Study of the Calix[4]resorcinarene--dopamine interactions in monolayers by measurement of pressure--area isotherms and Maxwell displacement currents.

The mechanisms of interactions between calix[4]resorcinarene and dopamine in monolayers formed at the air-water interface were studied by analyzing their mechanical, thermodynamic, and electrical properties evaluated from measurements of pressure-area isotherms and Maxwell displacement currents (MDCs). An increased concentration of dopamine in the water subphase resulted in an increase in the area per calix[4]resorcinarene molecule, an increase in the collapse pressure, and a shift in the monolayer phase transitions from the gaseous to the liquid state and from the liquid to the solid state toward higher molecular areas. A contactless method of recording MDCs enabled the monitoring of changes in the charge state of the monolayer-constituting molecules and the determination of a relationship between the phase state of the monolayer and the structural transitions of calix[4]resorcinarene. The changes of the MDC recordings started already in the gaseous state of the monolayer. On the basis of MDC values, we determined the normal component of the dipole moment of calix[4]resorcinarene, as well as that of its complex with dopamine. The dipole moment reached a maximum value of 1040 mD in the region of the phase transition from the liquid to the solid state of the monolayer. The results obtained suggest that the binding of dopamine with calix[4]resorcinarene depends on the orientation of the calixarene molecules in the monolayer. The calix[4]resorcinarene-dopamine interactions were also quantified in terms of the excess of Gibbs free energy, thereby allowing the evaluation of the energy of the calix [4]resorcinarene-dopamine bond, which was in the range from 1.95 to 8.54 kJ/mol depending on the surface pressure. This value implies weak interactions between these molecules.