Zeolites are hydrophilic due to the interaction of the dipole of the water molecule with
the electrostatic fields of the anionic aluminosilicate framework and the balancing nonframework cations. The molecular sieve silicalite, with a neutral Si02 framework and
no extra-framework cations, is hydrophobic. In contrast, the neutral aluminophosphate frameworks with no extra-framework cations are moderately hydrophilic, apparently due
to the difference in electronegativity between aluminum (1.5) and phosphorus (2.1). They
exhibit less affinity for H,O than the hydrophilic zeolites such as type A and type X zeolites yet substantially more than hydrophobic silicalite.