Abstract: In consideration of the similarity of insoluble monolayers and soluble monolayers,amolecular interaction parameter(β)was introduced to describe the nonideality of the mixedinsoluble monolayers. From the surface pressure-area isotherms of “6201”/C₁₈SO₄Na mix-tures,a high molecular interaction parameter which was closed to the critical value 2.0 wasobtained,which has further verified the previous guess that a phase separation took place ata higher surface pressure.

Key words: Insolube monolayers, Intermolecular interaction parameter, Nonideality, Sur-factant