Unraveling the Adsorption Mechanism of Rhodamine B onto Conjugated Polymeric Nanoparticles Using Isothermal Calorimetry and Monte Carlo Simulations

Abstract

We present herein a comprehensive study on the thermodynamics of rhodamine B (RhB) adsorption onto poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) nanoparticles (NPs) dispersed in water using the isothermal calorimetric titration (ITC) technique. ITC experiments were carried out over a wide temperature range (5–65 °C), and the resulting thermograms were subjected to a detailed thermodynamic analysis using different adsorption models. For this purpose, we developed a versatile computational code based on the Monte Carlo method. We found that the thermograms obtained at the different temperatures cannot be interpreted using the Langmuir, bi-Langmuir, or Brunauer, Emmet, Teller (BET) models. However, excellent thermogram fits were attained by applying the Langmuir model coupled with the formation of a RhB dimer on the surface of F8BT NPs. Using this model, the best fit to the experimental data at 25 °C yielded the following parameters for direct RhB adsorption on the F8BT NPs surface: equilibrium constant (Kl) of ∼2 × 107 M–1, enthalpy change (ΔHl) of ∼1 kJ mol–1, and entropy change (ΔSl) of ∼140 J K–1 mol–1. These results indicate that dye adsorption is predominantly an entropy-controlled process. The fit also provided parameters associated with the formation of a second dye adsorption layer, i.e., the formation of the RhB dimer on the F8BT NPs surface, revealing values of ∼1.7 × 105 M–1, ca. −20 kJ mol–1, and ∼30 J K–1 mol–1 for Km, ΔHm, and ΔSm, respectively. The latter two parameters are similar to those previously reported for RhB dimer formation in water (ΔHd = −12 kJ mol–1 and ΔSd = 22 J K–1 mol–1) supporting the chosen Langmuir-dimer model. The analysis of the thermograms also suggests that the NPs surface available for RhB adsorption is small, as compared to the total surface, and that it decreases with decreasing temperature. The latter observation is intriguing, especially since the F8BT NPs hydrodynamic diameter shows only a small decrease with decreasing temperature. To explain these results, we propose that the surface of the NPs harbors highly hydrated polar groups that reduce the accessibility of the dye for adsorption.