Is handmade soap healthier than regular soap?

In recent years, the atmosphere of a hand-made soap (or soap) has gradually become popular. Handmade soap is a very interesting DIY job, the difficulty is moderate, the conditions are not high, and it is closely related to daily life. Students who like to do it may wish to try it.

However, there are also articles on the Internet that claim that this cold-processed, hand-made "cold soap" is more healthful than industrial soap. For example, is it true that many natural beneficial ingredients, moisturizing factors, milder skin, etc. are retained?

What is soap?

To answer this question, you must first talk about what soap is. Simply put, whether industrial or handmade, soap is the product of a chemical reaction with an "oil" and a base. Specifically, it is a fatty acid sodium formed by a saponification reaction between a fatty acid and sodium hydroxide (NaOH, also known as caustic soda, caustic soda, which is a very basic substance). A simple reaction formula can be expressed as:

Fatty acid triglycerides of fatty aliphatic → + sodium hydroxide + Glycerol

The fatty acids mentioned here are actually a large category, which can be divided into at least two types of saturated fatty acids and unsaturated fatty acids. The former includes lard and butter, while the latter has olive oil, coconut oil and the like.

It is the fatty acid sodium that plays a role in washing off the stains in soap. Therefore, the quality of sodium fatty acid is directly related to the decontamination ability of soap. In actual production, the formula of soap (soap) is often very complicated, and the raw materials used are a mixture of several or even a dozen fatty acids in order to obtain a better decontamination effect and skin contact feeling.

What is the difference between “cold soap” and industrial soap?

So, since all have undergone the same saponification reaction, what is the difference between hand-made cold soap and industrial soap? Let's take a look at the typical preparation method of hand-cooled soap [1]: weigh a certain amount of base oil (ie fatty acid ester), sodium hydroxide and water; then slowly dissolve sodium hydroxide in water Stir well; after heating the base oil to 60 ° C, slowly pour into the above aqueous sodium hydroxide solution, stir while stirring, stir for 15-20 minutes after the completion of the pour, and maintain the temperature throughout the stirring process. At 40-50 ° C. Subsequently, the paste is poured into a mold and naturally cooled, placed in a cool and ventilated place, and can be demolded after 4-6 weeks.

The basic process of typical industrial soap manufacturing [2] is to add fatty acid esters, aqueous sodium hydroxide solution to the reaction vessel (that is, a large metal can), steam to boiling water, and in a powerful machinery The reaction is stirred for several tens of minutes to one hour, and the obtained product is washed with brine, layered, dried and cooled to obtain a soap base (ie, a piece of soap), and the soap base is dried, compressed, melted, and then added with spices and others. Some ingredients (such as disinfectants, moisturizers, etc.) are poured into the mold, demolded and polished to obtain soap.

Comparing the two processes, it is not difficult to see that the biggest difference lies in the temperature of the saponification reaction: the handmade soap is below 60 °C, and the industrial soap is usually around 90-105 °C. The difference in temperature directly affects the reaction rate. In most chemical reactions, the higher the temperature, the faster the reaction. Generally, for every 10 °C increase in temperature, the reaction rate is approximately doubled. That is to say, in the process of hand-made soap, the speed of the whole reaction is very slow, and the saponification reaction cannot be completed within twenty or thirty minutes, and the industrial soap-making process ensures that most of the raw materials are converted into products.

Another factor that affects the rate of reaction is the uniformity of the reactants. It is not difficult to understand that the oil and water poured into the beaker are generally divided into two layers, which are chemically called “heterogeneous”. If the two substances are to be in full contact and react, it is necessary. Strong mixing. Obviously, when hand-made soap is used to hold the glass rod to stir, the efficiency is far less than that of the mechanical stirring paddle driven by the motor in the industrial soap; if the electric drill is used as the agitator, it will be better, but at low temperature, the reactant The viscosity is large (paste), and the stirring effect is still not comparable to the reactor.

Will "cold soap" be less alkaline?

So, the reaction rate is slower, what does it mean?

This means that compared to industrial soaps, the hand-cured soap that has solidified after pouring into the mold has not yet fully reacted, and still contains a large amount of raw materials that have not yet reached the reaction, including the "base oil" and sodium hydroxide that you put in. Sodium hydroxide is a very alkaline substance that can cause irritation and chemical burns when it comes into contact with human skin. It can cause severe pain after contact with the eyes. In order to avoid this terrible consequence, all hand-made cold soaps will mention “putting for weeks to wait for maturity”, which is to let the residual raw materials continue unsuccessful saponification at room temperature. (For the chemical burn caused by sodium hydroxide, see "How did the horrible hickey in "The Fight Club" come from?")

Unfortunately, as mentioned earlier, the handmade soap in the "placement" is not as fully reactive as the saponification reaction at room temperature is slower than at 50 °C. More disadvantageously, at this point the soap has solidified into a solid, and the contact area between the oil and the sodium hydroxide is much smaller than in the liquid state, further reducing the rate of the reaction. Therefore, even after a few weeks, a small amount of sodium hydroxide may remain in the soap without reacting. How can you wash your face with this soap containing strong alkaline substances, how can it be milder than industrial soap?

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