Kinetic and thermodynamic investigation of rhodamine B adsorption at solid/solvent interfaces by use of evanescent-wave cavity ring-down spectroscopy
- Citation:
- Chen, M-S, Fan H-F, Lin K-C. 2010. Kinetic and thermodynamic investigation of rhodamine B adsorption at solid/solvent interfaces by use of evanescent-wave cavity ring-down spectroscopy. Analytical Chemistry. 82:868-877., Number 3
Abstract:
Evanescent-wave cavity ring-down spectroscopy is applied to investigate the adsorption behavior of rhodamine B at three different interfaces. The adsorption equilibrium constant (Kads) and adsorption free energy of rhodamine B at the silica/methanol interface are determined to be (1.5 ± 0.2) × 104 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) × 102 M-1 s-1 and (7.1 ± 0.2) × 10-3 s-1, from which Kads is obtained to be (1.45 ± 0.09) × 104 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) × 102 M-1 s-1 and (5.0 ± 1.0) × 10-3 s-1· The subsequent Kads is (3.6 ± 0.4) × 104 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 Kads of (2.5 ± 0.2) × 104 M-1 and (1.1 ± 0.2) × 105 M-1 for R+B- and RBH +, respectively. RBL is considered to have the same Kads value as R+B-. © 2010 American Chemical Society.
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