微量热法研究α-环糊精与新型表面活性剂的包结作用

在298.15 K下用微量热法研究了α-环糊精与3-烷氧基-2-羟丙基三甲基溴化铵在水溶液中的包结作用.实验结果表明,随着疏水链CnH2n＋1O中碳原子数目n的增加(n=7、8、12、14), 主-客体包合物的化学计量比由1 :1为主变为2 :1为主. 各包合物都相当稳定,对应于n=7、8、12、14所得实验稳定常数分别为,β1=1.95×103 dm3•mol－1、β1=2.62×103 dm3•mol－1、β2=3.06×106 dm6•mol－2、β2=13.75×106 dm6•mol－2.包合物的形成均是焓驱动过程.包合物的平衡常数随烷氧基(CnH2n＋1O)中碳原子数目n的增加而增大,而包合物生成过程的标准反应焓(ΔHΘ)和标准反应熵(ΔSΘ)都随n的增加而减小.从主、客体的微观结构及包合物形成前后表面活性剂离子憎水基团周围溶剂分子排列结构的变化出发对实验结果进行了讨论.

A Study on Interactions between α-Cyclodextrin and a Series of New Kind of Surfactants in Aqueous Solutions by Microcalorimetry

The interactions between α-cyclodextrin (α-CD) and 3-alkoxyl-2-hydroxypropyl trimethyl ammonium bromides (CnNBr), new surfactants synthesized in this laboratory, in aqueous solutions have been investigated with titration microcalorimetry at 298.15 K. Stoichiometric ratio of the host-guest complex changes from 1:1 to 2:1 as the number of carbon atoms(n) in hydrophobic chain, CnH2n＋1O, increases from 8 to 14. All the complexes are quite stable with the apparent experiential stability constants beingβ1=1.95×103 dm3•mol－1, β1=2.62×103 dm3•mol－1, β2=3.06×106 dm6•mol－2, β2=13.75×106 dm6•mol－2, respectively for n=7, 8, 12, 14. All the complexation processes are proved to be enthalpy beneficial but entropy unfavorable, and the absolute value of enthalpy changes(ΔHΘ) increases, while entropy change (ΔSΘ) decreases, with the enlarging of the number of methylene in the hydrophobic chain. The results are discussed in the light of hydrophobic interaction of α-CD with CnNBr and the iceberg structure formed by water molecules existing around the hydrophobic tail of surfactant.