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 Dbeta-mannitol using inverse gas chromatography (IGC). IGC was used to experimentally measure the dispersive components of surface free energy (gamma(S)(D)) and the specific components of free energy of adsorption (-DeltaGA(SP)) of Dbeta-mannitol by calculating the retention time of non-polar (n-alkanes) and polar (tetrahydrofuran and chloroform) probes, respectively. The results showed that Dbeta-mannitol surface is acidic in nature because the basic probe had more interaction with the surface as compared to acidic probe. Cerius(2) 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 Cerius(2) to model the adsorption of the same probes (n-alkanes, tetrahydrofuran and chloroform) on the Dbeta-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.