Application of molecular modelling to determine the surface energy of mannitol

In this paper, molecular modelling was used to investigate the nature of probe/surface interactions during the analysis of Dβ-mannitol using inverse gas chromatography (IGC). IGC was used to experimentally measure the dispersive components of surface free energy (γSD) and the specific components of free energy of adsorption (−ΔGASP) of Dβ-mannitol by calculating the retention time of non-polar (n-alkanes) and polar (tetrahydrofuran and chloroform) probes, respectively. The results showed that Dβ-mannitol surface is acidic in nature because the basic probe had more interaction with the surface as compared to acidic probe. Cerius2 software package was used to model the two morphologically important surfaces, which showed the presence of surface hydroxyl groups. Molecular dynamics simulations were performed in Cerius2 to model the adsorption of the same probes (n-alkanes, tetrahydrofuran and chloroform) on the Dβ-mannitol surfaces. The adsorption energies calculated from the simulation showed a close match to those determined experimentally. The calculated values are slightly higher for all probes except chloroform, but as a single perfect crystal was modelled without considering the effect of impurities, solvent and other physical factors this is not unexpected.