Thanks to Reddit for this.
Shampoo and body wash can actually be interchangeable, as anyone in an emergency situation knows. There's a slightly different pH required for both, but our bodies are quite adaptable, so using the more acidic of the two for both is acceptable, especially for the short time the detergent is on our skin and how rapidly it is rinsed away. Combination shampoo-body washes tend to be made a little thinner than a standalone body wash, so that things rinse away well.
The combination shampoo-conditioners don't condition your hair in the same way that regular conditioners do (which is by replacing oils that the detergent strips away). Combination shampoo-conditioners work on the notion that certain very acidic shampoo combinations leave the cuticle of the hair smoothed down instead of fluffed up. This smoothness feels like there have been oils or waxes added to the hair, but that actually hasn't happened. It just feels that way. (To feel your hair cuticles fluffed up instead of smoothed down, use plain bar soap to wash your hair. It's not pleasant, and is only bearable if you have your hair cut with clippers on the #1 (1/8" or 3mm) setting.)
Shampoo-conditioner combinations are just the shampoos that happen to leave the hair feeling so soft that you won't feel it necessary to add oils or waxes for a softer feeling.
It's possible, as one other poster mentioned, to capsule-ize the conditioner into something that doesn't burst open until the hair is rinsed, but there are a lot of unknown factors that make this approach less useful. The most important of these is the time that the shampoo is in the hair and scalp is an unknown entity. Some people shampoo up their hair first thing in the shower and don't rinse it off until the end of the shower. Others shampoo and rinse at the very end. If the capsules are waiting to dissolve, they're likely to break open long before the rinsing. If you shampoo and rinse very quickly, then the capsules may not have time to break, and will be rinsed away with the rest of the shampoo.
People with exceptionally dry hair will benefit from a shampoo-conditioner combo followed by
a conditioning treatment. The "built-in" conditioner is actually just a slightly lower pH than other shampoos.
Dishwashing detergent not labeled for hand soap purposes is not as acidic, and the surfactants tend to be stronger, and so it makes for a more irritating body wash and shampoo.
One exception to that: Liquid Octagon used to promote itself as an everyday/everything soap: from people to clothes to dishes to pets to floors. It didn't really do a great
job on any of them, but it did a passable/barely-acceptable job on everything.
And for the more technical ones:
To understand soap you have to understand polarity, and hydrogen bonding. Water is H2O, but it's better represented as HOH, because both hydrogens are bonded to an oxygen in the middle.
Oxygen is a very negative atom, and hydrogen is a very positive atom. This gives water a property called "polarity", where the hydrogen side is the positive pole and the oxygen side is the negative pole. Just like a magnet, the positive side of one water (the hydrogens) will "stick" to the negative side of another (the oxygen). This is an incredibly
important interaction called hydrogen bonding.
Oil, on the other hand, looks like .
It's a long chain of carbons surrounded by hydrogens--unlike water, there's no symmetry, and no negative-charged oxygen for the hydrogens to 'grab' onto.
This is why oil and water don't mix. Water is hydrogen bonding to itself--it almost acts like a magnet to itself, pushing oil out of the way to complete this hydrogen bonding. Oil only has positive ends, so it just repels itself and everything else.
Now, finally, onto soap. Soap is made by taking the last hydrogen off the rightmost end of that oil molecule. This can be done with lye, which is how you make soap: mix fat and lye. Once that last hydrogen (white ball) on the end is taken off, the red balls (oxygen) have their electrons freed up to hydrogen bond with water.
And this is why soap cleans oil, it bonds to water, but not so strongly that it pushes the oil out. It helps integrate nonpolar molecules (like oil) into a polar solution (like water)
And some more info:
shampoos and body washes are both surfactants. They clean the dirt and oil away. The main ingredient is usually SLS, sodium laurel sulfate, which is present in most soaps. Body washes are much harsher than shampoos, just as they are harsher than face washes, because the skin on your body can take much more of a beating than the skin on your face or your hair. Well, your hair can take it but it will become coarse and frizzy.
