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Want to save money on soaps and detergents?

April 24, 2010

Have you ever watched Fight Club?
Remember how Brad Pitt stole human fat to make soap?
And how he gave Edward Norton a chemical burn with lye? And then poured vinegar on him to neutralize the base?
Yeah, don’t do that.
Anyhow, I did watch that movie a few months ago, and since I’m usually up for the challenge of making my own anything, I thought it would be interesting to make my own soaps from scratch.
It’s not a difficult process, and the materials needed are not expensive at all, but it takes several days to make the soap. The quantities in the recipes are far too much for my personal use, and even too much for me to give as gifts, but investing all that time/money/effort/space/equipment for a smaller amount seems pointless when regular soap isn’t really that expensive.
Like Schrödinger’s cat and the principle of uncertainty, when you save money, you spend time and vice versa (bad joke, forget I said anything).
BUT, yesterday I saw this article, on how to save money on your laundry detergent, and that brought back my interest in the subject.

OK, let’s start at the beginning

What are detergents and soaps?
These are molecules with a dual nature- part hydrophilic and part hydrophobic. When in water, they dissolve only partially, and the hydrophobic part of the molecule clumps together to form a tiny oil bubble, which can solubilize other oily substances within it. When in oil, they do the opposite- the hydrophilic part of the molecule clumps up to form tiny bubbles and trap other water soluble molecules within it. The exact behavior of each detergent or soap depends on the balance between its hydrophobic and hydrophilic parts (HLB, hydrophilic lipophylic balance), the nature of the head group (hydrophilic part) and the nature of the tail group (hydrophobic part), but the general behavior is to form clumps or layers out of one part on the molecule, while the other part is soluble in its surroundings.
The difference between soaps and detergents is that soaps are ‘natural’ and detergents are completely synthetic. The reason it matters whether something is ‘natural’ or completely synthetic, is because it has a different impact on the environment and our health.
Soap would be composed of a naturally occurring fatty acid, such as is found in the membrane of cells and fatty tissues (that’s the fat Brad Pitt stole), and has an aliphatic tail (a hydrocarbon chain) and a carboxylic acid for a head:

The carboxylic acid is hydrophilic, because it is polar, similar to the water, and it can also form hydrogen bonds with the water. Unfortunately, that is not hydrophilic enough for the molecule to be soluble in water, because of its long tail. What you need is to force the head to be more polar, or have more hydrogen bonds with the water.
To make the head more polar, you can turn it into an ionic group, by changing the pH for example. The carboxylic group is an acid, and at a certain pH it will lose its hydrogen from the hydroxyl group and become negatively charged. In order to do that, we usually use lye (NaOH), a very strong base:

That’s the main component of bar soap.
But this head group is very sensitive to pH, hard water (containing a high concentration of Ca+2, Mg+2), and a high concentration of electrolytes in general. In their presence it will return to its acidic form and lose its function.
The larger the head group, the more hydrophilic it is, the less sensitive it is to its environment. So a better soap would be one made with KOH:

That’s the main component of liquid soap.

The next step to a less sensitive substance and a larger hydrophilic head would be to replace the carboxylic acid entirely with something much more polar or more strongly ionic. That would make it a detergent, because the process of forming such a molecule is synthetic.
An ionic detergent may have a sulfate (SO4-3) head or a sulfonate (SO3-2) head. The difference is the ester bond versus the ether bond of the head to the tail:

(sodium lauryl sulfate, SLS, SDS, or sodium dodecyl sulfate, is a detergent and an emulsifier used in toothpaste, shampoo and other things. I think I’ll give toothpaste its own post)
The ester bond is sensitive to pH, and so is sensitive to the presence of water, whereas the non-ester bond is not. That’s why sulfate detergents are usually solids, and sulfonate detergents are usually liquids. The liquid detergents are less sensitive, more efficient, and you need less of them to clean whatever it is they are designed to clean.

To add more hydrogen bonds to the head, you can react the carboxylic acid with ethylene oxide, in a chain reaction, which will produce non-ionic detergents:

This is a chain reaction which cannot be controlled, and the end result is a mixture of molecules with different lengths and different behaviors. You can direct the reaction toward a certain average length depending on the initial starting concentration of the reactants, but you will still get a mixture, not a pure substance. These materials are usually toxic if swallowed.
These detergents are more stable and more efficient then the previous ones, but you can see that again we have an ester group, which is sensitive to pH. To make the detergent even more stable, we need an ether bond.
The ether bond cannot be produced by the carboxylic acid, so instead of a fatty acid, we need to start with a fatty alcohol:

Now we have one of the better detergents, which is very stable, easily soluble in water, you need only a small amount of it, and it works best in cool temperatures, so you don’t even need to waste money on heating the water.

