## 1 Introduction  
Saponified fatty acids (soaps) are common anionic surfactants that are widely used as detergents and emulsifiers. Among fatty acid soaps, lauric acid soap is often used as a body cleanser, shampoo, dishwashing detergent, and laundry detergent, owing to its good foaming capacity and strong detergency. However, surfactants have been reported to affect the skin barrier function [1; 2; 3]. The skin barrier consists of keratin proteins and intercellular lipids. Keratin, a cytoskeletal protein, is composed of a group of intermediate filaments composed of keratinocytes, which are differentiated and enucleated epidermal cells. Intercellular lipids have a lamellar structure in which the lipid molecules, mainly composed of ceramide, fatty acids, and cholesterol esters, are regularly arranged. Stratum comeum cells and intercellular lipids form a structure known as the brick and mortar model, establishing a skin barrier function [4; 5; 6]. Previous studies on the effects of surfactants on the skin have reported a correlation with the alkyl chain length of surfactants [7; 8; 9]. [PERSON] et al. evaluated the relationship between the carbon number of fatty acid surfactants and irritancy to skin (using skin roughness and degreasing power in the patch test as evaluation scales), and revealed that C12 alkyl anionic surfactants caused the strongest skin irritation [10]. In addition, it has been reported that the skin barrier function is reduced due to the formation of a stratum corneum permeation path for a specific molecule, thereby increasing the skin permeability of the molecule. Using this phenomenon, the skin barrier function has been evaluated by measuring the invasiveness of substances or transepidermal water loss (TEWL) [6; 11; 12; 13]. In vitro skin permeation assays, using the Franz diffusion cell system, are the most commonly used methods for assessing the skin absorption of drugs [14]. [PERSON] et al. developed a transmission index (TI) method to evaluate the effect of anionic surfactants on the skin barrier function, using the skin permeation rate of methylparaben as an index [15]. Some previous studies have reported interesting findings on the effects of amphiphiles on the skin barrier function. Ionic liquids are amphiphilic substances similar to surfactants. Several interesting reviews of ionic liquids for enhancing drug skin permeation have been summarized [16; 17]. [PERSON] et al. has reported the skin penetration effect by water-in-oil (W/O) or oil-in-water (O/W) emulsion, consisting of the hydrophobic ionic liquid or by the hydrophilic ionic liquid [18]. Some ionic liquids are formed using fatty acids as raw materials and are known to significantly enhance the skin permeability of various drugs. Medrx Co. Ltd., has commercialized an ionic liquid composed of fatty acids and amines with high biocompatibility as a skin permeation enhancer [19]. [PERSON] et al. investigated the mechanism by which fatty acid-aliphatic amine ionic liquids enhance the skin permeation of drugs [20]. Their results have shown that the skin penetration-promoting effect of drugs differs depending on the composition of the ionic liquid. While the ionic liquid composed of octanoic acid and triisopropanolamine markedly enhanced the skin permeability of hydrophilic model drugs, the ionic liquid derived from isosteric acid, a branched-chain C18 fatty acid, has an inhibitory effect on the skin permeability of hydrophobic drugs [20]. These results suggest that the structure of the alkyl chain of the amphiphile, including the surfactant, affects the skin barrier function. Hence, we investigated the effect of the alkyl structure of fatty acids on the skin barrier function using fatty acid soaps, which are common and simple amphiphiles. C18 fatty acids were used as the raw materials for the preparation of the fatty acid soaps.