Shampoo bars made of “all natural” ingredients are all the rage in the hair- and skin-care markets. These handmade soaps and shampoo bars are especially gaining popularity in the curly-hair community because they tend to be free of sulfates and silicones and are made from moisturizing oils and gentle cleansers.

Many people report that they are extremely pleased with the results they are getting, citing benefits such as increased softness, better curl formation and, in some cases, elimination of the need to use conditioner.

However, not all users have had such pleasant experiences, and there is some confusion over what the advantage is of shampoo bars over traditional shampoos or low-sulfate or sulfate-free cleansers. There is also some debate about whether the shampoo bars should be followed up with a vinegar rinse, a conditioner, or both.

As usually do, I will delve into the basic chemistry of shampoo bars to discover what answers lie beneath the surface.

What is a Shampoo Bar?

Soap molecules used in shampoo bars are similar to some of the more familiar hair cleansers such as sodium lauryl sulfate in that they are anionic (negatively charged) surfactants. The difference is that the polar head group of the molecule is a carboxylate, rather than a sulfate (R-COO-Na+ vs. R-OSO3-Na+), which results in a milder surfactant. They are formed by reacting a fat (triglyceride) with a strong base, either sodium hydroxide (lye) or potassium hydroxide (potash), in a process called saponification. In this reaction, the fatty acids are cleaved from the triglyceride backbone and in a two-step chemical reaction soap molecules are formed, along with water and glycerin. The amount of strong base needed is calculated based upon published saponification values for the fats being used in the process. Although the source of fats is natural, there is still a chemical reaction and modification that must be done to get a useful derivative.

Typically, an excess of oils is added to the mixture prior to the mixing of fats and base. This provides two benefits:

1. The lye is completely consumed in the chemical reaction, which makes certain the final product doesn’t burn or irritate skin or damage hair.

2. The excess oils act as “superfatting” agents in the shampoo bar, which contribute to mildness and an overall luxurious feel to the soap. These oils act as moisturizing and conditioning agents, much as they would in a regular shampoo or conditioner.

Schematic of the chemical reaction for soap making (saponification). Schematic of the chemical reaction for soap making (saponification).

Most handmade soap makers use a “cold process,” where the main source of heat used is from the exothermic reaction itself (unless the oils or fats need to be pre-melted). The lye or potash is added slowly to water, which quickly becomes hot. It is set aside for a few minutes to cool slightly while the oils are mixed separately. The basic solution is then mixed with oils and stirred until it begins to thicken. Essential oils and colorants can be added at this time, and then the soap is poured into molds. After it cools for a few hours, it can be removed carefully from the molds and cut into bars if needed. These individual shampoo bars are then covered and left to “cure” on racks for a few weeks. This ensures that all of the lye is gone and that the soap is hard.

You may note that in this process, glycerin, a byproduct of the saponification reaction, is left to add humectant and lubricative properties to the soap. It is important to be aware of this because it can potentially be problematic for those with colored hair, especially if the hair was colored recently, if temporary dye was used or the if hair color was heavy in red dye. The humectant properties of glycerin can be a boon or curse for curly hair also, depending upon the hair type, condition of the hair, and environment in which the product user lives.

Soaps are classified as gentle cleaners due to being less efficient at removing oil from the hair when compared to some of the synthetic surfactants. This is a beneficial property in a cleanser for those of us with hair already prone to being dry. The excess oils in a superfatted soap act as emollients and moisturizers to replace oils removed from the hair during the cleansing process. Curly hair doesn’t typically have much oil from the scalp distributed down the hair shaft in the first place, so it needs this extra moisture added in a cleansing routine.

The properties of any particular soap may vary greatly, depending upon which oil or combination of oils is used to make it. Coconut oil is admired for its luxurious, foamy texture. Olive oil (castile soap) is considered to be unparalleled for skin with any types of eczema or psoriasis problems and is very gentle with hair. Evening primrose oil and calendula oil, while expensive, can also add healing and moisturizing properties to the soap. Jojoba oil is very similar in composition to human sebum, so it is great at dissolving old sebum, cleansing the scalp gently and replacing some of the natural oils. Shea butter is prized for being an excellent moisturizer, and soaps with this ingredient included can leave the hair and skin feeling soft.

