Chen Yinshan, Zhang Wei, Yu Lian
School of Pharmacy and ‡Department of Chemistry, University of Wisconsin-Madison , Madison, Wisconsin 53705, United States.
J Phys Chem B. 2016 Aug 18;120(32):8007-15. doi: 10.1021/acs.jpcb.6b05658. Epub 2016 Aug 5.
Surface-grating decay has been measured for three organic glasses with extensive hydrogen bonding: sorbitol, maltitol, and maltose. For 1000 nm wavelength gratings, the decay occurs by viscous flow in the entire range of temperature studied, covering the viscosity range 10(5)-10(11) Pa s, whereas under the same conditions, the decay mechanism transitions from viscous flow to surface diffusion for organic glasses of similar molecular sizes but with no or limited hydrogen bonding. These results indicate that extensive hydrogen bonding slows down surface diffusion in organic glasses. This effect arises because molecules can preserve hydrogen bonding even near the surface so that the loss of nearest neighbors does not translate into a proportional decrease of the kinetic barrier for diffusion. This explanation is consistent with a strong correlation between liquid fragility and the surface enhancement of diffusion, both reporting resistance of a liquid to dynamic excitation. Slow surface diffusion is expected to hinder any processes that rely on surface transport, for example, surface crystal growth and formation of stable glasses by vapor deposition.
已对具有广泛氢键的三种有机玻璃（山梨醇、麦芽糖醇和麦芽糖）进行了表面光栅衰减测量。对于1000nm波长的光栅，在所研究的整个温度范围内，衰减是由粘性流动引起的，涵盖了10(5)-10(11) Pa·s的粘度范围，而在相同条件下，对于分子尺寸相似但没有或只有有限氢键的有机玻璃，衰减机制从粘性流动转变为表面扩散。这些结果表明，广泛的氢键会减缓有机玻璃中的表面扩散。这种效应的产生是因为分子即使在表面附近也能保持氢键，所以最近邻分子的损失不会转化为扩散动力学势垒的成比例降低。这种解释与液体脆性和扩散的表面增强之间的强相关性一致，二者均反映了液体对动态激发的抗性。预计缓慢的表面扩散会阻碍任何依赖表面传输的过程，例如表面晶体生长和通过气相沉积形成稳定玻璃。
