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

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 (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(-).