Kinetic and Thermodynamic Investigation of Rhodamine B Adsorption at Solid/Solvent Interfaces by Use of Evanescent-Wave Cavity Ring-Down Spectroscopy

摘要

Evanescent-wave cavity ring-down spectroscopy is applied to investigate the adsorption behavior of rhodamine B at three different interfaces. The adsorption equilibrium constant (K_(ads)) and adsorption free energy of rhodamine B at the silica/methanol interface are determined to be (1.5 +- 0.2) X 10~(4) M~(-1) and -23.8 +- 0.4 kJ/mol by use of a Langmuir isotherm model. A Langmuir-based kinetic model is also developed to determine the corresponding adsorption and desorption rate constants of (1.02 +- 0.03) X 10~(2) M~(-1) s~(-1) and (7.1 +- 0.2) X 10~(-3) s~(-1), from which K_(ads) is obtained to be (1.45 +- 0.09) X 10~(4) M~(-1), in agreement with the value determined under equilibrium conditions. Similarly, when rhodamine B is at the chlorotrimethylsilane-immobilized silica/methanol interface, the adsorption and desorption rate constants are determined to be (1.7 +- 0.2) X 10~(2) M~(-1) s~(-1) and (5.0 +- 1.0) X 10~(-3) s~(-1). The subsequent K_(ads) is (3.6 +- 0.4) X 10~(4) M~(-1), which is larger than that at the silica/methanol interface. The former adsorption is dominated by hydrophobic interaction, while the latter is subject to electrostatic attraction. When rhodamine B is at the silica/water interface, there exist three chemical forms, including zwitterion (R~(+)B~(-)), cation (RBH~(+)), and lactone (RBL). A combination of double-layer and Langmuir competitive models is used to fit the adsorption isotherm as a function of solution pH, yielding K_(ads) of (2.5 +- 0.2) X 10~(4) M~(-1) and (1.1 +- 0.2) X 10~(5) M~(-1) for R~(+)B~(-) and RBH~(+), respectively. RBL is considered to have the same K_(ads) value as R~(+)B~(-).