Effect of external electrostatic charge on condensed phase domains at the air-water interface: Experiment and shape equation analysis

The effect of external electrostatic charge on the shapes of liquid condensed (LC) phase domains in monolayer at the air/water interface was investigated. For this reason the thermodynamic properties, domain size, and spontaneous polarization were analyzed by surface pressure-area isotherms, Brewster angle microscopy (BAM), and Maxwell displacement current technique. The analysis indicated magnesium ions preferred to bond with dipalmitoyl phosphatidylcholine negative head group in liquid expanded phase and/or at domain boundary at low ion concentration and got an access to binding with molecules inside of the LC domains for higher ion concentration. Domain size increase characterized by BAM was discussed in respect to the shape equation on the basis of electrostatic energy contribution. Although molecular repulsive force increased by adding of ions into subphase, the growth of domain size exceeded this tendency. Following shape equation analysis it was suggested that this effect corresponded to change in dipole moment orientation represented by increase in spontaneous polarization in normal projection. This demonstrated impact of local electrostatic field on molecular dipoles and free energy of LC domains.