The molecular density at the air-water interface is manipulated with moveable barriers in a Langmuir-Blodgett
(LB) film balance. A modern film balance is sealed with Polytetrafluorethylene (PTFE or Teflon®).
The water subphase is temperature controlled. By moving PTFE barriers the film density decreases
(compression) or increases (expansion), which changes the surface tension
γlv of the air-water interface.
The quasi two-dimensional lateral or surface pressure Π of the water surface can be calculated according to
Π = γ0(H2O) – γlv
As a reference the surface tension of the pure water with γ0 (H2O) ≈ 73 mN/m at 20°C is chosen. Plots of the surface pressure Π versus the molecular area of the amphiphiles at constant temperature T (isotherms) are characteristic for the chosen amphiphile. The plot shows the surface isotherm for DMPE at 20°C.
Isotherm of DMPE at 20°C
At high molecular areas the monolayer exhibit a quasi two-dimensional gas state. The pressure increases at
78 Å2/molecule with further film compression into the liquid-expanded (LE) phase,
where the amphiphiles are in contact without molecular ordering.
The plateau at Πc = 5 mN/m is a phase transition from the LE-phase
to the liquid-condensed (LC) phase. In the LC-phase DMPE exhibit a short-range translational order
of the headgroup positions. Simultaneously, the alkyl chains of the hydrophobic molecular part show a long-range order
in their orientation, as indicated in the insets.
A monolayer can be transferred to a flat solid support, as e.g. a hydrophilic silicon substrate, by an upstroke of the immersed substrate through the monolayer. Since the substrate is hydrophilic, the headgroup of the monolayer will orient towards to the solid surface, whereas the hydrophobic alkyl chains are exposed to the air.
Hydrophilic Langmuir-Blodgett transfer
At high transfer speeds the quality of the transferred monolayer will largely be dominated by hydrodynamics: the underlying water layer has to be removed from the monolayer in contact with the solid surface. At low transfer speeds, the monolayer quality after transfer will be increasingly dominated by molecular interaction between the monolayer and the solid surface. This has been shown by fluorescence microscopy and precise contact angle measurements, both for very low transfer speeds close to the equilibrium height of the water meniscus adjacent to the three-phase contact line [1,2].
Fluorescence image (top view) of a hydrophilic Langmuir-Blodgett transfer
of a DMPE monolayer onto a silicon substrate
The monolayer condenses in a pressure range of 2 to 5 mN/m, when it is transferred from the LE-phase onto a negatively charged flat silicon substrate, owing to the substrate-layer interaction (red line in the isotherm above). This can be achieved by an electrostatic interaction between the negatively charged silicon surface and the dipolar headgroup of DMPE. To get more insight into the substrate-layer interaction, the Monolayer Particle Interaction Apparatus (MPIA) has been constructed.