Conditioners add moisture back to your hair when shampoo has stripped it all away. The main ingredient in most conditioners are silicones, usually dimethicone and any other ingredients ending in -cone. These ingredients coat the shaft of the hair, allowing the hairs to slip past each other and giving the hair shine. The problem with silicone is it is very difficult to remove, and is only removable by sulfates, making shampoo and conditioner a cycle whereby if you use a silicone based conditioner, you have to use a sulfate based shampoo to remove it. This why you now see sulfate free products, because there are large movements (such as no poo) against using these products in your hair.
2-in-1 or 3-in-1 products are not going to give you the same results as using separate products. They will definitely not be able to adequately condition your hair and may be too harsh of a cleanser for your hair.
And some more detail:
Head and Shoulders
Active ingredient: Pyrithione zinc 1%
Inactive ingredients: water, sodium laureth sulfate, sodium lauryl sulfate, cocamide MEA, zinc carbonate, glycol distearate, dimethicone, fragrance, cetyl alcohol, guar hydroxypropyltrimonium chloride, magnesium sulfate, sodium benzoate, magnesium carbonate hydroxide, ammonium laureth sulfate, benzyl alcohol, sodium chloride, methylchloroisothiazolinone, methylisothiazolinone, sodium xylenesulfonate, blue 1, red 4
Head & Shoulders Smooth & Silky Dandruff Conditioner
Active Ingredients: Pyrithione Zinc (0.5%, Anti Dandruff)
Inactive Ingredients: Water, Stearyl Alcohol, Cetyl Alcohol, Stearamidopropyl Dimethylamine, Dimethicone, Glutamic Acid, Benzyl Alcohol, Phenoxyethanol, Fragrance, Methylparaben, Propylparaben, Citric Acid
Head & Shoulders Dandruff Shampoo Plus Conditioner
Active Ingredients: Pyrithione Zinc (1%)
Inactive Ingredients: Water, Sodium Lauryl Sulfate, Sodium Laureth Sulfate, Glycol Distearate, Zinc Carbonate, Sodium Chloride, Fragrance, Sodium Xylenesulfonate, Dimethicone, Cocamidopropyl Betaine, Sodium Benzoate, Guar Hydroxypropyltrimonium Chloride, Magnesium Carbonate Hydroxide, Methylchloroisothiazolinone, Methylisothiazolinone, Blue 1, Red 33
You can see the surfactants sodium lauryl and sodium laureth sulfate pop up in both mixes that have shampoos. These are the amphiphlic substances (i.e. substances that have a charged group sulfate and a hydrophobic group lauryl). These mix with oils to break them up and make them more hydrophilic (water loving) so they can be washed away. You can see the conditioner mainly has amino acids, acids, and fatty alcohols in it to keep the pH low and replace the oils from your scalp with other fatty alcohols. The 2 in 1 contains the surfactants, since without them it can’t clean away the oils in your hair, but you can see it doesn’t use the same fatty alcohols to replace the oils you lose. It uses other materials such as dimethicone (a silicone polyol that coats the hair and reduces moisture loss) and Cocamidopropyl Betaine(an antistatic agent) since just normal fatty acid replacements would be washed away by the lauryl sulfate type surfactants needed in the shampoo part. Body wash is similar to other shampoo type mixtures.
Both shampoos and body washes use chemicals called surfactants that help make oils (that are hydrophobic or water hating) mix more readily with water. This helps them wash away these oils so you feel clean. Conditioners try and replace these oils or prevent the side effects of losing these oils such as water loss leading to dry less shiny hair. Mixed shampoos and conditioners can't simply mix surfactants and plain oily agents like those found in your hair as they would use up the surfactants in the shampoo or just get washed away. Combined shampoo and conditioners therefore use other moisturizers (water retaining substances) that help keep the hair shiny and prevent dryness. 3-in-1 products are more of a marketing gimmick.
And why not some more, right.
I'm going to try and keep this as short and simple as possible. Shampoos(most brands) are very similar to soap. The difference between traditional soap and shampoo is that shampoo is a lot gentler on your hair preventing the sebum protecting your hair from becoming brittle, from being completely washed off. Conditioner on the other hand moisturizes and helps keep the hair more manageable. Conditioners also have a lower pH than shampoo and are more acidic, this being that it helps promote development of amino acids and keratin that makes the hair look healthier and more desirable. The 2-in-1s sold that are said to do the same job may not work as well as individually applying each bottle. Hope this helped slightly.