Van der Waals constants (such as 'a' and 'b' in the van der Waals equation of state) are well-established physical parameters that differ characteristically among various gases depending on their molecular size and intermolecular forces.
The claim that van der Waals constants vary predictably among different gases is a fundamental thermodynamic principle taught in introductory chemistry and physics. It represents common knowledge about real gas behavior, so no specific paper citation is necessary.