Medium effect (transfer activity coefficient) of methanol and acetonitrile on β-cyclodextrin/benzoate complexation in capillary zone electrophoresis
Medium effect (transfer activity coefficient) of methanol and acetonitrile on β-cyclodextrin/benzoate complexation in capillary zone electrophoresis
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DOI：
10.1021/ac026407z
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发表时间：
2003-04-01
影响因子：
7.4
通讯作者：
Kenndler, E
中科院分区：
文献类型：
--
作者：
Porras, SP;Sarmini, K;Kenndler, E
缔合常数，K-C，来自于电泳迁移率的阴离子溶质（7苯甲酸酯与羟基或氯取代基），通过毛细管区带电泳在不同的溶剂系统，由水与高达20%（v/v）的甲醇或乙腈的二元混合物，分别。发现缔合常数随有机溶剂浓度的增加而减小。应用传递活度系数（或介质效应）的概念，分析了有机溶剂对苯甲酸酯与β-环糊精的K-c的影响。这一概念使得能够评估参与不同溶剂中络合平衡的各个物种的贡献的意义：苯甲酸根离子、β-环糊精和阴离子苯甲酸根-β-环糊精络合物。根据苯甲酸在不同混合溶剂中的酸度常数的变化以及相应的质子和分子酸的转移活度系数计算了介质对苯甲酸的影响。β-环糊精的转移活性系数由其在不同溶剂中饱和时的溶解度决定。以这种方式，对于从水转移到相应的混合溶剂中，可以估计络合平衡中涉及的每种物质的标准转移自由能Δ G（t）（0）。发现有机溶剂不显著影响苯甲酸根阴离子的DeltaG（t）（0）。然而，有机溶剂在β-环糊精和络合阴离子的稳定性方面发挥不同的作用：而乙腈的加入对阴离子络合物的Δ G（t）（0）几乎没有影响，K-c的降低是由β-环糊精的增强稳定性引起的（由其更好的溶解度反映）。另一方面，甲醇的加入降低了β-环糊精的溶解度，因此Δ G（t）（0）为正值。因此，对甲醇溶液中K-c的总体影响必须与苯甲酸酯-β-环糊精复合物的显著去稳定化有关。
Association constants, K-c, were derived from the electrophoretic mobilities of the anionic solutes (seven benzoates with hydroxy or chloro substituents) by capillary zone electrophoresis in different solvent systems, consisting of binary mixtures of water with up to 20% (v/v) methanol or acetonitrile, respectively. The association constants expectedly are found to decrease with increasing organic solvent concentration. The effect of organic solvents on the K-c of the benzoates with beta-cyclodextrin was analyzed applying the concept of the transfer activity coefficient (or the medium effect). This concept enables the evaluation of the significance of the contributions of the individual species involved in the complexation equilibrium in the different solvents: the benzoate ion, beta-cyclodextrin, and the anionic benzoate-beta-cyclodextrin complex. The medium effect on benzoate was calculated from the change in acidity constant of benzoic acid in the different mixed solvents and the corresponding transfer activity coefficients of the proton and the molecular acid. The transfer activity coefficients for beta-cyclodextrin results from its solubility at saturation in the different solvents. In this way, an estimation of the standard free energy of transfer, DeltaG(t)(0), of each species involved in the complexation equilibrium was possible for the transfer from water into the respective mixed solvent. It was found that the organic solvents do not significantly affect DeltaG(t)(0) for the benzoate anion. However, the organic solvents play a different role concerning the stabilization of beta-cyclodextrin and the complex anion: whereas the addition of acetonitrile has nearly no influence on DeltaG(t)(0) of the anionic complex, the reduction in K-c is caused by the enhanced stabilization of beta-cyclodextrin (reflected by its better solubility). Addition of methanol, on the other hand, lowers the solubility of beta-cyclodextrin, thus giving positive values for DeltaG(t)(0). Thus, the overall effect on K-c in methanolic solutions must be related to the pronounced destabilization of the benzoate-beta-cyclodextrin complex.