents that contain fabric softener (eg Cold Power + Cuddly, Biozet + Softener, Surf 2 in 1) work differently. The fibres repel each other, which causes everything to stand up and become fluffy. They don’t reduce absorption. These types of detergents can be used continuously without any issue.
For more information on what is in detergents and the function of the raw materials see our information on Mainstream Detergents, Fragrance in Detergent, Enzymes, Optical Brighteners and Plant based, Sensitive & “Eco” Detergents, and the WashWise information sheet on Laundry Detergent Ingredients.