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As a teacher of soap-making classes, this question comes up often. What is the difference between soap and detergent and which is “better?” The simple answer is that soap is made with natural oils and fats and that detergents are synthetic and typically petroleum-based.
Soap and detergent are both surfactants (short for “surface active agents”). They clean by reducing the surface tension of water with their high polarity – one end of a soap or detergent molecule is attracted to organic material (dirt and grease) and the other end dissolves in water. Both work by surrounding dirt and then rinsing away in water.
Soap is considered a more natural surfactant than detergent. Soap is a salt of a fatty acid, created by combining natural fats and oils and a strong base (sodium hydroxide for bar soap and potassium hydroxide for liquid soap). Both vegetable oils and animal fats can be used to make soap; vegetable-based, “castile” soap tends to be milder to the skin. Tallow or other animal fats are commonly used to make conventional soap and these soaps can be more drying and thus less ideal for sensitive skin. Some commercial soap-makers also extract and sell glycerine, a natural by-product of the soap-making process. Glycerine is a natural humectant, so the removal of glycerine can make commercial soap more drying. Using animal fat for soap-making is arguably a good use of by-products from the meat industry, however. Goat’s milk soap can be made with vegetable oils or animal fat by replacing some or all of the water with milk for the lye solution. Soap is naturally alkaline (pH 9.5-10) and so by nature requires no preservation to prevent microbial growth.
Detergents are synthetic, mostly petroleum-based surfactants; chemically they are long carbon chains with a polar portion that dissolves in water similar to soap. Like soap, they can be solid or liquid. Phosphate-based detergents have a bad rap for polluting rivers and streams by causing algal blooms that deplete oxygen and thus kill fish. Some detergents are too harsh for skin while others are used to create mild liquid facial cleansers. While some detergent-based cleansers can be mild, detergents are void of any nutrients that a natural vegetable oil-based soap would contain. Detergents work better for laundry in hard (well) water as calcium and magnesium ions don’t interfere with detergents as much as they do with soap, though a well-formulated coconut-based laundry soap can work well in hard water. Detergents are inexpensive to make, which is why most commercial liquid cleansers (laundry, dish, facial washes, etc.) are detergents. The pH of liquid detergent is neutral or slightly acidic. This is thought by some to be a plus for facial washes because the pH of facial secretions is also slightly acidic. However, this ideal pH is also ideal for bacterial growth so detergent-based facial cleansers are preserved with broad-spectrum preservatives that can cause skin irritation. Detergent-based facial washes are often a combination of surfactants, alcohols, fragrances and preservatives, all which may irritate skin and pollute rivers. In my experience and opinion, a well-formulated facial bar soap (super-fatted well with a non-comodegenic oil) is a superior choice.
A few common soap names: Sodium or potassium palmate, cocoate, tallowate, palm kernelate, olivate. Label may also simply read “olive oil, palm oil, …sodium hydroxide, etc.” or “Saponified oils of olive, …”
A few common detergent names: Sodium laureth sulfate, cocoamido propyl betaine, lauramide DEA, sodium cocoyl isethionate, disodium laureth sulfosuccinate, triethanolamine, sodium lauroyl sarcosinate, sodium olefin sulfonate, Sodium Lauryl Glucosides Hydroxypropyl Sulfonate.