Characterization of the Chemical and Physical Properties of Bar Soap Made with Different Concentrations of Bentonite as a Filler

The use of bentonite in cosmetics is considered beneficial for absorbing dirt and moisturizing and detoxifying the skin. In this study, different amounts of bentonite (10, 12.5, 15, 17.5, and 20 g) were used as a filler in 5 separate formulations of natural bar soap having coconut oil as the primary ingredient. Each type of bentonite soap was analyzed in accordance with the Indonesian National Standards (SNI). The free alkali content for each of the five preparations of bentonite soap was zero, the moisture content ranged from 0.88 to 1.15 %, the pH ranged from 8.7 to 8.8, the total free fatty acid content ranged from 0.28 to 0.72, and the foam or bubble stability was above 70%, while the soap’s hardness ranged from 1.49 N/m² to 1.54 N/m². All five soap types fulfilled SNI standards, but the soaps containing 15 g and 17.5 g of bentonite had a far better appearance. The chemical compositions of the bar soap composed of 15 g bentonite (BS 15) and the bar soap composed of 17.5 g bentonite (BS 17.5) were calculated using material balance and compared by Energy Dispersive X-ray (EDX) analysis. The EDX analysis revealed that both BS 15 and BS 17.5 contained the Si and Al metals originating from the bentonite. No heavy metals considered harmful to skin health (e.g., Hg, Pb, As, or Cd) were detected. Scanning electron microscopy (SEM) of the topography and texture of BS 15 and BS 17.5 revealed a more even distribution of Si and Al throughout BS 17.5 than in BS 15. BS 17.5 was therefore deemed the best formula as it meets SNI standards and has a better appearance and a more even mineral distribution.

Bar soap; Bentonite; Characterization; SEM-EDX; Stoichiometry

Current trends in consumer preference indicate an increased demand for natural ingredients in skin care and cosmetic products. This has led to an increase in the economy of small-scale homemade (artisanal) industries that offer a wide range of products with a variety of natural ingredients. One skincare product with natural ingredients that is now in vogue is natural bar soap. The global market for bar soap has been studied in key regions, including North and South America, Europe, China, Japan, India, the Middle East, and Africa. The market for bar soap in the Asia-Pacific Region is likely to show remarkable growth.

    Bar soap made from natural ingredients, such as vegetable oil, does not dry out the skin because this soap type contains glycerin. Natural herbal soap ingredients are generally vegetable or saponification oil (even animal fat), glycerin, natural fragrance, various organic additives, and sodium hydroxide. One soap-making process involves oil saponification, a chemical reaction that occurs between fats like coconut oil (C₃₃H₆₂O₆) and alkali or sodium hydroxide (NaOH) to producing glycerol (C₃H₈O₃) and soap (Vidal et al., 2018). Chemically, soap is symbolized by the general formula RCOONa, in which R is a long-chain alkyl group consisting of 12 to 18 carbon atoms.

Natural herbal soaps usually contain a mixture of coconut oil, palm oil, olive oil, rice bran oil, and sunflower seed oil (Araseretnam and Venujah, 2019). The distribution of saturated and unsaturated fatty acids determines the hardness, aroma, cleansing, power, foaming, and moisturizing ability of the soap. The properties of the resulting soap are also further influenced by the additives or fillers that are used. One common filler used in soap-making is bentonite clay, a natural clay that is used as a traditional skin regimen and medicine in many cultures (Moosavi, 2017).

Bentonite clay acts as a detoxifying agent due to its polycationic property, which promotes the absorption of negatively charged toxins (Williams et al., 2009). Bentonite, better known as grey clay powder, has been used for centuries for medicinal purposes. Because of its active properties, this mineral is able to fight lipodystrophy, a disease that causes humans to lose fat tissue in body parts, giving the appearance of aging, acne, and cellulite (Carretero and Isabel, 2002). Clay powder can also be formulated into topical applications to serve as a dermatological protector. The active properties of clay powder are what give it the ability to provide opacity, cover up blemishes, and remove shine from the skin. Its activity comes from the two prevalent metal oxides, SiO₂ (silica), which ranges in content from 48% to 50%, and Al₂O₃ (alumina), which ranges in content from 14.5% to 15%. Other less prevalent metal oxides include Fe₂O₃ (iron), CaO (calcium), K₂O (potassium), Na₂O (sodium), Ti₂O (titanium), and P₂O₅ (phosphorous); these are all found in relatively small amounts (Shah et al., 2013; Nweke et al., 2015).

