The ion exchange mechanism will not simply fill the inner cavities up to the rim with cations; rather than substitute already present ions by others.
For water softening, if the the free / accessible diameter of the pores of zeolite(s) is at least 3 Å, more relevant properties for ion exchange for a potential application are, for example,
- the theoretical number of sites where ions may be exchanged per formula unit (exchange capacitity)
- the strength the ions to be released (in exchange to the incoming ones) are bound to the framework
- the strength the newly bond ions are bond to the frame (often you want to regenerate / recycle the zeolite)
- if there is selectivity in binding ions
- the exchange rate between incoming and released ions
- resilience of the framework under the parameters of use (temperature, pressure, acidity of the medium; mechanical stress by weight / fluid flux; repeated regeneration)
- the absence of toxic / harmful products relased during its deployment
- cost in production and waste disposal of this material
The use of synthetic zeolites of different diameters accessible (3, 4, 5 Å, to mention just a few) often aims to benefit from their reactivity as Brønsted / Lewis acid as well as their selectivity due to the geometrical constrain imposed. Hence a 3 Å-zeolite may be used to trap selectively remaining small quantities of water out of ethanol used as biofuel, for example.