In recent years, a trend of Hand-made Soap (or soap) has gradually become popular. Making soap by hand is a very interesting DIY job, with moderate difficulty, low requirements, and closely related to daily life. Students who like to do it may as well try it.
However, there are also some articles on the Internet claiming that this kind of “cold soap” made by hand with cold technology is more beneficial to health than the soap produced by industrialization. For example, it retains many natural beneficial ingredients, is rich in moisturizing factors, and is more gentle on the skin. Are these claims true?
what is soap?
To answer this question, we must first talk about what soap is. Simply put, soap, whether industrial or handmade, is the product of chemical reaction between “oil” and alkali. To be exact, it is sodium fatty acid produced by saponification reaction between fatty acid and sodium hydroxide (NaOH, also known as caustic soda and caustic soda, which is a very basic substance). With a simple reaction formula, it can be expressed as:
fatty acids glycerol triglycerides + sodium hydroxide → sodium fatty acids + glycerol
The fatty acids mentioned here are actually a large category, which can be divided into saturated fatty acids and unsaturated fatty acids. The former includes lard and butter, while the latter includes olive oil and coconut oil.
It is fatty acid sodium that plays a role in washing away stains in soap. Therefore, the quality of fatty acid sodium is directly related to the decontamination ability of soap. In actual production, the formula of soap (soap) is often very complex, and the raw materials used are mixtures of several or even a dozen fatty acids, in order to obtain better decontamination effect and skin contact feeling.
what is the difference between “cold soap” and industrial soap?
Then, since they have all gone through 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 manual cold soap : first weigh out a certain amount of base oil (i.e. fatty acid ester), sodium hydroxide and water; Then sodium hydroxide is slowly dissolved in water and stirred evenly; After heating the base oil to 60 ° C, slowly pour it into the above aqueous sodium hydroxide solution, and stir it continuously while pouring. After pouring, stir it for 15-20 minutes, and keep the temperature between 40-50 ° C during the whole mixing process. Then, pour the paste into the mold for natural cooling, place it in a cool and ventilated place, and demould it after 4-6 weeks.
The basic process of typical industrial soap making  is to add the fatty acid ester and sodium hydroxide aqueous solution to the reaction kettle (that is, a large metal pot), heat it with steam until the water boils, and react for dozens of minutes to an hour under strong mechanical stirring. The obtained product is washed with salt water, layered, dried and cooled to obtain soap base (i.e., broken soap). After drying, pressing and melting the soap base, Add spices and other ingredients (such as disinfectants, moisturizers, etc.), pour them into the mold, demould and polish them to get soap.
Comparing the two processes, it is not difficult to see that the biggest difference lies in the saponification reaction temperature: manual soap is below 60 ° C, while industrial soap is often around 90-105 ° C. The temperature difference directly affects the reaction rate. In most chemical reactions, the higher the temperature, the faster the reaction. Generally speaking, every 10 ° C increase in temperature will increase the reaction rate by about twice. That is to say, in the process of making soap by hand, the speed of the whole reaction is very slow, and the saponification reaction can not be completed in 20 to 30 minutes, while the industrial soap making process can ensure that most of the raw materials are converted into products.
Another factor affecting the reaction rate is the homogeneity of reactants. It is not difficult to understand that the oil and water poured into the beaker are generally divided into upper and lower layers, which is called “heterogeneous” chemically. If the two substances are to be fully contacted and reacted, strong stirring is required. It is obvious that when making soap by hand, holding a glass rod to stir, its efficiency is far from being compared with the mechanical stirring paddle driven by a motor in industrial soap making; It would be better to use an electric drill as a stirrer, but at low temperature, the viscosity of the reactants is large (in paste form), and its stirring effect is still not comparable to that of the reactor.
will “cold soap” be less alkaline
So, what does the slower reaction mean?
This means that compared with industrial soap, the manual soap poured into the mold and solidified has not yet completely reacted, and still contains a large number of raw materials that have not yet reacted, including the “base oil” and sodium hydroxide you put in. Sodium hydroxide is a very alkaline substance. It will cause irritation and chemical burns when it comes into contact with human skin. It will also cause severe pain when it comes into contact with eyes. In order to avoid such terrible consequences, all hand-made cold soaps will mention “waiting for maturity for several weeks”, which is actually to let the residual raw materials continue the unfinished saponification reaction at normal temperature. (for details on the chemical burns caused by sodium hydroxide, see “how did the horrible kiss in fight club come from?”)
Unfortunately, as mentioned earlier, the hand soap in “placing” is not as completely reactive as people would like, because the saponification reaction at room temperature is slower than that at 50 ° C. More disadvantageous is that the soap has solidified into a solid, and the contact area between the oil and sodium hydroxide is much smaller than that in the liquid state, further reducing the reaction rate. Therefore, even after a few weeks, there may still be a small amount of sodium hydroxide left in the soap without reaction. How can washing your face with this soap containing strong alkaline substances be milder than industrial soap?