The alpha effect is also dependent on solvent but not in a predictable way: it can increase or decrease with solvent mix composition or even go through a maximum.  At least in some cases, the alpha effect has been observed to vanish if the reaction is conducted in the gas phase, leading some to conclude that it is primarily a solvation effect. However, this explanation has limitations since similar alpha effects could be found in different solvent systems and also because the solvation affects both the basicity and the nucleophilicity of the nucleophile.