The Drawbacks of Shampoo Bars

When used in soft water, soap can generate a nice lather and leave hair feeling very soft and clean. In fact, in really soft water and after using an extremely moisturizing soap, the soft and slippery texture of our skin and hair can feel so foreign to us that we may continue rinsing repeatedly in an attempt to remove the perceived residue.

Unfortunately, soap’s effectiveness is significantly reduced when used in hard or acidic water. The reason for this is that the carboxylate group on the soap molecule interacts preferentially with the metallic ions that are so prevalent in hard water (usually calcium, iron, and/or magnesium). The result is the formation of a precipitate, which leaves an insoluble film on whatever surface comes into contact with it, including the hair. This film can be very difficult to remove and leaves the hair dull, lifeless, tangled, and dry. The soap lathers less and cleanses less effectively for the same reason: two soap molecules are removed from action by each magnesium or calcium ion when the complex is precipitated from the solution, so there is less soap available for cleansing. That squeaky clean feeling you may get after using a bar soap is actually the feel of organic/mineral deposits on your hair shaft. This deposit left on the hair can also attract dirt, making hair greasy and dirty. This problem was one of several driving forces for the development of synthetic surfactants such as sodium laurel sulfate.

Another potential hazard of the shampoo bars and soaps is that they typically have a pH in the 8 to 9 range, which is substantially more basic than the natural level for hair. This can result in a temporary breakage of disulfide bonds in the keratin protein of the hair, which can disrupt curl formation and cuticle structure. The basic environment softens the hair, swells it, and leaves it with a ruffled cuticle. This rough surface is not only a source of potentially damaging entanglements and breakage, but also is unattractive because it reduces the shine and gloss of hair tremendously. Swelling of the hair also enables larger colorant molecules to escape, which can shorten the lifetime of a coloring application.

Fortunately, there are a few things you can do to counteract these two effects. Some soap makers put various additives in their soaps that help to keep the soap molecules from binding with hard water metals (sodium silicate, sodium carbonate, borax). However, in the “all-natural” products this is not likely, so it is important to take some steps after shampooing. Rinsing with a mildly acidic solution will help dissolve the soap scum deposit from your hair, shrink the hair shaft diameter, flatten the cuticle and increase the shine and smoothness of your hair. White vinegar, apple cider vinegar, or dissolved citric acid or vitamin C (ascorbic acid) all have sufficiently low pH to help return hair to its preferred pH pf approximately 4-5. Using a clarifying shampoo with EDTA in it can also help remove build up, but would also involve use of a more harsh surfactant, so might be best done only occasionally.

You can also try washing your hair in bottled, purified water when you use your soap, which would make this step necessary less often. Another option is getting a showerhead filter, which is generally less expensive than a whole-house water softener. Even if you take these steps, it is wise to do a mild acidic rinse, due to the basic properties of the soap. Also, dirt on your hair can have minerals in it, which can then create soap scum, so you can’t avoid the need for the low pH rinse entirely.

Depending upon the composition of the soap you are using, the condition of your hair, and the type of water you have, you may find you need to use less conditioner than when you use other cleansers. Experimentation will help you figure out what helps your hair look and feel its best.

How can I incorporate shampoo bars into my hair care routine?

Look for one with the plant-derived oils which you prefer, or buy a few bars and try different recipes.

Lather the bar in your hand, not on your head. Your hair is as fragile as a cashmere sweater, and needs very careful handling at all times.

Use soft water to wash your hair with soap bars whenever possible.

Follow up with a mildly acidic rinse to restore the natural pH of your hair and to impart that shiny, glossy surface we all desire.

Use detangler or conditioner to your own personal tastes. In other words, if you still feel you need it, go ahead! I personally wouldn’t skip that step unless my hair looked weighed down or limp.

Give it a few tries. I have read that it can take some time for your scalp and hair to adjust, just as it often does when you go to a low shampoo or shampoo free routine.

Report your results to us!

I plan to head to my local health food store soon for some shampoo bars, or maybe don my chemist gear and make some myself!

Additional reading about pH and hair is available here.