phenomenon is not limited to the formation of a theoretical ‘‘monolayer’’ of the adsorbed impurities. This is clear when considering that the adsorption of MG alone exceeds the monolayer value (without taking into account the adsorption of the other impurities). Then the adsorption phenomenon is more complex and must include multilayer formation.
The chemistry on the silica surface depends on the nature of attractive forces existing between the adsorbate and the adsorbent. This interaction may be due to chemical bonding, hydrogen bonding, hydrophobic bonding or van der Waals forces. Hydroxyl groups play an important role on the silica surface because hydrogen bonding is the most common mechanism of adsorption. Parida et al. [24] postulated that due to the presence of silanol groups at the surface silica is mostly embedded with water in a multilayer. The authors found differences between the characteristics of the first layer of adsorbed water and the subsequent layers. Yamauchi and Hondo [25] suggested that water would settle only on part of the silanols at first as SiOH–OH2 complexes by an H-bonding mechanism. A second water molecule would be added on the previous silanol water complexes in the form of SiOH–OH2–OH2.
Garrone et al. [26] compared the adsorption of water on the silica surface with that of other molecules of similar size (CO, H2O, N2O and NH3). They found evidences indicating that the main interactions of silanol groups (SiOH) with the O end of CO, with the O end of N2O, with the O atom in H2O, and with the N atom in NH3, are due to H-bonding. This H-bonding could be extended in the pore space involving more adsorbate molecules. Mizukami et al. [27] proposed the formation of surface molecular macroclusters for adsorption of ethanol on the silica surface and proposed that the H-bonding between the silanol group and the OH group of ethanol is responsible for adsorption.