Clustering of caffeine in water and its adsorption in activated carbon: Molecular simulations and experiments

H Ram├ęzani and I Ellien and Z El Oufir and N Mathieu and S Delpeux and SK Bhatia, COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS, 673, 131645 (2023).

DOI: 10.1016/j.colsurfa.2023.131645

Motivated by the importance of caffeine removal from wastewater, the clustering and adsorption of caffeine in activated carbon are studied using Molecular Dynamics and Grand Canonical Monte Carlo simulations, and the results validated by experiments showing similar clustering mechanisms in the large pores as well as high caffeine concentrations in water. The mechanism of caffeine clustering as a function of concentration from high concentration up to solubility threshold is described by the creation of hydrogen bonds between dimers (hydrogen of methyl group and oxygen of caffeine) and between dimers and another caffeine molecule (hydrogen bonds between the hydrogen of methyl group and nitrogen of another caffeine molecule). The caffeine solvation pattern shows strong interaction between water and nitrogen atoms of the caffeine molecule. Analysis of the unfolding of caffeine aggregates shows that the solvent accessible surface area varies from 120 to 350 & ANGS;2/caffeine molecule (solvent accessible surface area of a single caffeine molecule). Caffeine dimers and more complex forms are also observed in the GCMC simulations, and show that the same mechanism behind the caffeine clustering produces caffeine stacks in the large pores of nanoporous carbon (Activated Carbon Cloth (ACC)). For all molecular simulations, we have utilized our new computed force field for the caffeine molecule as the most accurate force field for both MD and GCMC. This new force field is examined by neutron scattering experiments in the literature for MD simulations and by adsorption isotherm and X-ray scattering experiments for GCMC simulations. According to the X-ray intensity measurements on ACC before and after adsorption, and molecular simulation results, we conclude that this new force field is more accurate than similar force fields in the literature as it produces satisfactory results for both MD and GCMC simulations.

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