The KSV NIMA Surface Potential Sensor is an instrument that offers complementary data on the packing and orientations of surfactant molecules within the sampling region of the Langmuir film. Whereas the surface pressure is measured as a change in the surface tension of the air-water interface, the Surface Potential Sensor measures the potential difference above and below the film and is sensitive to the sum of all the individual dipole moments.
KSV NIMA SPOT measures surface potential by the vibrating plate capacitor method:
Changes in surface pressure are only detected once a closely packed monolayer begins to form. Changes in the surface potential, however, can be measured as soon as molecules with dipoles appear at the surface. As the orientation of the molecules change during compression, the alignments of the dipoles cause a large change in the surface potential. This is clearly demonstrated in the plot shown below.
The figure displays surface pressure-area (violet) and surface potential- area (light blue) isotherms of an antiparasitic drug monolayer on an air-buffer solution interface. An unusual surface pressure-area transition was observed at mean molecular area of 140 Å2, but no transition was shown in the surface potential-area isotherm. This suggests that the transition is not a phase transition but instead the drug could undergo aggregation, dimerization or conformational change at this mean molecular area.