## Materials and Methods  
### Preparation of the Soaps  
Fatty acid soaps were prepared by saponification reaction between the fatty acid and sodium hydroxide. The C18 fatty acids were prepared based on the classification of the alkyl structure. Stearic acid was prepared as a saturated straight-chain fatty acid. The optical isomeric mixture of isostearic acid was prepared as a saturated branched-chain fatty acid. Two types of C18 fatty acids were prepared as unsaturated fatty acids. Oleic acid was prepared as a monounsaturated fatty acid, while linoleic acid was prepared as a di-unsaturated fatty acid. For comparison with these sodium soaps, sodium laurate was prepared from lauric acid. All reagents were of first or special grade and purchased from Wako Pure Chemical Industries (Tokyo, Japan). A 1.0 mol/L sodium hydroxide aqueous solution was prepared by dissolving solid sodium hydroxide (Sigma-Aldrich, St. Louis, MO, USA) in deionized water. A 5 mL solution of 1 mol/L sodium hydroxide (5 mmol NaOH) was stirred on a hot plate stirrer (AS ONE Co., Osaka, Japan) maintained at 70 \({}^{\circ}\)C. Then, 5 mmol fatty acid was added to it and the mixture was stirred until it was completely homogenized. The resultant saponified fatty acid was adjusted to a 1% concentration by adding deionized water. Skin Permeation Experiments and Evaluation of the Effect of Fatty Acid Soap on the Skin Barrier Function  
The effect of different fatty acid soaps on the skin barrier function was assessed by the TI method, based on a previously described method [15]. The skin permeability assay was performed using the Franz diffusion cell system (vertical reservoir, 8 mL; permeation area, 1 cm\({}^{2}\)) (PermGear, Hellertown, PA, USA). The reservoir chamber was filled with saline and warmed up to 37 \({}^{\circ}\)C. Laboskin(r) (Hoshino Laboratory Animals Inc., Ibaraki, Japan; certificated by the Japanese Society for Laboratory Animal Resources); the back skin from a 7-week-old male Hos:HR-1 hairless mouse, was divided into four parts and used as the transmission skin sample. There were no statistically significant differences in the head and tail sides as well as the right and left sides of the midline. A 1 mL 1% fatty acid soap solution or control sample (deionized water) was applied to the external surface of the skin for 1 h. After treatment, the test or control sample was removed and the skin was thoroughly washed five times each with 1 mL of deionized water followed by 2 mL of deionized water. Methylparaben was purchased from Ueno Fine Chemical Industry Ltd. (Osaka, Japan). One hundred milligrams of methylparaben was accurately weighed using an electronic analytical scale AUX120 (Shimadzu, Kyoto, Japan) and dissolved in deionized water. To ensure complete the dissolution of methylparaben, the solution was stirred on a vortex mixer, followed by ultrasonication for 15 min. It was then diluted with 100 mL of deionized water to obtain a 0.100% (w/v) solution. After skin stimulation with soap, 1 mL of the 0.100% methylparaben solution was applied to the skin after washing twice. The donor cup was sealed with parafilm to prevent the evaporation of the methylparaben solution. The reservoir solution was stirred at approximately 500 rpm, using either 6 or 12 stations multi magnetic stirrer (AS ONE). The reservoir solution in the receptor cup was mixed 10 times with a micropipette. After 1 h, 200 \(\upmu\)L of the sample was withdrawn and an equivalent volume of saline was replaced. The process was repeated after every hour for 6 h.  
The concentration of methylparaben in the collected reservoir solution was measured by High Performance Liquid Chromatography (HPLC) method using an LC-20 AB HPLC system (Shimadzu) with a Capcell Pak(r) 5 \(\upmu\)m C18 MG-II column (internal diameter: 2.0 mm \(\times\) 150 mm) (Shiseido, Tokyo, Japan). An accurately diluted sample of 0.002% methylparaben was used as the standard.