## 5 Conclusions  
We introduce a new type of cell for liquid-liquid two-phase systems for NR measurements. The cell can keep an ideal liquid-liquid interface owing to a PTFE frame at the bottom, which also enables obtaining fine NR profiles regarding the interfacial structure. Using this new cell, we analyzed the adsorption structure of SDS molecules between toluene-\(d_{8}\)-D\({}_{2}\)O and air-D\({}_{2}\)O interfaces by the NR technique. A disordered configuration would be indicated for the SDS molecules in the SDS layer of the toluene-\(d_{8}\)-D\({}_{2}\)O system, unlike those of the air-D\({}_{2}\)O system. Additionally, for air-liquid interface reflection measurements, the use of deuterated solvents was preferable to obtain lower air-liquid interfacial roughness and low-background NR data. This work indicates an effective method to understand the interfacial phenomena in liquid-liquid systems; therefore, the obtained results contribute to improving the analytical method of interfacial structures, leading to enhancing knowledge of chemical reactions in two-phase systems. The following are available online at [[https://www.mdpi.com/article/10.3390/app12031215/s1](https://www.mdpi.com/article/10.3390/app12031215/s1)]([https://www.mdpi.com/article/10.3390/app12031215/s1](https://www.mdpi.com/article/10.3390/app12031215/s1)), Figure S1: Photos of a new type of cell with the PTFE frame. Figure S2: \(RQ_{2}\)4 vs. \(Q_{2}\) profiles for the air-organic, air-\(\mathrm{D_{2}O}\), and air-\(\mathrm{D_{2}O}\)-SDS samples. Figure S3: (A) The fitting curves of the air-\(\mathrm{D_{2}O}\)-SDS sample as calculated using the model shown in Tables S3 and S5, and no SDSs adsorbs model. (B) The SLD profiles of the air-\(\mathrm{D_{2}O}\)-SDS sample as calculated using the model shown in Tables S3 and S5, and no SDS adsorbs \(d_{\mathrm{S}}\)-\(\mathrm{D_{2}O}\)-SDS sample as calculated using the model shown in Tables S3 and S5, and no SDS adsorbs model. (B) The SLD profiles of the toluene-\(d_{\mathrm{S}}\)-\(\mathrm{D_{2}O}\)-SDS sample as calculated using the model shown in Tables S3 and S5, and no SDS adsorbs model. Table S1: The list of the constrained parameters for the model analysis of air-\(\mathrm{D_{2}O}\)-SDS and toluene-\(d_{\mathrm{S}}\)-\(\mathrm{D_{2}O}\)-SDS samples. Table S2: The parameters used to fit the measured NR profiles from the air-\(\mathrm{D_{2}O}\)-SDS and the air-\(\mathrm{D_{2}O}\)-SDS, toluene-\(d_{\mathrm{S}}\)-\(\mathrm{D_{2}O}\), and toluene-\(d_{\mathrm{S}}\)-\(\mathrm{D_{2}O}\)-SDS solution interfaces. Table S4: The list of the constrained parameters for the model analysis of air-\(\mathrm{D_{2}O}\)-SDS and toluene-\(d_{\mathrm{S}}\)-\(\mathrm{D_{2}O}\)-SDS samples. Table S5: The parameters used to fit the measured NR profiles from the air-\(\mathrm{D_{2}O}\)-SDS, and toluene-\(d_{\mathrm{S}}\)-\(\mathrm{D_{2}O}\)-SDS solution interfaces. Conceptualization, K.A.-S., N.L.Y., R.M. and H.N. ; methodology, K.A.-S., N.L.Y. and R.M. ; validation, K.A.-S., N.L.Y. and Y.U. ; formal analysis, K.A.-S.; investigation, K.A.-S., Y.U., R.M. and H.N. ; resources, K.A.-S. and H.N. ; data curation, K.A.-S.; writing--original draft preparation, K.A.-S.; writing--review and editing, K.A.-S., N.L.Y., Y.U., R.M. and H.N. ; supervision, K.A.-S. and N.L.Y. ; project administration, K.A.-S. and N.L.Y. ; funding acquisition, K.A.-S. and H.N. All authors have read and agreed to the published version of the manuscript. The authors acknowledge financial support from the Japan Society of the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (B) 17H03438, (B) 20H02497, and (C) 21K05121. Not applicable. The data presented in this study are available on request from the corresponding author. We thank [PERSON] (JAEA) for his support in the development of the sample cell. The NR samples were prepared at the User Experiment Preparation LabIII (CROSS). The neutron experiments were conducted at the BL16 SOFIA and BL17 SHARAKU apparatus in J-PARC, Tokai, Japan (Proposal Nos.