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

• Published in: Analytical chemistry (Washington) Vol. 82; no. 3; pp. 868 - 877
• Main Authors: Chen, Ming-Shiang, Fan, Hsiu-Fang, Lin, King-Chuen
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.02.2010
• Subjects: Adsorption; Analytical chemistry; Chemistry; Desorption; Exact sciences and technology; Isotherms; Methanol; Reaction kinetics; Silica; Solvents; Spectrometric and optical methods; Spectroscopy; Spectrum analysis; Thermodynamics; Water; Rhodamine; Equilibrium constant; Adsorption isotherm; Immobilization; Hydrophobic interaction; Zwitterion; Silica; Ring laser; Chemical enrichment; Cavity resonator; Rate constant; Double layers; Methanol; Adsorption energy; Lactone; Desorption; Equilibrium condition; Langmuir layer; Langmuir isotherm; Kinetic model; Absorption spectrometry; Adsorption; Evanescent wave; Xanthene dye
• ISSN: 0003-2700; 1520-6882; 1520-6882
• DOI: 10.1021/ac9020209

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) × 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 K ads 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 K ads 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 K ads 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 K ads value as R+B−.