Fan Hsiu-Fang, Hung Chien-Yi, Lin King-Chuen
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan.
Anal Chem. 2006 Jun 1;78(11):3583-90. doi: 10.1021/ac0520588.
Evanescent wave cavity ring-down absorption spectroscopy is applied to measure the thermodynamic properties of the surface adsorption for neutral trans-4-[4-(dibutylamino)styryl]-1-(3-sulfopropyl) pyridinium (DP) and charged trans-4-[4-(dibutylamino)styryl]-1-methylpyridinium iodide (DMP+ I-) at the silica/CH3CN interface, where the interfacial density is determined by measurement of absorbance. The bulk concentration dependence of the surface density may be characterized with a Langmuir isotherm model, which yields saturated surface density, equilibrium constant, and free energy of adsorption of (7.0 +/- 0.3) x 10(13) cm(-2), (1.3 +/- 0.2) x 10(4) M(-1), and -23.5 +/- 0.4 kJ/mol for DP and (8.9 +/- 0.3) x 10(12) cm(-2), (2.6 +/- 0.7) x 10(4) M(-1), and -25.2 +/- 0.6 kJ/mol for DMP+ I-, respectively. The surface density of the isolated silanol groups may then be estimated in terms of the molecular probe results. The absorption contribution from the bulk solution is a factor of approximately 10(1)-10(2) smaller than the total absorbance measured such that subtraction of the bulk contribution leads to negligible change of the thermodynamic properties. The DP is adsorbed to the SiOH sites by forming hydrogen bonds, while the DMP+ cation is bound to the SiO- sites by electrostatic attraction. Surface forces are also probed by addition of triethylamine (TEA), which is competitive with DP for the silanol sites. When the TEA concentration is increased, the DP surface density is found to decrease, whereas the DMP+ surface density increases. The obtained thermodynamic properties are generally consistent with those measured by second harmonic generation spectroscopy. However, when a tetramethylammonium ((CH3)4N+ Cl-) salt is added, the DMP+ cation behaves differently between these two methods. Formation of an electrical double layer may account for the difference.
采用倏逝波腔衰荡吸收光谱法测量了中性反式-4-[4-(二丁基氨基)苯乙烯基]-1-(3-磺丙基)吡啶鎓(DP)和带电荷的反式-4-[4-(二丁基氨基)苯乙烯基]-1-甲基碘化吡啶鎓(DMP⁺I⁻)在二氧化硅/乙腈界面处的表面吸附热力学性质，其中界面密度通过吸光度测量来确定。表面密度对本体浓度的依赖性可用朗缪尔等温线模型来表征，该模型给出DP的饱和表面密度、平衡常数和吸附自由能分别为(7.0±0.3)×10¹³ cm⁻²、(1.3±0.2)×10⁴ M⁻¹和 -23.5±0.4 kJ/mol，DMP⁺I⁻的分别为(8.9±0.3)×10¹² cm⁻²、(2.6±0.7)×10⁴ M⁻¹和 -25.2±0.6 kJ/mol。然后可根据分子探针结果估算孤立硅醇基团的表面密度。本体溶液的吸收贡献比测得的总吸光度小约10¹ - 10²倍，因此减去本体贡献导致热力学性质的变化可忽略不计。DP通过形成氢键吸附到SiOH位点上，而DMP⁺阳离子通过静电吸引结合到SiO⁻位点上。还通过添加与DP竞争硅醇位点的三乙胺(TEA)来探测表面力。当TEA浓度增加时，发现DP表面密度降低，而DMP⁺表面密度增加。所获得的热力学性质通常与通过二次谐波产生光谱法测得的性质一致。然而，当加入四甲基氯化铵((CH₃)₄N⁺Cl⁻)盐时，DMP⁺阳离子在这两种方法中的行为有所不同。电双层的形成可能解释了这种差异。
