A statistical physics–based physicochemical study of l-phenylalanine adsorption on activated carbon

Abstract

Context

In this comparative study of the adsorption of l-phenylalanine (l-Phe) on two modified low-activated carbons (ACK and ACZ) at four temperatures (293–313 K), steric and energetic characteristics of adsorption were investigated. An advanced statistical physics multilayer model involving single-layer and double-layer adsorption scenarios was developed to interpret the l-Phe adsorption phenomenon. Modeling results indicate that two and three l-Phe layers were arranged depending on the tested adsorption systems. The estimated number of l-Phe molecules per leading adsorption site varied from 1.71 to 2.09 and from 1.76 to 1.86 for systems l-Phe-ACK and l-Phe-ACZ, respectively. The results show that a multimolecular adsorption mechanism might connect this amino acid molecule on ACZ and ACK surfaces in a non-parallel location. These parameters changed as follows, according to the adsorbed quantity at saturation analysis: Q_asat (l-Phe-ACK) ˃ Q_asat (l-Phe-ACZ). This indicates that ACK material was more efficient and valuable for l-Phe adsorption than ACZ. It was also shown that the adsorption capacity decreases as the temperature increases, proving the exothermicity of the adsorption process. This analytical substantiation is confirmed by calculating the binding energies, suggesting the occurrence of physical bonds between l-Phe amino acid molecules and ACK/ACZ binding sites and among l-Phe–l-Phe molecules. Pore size distribution was interpreted and calculated by applying the Kelvin theory to data from single adsorption isotherms. All used temperatures depicted a distribution of pores below 2 nm. The docking analysis involving l-Phe and the ACZ and ACK adsorbents reveal a significant resemblance in how receptors detect ligands. Consequently, the findings from the docking process confirm that the calculated binding affinities fall within the spectrum of adsorption energy.

Methods

This study analyzed the adsorption capacity of the l-Phe through a model proposed by statistical physics formalism. Molecular docking was used to determine the various types of interactions between the two activated carbons. Two aspects, including orientation of l-Phe on the site, number of molecules per site n, interaction energy, density of receptor site, and adsorption capacity, were discussed to elucidate the influence of activation on the two adsorbents.