Soap vs Shampoo…. what’s the difference?

Soap vs Shampoo…. what’s the difference?

Feb 06, 2022

Barbara Marengo

Last year one of my clients asked me for a shampoo bar. She has short brown hair which she washes daily and wanted an easy to use solid bar of soap, for her hair!

Looking at her face as I tried to explain that she should rather use a natural but traditional shampoo (liquid detergent in a plastic bottle) I realized that I was way out of touch. Our customers want natural products which are plastic free. Liquid shampoo in a single use plastic bottle was not a solution, but I was not convinced that a bar of soap was the answer. And I was not convincing her that it wasn’t.

The thing is, as an experienced and qualified formulator of both cold pour soap and liquid shampoo, I know that natural soap (solid or liquid) is not ideal for washing your hair. There are lots of people who happily wash their hair with soap. Some follow up with an acid rinse using apple-cider vinegar to restore the pH and condition. But mostly these are customers who do not invest hundreds of dollars annually colouring or heat styling their hair. Soap will not work for chemically treated hair which has been damaged and or very dry curly hair. Hair with these needs require additional care when cleansing. 

Here’s why I wouldn’t wash my hair with soap.

  1. Your scalp’s pH is different from the rest of your body. Depending on your age and your general health of your scalp usually has a pH of around 3.5 – 5. Soap is highly alkaline. While soap works to clean your hair and scalp, it also damages the keratinous scales which form your hair shaft and will leave your hair dry and frizzy.
  2. Your hair is negatively charged and needs a positively charged product in order to leave it shiny and strong. Soap is anionic and thus negatively charged.
  3. Soap will dry out your scalp damaging your scalp’s microbiome. While your skin’s pH balance recovers quickly, your scalp will need to produce more sebum to achieve this, leading to a chain reaction of dry scalp and oily hair. 

Soap is the product of a fat and an alkali and carries a pH of between 8-12. The variety of fats used and the curing time will reduce the pH making your product a gentler bar, but you cannot reduce the pH below 8 without losing the very nature of what makes a soap bar, well…soap. And as an anionic surfactant, soap carries a negative change.  Interestingly, sulphates are also anionic and tend to be alkaline. Think SLS and SLES!

Remember that cool experiment from school where we use to measure static in your hair with a balloon? If the surface of the balloon is positively charged, and our hair is negatively charged, then your hair will stick to the surface of the balloon. If you rub the balloon and it becomes negatively charged, then the static between them will leave your hair mid-air. It’s the same with your shampoo. If your shampoo creates a build-up of static electricity in your hair, you will be left with frizzy, dry fly-away hair. Each strand trying to get away from the one next to it. What you cannot see with the human eye is that the keratinous scales on the hair shaft have been lifted and the core of the hair shaft will have absorbed the water and soap solution, damaging your hair strand from the inside by causing it to swell with water and stretch with the added weight. Add to this heat from styling and exposure to the sun and your hair will quickly lose its shine and strength. The bottom line: the true nature of soap is fundamentally antagonistic to the needs of your hair and scalp.

So, what can shampoo do that soap cannot?

 As a formulator, your mission starts with understanding your clients' needs and creating the best solution to meet them. I discovered that there are a number of naturally derived plant-based surfactants – some of which are biodegradable and ecofriendly. Advancement in green technology in the beauty industry has led to exciting new developments in plant-based ingredients, allowing for the creation of a soap free solid shampoo bar, which is naturally derived from plant oils. These surfactants are either non-ionic (do not carry a charge), amphoteric (can be either positive or negative, depending on the pH) or cationic (positively charged). Ideally, you want to create a blend of surfactants with an acidic-neutral pH of between 3.5-5. The reason for the blend is that you can create a synergy of ingredients that is gentle and effective. Ideally you want to use a blend of non-ionic, cationic and amphoteric surfactants.

To create a solid shampoo bar, you need to either purchase a recipe from a trusted chemist, or develop an in-depth understanding of formulating for haircare. This is not as simple as making cold or hot process soap, where regardless of the blend of oils, you will always end up with an anionic, alkali product.  The addition of oils and butters as well as fatty acids will give your shampoo bar properties which your soap bar cannot.


Here’s what you can create in a shampoo bar, that you cannot with soap:

  1. pH can be adjusted to maximize the efficacy of the ingredients without compromising the integrity of the solid bar – ie bar can be neutral – acid, and still hard.
  2. Blend of positively charged surfactants to gently cleanse even the most sensitive scalp
  3. Support the microbiome of the scalp, nourishing and feeding the hair follicles for optimum growth and nourishing the hair fibres for strength and shine using a compliment of natural oils, butters and botanical actives which are not damaged by the pH of the product.

 I am proud to say that I have developed a range of 5 solid naturally derived shampoo bars and conditioner bars, to better suit my client’s needs. Thanks to my customer who raised the bar, we now create haircare which does not require a plastic bottle, and is convenient and effective.

Solid soap free shampoo in a bar which leaves your hair shiny, healthy and strong, while gently cleansing and nourishing the most sensitive scalp.


Barbi Marengo is a student of Formula Botanica studying towards her Advanced Diploma in Organic Skincare and Haircare. She has been formulating cold process vegan soap since 1998 and. solid shampoo bars since May 2020.