How does the cold water thing work?
The solid substances need heating to dissolve completely. It’s about entropy, the thermodynamics of mixing, which is a different post.
Non-ionic detergents are usually liquid, so they are already in the right phase and don’t need heating. In fact, heating will actually damage them. They have a certain conformation in cold water, and a different conformation in hot water. It’s about thermodynamics again, but this time I’ll explain a bit: large molecules have many different ways of moving and shaping themselves. Each new shape has a different level of energy, meaning it takes a different amount of energy to move into that particular shape and maintain it. Heat is one form of energy the molecule can use to move and change its shape (conformation).
Non-ionic detergents, with their long head group, twist into a helical shape. The ‘cold’ conformation, the one that requires less energy, is the water soluble one, in which the oxygen atoms in the head turn outwards, in a way that lets them form hydrogen bonds with the water. In the ‘hot’ conformation, the helix twists so that the oxygen atoms now face inwards, and cannot form bonds with the water.

After you fiddled with the head group (and there are many more ways to do that), you can also fiddle with the tail group, making it more or less hydrophobic, longer, shorter, more twisting, give the molecule two heads or two tails, and many more options. The only thing that you need to maintain is the dual nature of the molecule, so it will be able to sit in the interface between two different solvents.

How do soaps and detergents clean?
This is where the dual nature of the soaps and detergents becomes important.
After you had a nice greasy meal, you want to wash your hands with water. But the water doesn’t dissolve the grease, and therefore, when the water flows away, the grease remains.
Soap is able to dissolve in water, but forms tiny bubbles of oil, which can trap the grease. When the water flows away, the bubbles flow with it, and the grease that is trapped in them is removed. Same thing happens when you wash your clothes, your floor or anything else, when you use soap or detergent.
But that’s not all! Soap and detergents also have some anti-bacterial properties. Since cell membranes are by and large composed of molecules similar to soap, they may be damaged by it, and the cells may die. That may be one of the reasons that harsh soaps or detergents damage your skin. It also damages many different bacteria, but not all bacteria.

Why does the washing machine need to move so much?
Water has a large surface tension, which means water molecules prefer to stick together, rather than mix with other substances. It also means water will usually not penetrate the thick weave of your clothes, because going through those tiny pores will force the water to break into tiny beads, which requires an investment of energy in order to break the surface tension.
Detergents are supposed to reduce water surface tension and help the water soak the clothes, in order to get to the dirt. Older detergents were less efficient in doing so, and the washing machine needed to invest physical force to break the water into tiny beads, which would then penetrate the clothes.
The more efficient the detergent, the less mechanical force is required in the washing.

How does all this tie to your household’s budget?
Your washing machine is designed to twist and turn, no matter which detergent you use, so you don’t have to buy the expensive stuff. In fact, if your clothes aren’t that dirty or don’t have any grease stains on them, you don’t need any detergent at all. The water is enough to dissolve any non-greasy dirt, and the washing machine’s gyrations are enough to force the water to meet that dirt.
Same goes for floor washing detergent and hand soap.
Only if the dirt is greasy, or particularly smelly (scent molecules are usually ‘oily’), you will need detergent to wash it off.
As for bacteria, soap doesn’t remove all of it, and washing your hands or clothes with water alone is still very effective in removing most of it. There is a difference between using and not using soap as far as bacteria go, but not a large difference.
(On the other hand, always washing your hands with soap is a ‘better safe than sorry’ situation, because you do come in contact with a lot of bacteria and greasy, without even noticing).
Reduce your soap and detergent consumption, and you will reduce your expenses.
Since detergents aren’t great for the environment, reducing your consumption of them will also benefit the planet.
To reduce your expenses even more, you can make your own bar soap, liquid soap, shampoo, laundry detergent, moist towels

Aluminum foil in the dryer?!?!?!
Yes indeed!
If we’re already talking about saving money with our laundry, let’s talk about the dryer.
I live in a very sunny country, so for most of the year we hang-dry our clothes in the sun. It’s both free and eco-friendly.
If you can’t hang-dry your clothes, you’re probably using a dryer, and your clothes get static if you don’t use that special scented paper. Personally, I like the smell of it, but I don’t like the price of it.
There are two ways to reduce the cost: use the spacial paper more than once, or use Aluminum foil, scrunched up in to a ball (and use it more than once, of course). The Aluminum foil will prevent the static just like the special paper!

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