If there exists a difference in the electrostatic potential between two surfaces and these are put in movement relative to each other there will be a current flowing through an external circuit. This kind of “condensator” was used by Volta and Kelvin to study the differences in Volta potential between various metals. This “condensator” is often also called the Kelvin probe. Yamins and Zisman applied this method to study monomolecular films.1 Since then this method has widely been used for determining the surface potential of monolayers. The design of KSV SPOT is based on these main ideas. The main application area for KSV SPOT is in monolayer studies, but due to its stand-alone property it is also suitable for other areas where surface potential measurements are required.
Insoluble monolayers at the gas/liquid interface are generally characterized by measuring the surface pressure ()- area (A) isotherm. The - A isotherm is characteristic for any given ampiphile2 or mixtures of ampihphilic molecules. Many substances show a variety of different monolayer phases.3,4 The major phases are easily recognized in the isotherm as sharply separated regimes with different compressibility. Although the measurement of as a function of A can give valuable information about monolayers (area per molecule, collapse pressure, phase transition pressures and temperatures, mixing behavior of two or more amphiphiles etc) it may not reveal all of the desired information.
A complementary and a more sensitive way to characterize a floating monolayer is to measure the changes in the surface potential, V.5 This is possible due to the fact that an insoluble monolayer at the gas/liquid interface changes the surface potential through this interface. This change equals the Volta potential between the surface of the liquid and that of the metal probe. Normally only the changes in V due to the presence and changes in the state of the monolayer are measured. In this case the clean liquid surface is fixed to the value 0 mV, and the spread monolayer gives the obtained V. V can be used to determine the composition of the monolayer, the dissociation degree of an ionisable monolayer, the orientation and reorientation of the monolayer molecules at the interface during compression.3,5,6,7,8 By measuring V one can also reveal the interaction between the molecules in the monolayer much earlier than by measuring . The type of deposition that has occurred for LB films (X, Y or Z deposition) has also been determined by measuring the V.9
