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I am reading about soaps , micelles and all that. I understand how soap behaves in water contaminated by oil. Soap helps in emulsifying the oil into water.

Let us suppose now we have dirt(i.e oil, grease etc) not alone in water but present on a some cloth. Why can't we put the cloth just in water to get it clean ? What precisely is the need for agitation in a washing machine or by any other method ?

I searched and found that agitation produces foam but according to this post foam does not alter the washing process anyhow.

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  • $\begingroup$ Agitation or mechanical scrubbing[either by hand or machine] helps in the emulsification of the grease or oil patch $\endgroup$ – Anne simon Jan 26 '17 at 16:11
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When you wash a piece of cloth, even without soap, it's better to agitate. The vibration, friction, etc. will help speed up the process of mixing the dirt/grease with water.

Even the very definition of "agitation" uses the example of washing. From Vocabulary.com

Agitation is the act of stirring things up, like the agitation of a washing machine that moves the water, detergent, and clothes around and gets the dirt out

And as you rightly pointed out from another question, "the froth has little or no effect on the detergent action".

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The modern definition of washing is "washing can be defined as both the removal by water or aqueous surfactant solution of poorly soluble matter and the dissolution of water-soluble impurities from textile surfaces". This is in the book "Laundry Detergents" written by E. Smulders. There is a whole chapter on the physical chemistry of the washing process. You can read that.

What is the purpose of the washing machine? Washing machine provides the mechanical agitation and the modern ones also heat the water (esp. the European style front loaders). This is akin to using a stirrer in a chemical reaction or using a stirrer to dissolve difficult to dissolve solids in a given solvent and using heat to accelerate this process. The purpose of agitation is to constantly bring the clothes and fibers in contact with fresh and (ideally concentrated) detergent solution. This is exactly why the front loaders are far better than these vertical type washers commonly seen in the US and Asian countries. Front loaders uses a concentrated solution (a smaller volume) vs. gallons of water in vertical machines. With apologies, to other manufacturers, not all washers were created equal (just like cars).

Now there is not only oily dirt but there is physical dirt as well. You need mechanical action to dislodge it from the twisted and woven fibers of the clothes. German washers especially Miele, Siemens, Bosch outperform in cleaning tests on the so-called washing grades of A, B, C, D, E etc, so they way you agitate also matters and how long you agitate.

If you let the clothes soak in a detergent solution, you are simply relying on diffusion process to help emulsify or dissolve the oily dirt. So imagine the difference, an oily spot soaked in detergent vs. an oily spot constantly being exposed to fresh detergent solution. Consider the analogy of copper sulfate crystals sitting at the bottom of a beaker vs. a stirrer continuously shaking the crystals. It takes a couple of months to make the entire solution blue (diffusion alone). A stirred solution takes a few minutes to dissolve the crystals. This is why soaking alone is an inefficient process. So much so for advocacy of washing machines or the need of a proper mechanical agitation. Some manufacturers also tried ultrasonication but apparently this no better than an ordinary washer.

The rest of magic of cleaning clothes comes from the detergent chemistry-which is far too complex to discuss in a single post. You have surfactants, bleaching agents, bleach catalysts, enzymes, water softeners, builders, foam regulators, EDTA, optical brighteners, anti-sudsing agents, soil anti-redeposition agents, perfume and God knows what not. Finally you have to protect your washing machine from corrosion so detergents also have anti-corroding agents.

Last but not the least, they have to ensure that none of these chemicals are carcinogens, mutagens, skin irritants and they don't damage the sewer system and these days the environment. Salute and hats off to detergent chemists who make our clothes clean everyday.

Kirk Othmer Encyclopedia and Ulmann's Encyclopedia of Chemical Technology will be great resources for detergent chemistry. They have chapters upon chapters on these topics.

