###### Abstract  
Knowledge of interfacial structures in liquid-liquid systems is imperative, especially for improving two-phase biological and chemical reactions. Therefore, we developed a new sample cell for neutron reflectometry (NR), which enables us to observe the layer structure around the interface, and investigated the adsorption behavior of a typical surfactant, sodium dodecyl sulfate (SDS), on the toluene-\(d_{\mathrm{S}}\)-D\({}_{2}\)O interface under the new experimental conditions. The new cell was characterized by placing the PTFE frame at the bottom to produce a smooth interface and downsized compared to the conventional cell. The obtained NR profiles were readily analyzable and we determined a slight difference in the SDS adsorption layer structure at the interface between the toluene-\(d_{\mathrm{S}}\)-D\({}_{2}\)O and air-D\({}_{2}\)O systems. This could be owing to the difference in the adsorption behavior of the SDS molecules depending on the interfacial conditions. neutron reflectometry; liquid-liquid interface; air-liquid interface; adsorption structure; sodium dodecyl sulfate +
Footnote †: journal: _Sci._  
absorption fine structure (TR-XAFS) method developed at BL39 XU of SPring-8 is useful, in which the enrichment of bromide ions was observed at an \(n\)-heptane-water interface in the presence of dimethyldipalmitylammonium ions [5]. These developments are important not only from a technological point of view, but also because they provide information on molecular-level structures at the interfaces. In the field of solvent extraction, the interfacial structure and potential of two different lipophilic extractants were determined using a combination of X-ray and neutron reflectivity experiments [1]. The findings revealed that the extractant-enriched interface could repel or attract hard trivalent cations depending on the ligand's nature. [PERSON] et al. combined NR and electrochemical characterization techniques to investigate the effects of applied electric fields and electrolyte concentration on the interfacial structure of the interface between two immiscible electrolyte solutions (ITIES), including the impacts of an adsorbed lipid layer [6]. This is the first NR-electrochemistry research study related to the interfacial structure of the ITIES. On the other hand, [PERSON] et al. have explained how the considerable accumulation of surfactant contaminants at bare oil/water can be studied using X-ray and neutron reflectometry with appropriate contrast settings [7]. These findings show that surfactant impurities may be a limiting factor in studying fundamental oil/water interface processes. Among the above recent techniques, NR is quite suitable for the analysis of organic molecules distributed across an organic-water interface, since the contrast between solvent and scatterer can be controlled by isotope labeling, such as using deuterium to replace hydrogen in organic molecules [8; 9; 10; 11]. However, in general, deuterated organic molecules are expensive and not readily available; therefore, a smaller-size cell that can provide an ideal liquid-liquid interface is valuable to reduce their usage. The development of a universal experimental apparatus that can handle a wide range of solvent combinations, including water/oil, oil/water, oil/acid and oil/base, is another motivation for a new cell design. We have studied the improvement of a sample cell which was originally proposed in a TR-XAFS study [5], while the cell is not suitable for neutron experiments; some components need to be replaced. Considering that the NR measurement for the adsorption behavior of an organic molecule on the interface in the liquid-liquid system is now developing, it is worthwhile to provide a method for measurement and analytical results using the new cell by NR.