## 4 Discussion  
In this study, the effect of the structural differences in the various isomers belonging to the hydrophobic group of anionic surfactants on the skin barrier function was evaluated using the TI method. This method was developed for the purpose of the easy analysis of the effect of surfactants on the skin barrier function. It is an effective method for the comparative study of surfactants, provided the drug has a steady state in skin permeation (Figure 1). The TI value of the sodium laurate soap was approximately 4 (Table 1), which was different from the value obtained in our previous study (2.53) [15]. Of note, the sodium laurate soap assessed in the previous study was a cosmetic raw material-grade industrial product. On the other hand, the sodium laurate soap in this study was prepared using reagent-grade lauric acid and sodium hydroxide. Thus, it was assumed that the differences in purity and neutralization rates of the fatty acids led to the different TI values. From a qualitative point of view, previous studies have shown that sodium laurate soap has a significantly higher TI value than other anionic surfactants, indicating that the effect of sodium laurate soap on the skin barrier function is strong. Previous studies have reported that C12 fatty acid surfactants cause strong skin irritation [10]. Based on the fact that the surfactants with strong skin irritation have high TI values, it was suggested that the TI value was related to skin irritation. The TI values are expressed as the ratio of \(Flux_{ss}\) of surfactant-treated skin to control-treated skin. The standard error (SE) was calculated from the unbiased variance of the combination of the \(Flux_{ss}\) (\(n=35\)) of control and \(Flux_{ss}\) (\(n=6\)-\(10\)) of the test sample. The significant differences in the TI values between the C18 fatty acid soaps suggested that the effect of sodium fatty acid soaps on the skin barrier function was affected not only by the carbon number of alkyl structures, but also by their alkyl structures. When the fatty acids were arranged in increasing order of their respective TI values (saturated straight chain \(<\) saturated branched chain \(<\) monounsaturated \(\sim\) < di-unsaturated), the sequence corresponded to the respective melting points of the fatty acids. The melting points of the saturated straight-chain stearic acid, saturated branched-chain isosteric acid, monounsaturated oleic acid, and di-unsaturated linoleic acid are 70, 16, \(\leq\)15, and \(-5\)\({}^{\circ}\)C, respectively. There is a relationship between the melting point and solubility of a substance, with substances with higher melting points generally being less soluble [23]. In soaps, generally, the Krafft point is proportionally affected by the melting point. The Krafft point increases as the number of carbon atoms in the alkyl chain increases. At the same time, if the hydrophilic groups are the same, the solubility in the water decreases as the carbon number increases. When an alkyl structure includes unsaturated groups, the Krafft point decreases and the water solubility increases. In addition, the solubility tends to increase as the hydrophilic group approaches the center of the alkyl chain [24]. The difference in the solubility of the sodium soap was also confirmed by the change in the dissolution state during the soap preparation as well as the properties of the prepared soap. Based on these facts, it was assumed that the melting point, Krafft point, and the solubility of the raw fatty acid were the main factors affecting the skin barrier function. The relationship between the carbon number of surfactants and skin irritation is well known [25; 26]. It can be said that their report is a mention to one aspect of the relationship between the melting point of fatty acids and the properties of surfactants on the skin. The temperature conditions of the experiment are important because the solubility is affected by temperature.