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

摘要

The effect of external electrostatic charge on the shapes of liquid condensed (LC) phase domains inmonolayer 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 analysisindicated magnesium ions preferred to bond with dipalmitoyl phosphatidylcholine negative headgroup in liquid expanded phase and/or at domain boundary at low ion concentration and got anaccess to binding with molecules inside of the LC domains for higher ion concentration. Domainsize increase characterized by BAM was discussed in respect to the shape equation on the basis ofelectrostatic energy contribution. Although molecular repulsive force increased by adding of ionsinto subphase, the growth of domain size exceeded this tendency. Following shape equation analysisit was suggested that this effect corresponded to change in dipole moment orientation represented byincrease in spontaneous polarization in normal projection. This demonstrated impact of localelectrostatic field on molecular dipoles and free energy of LC domains.