Principal researcher at MPI-P:
Film balance construction:
We have built-up a novel technique to study via force-distance curves the interaction between a micro-sized colloid particle,
attached to the end of an Atomic Force Microscope (AFM) cantilever, and a monomolecular film (monolayer)
of insoluble amphiphilic molecules (amphiphiles) at the air-water interface in a Langmuir-Blodgett (LB) film balance .
Such molecules consist of a polar hydrophilic headgroup, soluble in the water, and a hydrophobic,
i.e. water-insoluble part, which is exposed to the air. A commonly investigated class of amphiphiles is the class of lipids.
Their hydrophobic part consists of one, two or more alkyl chains. For a brief review on monolayer phases and their investigation
with the Langmuir-Blodgett technique, please click
Monolayer Particle Interaction Apparatus (MPIA)
To evaluate our novel technique, we measured force-distance curves between a negatively charged, hydrophilic silicon bead and a monolayer of the well-known lipid DMPE. It is a bilayer-forming lipid in human cells and its thermodynamic properties as a monolayer at the air-water interface has been extensively studied. For details click here. The isotherm of DMPE at the air-water interface at 20°C and the force-distance curves, measured at two surface pressures with a particle speed of several 10 μm/s, are shown below.
Isotherm of DMPE at 20°C
Force-distance curves between a silicon particle
and a DMPE monolayer at 20°C
During approach (a) there is no force until the particle jumps into contact with the monolayer (insert, b ->
c -> d). Upon further particle movement the air-water interface is distorted;
the slope reflects the stiffness of this interface. When the direction of the particle movement is inversed,
the maximum force (e) is reached (load). During retraction the monolayer adheres to the particle surface,
until it jumps out of the interface (f) to a force of zero again (g).
From the particle radius R and the jump-in distance D of the particle into contact with the monolayer, the contact angle Θ can be calculated according to
CosΘ = (R - D) / R
Only for DMPE in the LE-phase a contact angle could be measured.
Additionally, the adhesion Fadh to the monolayer decreases with increasing
surface pressure and reaches it lowest value in the LC-phase. This we interpret as the decreasing tendency of the monolayer
to be distorted by the approaching particle, owing to the increasing layer stiffness.
Accordingly, we observe an increasing slope between d and e with increasing surface pressure of the monolayer.
This leads us to the conclusion that in the LC-phase the mechanical properties of the monolayer
determine the layer-particle interaction, whereas in the LE-phase it is rather determined by the molecular forces.
Currently, we evaluate the influence of the particle speed on the force-distance curves. Further projects include the investigation of different interaction forces as electrostatic, van-der-Waals or steric forces between monolayer and particle, as realized by different amphiphiles and/or different surface properties of the particle.
This project has been funded by the Deutsche Forschungsgemeinschaft (DFG) within the Priority Program (Schwerpunktsprogramm) SPP 1052 “Wetting” (GR2003/1-1).