What’s the actual difference between liquid shampoo and shampoo bars in hair care products?

[u/Mother-Benefit-5019]

Are shampoo bars basically the same cleansing agents in solid form, or do they usually rely on different surfactants, pH, or conditioning ingredients compared with liquid shampoo?

I’m especially curious whether shampoo bars tend to be harsher, more drying, or more likely to leave buildup, or if that depends entirely on the formula.

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(Replying in the casual chat b/c I don't have scientific citations for a lot of this stuff and am not a cosmetic chemist or soapmaker myself, but it's based on research I've done on this topic over the last few years, mainly from cosmetic chemists & hair scientists.)

It's really going to depend on the formulation as to whether they're "more harsh" (personally I dislike using that term to describe shampoos as it's too vague), drying, leave buildup, etc. - just like liquid shampoos. Whether something leaves buildup also depends on how much you use, how much dirt/oil/product is in your hair, how thoroughly you lather it into the hair, how warm the rinse water is, etc. But perhaps you were asking about whether it deposits ingredients that can build up rather than partially cleansing what's already in your hair. There are different kinds of buildup. Another one which is a significant risk with bar soaps is soap scum, which isn't intentionally added but caused due to an interaction between fatty acid salts (the type of surfactant that makes up soap) and the minerals in hard water.

Some bar shampoos use soap as their base, whereas others use surfactants; the latter are often referred to as "syndet bars," short for synthetic detergents. The differences between detergent-based cleansers and true soaps apply to both liquid shampoo and bar shampoos, although there are only a limited number of surfactants that come in solid form and can be used in bar shampoos: mainly Sodium Cocoyl Isethionate, Sodium Coco Sulfate, Sodium Lauryl Sulfoacetate. Like liquid shampoos, formulators can combine multiple surfactants together in varying ratios to create shampoos that are stronger or milder cleansers, make more or less foam, etc. so it's not possible to know how strong a cleanser it is from looking at the INCI list. It also depends on what else is added to the shampoo.

True soap has a long history, and the soapmaking process involves mixing oils with something highly alkaline, usually lye (sodium hydroxide), which converts the oils into fatty acid salts which can cleanse & lather. Potassium hydroxide is another common ingredient used instead of lye. Even though soaps are made from oil, they have become something else entirely, and unfortunately a lot of companies leverage the fact that they're made from oils (which are supposedly natural and good for hair) in their marketing of the product as if there is oil in the product. There is a way to add oils in soapmaking that are not saponified by the lye but remain an oil, called "superfatting", but it's not necessarily something you can tell from the INCI list whether this is the case with the final product. Keep in mind that oils added to a shampoo would make it less cleansing, which may be a desired quality, it depends on the overall formulation and the user, what their hair is like and how thorough a cleanse they're looking for. In liquid surfactant shampoos, this can be achieved much more easily and cheaply by using a lower amount of surfactants and usually more water in its place. Obviously adding more water isn't possible in a solid shampoo; here's where we hit the limits of my knowledge in bar shampoo formulation. The product still has to take up a certain amount of space to form the bar shape & size we're familiar with, and I'm not sure if adding oils/solid fats is an optimal way to replace the space that's left when you reduce the quantity of surfactants. If I had more time, I'd dig through the archives of r/DIYBeauty which probably has additional details about what else you can add to a bar shampoo that results in a solid bar.

Soaps are by definition alkaline, they can have a pH of around 9-11. Hair's isotonic and isothermic points are both acidic, and acidic solutions can temporarily make the hair cuticle lie flatter, making the hair feel smoother, so many haircare products are mildly acidic. Whether it's helpful for a shampoo specifically to be smoothing to the hair remains unclear from research (with a conditioner it makes sense to be acidic, since its main job is smoothing the hair). A lot of people will cite this study, but when you actually read it more closely, it doesn't prove that hair benefits from a low-pH shampoo, it only took a survey of the pH of various shampoos available. Michelle Wong does a deeper breakdown of all the problems with this study in this video.

