## 3 Results
### Evaluation of the Effect of Sodium Fatty Acid Soaps on the Skin Barrier Function  
The effects of the various fatty acid soaps on the skin barrier function were evaluated. Figure 1 shows the skin permeation profile of methylparaben in skin samples treated with the five fatty acid soaps and the control (deionized water). The change in the skin permeability of methylparaben showed a high correlation with the permeation time for all samples (R \(>\) 0.94), confirming that the skin permeation profile from 1 to 6 h was at a steady state. Based on this observation, the effect of fatty acid soap on skin barrier function was evaluated using the TI method. The slopes of the time-concentration scatter plot for each fatty acid soap and control-treated sample are shown in Figure 2 as \(Flux_{ss}\). The TI values of the five fatty acid soaps are shown in Table 1. The highest \(Flux_{ss}\) value was observed for sodium laurate (6.08 \(\pm\) 0.27). It was observed that the \(Flux_{ss}\) of C18 fatty acid soap varied depending on the alkyl structure as follows: stearic acid (1.67 \(\pm\) 0.05) \(<\) isostearic acid (2.38 \(\pm\) 0.10) \(<\) oleic acid (4.44 \(\pm\) 0.22) \(<\) linoleic acid (5.18 \(\pm\) 0.1),saturated straight-chain fatty acid \(<\) saturated branched-chain fatty acids \(<\) monounssaturated fatty acids \(<\) diunsaturated fatty acids. Figure 1: Time-dependent change in the permeation of methylparaben in skin stimulated with each surfactant and control (water). The skin permeation rate of methylparaben from 1 to 6 h was constant, indicating that the skin permeation was in a steady state. Each point value represents mean \(\pm\) standard error, n = 35 (control), n = 6–10 (sample). Figure 2: The Fluxss of the skin samples stimulated by each surfactant and control (water). Each point value represents the mean \(\pm\) standard error, \(n\) = 35 (control), \(n\) = 6–10 (sample). The white bar represents the Fluxss of the water-treated sample (control). The shaded bar represents the Fluxss of the sodium laurate-treated sample. The Fluxss of each structurally isomeric C18 fatty acid soap (sodium isostearate, sodium oleate, and sodium linoleate)-treated sample is represented by gray bars and the name of each fatty acid soap is described below the horizontal axis. The measured value, represented by symbol (*), shows the statistically significant difference against the control; \(p<0.05\). The non-statistically significant difference observed against the control is represented by \(\dagger\) and against sodium laurate is represented by \(\dagger\)\(  
Figure 1: Time-dependent change in the permeation of methylparaben in skin stimulated with each surfactant and control (water). The skin permeation rate of methylparaben from 1 to 6 h was constant, indicating that the skin permeation was in a steady state. Each point value represents mean \(\pm\) standard error, n = 35 (control), n = 6–10 (sample). In our previous study, we observed that surfactants with a significant difference in \(\it Flux_{\rm ss}\), with respect to the control, had an effect on the skin barrier function [15]. Since the TI value of stearic acid soap was \(1.10\pm 0.08\), which was not significantly different from the control \(\it Flux_{\rm ss}\), we concluded that it did not affect the skin barrier function. The TI value of the isostearic acid soap was \(1.56\pm 0.10\), which was significantly different from the control \(\it Flux_{\rm ss}\) (p \(<\) 0.05). The TI values of the two unsaturated C18 fatty acid soaps, prepared using oleic acid and linoleic acid as raw materials, were \(2.92\pm 0.07\) and \(3.40\pm 0.07\), respectively, and were significantly different from the control \(\it Flux_{\rm ss}\). The TI value of the lauric acid soap was extremely high at \(3.99\pm 0.10\). Furthermore, the \(\it Flux_{\rm ss}\) values of the oleic acid and linoleic acid soaps were not significantly different from those of lauric acid soap (p \(>\) 0.1). There was no significant difference in the TI values of oleic acid soap and linoleic acid soap (p \(>\) 0.1).