Project III - Monolayer Particle Interaction Apparatus (MPIA)

Principal researchers at MPI-P:	Dr. Karsten Büscher, Dr. Michael Kappl
Film balance construction:	Dr. Hans Riegler

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 [1]. 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 here.

Monolayer Particle Interaction Apparatus (MPIA)
MPIA combines a temperature-controlled Langmuir-Blodgett (LB) film balance with the AFM-like colloidal probe technqiue. It measures the forces between a colloidal probe attached to a cantilever and a monomolecular film (monolayer) at the air-water interface by approaching the headgroup layer from below. The monolayer is spread on top of a water subphase in a LB film balance. The molecular density is controlled by two barriers compressing or expanding the monolayer. The surface pressure Π is measured with a Wilhelmy system, a filter paper immersed into the subphase. Π is the difference between the surface tension of the pure water surface and that in presence of the monolayer. A fluorescence microscope ensures the visualization of the different monolayer phases, after a fluorescence dye has been added to the monolayer. Since we want to study the effect of the approaching and retracting particle on the monolayer, the microscope objective has to be aligned with the particle. For lateral observation neither the objective, nor the particle can be moved laterally. The monolayer itself has to be moved instead. Therefore, the movement of the barriers (parallel or anti-parallel) is decoupled.
For measurement of the force-distance curves, the cantilever is attached to a piezo translator to move it up (approach) and down (retraction). A laser beam is deflected from the back-side of the cantilever to a position sensitive detector (PSD), which measures the deflection as a voltage.
The inset shows a zoom-in of the particle after jump into the air-water interface. From the jump-in distance D and the particle radius R the contact angle Θ between monolayer and particle surface can be calculated (see main text below!). The length scale of the amphiphiles is not to scale. They are about 5000 times smaller than the particle, so that the particle surface appears rather flat.

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
The lipid DMPE (1,2-Dimyristoyl-sn-glycero-phosphoethanol- amine) exhibits different phases as a monomolecular film (monolayer) at the air-water interface. The plateau hints at a phase transition from a non-ordered liquid-expanded (LE) phase to an ordered liquid-condensed (LC) phase (details). The black dots show the points, where the force-distance curves have been measured.

Force-distance curves between a silicon particle and a DMPE monolayer at 20°C
Here, representative force-distance curves have been measured at 0.5 mN/m in the LE-phase and at 10.0 mN/m in the LC-phase of DMPE (see black dots in the isotherm above). The force is normalized to the radius of the silicon particle (R = 5 µm), the x-axis shows the piezo displacement. The distance from the monolayer is given by setting the piezo displacement to zero at a force of zero after the jump-in of the particle. The arrows showing to the left indicate the approaching particle, those showing to the right the retracting particle.
The inset is a zoom-in of the force-distance curve measured at 0.5 mN/m. The jump-in distance D can be recognized. For a detailed explantion, see main text.

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

Only for DMPE in the LE-phase a contact angle could be measured. Additionally, the adhesion F_adh 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).