The solubility of metal halides depends on their nature, lattice enthalpy and hydration enthalpy of the individual ions. Amongst fluorides of alkali metals, the lowest solubility of LiF in water is due to
Since lattice enthalpy decreases much more than the hydration enthalpy with increasing ionic size for alkali metals, the solubility of LiF is the lowest in water due to highest lattice enthalpy.