By Dr. Carol Fassbinder-Orth

When one hears the word “soap”, most people think of bar soap or foaming hand soap. When one hears the word “detergent” most people think of laundry or dish detergent. However, detergents are also the primary active ingredient in most shampoos, body washes, bubble baths, and facial cleansers. Here I explain the science behind soaps, detergents, and liquid vs solid shampoos to help consumers make more informed decisions.

Basic Definitions

Soap: A compound made of natural fats, oils, or butters and a strong alkali such as sodium hydroxide (also called caustic soda), or potassium hydroxide (also called caustic potash). Potassium-based alkali soaps are more water soluble and make a softer soap (Example: liquid castile soaps). Sodium-based alkali soaps make a less water soluble product and make a harder soap (Example: bar soap).

Detergent: Synthetic surfactant used as a cleansing agent. Detergents are distinguished based on their chemical properties, and can generally be classified as anionic, cationic, and non-ionic detergents.

WHAT ARE THE DIFFERENCES BETWEEN SOAPS AND DETERGENTS?

It can be very confusing to dissect all the literature available to us on the products we put on our skin and hair and use in our household cleaning. Marketing programs always highlight the most sellable aspects of a product, and make claims of “all-natural” ingredients or highlight high quality ingredients that actually make up a minuscule part of the overall product. Here are the similarities and differences between soaps and detergents in your personal care and household cleaning products:

Similarities:

Soaps and detergents are amphipathic cleaning agents, meaning they have a polar (water loving) end and a nonpolar (fat loving) end. This amphipathic quality helps these cleaning agents emulsify fats and grease from surfaces, including your skin and hair.

Differences:

Source: Soaps are made from natural products: fats, oils, butters and alkalis, whereas detergents are primarily synthetically-made compounds. However, there are new detergents that are being manufactured from natural sources, such as coco glucoside.

Cleansing power: Detergents are usually superior to soap in terms of grease, dirt, and oil removal power from any surface. This can be beneficial for a household cleaning product, but can be overly drying for a personal care product.

Biodegradability: Soaps are readily biodegradable in the environment, and pose minimal risk to aquatic life. Some detergents, such as cationic detergents, are highly persistent in the environment and pose serious risk to aquatic wildlife (Scott and Jones, 2000).

Health Impacts/ Hazards: Several recent peer-reviewed scientific studies have reported negative human and wildlife health impacts of specific detergents, such as ammonium lauryl sulfate (Park et al., 2019, Lechuga et al., 2016). Detergents such as sodium lauryl sulfate are also known to increase negative charges on the surface of hair, increasing frizz (Martins et al., 2019). No known health hazards have been reported for soap, although soap will leave a soap scum on surfaces (including hair and skin) if used with hard water.

SOAP

Soap is formed through the reaction of fats, butters, or oils with an alkali. This process is called saponification. Fats, butter, and oils contain triglycerides, which are molecules that have a glycerol backbone and 3 fatty acid tails. When triglycerides react with alkali, the result is a fatty acid salt (e.g. Sodium palmitate). Soap makers often include multiple types of butters and oils in their recipe because of the diversity of fatty acids they contain. Some fatty acids will not saponify, and so remain as a fatty acid in the soap, and contribute to the moisturizing potential of the soap. Others will produce fatty acid salts that result in a hard soap, or others may generate a luxurious lather. Soaps are an alkaline product (high pH). This may transiently change the pH of the skin, but is not known to adversely affect the physiology of the skin or hair. Most of our bar soaps use sodium hydroxide as the source of alkali to produce a hard bar of soap. We use a dual lye (both sodium hydroxide and potassium hydroxide) to produce a high lather Shaving Soap, and only potassium hydroxide to produce a liquid castile hand soap called Foaming Honey Hand Soap.

COLD PROCESS SOAP: Soap can be made through a cold process or hot process technique. In the cold process technique, oils and butters are combined with lye at about 120 degrees Fahrenheit, and poured into molds once the soap has thickened to a “trace”. Soap cures in molds for about 48 hours, where it completes most of the saponification process (ensuring that no free lye remains in the soap). The bars are then cut and the soap must cure for several weeks to allow the excess water to evaporate and generate a hard bar that lasts a long time. Some types of soap, like our Aleppo Soap, cures for more than 9 months! Examples of soaps made by the “Cold Process” method include our Hand and Body Soaps, Facial Soaps, and Shampoo Bars.

