KSV SPOT
KSV SPOT Manual
Rev. 1.0
5
3. General introduction
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 (
π
)- area (A) isotherm. The
π
- A isotherm is characteristic for any given
ampiphile
2
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
π
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
π
. The type of deposition that has occurred for LB films (X,
Y or Z deposition) has also been determined by measuring the
∆
V.
9
1
Yamins, H.G. and Zisman, W.A.,J. Chem. Phys. (1933), 1, 656.
2
Amphiphile = A substance containing a hydrophilic (water loving) and a hydrophobic (oil loving) part. The
hydrophilic part can for example consist of an alcohol, carboxylic acid, amine or phosphorylcholine groups, and
the hydrophobic group(s) is generally composed of hydrocarbon chains.
3
Binks, B.P. Adv. Coll. Interf. Sci. (1991), 34, 343.
4
Roberts, G.G. Langmuir-Blodgett Films, Plenum Press (1990).
5
Gaines, G.L. Insoluble Monolayers at the Liquid-Gas Interface, Wiley-Interscience (1966).
6
Lindén, M. PhD thesis, Åbo Akademim Univerity, Finland (1996).
7
Vogel, V. And Möbius, D. J. Coll. Interf. Sci. (1988), 126, 408.
8
Phillips, M.C., Cadenhead, D.A., Good, R.J., King, H.F. . J. Coll. Interf. Sci. (1971), 37, 437.
9
Christie, P., Jones, C.A., Petty, M.C., Roberts, G.G. J. Phys. D. (1986), 19, L167.
