what we expect here would be the tertiary amine to have the greatest basicity and the least would be the primary amine.
But through experiments it has proven that secondary amines to have the greatest basicity. the order of basicity is as follows.
NH3 < primary amine < secondary amine
The basicity of the tertiary amine is approx. equal to that of primary amine.
The reasons behind this is:
Solvation of ions
3 + inductive effect
When the alkyl group attached increases the basicity reduces. It is because the size of the alkyl group is so large that it prevents an attack from Hydrogen ion. So according to this theory decreasing order would be
Primary amine > Secondary amine > Tertiary amine
But there's another thing you've got to consider. That is the solvation of ions. Amines dissolve in water to form protonated amines thus increasing the number of possibilities of hydrogen bonding. More the H bondings, greater the dissolution in water. Higher the H bonding, greater is their stability and greater is their basicity
Experiments have indicated that primary amines are the ones who have more H bonds and tertiary amines are the least.
So by combining all those reasons scientists have shown that basicity of amines vary as follows.
NH3 < primary amine ~ tertiary amine < secondary amine