Fabric softener and absorbency

Why do towels dry you less well after the wash that made them feel softest?

A towel washed with softener comes out plush, pleasant against the skin, and noticeably worse at its job. It skates over wet skin, pushing water around rather than taking it up, and needs more passes to do what it used to do in one.

The odd part is that softness and absorbency feel as though they should go together. A soft towel seems like a better towel. And the product is sold as a treatment for the fabric, not against it. So either the softening and the failure are unrelated coincidences, or they are the same event described from two sides — and if it is the second, we ought to be able to say what that event is.

Start with how a towel works at all. It does not absorb the way a sponge does. A cotton towel is a mass of loops, each loop a bundle of fibres, and water is drawn into the spaces between and within them by capillary action: water wets the fibre surface, and surface tension pulls it along the narrow channels. Two things are needed for that. Narrow channels — supplied by the loop pile and the fibre structure. And a surface the water actually wants to wet, which cotton provides, being covered in hydroxyl groups that hydrogen-bond readily with water.

So absorbency is not about the towel having room for water. It is about the water being pulled in. Remove the pull and the room is useless.

Now ask what a softener is, chemically. The classic softeners are cationic surfactants — molecules with a positively charged head and one or two long fatty tails. Cotton fibres in wash water carry a slight negative surface charge, so the charged heads are attracted to the fibre and stick, leaving the fatty tails pointing outward.

Follow that. The fibre is now coated in a layer of outward-facing hydrocarbon tails. Hydrocarbon is not wetted by water — that is what makes waxes and oils water-repellent. The molecule has therefore taken a surface that water was eager to spread across and given it a surface water would rather avoid.

And now both effects follow from the same fact. The tails lubricate: fibres slide past one another instead of catching, the pile stands loose rather than matted, and the fabric feels soft and drapes well. The very same tails repel water, so the capillary pull that drew water into the channels is weakened. The towel is softer because it is coated, and less absorbent because it is coated. It is one change with two consequences pointing in opposite directions.

That also explains the timing people notice. The effect accumulates. Each wash deposits a little more, and the coating is not fully removed by ordinary washing, so a towel that was merely soft after one treatment becomes genuinely poor after a dozen — a slow drift rather than a single bad wash.

And it explains why the failure is specific to towels. A shirt is not asked to absorb anything; for a shirt, the coating is nearly all benefit. The trade-off only bites where wetting is the function.

The towel is not "clogged", and this is the most common wrong mechanism. People often describe softener as blocking the spaces or filling the gaps, by analogy with limescale or detergent residue. That is not what is happening: the channels are still open and the towel still has all its volume. What has changed is the surface energy of the fibres. You can see the difference in the failure mode — a clogged towel would be stiff and heavy, whereas a softened one is light and fluffy and still fails. Absorbency is a wetting property, not a volume property.

Not all softeners work the same way, and the newer ones muddy the story. The account above is cleanest for traditional cationic softeners. Silicone-based finishes soften by a related route and also reduce wetting; some modern formulations use hydrophilic silicones designed to soften with less absorbency penalty, and dryer sheets deposit a coating by melt transfer rather than from solution. So the strength of the effect varies by product, and I would not claim it is uniform across everything sold under the name.

Some of the folk remedies work, and for the reason the account predicts. Washing with vinegar helps because acid conditions reduce the deposition and help strip existing coating; a hot wash with detergent and no softener removes it gradually, since detergents are built to remove exactly this kind of fatty deposit. What does not help is anything aimed at the fabric's structure, like extra tumbling — which is a useful check on the account, because a clogging model would predict mechanical agitation to help and a coating model would not.

Hard water complicates the picture in the opposite direction. In hard water, calcium and magnesium react with soap residues to leave deposits that stiffen towels and also impair absorbency. So a stiff, scratchy, poorly absorbing towel and a soft, fluffy, poorly absorbing towel can arrive at the same failure by opposite routes. If a towel is both harsh and unabsorbent, softener is probably not the culprit.

The falsification test. If reduced wetting is the mechanism, then a treated towel should show a measurably longer time for a water drop to be taken up than an untreated one, while retaining essentially the same mass and thickness. Drop a bead of water on each and watch. If the treated towel absorbed the drop as fast but simply held less water in total, the account would be wrong and something about capacity rather than wetting would be doing the work.

• Why silicone softeners can be engineered to soften with less absorbency penalty.
• How surface energy governs wetting generally, from raincoats to non-stick pans.
• Why hard water produces a stiff towel that also fails to absorb, by an unrelated route.