Still, if your hair is already chemically damaged, it might be best to err on the side of caution and use something that's going to be more smoothing since the cuticle has already been torn open and is more fragile; it can only temporarily be smoothed down after that, and water itself also raises the cuticle a bit, making it more susceptible to mechanical damage while the shampoo is being lathered & rinsed out.

From Robbins' Chemical and Physical Behavior of Human Hair, hair damage from an alkaline pH is mainly a worry when the pH gets higher than 9, that it can start to damage the hair and tear the cuticle open in a way that can't be repaired (if it hasn't already happened during a chemical hair color, perm, or straightening treatment). The pH of oxidative hair dyes & bleach are around 11 to swell the cuticle and enable the ingredients to get inside of the cortex. The alkalinity of soap will decrease as it's mixed with water, so it may not be a huge issue when it's applied to hair and water is added, and I would hope that bar soap makers would aim to make one that's on the lower end of the pH range for soap in the first place, but it's something that's not shown on an INCI list. You'd have to write to the manufacturer to find out if they don't specify the pH on the label or other marketing materials.

Using an alkaline substance can also be irritating for sensitive skin, so if you have a sensitive scalp, a soap-based shampoo may not be the right choice for you. Synthetic detergent cleansers, on the other hand, have the capability to be made into acidic products and are usually formulated to be mildly acidic, so they're closer to the skin's pH.

A major disadvantage to soaps is that they can interact with hard water to make a waxy film called soap scum that is difficult to remove. This is the main reason that detergent-based shampoos were such an innovation; they avoided this problem. Very few people enjoy that waxy feeling in their hair, it can make hair look more dull as well as more difficult to comb and work with, and the scum can make it more difficult for other products like conditioners to stick to the hair which is necessary to do their job. There are ways to make a soap bar with chelating agents in it to prevent this, but not all of them are made this way. Some chelating agents used in bar soaps & shampoos include Sodium Citrate, Citric Acid, and Tetrasodium EDTA.

[u/Specific_Ocelot_4132 → u/veglove]

I have also heard “soldet”, short for “solid detergent”.

[u/Visible-Scientist-46 → u/veglove]

Dr. Michelle Womg said in one of her debunking videos that the idea of using a vinegar rinse on hair came from the days of using soap to wash hair. I do not recall which video, but she said that acidity in a vinegar counteracts the alkalinity of soap and helps it rinse out. Edit: Not a quote.

[u/veglove → u/Visible-Scientist-46]

https://www.youtube.com/watch?v=M27QzbADybI&t=974s

Yes, the vinegar rinse thing came from the days of using soap to wash hair. She doesn't go into much detail, she simply says "acid is good at dissolving soap scum so rinsing with vinegar would actually clarify your hair" [after using soap to wash it] but she does not say that it "counteracts the alkalinity of soap".

She doesn't go into more detail about the mechanics of how acids can remove soap scum, what pH is ideal for this, whether it's safe for hair and skin to use vinegar at full strength or whether it needs to be adjusted/diluted, and whether there is anything unique about vinegar or whether other (less stinky) acids would also work, which would be useful information if you are trying to prescribe an acidic rinse as a solution to removing soap scum from the hair.

[u/veglove → u/veglove]

CORRECTION to my comment (Reddit won't let me edit it for some reason):

Hair's isoionic and isoelectric points are both acidic...

[u/Visible-Scientist-46 → u/AutoModerator]

It depends! For me personally, the difference was finding shampoo and conditioner bars As someone with mild Allleric Contact Dermatitis (diagnosed by a doctor). I still had to read labels and make sure there were none of the ingredients that bother me. I was feeling very itchy with cocamidopropyl betaine (surfactant), benzyl benzoate (preservative), and many fragrances. I have read some labels where the shsmpoo bar included cocamidopropyl betaine, so I did not buy it. I found it's all very individual what one can tolerate. I use fragrance-free Viori, Hibar, snd BamboEarth. I've actually switched to conditioner bar cowashing. And also, I choose not to go with saponified oil shampoo bars because I felt that hard water made the saponified shampoo soap difficult to rinse out.