Structural study of hydrated ions in confinement

本研究采用差示扫描量热法、红外光谱、X射线衍射和准弹性中子散射等方法，对水、甲醇、乙腈和几种电解质水溶液在具有亲水表面的MCM-41和具有疏水表面的活性炭纤维中的结构和动力学进行了研究。(1)对于MCM-41 C10（孔径为21 μ m）、C14（28 μ m）和C18（37 μ m），单层水以2.6 μ m的O-O距离与表面硅醇基团（SiOH）牢固结合，导致毛细管冷凝水中的氢键网络扭曲。水分子的平移扩散系数随着孔径的减小而减小，并且被发现约为本体水的一半。(2)甲醇分子被限制在MCM-41中的C10和C14与硅醇基团形成强氢键。甲醇分子在表面上的结构和动力学对孔径和温度的依赖性很小。另一方面，毛细管冷凝的甲醇分子与本体一样形成链状结构，但其氢键网络非常扭曲。(3)乙腈分子在MCM-41 C14中以偶极-偶极相互作用与表面硅醇基团发生相互作用，其N-O距离约为3 π。乙腈分子在孔中的平移扩散比在本体中的扩散受到更大的阻碍，其扩散活化能约为7-12 kJ/mol，远小于水分子的扩散活化能。(4)在229 K左右，MCM-41 C10中的毛细管冷凝重水表现出从低密度水到高密度水的动态交换。(5)在MCM-41 C10、C14、C18和ACF中的电解质水溶液的熔点随着离子表面电荷密度的降低而降低。离子的第一水合层结构与体相类似。

In this study, the structure and dynamics of water, methanol, acetonitrile, and several aqueous electrolyte solutions confined in well-controlled MCM-41 with hydrophilic surface and activated carbon fibers with hydrophobic surface have been investigated by differential scanning calorimetry, FTIR, X-ray diffraction and quasi-elalstic neutron scattering.(1) For MCM-41 C10(pore diameter 21 Å), C14(28 Å), and C18(37 Å), monolayered water are strongly bound to the surface silanol groups (SiOH) with an O-O distance of 2.6 Å, resulting in distorted hydrogen bonding network in capillary-condensed water. The translational diffusion coefficients of water molecules decrease with decreasing pore size and were found to be about half to bulk water.(2) Methanol molecules confined in MCM-41 C10 and C14 form strong hydrogen bonds with the silanol groups. The structure and dynamics of the methanol molecules on the surface show little dependency on pore size and temperature. On the other hand, capillary-condensed methanol molecules form a chain-like structure as in bulk, but its hydrogen-boned network is very distorted.(3) Acetonitrile molecule confined in MCM-41 C14 interact with the surface silanol groups by dipole-dipole interactions with an N-O distance of about 3 Å. Translational diffusion of acetonitrile molecules in the pores is more retarded than that in bulk, and its activation energy was found to be about 7-12 kJ/mol, much smaller than that of water.(4) Capillary-condensed heavy water confined in MCM-41 C10 has shown a dynamical cross-over at around 229 K from low-density water to high-density water.(5) The aqueous electrolyte solutions confined in MCM-41 C10, C14, C18, and ACF showed a decrease in melting point with lowering surface charge density of ions. The structure of the first hydration shell of the ions is similar to that in bulk.