NCAFM2023 Programme Booklet

Thursday 1420 - 1440

MOLECULAR-SCALE ANALYSIS OF SURFACTANT CRYSTAL-WATER INTERFACE STRUCTURES BY FM-AFM AND MD SIMULATION Kazuki Miyata 1-4* , Haohui Zhang 2 , Itsuki Hase 3 , Yoichi Kumagai 4 , Takumi Yoshino 3 , Ryota Hashimoto 5 , Keisuke Miyazawa 1,3,4 , Takahiko Ikarashi 2 , Atsunori Morigaki 5 , Ygor Morais Jaques 6 , Adam S. Foster 6 , Yasushi Kakizawa 5 , Takeshi Fukuma 1-4 1 Nano Life Science Institute (WPI-NanoLSI), Kanazawa University, Kanazawa 920-1192, Japan 2 Division of Nano Life Science, Kanazawa University, Kanazawa 920-1192, Japan 3 Division of Electrical Engineering and Computer Science, Kanazawa University, Kanazawa 920-1192, Japan Email: k-miyata@staff.kanazawa-u.ac.jp Surfactants are used in a wide range of applications, including pharmaceuticals and detergents, as aids to dissolve substances. To maximize the immediacy and efficacy of these applications, it is expected to control the dissolution of surfactant crystals at the molecular level. So far, the physical and chemical properties of its crystal surface have been investigated by experimental methods such as X-ray scattering, X-ray diffraction, and Fourier-transform infrared spectroscopy. However, due to the difficulty of direct visualization in real space at subnanometer-scale resolution, the surface structure at molecular level and its dynamic events during the dissolution have remained elusive. For revealing them, here we demonstrated molecular-resolution imaging of surfactant crystal-water interface by FM-AFM. In this study, we measured the sodium salt of α -Sulfonated Fatty Acid Methyl Ester (MES), one of the crystalline anionic surfactants, possessing a residual palmitic acid structure (C16MES-Na) (Fig. (a)). The hydrophilic groups of the salt consist of sulfonic acid (SO 3 ) and methyl ester. The C 16 MES-Na powder was placed in pure water at a concentration of 10% and completely dissolved by heating. After leaving it for a few days, the molecules bind to each other, crystallize, and precipitate. We performed FM-AFM imaging of its precipitate crystal surface in C 16 MES-Na saturated solution and succeeded in visualizing the molecular-scale well-aligned dotted patterns (Fig. (b)). The arrangement of these patterns shows good agreement with previously reported unit cells as measured by X-ray crystallography. To investigate its imaging mechanism in detail, we also performed the molecular dynamics (MD) simulation with a model of C 16 MES-Na crystal structure and enough water molecules (Fig. (c)). The obtained water density distribution map was converted to a force map and then to a frequency shift (Δf) map using the solvent tip approximation (STA) model. In addition, we generated FM-AFM images from the Δf map by a virtual AFM simulator for various Δf setpoint, and among them, we found the best agreement image with the experimentally obtained image (Fig. (d)). The location of the molecular-scale protrusions and the average tip height in that image suggest that the origin of these patterns is the hydration structure on SO3 group. Based on the knowledge of the FM-AFM imaging mechanism at the terrace, we aim to understand the surfactant crystal dissolution process by visualizing the step edge structures by high-speed FM-AFM. 4 Division of Frontier Engineering, Kanazawa University, Kanazawa 9201192, Japan 5 Research and Development Headquarters, Lion Corporation, Tokyo 132-0035, Japan 6 Department of Applied Physics, Aalto University, Helsinki FI00076, Finland

Fig. (a) [Left] Schematic model of a C 16 MES molecule. [Lower right] Previously reported unit cell of C 16 MES crystal as measured by X-ray crystallography. (b) FM-AFM image of C 16 MES-Na surface. (c) MD simulation models omitting water molecules. [Left] side view and [Right] top view near the surface. (d) FM-AFM image generated by a virtual AFM simulator. Yellow points are the positions of SO 3 groups.

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