Shaving Soap made using a dual lye hot process method. Notice the high amount of lather generated from this formulation.

HOT PROCESS SOAP: In hot process soap making, the oils and lye are combined and kept at about 150 degrees Fahrenheit in a slow cooker to speed up the saponification process. Once a thick trace is met, soap is poured into molds and then cut a few days later. Examples of soaps made by the “Hot Process” method include our Shaving Soap and our Foaming Honey Hand Soap.

Castile foaming hand soap made with a hot process method

SUPERFATTING: Different applications call for different soap formulations. A soap that one would use for cleaning dishes should remove all the oils and grease from the dishes. However, a soap you use on your face that removes all the oil would be considered drying. Therefore, soap makers use a technique called “superfatting” to increase the moisturizing characteristics of a soap for personal care products. Superfatting refers to the percentage of fats that remain unsaponified (not soap) after the saponification process. The higher the superfat, the more moisturizing the soap will be. We make our soaps with a range of 5% superfat in our Hand and Body Soaps to 8% superfat for our anti-aging Tremella aeterna Facial Soap.

DEALING WITH SOAP SCUM: Soaps react with Calcium and Magnesium in hard water to form soap scum, a stubborn scaly buildup that can form on just about any surface. Detergents do not form soap scum in hard water because of differences in the chemical structure between soaps and detergents. The polar end of detergents (usually a sulfonate) is less likely than the polar carboxylate of soap to bind to calcium and other ions commonly found in hard water, and therefore less likely to form a soap scum film. Acids like vinegar can be used as a cleaning product in your bathroom to help remove the scum (pour vinegar down your drains monthly to prevent soap scum buildup), and can also be used on your hair to remove an soap scum that may be present. We make a hard, cold process shampoo bar that includes apple cider in the bar to reduce the buildup of residue on your scalp caused by the reaction of soap with minerals in hard water.

How do you know if you have hard water? Check with your city. Sometimes the water hardness report is included in yearly water reports from your water supplier. Call if you aren’t sure, as this information is publicly available to you as a customer. If you live on well water, water test kits are available at home improvement stores. Water softeners decrease the levels of Calcium and Magnesium in your water, helping to prevent or reduce the buildup of soap scum.

DETERGENTS

There are three main types of detergents: anionic, cationic, and non-ionic detergents.

Anionic

Anionic detergents are commonly used in personal care products, such as shampoos, body and facial washes, and also in laundry and dish washing products. They create a lot of foam when introduced into water, and contain a negatively charged head (often a benzenesulfonate head group). Examples include: ammonium lauryl sulfate, sodium lauryl sulfate, sodium cocoyl isethionate, sodium coco sulfate, sodium lauryl sarcosinate, and sodium myreth sulfate. These sulfates can cause skin or scalp irritation in some people (like me!) because they can penetrate into the epidermis a significant amount. Other common anionic detergents include sulfonates and gluconates. Biodegradability varies according to chemical characteristics.

Cationic

Cationic detergents are commonly used in laundry cleaning and softening products and often contain a quaternary ammonium salt group. Examples include: Dimethyldioctadecylammonium chloride and alkyl hydroxyethyl dimethylammonium chloride. Some cationic surfactants are persistent in the environment (do not biodegrade quickly), and can be very irritating to the skin.

Non-ionic

Non-ionic detergents are a large class of detergents that are used in a wide variety of applications, including laundry detergents and personal care products. Examples include: ethoxylates, alkoxylates, cocamides, and glucosides. They have a lower foaming potential than anionic detergents, and are often used in conjunction with anionic detergents in laundry detergents to maximize cleaning power while minimizing foaming. A new detergent that is manufactured from coconut or sustainably sourced palm oil is called coco glucoside, and is considered a natural product by the Natural Product Association.

SHAMPOO: LIQUID VS. SOLID BARS

Did you know that there are three different types of shampoos? Everyone is familiar with liquid shampoo, but there are also hard soap-based shampoo bars and hard syndet detergent shampoo bars.

