Azeotropic transformation in mixed monolayers of octadecylurea and hexadecylurea

The surface pressure of mixed monolayers of octadecylurea and hexadecylurea has been measured as a function of mean area per molecule at various temperatures and compositions. The surface pressure of the phase transition obtained was observed to decrease both with an increase in temperature and with an addition of another component. With the aid of the thermodynamic method developed previously, the apparent molar entropy and energy changes associated with the phase transition were found to be positive. These positive values were explained by the rupture of the intermolecular hydrogen bonding. Furthermore, the activity coefficients of film-forming components with reference to their respective pure components were considered in connection with the mutual interaction between octadecylurea and hexadecylurea molecules. It was concluded that the system exhibits the negative azeotropy as a result of the difficulty in forming the hydrogen bonding in the mixed monolayer.