The density of bentonite is higher than that of water, so it is more stable and can maintain its form for a longer time than it can in the bentonite soap that is produced (Asad et al., 2013). Much research has already been done on the production of detergent formulas that contain soil additives such as clay, bentonite, or even kaolin. Patents (Dahlan, 2010) have been granted for clay-containing detergents for external use and for the cleaning of equipment contaminated with impurities; these detergents range in pH from 5.5 to 6.5. Research has also been conducted aimed at the optimization of bentonite use as a filler for liquid detergents used as agents to clean extreme "najis mughalladhah" (Wati et al., 2019). Najis Mughalladhah in the Islamic religion is a form of ritual uncleanliness originating from dogs and pigs, and this uncleanliness requires special treatment for purification or cleansing.

Generally speaking, the main raw material for solid bath soap is currently sodium lauryl sulfate (SLS), which acts as a detergent. However, SLS is produced from petroleum, so this is not a sustainable industrial process. Therefore, industry must develop an adequate and appropriate strategy to maintain long-term competitiveness for soap making. Tatiana and Mikhail (2020) discuss the relationship between the innovative strategy recommended for industry life cycle stages and the specification of regional and territorial factors. The industry for skincare products with natural ingredients can be developed in Indonesia because the industry lifecycle sustainability can be maintained, especially when considering that Indonesian territories are the largest global producers of vegetable oils, including coconut and palm oils.

Studies of important implications of the development and success of products have identified six standard factors that influence the success of a variety of products; these are price, product performance, brand, esthetic design, service, and marketing (Setyaningrum et al., 2020). A viable esthetic design for bath soap is influenced by the soap’s physical properties, such as its hardness, and the compatibility of the bentonite filler with the solid soap.

        Soaps prepared by the addition 10–20 g bentonite filler showed characteristics that fulfill the SNI standards. The five tested soap formulas had zero free alkali content and a pH ranging from 8.7 to 8.8, indicating that the resulting bar soaps are extremely safe to use on the human body. The moisture content was relatively low and the free fatty acid content was also low, while the foam showed good stability. However, of these five soap formulations, the soap with 10% and 12.5 g bentonite appeared to be softer, whereas the soap containing 20 g bentonite showed a rapid onset of the trace process, to the point that the soap was difficult to mold. These separate factors affected the performance of the soap shape, so only two soaps, BS 15 and BS 17.5, with 15 and 17.5 g of bentonite filler, respectively, were analyzed further. Calculation of the soap composition using material balance theory and EDX analysis revealed that the addition of bentonite increases the mineral content of the soap preparation. A comparison of the amounts of C, O, Na, Si, and Al indicated the same trend. The EDX analysis clearly showed that the resulting soap only contains the metals Si and Al, which come from the added bentonite and are not harmful to skin health. No harmful heavy metals (such as Hg, Pb, Cu, or others) were found. Analysis of the morphology using SEM confirmed that the soap formulation is compatible with the addition of either amount of bentonite. The BS 17.5 had a more even and homogeneous distribution of mineral particles on the soap surface compared to the BS 15 soap; therefore, the BS 17.5 soap was deemed the best formula soap. It had a better appearance and it met all the parameters required by the SNI standards. The presence of Si and Al metals from the bentonite also means that this soap can be used as an agent for the cleansing of extreme najis mughalladhah. Therefore, further research should be conducted to obtain additional basic and accurate evidence. Aside from this, but no less important, is a need for further study of the effects of glycerin as a byproduct of the saponification process and of the cytotoxicity of bentonite soap and its capacity to hydrate the skin. Future research should aim at determining the safety and skin health benefits of bentonite clay soaps.

    This work was supported by the TALENTA USU program with contract number 4167/UN5.1. R/PPM/2017; April 1, 2019.