水和甲醇混合溶剂氢键相互作用及对高分子链溶解能力的理论研究

用密度泛函理论对水和甲醇混合溶剂体系的氢键结构进行了详细研究.通过构象和频率分析发现在水团簇中五聚体和六聚体环状结构最为稳定,同时发现一个全新的特征,即甲醇分子能与水五聚体和六聚体形成双氢键.根据各相互作用的稳定化能,分析了水和甲醇混合溶剂对PNIPAM溶解能力的影响,并对实验现象给予了合理解释.

Hydrogen-bonding Interaction in Water/Methanol Mixed Solvent and Theoretical Studies on Solubility of Polymer Chain

Theoretical study on the structure of hydrogen bond was carried out by using density functional theory. Geometries were optimized at B3LYP/6 31G * and single point calculations were performed at B3LYP/6 31+G * level. Stabilization energy was obtained by taking basis set superposition error correction into account. After the investigation of various minimum energy geometries, single ring structures of water pentamer and water hexamer were found most stable in liquid water. The calculations show that these single ring structures have no imaginary frequency. Adding the methanol molecules to the ring like water cluster, geometries of water methanol complexes were optimized. No imaginary frequencies were found in these structures. A significant and novel feature revealed firstly in this work is the perfect double hydrogen bond between methanol molecules and the ring like water complexes. In water and methanol solvent, the polymer chain, PNIPAM, can form hydrogen bond with solvent molecules. It can stretch to coil shape and be soluble. But when the two solvents are mixed, water and methanol molecules will form stable comlexes because of strong double hydrogen bonding. The active point of hydrogen bonds of mixed solvent will be shielded; and the solvent becomes non polar. So, the polymer chain will overlap immediately because of the formation of intra molecular hydrogen bonds and consequently perform a coil to globule transition.