Here's why:

* Chemical Structure: DAP has two ammonium ions (NH₄⁺) per phosphate ion (PO₄³⁻), while MAP has only one. The additional ammonium ion in DAP contributes to greater water solubility.

* Ionic Strength: The presence of more ammonium ions in DAP increases the ionic strength of the solution, leading to stronger interactions with water molecules and higher solubility.

* Hydrogen Bonding: Both DAP and MAP can form hydrogen bonds with water molecules. However, the presence of two ammonium ions in DAP allows for more hydrogen bonding, further enhancing its solubility.

Solubility Data:

* DAP: Approximately 40% by weight in water at 25°C.

* MAP: Approximately 18% by weight in water at 25°C.

In summary: DAP's higher ionic strength and greater capacity for hydrogen bonding result in a significantly higher solubility in water compared to MAP.