Brewster Angle Microscopy
During the last decade Brewster angle microscopy has provided a valuable insight into the phase behavior of insoluble monolayers at the air-water interface. Amphiphilic monolayers are quasi two-dimensional systems and possess a large number of phases. Each phase may exhibit a different orientation, tilt azimuth or rotational degrees of freedom of the molecules. These features can be visualized by means of Brewster angle microscopy. The typical domain sizes are in the order of 20- 200 micrometers.
The existence of a Brewster angle is a pecularity of p-polarized light. The reflectivity coefficient rp vanishes at the Brewster angle and hence no light is reflected there. A monolayer at the air-water interface yields a certain amount of reflected light, which is sufficient to produce an image. The internal structure of domains can be detected. This technique is in many respects superior to fluorescence microscopy, since it does not require any label which could have an undesired impact on the systems.
Figure 1: The reflection coefficient of p-polarized light vanishes at the Brewster angle
Figure 2: Domain with an internal structure
|BAM image of domains of the glycerol ESD-16 recorded at the air-water interface. The domains are circular in shape and possess a sevenfold internal symmetry. Each segment has a uniform reflectivity and in the majority of cases there is a common point of intersection in the center of the domain. The contrast between the segments is due to the different organization of the tilt azimuth within each segment. Consequently, every single segment exhibits different optical properties.|
Figure 3: The Multiskop in BAM mode with the LB trough
Example of our own research
Transition state between oil-water and air-water interfaces:
Insoluble monolayers at the air-water interface are exposed to a gas phase containing organic hydrocarbons. The hydrocarbons are partly incorporated within the monolayer, leading to changes in the orientational order and the formation of new phases with a different morphology. The transition state resembles features of the air-water and oil-water interfaces and the control of the hydrocarbon partial pressure allows continuous tuning between both interfaces. The influence of the chemical nature of the hydrocarbon and the effect of the partial pressure of the hydrocarbon on the monolayer structure are assessed.
M. Harke, H. Motschmann
On the Transition State between the Oil-Water and Air-Water Interfaces
Langmuir, 14, 313 (1998)