Dutta Chayan, Mammetkuliyev Muhammet, Benderskii Alexander V
Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA.
J Chem Phys. 2019 Jul 21;151(3):034703. doi: 10.1063/1.5066597.
We present a measurement of molecular orientation of water at charged surfactant aqueous interfaces as a function of surface charge density. The polarization dependent spectral line shapes of the water bend mode were measured by vibrational sum-frequency generation at the positively charged surfactant cetyltrimethylammonium bromide (CTAB)/water interface and negatively charged surfactant sodium dodecyl sulfate/water interface. Orientational analysis using the water bend mode as a vibrational probe, within the electric dipole approximation, reveals structural differences between these surfaces and quantifies how different hydrogen bonded species re-orient around the surfactant head groups as the surface charge density changes. As the concentration of the positively charged surfactant (CTAB) increases, the surface water molecules with free-OH groups reorient their hydrogen away from the bulk water and the C axis closer to the surface normal. This suggests that these free-OH molecules are in general located above the positively charged head groups of CTAB, and thus, the charge-dipole interaction pulls their oxygen "down" and pushes hydrogen "up." On the contrary, water molecules with two donor hydrogen bonds re-orient their hydrogen toward the bulk water, likely because most of these molecules are below the CTAB surfactant head groups.
我们给出了作为表面电荷密度函数的带电表面活性剂水相界面处水分子取向的测量结果。通过在带正电的表面活性剂十六烷基三甲基溴化铵（CTAB）/水界面和带负电的表面活性剂十二烷基硫酸钠/水界面处的振动和频产生来测量水弯曲模式的偏振相关光谱线形状。在电偶极近似下，使用水弯曲模式作为振动探针进行取向分析，揭示了这些表面之间的结构差异，并量化了随着表面电荷密度变化，不同氢键物种如何围绕表面活性剂头基重新取向。随着带正电表面活性剂（CTAB）浓度的增加，具有游离 - OH基团的表面水分子将其氢重新取向远离本体水，且C轴更靠近表面法线。这表明这些游离 - OH分子通常位于CTAB带正电的头基上方，因此，电荷 - 偶极相互作用将它们的氧“向下”拉并将氢“向上”推。相反，具有两个供体氢键的水分子将其氢重新取向朝向本体水，可能是因为这些分子中的大多数位于CTAB表面活性剂头基下方。
