Has anyone ever determined whether fabric that has been saturated with soapy water is more likely to repel the clear rinse water, or if the rinse water is more likely to drive out the soapy water?
I figure this must have been studied for the purposes of marketing laundry products, but I would like to know what the answer is, if anyone can provide it.
And in the same vein: warm vs. cold for rinse…seems like cold is actually better. Do we know why?
I also know that old-fashioned soap does not clean, it breaks the surface tension of water for water to clean, but detergent is different and actually does clean. How does that work?
The spin cycle is there to force as much of the soapy liquid out as possible before the rinse, so that the soapy bit is diluted by the rinse water. I think some rinse cycles also agitate so to facilitate dilution. In the old days, they ran the clothes through a ringer to squeeze as much liquid out as they could.
Naah, he meant the clothes were slid into a mechanical bell so the clapper part banged against the gong part with the cloth in between. Which pounded the excess water out, one little spot at a time.
In (lazy)reply to: I also know that old-fashioned soap does not clean…
A brief history of soaps and detergents
Old-fashioned soap worked because it was made up of chemicals called ‘surfactants’, which lowered surface tension in water and broke down fatty materials—in other words, they decreased the fabric’s hold on the dirt and they also dissolved some dirt particles. In the early 20th century, as people became more familiar with bacteria as a cause of disease, antiseptic chemicals such as ‘carbolic’ (phenol) were added to household washing materials.
These original soaps did not degrade in the environment and their residues remained in waterways and water treatment plants. By the 1950s, drains and rivers often carried persistent mounds of foam, and the water became toxic to small organisms living in the water.
Manufacturers therefore sought to make washing powders ‘biodegradable’—to decompose naturally as soon as possible after use. Water hardness is a significant factor in the effectiveness of modern detergents; the harder the water, the more detergent is required. The ions in ‘hard’ water, particularly of calcium and magnesium, bind to the surfactant components and leave an unpleasant scum on the water—and clothes.
So manufacturers added ‘builders’, which bind to and essentially remove these ions and, in doing so, ‘soften’ the water. (You can tell the difference between water that has been passed through a water softener and water that hasn’t and remains hard, by the ‘sudsiness’ of soap used in the shower).
Phosphates have been the builder most commonly used in detergents. However, because excess phosphates cause problems in our waterways, some detergent manufacturers have developed ‘phosphate-free’ detergents.
From Redirect Notice (.pdf file from EPA of South Australia.)
This bit isn’t right. Both soap and detergent are surfactants. They lower the surface tension, by which is meant the energy at the interface between the wash liquor and the air. They lower other interfacial tensions, too, particularly between the wash liquor and hydrocarbon oils. By a process called “roll up” a surfactant solution can advance the solution solid interface and pry or roll a droplet of oil up into a separated droplet floating in the solution. By a process called “emulsification” a surfactant solution can make a separated droplet stable and keep it suspended (a suspension of oil droplets in water is an emulsion). Also, the liquid solid interfaces can conform to, and have better affinity for, both clothing fibers and solid particles that are otherwise clinging to them by for example van der Waals force.
1967, Chem 105 Lab. Grad. student TA is full of himself and expounds a great deal. Whether or not he knew what he was talking about is an open question to this very day.
According to him:
Soap - reduces surface tension; makes water “wetter”
Detergent - bi-polar molecule. One end is attracted to water, the other to anything but water.
It best to dissolve something into liquid… it would be hard to dissolve something into a solid.
The aim is to dissolve oils, starch, proteins and the colours they contain (such as blood, or plant matter …) into the water.
So
Dissolve the stuff into water with detergent by agitating for a while
drain the water (spin for greater effect)
Rinse… agitate… to ensure the wetness in the fabrics is changed for fresh(er) water… this dissolves detergent and dirt into a greater volume of water … basically by the way the contaminant disperses through the water to form equal concentration in all that water…
Have you ever seen a washing machine with an option for a hot rinse?
I thought it was crazy until I tried it… and I realized that it works. Things still feel soapy if you rinse in warm or hot water, they feel squeeky when you use cold.
Hot water has higher solubility than cold water so it goes without saying that detergent will be dissolved, and hence be rinsed away better. If you think warm-rinsed clothes feel soapy, what you’re feeling is the effect of the dissolved detergent in the water on your skin. I’m not sure what you mean by your cold-rinsed clothes feeling “squeeky” (sic), but if you mean it feels “harder” (less slippery), that’s because detergent is not in the water and left on your clothes.
I’ve heard that the common wisdom is that cold water should be used on stained clothes, because the hot water can help move the stain into the fabric of the cloth, where it’s almost impossible to get out. Cold water has a better chance of removing the stain molecules without carrying them into the fabric.
Not an open question; he didn’t know what he’s talking about.
Soaps and detergents both reduce surface tension (surfactants) and are bi-polar molecules that produce micelles with the oil droplets in the center.
The main difference is that soaps are prone to reacting with the calcium and magnesium ions in hard water, and creating soap scum, which both reduces the cleaning ability of soap and also creates more crud in the water.
Detergents are much less sensitive to water hardness, and I suspect that their reaction products aren’t as problematic as soap scum (calcium/magnesium stearate).
It depends on what caused the stain and how fast you treat it. For blood or other protein-based stains, use cold water. For grease or oil-based stains, use hot.