Liquid Shampoo: Liquid Shampoo is the liquid foamy concoction that can be used on your hair and many times, your body. Most commercial formulations of shampoo contain an anionic detergent such as sodium laureth sulfate, although some may use more gentle, naturally derived, non-ionic detergents such as coco glucoside. Liquid shampoos contain water in addition to all their other ingredients. Water availability is a term used in microbiology to describe the potential for microbes to grow in a particular substance. If a personal care product contains water and anything that a bacteria or fungus can use for food (sugar, oil, or protein), bacteria and fungi can grow in it if it doesn’t contain a strong preservative. Therefore, all liquid shampoos, body washes, or lotions- no matter how “all natural” they are-must contain preservatives to prevent bacterial and fungal growth. Sometimes companies will hide their preservatives by including ingredients such as aloe vera gel or a fruit extract. Both of these liquids will contain a preservative, but because the preservative is considered a secondary ingredient, companies are not required to list preservatives separately in their ingredient list. Example synthetic preservatives that are commonly used in liquid shampoos include: sodium benzoate, potassium sorbate, imidazolidinyl urea, and DMDM hydantoin. Natural preservatives vary in effectiveness. Some companies, like Tom’s of Maine, use natural preservatives (e.g. sodium gluconate) in their formulations, and hopefully more will follow suit.

Syndet Shampoo Bars: If you are looking to reduce plastic, but keep the same basic feel of detergent-based shampoos, syndet shampoo bars are the way to go. Syndet stands for SYNthetic DETergent. Most are made with “noodles” of sodium lauryl sulfate or sodium coco sulfate (a gentler detergent). Like liquid shampoo, syndet shampoo bars are effective at cleansing, but conditioners are often needed to help moisturize your hair.

Hard soap shampoo bar made with beer and apple cider vinegar

Soap Shampoo Bars: Soap shampoo bars are the all natural solution that reduces plastic use and also eliminates detergent usage. Soap shampoo bars are made using traditional cold or hot process methods. The oils included are often those that generate a good lather and have good cleansing abilities (coconut and castor oil, for instance). Like soap used on the body, if you have hard water, you could have a soap scum residue left on your hair. To prevent this, an apple cider vinegar rinse is often recommended for soap shampoo bars. At Bountiful Blossoms Bee Company we include beer in our recipe which increases lather and provides vitamins, amino acids, and minerals for the hair. We also include apple cider vinegar in the bar to reduce the pH of the bar and prevent soap scum buildup. Because less of the oils are stripped from your hair and the shampoo bars often contain nourishing oils (like jojoba or argan oil), conditioners are generally not needed for shampoo bars.

Adjustment time: It can take anywhere from 1 week to several months for your hair and scalp to adjust to a new shampoo. Give it time!

SHAMPOO SUMMARY: Liquid shampoos are convenient and produce great lather but can be irritating, contribute to frizzy hair, and generate plastic waste. Syndet hard shampoo bars reduce the plastic waste and provide strong cleansing power, but can still be irritating or drying for some people. Soap shampoo bars are all natural and gentle on the hair but some formulations can leave a soap scum residue on the hair, especially if you have hard water.

SOAPS vs. DETERGENTS FINAL POINTS

Soaps are made with natural products that are gentle on the skin and hair. They may not be for everyone, especially if you have hard water. Detergents can vary from naturally-derived, gentle, biodegradable compounds to synthetic, irritating, and nonbiodegradable.

Questions? Ask below!

References

Lechuga, M., Fernández-Serrano,M., Jurado, E. Núñez-Olea, J., Ríos, F. Acute toxicity of anionic and non-ionic surfactants to aquatic organisms. Ecotoxicology and Environmental Safety. 2016. 125:1-8. https://doi.org/10.1016/j.ecoenv.2015.11.027.

Martins, Giselle, and Maria Fernanda Reis Gavazzoni Dias. 2019. “Hair Cosmeceuticals.” In Alopecia, 285–93. Elsevier. doi:10.1016/b978-0-323-54825-0.00027-2.

Park EJ, Seong E, Kim Y, Lee K. Ammonium lauryl sulfate-induced apoptotic cell death may be due to mitochondrial dysfunction triggered by caveolin-1. Toxicol In Vitro. 2019;57:132‐142. doi:10.1016/j.tiv.2019.02.021

Scott MJ, Jones MN. The biodegradation of surfactants in the environment. Biochim Biophys Acta. 2000;1508(1-2):235‐251. doi:10.1016/s0304-4157(00)00013-7

The Science of Soaps and Detergents