Cluster adsorption of histidine enantiomers on carbon nanotubes from aqueous solutions

The study is devoted to the analysis of adsorption isotherms of histidine enantiomers on carbon nanotubes mkNANO MKN-SWCNT S1 from aqueous solutions in the temperature range 25-80°C. Histidine enantiomers from Sigma Aldrich were used as amino acids, and mkNANO MKN-SWCNT S1 carbon nanotubes (Canada) were used as an adsorbent. Experimental data obtained by the construction of isotherms, were used to calculate the separation coefficients of enantiomers on CNTs, the values of separation coefficients were higher than for other sorbents. The interpretation of the isotherms was based on the cluster adsorption model, providing very good agreement between theory and experiment (R2=0.994-0.999), which showed that L- and D-histidine were sorbed on the surface of a nanotube in the form of monomers and clusters. Three characteristic regions were identified on the adsorption isotherms: concentration range in which only sorbate monomers were fixed on the surface of the nanotube; concentration range in which sorbate molecules were fixed on the nanotube only in the form of clusters; concentration range in which monomers and clusters were present on the surface of the sorbent. The dependence of these isotherm regions on temperature was analysed.