The calculations presented up to this point made the simplifying assumption that no coadsorption of water occurs when adsorbing TCS from aqueous solution. While this simplification might appear reasonably justified for all-silica zeolites, which are known to be highly hydrophobic, the introduction of framework Al atoms and charge-balancing protons will increase the hydrophilicity. The relative affinity towards water and organic molecules such as phenol has sometimes been evaluated by comparing interaction energies computed for adsorption complexes of individual molecules. However, when looking at molecules of vastly different size, adsorption energies obtained for single molecules will (in almost all cases) be much less negative for water, simply because the contribution of dispersion interactions is much larger for a bulky organic molecule like TCS. For this reason, it appears to be more plausible to compare TCS adsorption energies to those obtained for a cluster of water molecules that is similar in size to TCS. An estimation of the molecular volumes based on the crystal structures delivered molecular volumes of 32.5 Å 3 for H2O (hexagonal ice ) and 302 Å for TCS, corresponding to a ratio of about 1:9.3. Moreover, experimental investigations on protonated zeolites have shown that the typical size of water clusters in the vicinity of the acid sites (framework protons) is in the range of eight H2O molecules (for MFI-type ZSM-5 ). For these reasons, an adsorption of eight water molecules per simulation cell was assumed in calculations aimed at a comparison of TCS and H2O adsorption energies and in studies of TCS+H2O co-adsorption. Additional results obtained for 1, 4, and 12 H2O molecules per cell were evaluated to investigate the impact of cluster size on the adsorption energy.