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    $\begingroup$ I was just about to post "Doesn't anybody here know how soap works?" in meta, but you've come through with an answer, thank you! I'm curious about "...emulsify or dissolve the oily dirt." Are there some aspects of removing "oily dirt" when washing clothes are are indeed more like dissolution than emulsification? $\endgroup$ – uhoh May 25 at 5:29
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    $\begingroup$ May be I used the words loosely. See how soap "dissolves" oil. I am using dissolve in a loose sense. If you have fruit stains, you are then actually dissolving those stains and also bleaching them. Enzymes may destroy protein type stains. $\endgroup$ – M. Farooq May 25 at 5:32
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    $\begingroup$ In older times, when clothes were boiled in lye, you would actually soapnify the oil stain by coverting it into soap :-) Many washermen in South Asia still practised this for large sheets. The German machines have a Kochwasche program- which is almost the same as cooking the laundry. Literally it washes clothes in boiling hot water. $\endgroup$ – M. Farooq May 25 at 5:37
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    $\begingroup$ sure, for example I think that the membranes of dead skin cells are certainly broken down by the detergent releasing the contents of the cells, which themselves are broken down. Some of that organic material could be said to be emulsified while certainly some proteins and nucleic acids will just go into solution. $\endgroup$ – uhoh May 25 at 5:38
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    $\begingroup$ Right, it has a fascinating chemistry. I had fascination with clean clothes and with detergents and read a lot about them since I was in school from those Encyclopedia :-) $\endgroup$ – M. Farooq May 25 at 5:39
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A grease/oil spot of about a milligram would make a droplet about 1 mm in diameter. This is too large to stay suspended in water - it would float to the surface, where it would combine with any other tiny droplets, even if these droplets were coated with surfactant. When you withdraw the clothing from the dirty water, or drain the water thru the fabric, the oil/grease would be trapped again on the fabric.

However, if you agitate all the 1 mm droplets, each covered with a monolayer of surfactant, in the presence of more surfactant in the water, the droplets will deform, elongate, and split, generating more surface area (the extra surfactant molecules in the water will rush to coat the oil/grease). As agitation continues, the tiny droplets become smaller and smaller, always covered with a monolayer of surfactant, because you put enough into the water at the beginning of the washing cycle.

Finally, the droplets become small enough that their buoyancy is negligible compared to Brownian motion (maybe 0.005 mm or less) and they act like large dissolved molecules. At this size, the particles are very difficult to filter out of the water using ordinary fabric, which is good, because you don't want to trap them, you want to drain them away.

One way to make an emulsion of small particle size is to add an organic acid (e.g., oleic) to an oil (e.g., canola), then pour this into a sodium hydroxide solution. The acid saponifies in intimate contact with the oil and the oil disperses into small droplets coated with oleate anions. Sodium ions are also in solution. This happens spontaneously thru a chemical reaction.

However, if you drop canola oil into a sodium oleate (a surfactant) solution, it floats without significant emulsion forming. This is typical of ordinary oil/grease - you don't have a readily available chemical reaction to form small droplets. But if you shake it you break the oil into tiny droplets, and you will get an emulsion that is stable (i.e., doesn't separate into two layers) for a while. The emulsion will be stable longer if the droplets are smaller. You have substituted mechanical energy for the chemical energy, and the surfactant acts to maintain the stability.

If you have insufficient surfactant, the droplets won't get small enough to become stable in water. If you use excess surfactant, you will get smaller particles and a more stable emulsion, and lots of foam.

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Soap can mix with both water and with oil. Why? The soap molecule has two different ends, one that is hydrophilic (polar head) that binds with water and the other that is hydrophobic (non-polar hydrocarbon tail) that binds with grease and oil.

enter image description here

When greasy dirt or oil is mixed with soapy water, the soap molecules arrange themselves into tiny clusters called micelles.

enter image description here

The water-loving (hydrophilic) part of the soap molecules sticks to the water and points outwards, forming the outer surface of the micelle.

enter image description here

The oil-loving (hydrophobic) parts stick to the oil and trap oil in the center where it can't come into contact with the water.

With the oil tucked safely in the center, the micelle is soluble in water. As the soapy water is rinsed away the greasy dirt goes along with it. Micelle Picture from Wikipedia

So basically, the soap is attracted to the fat/oil/grease because of its fat-loving side but then tears up the grease by pulling it into the water using its water-loving side. Sounds kind of like a football play--surround the oil particles and move them away from one another.

enter image description here

Ever wonder why it is easier to clean dirty, greasy hands (and other things) in hot or warm water rather than cold water? It is because the fats and oils soften or melt in hot water, which allows them to attach more readily to the hydrophobic end of the soap molecule. In turn, that makes it easier to rinse away.

Credits: entire answer is copied from https://www.chagrinvalleysoapandsalve.com/blog/posts/how-does-soap-work/ Without any edits

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