Imaging of thin films using Brewster Angle Microscopy

[The method] [The apparatus] [The project] [Some images] [Links] [Contacts]

1. The method

The imaging of thin films at the gas/liquid interface is generally considered to be of great importance in the study of such systems. There are many techniques that can been applied (e.g. fluorescence microscopy, AFM to LB films, etc.). Many of these techniques require the use of probe compounds in the system (such as a fluorescent dye for instance) or an appropriate modification of if (as in the case of AFM where an LB film has to be formed). Although still useful there is always the uncertainty of whether the system under study is in its original state or not.

Brewster Angle Microscopy is a technique that allows the in situ study of thin films at the gas/liquid or solid/gas interfaces. The basis of this method is illustrated at the following scheme (the Brewster angle):

The Brewster angle

For a beam of p-polarised light (p- parallel to the plane of incidence) there is an angle of incidence theta at which no relfection occurs. This is called "Brewster angle" and is satisfying the relationship tan(theta) = n_subphase / n_air, where n is the refractive index of the corresponding phase.

Introducing a thin film in between the two phases the optical properties of the system change so that a small amount of the incident intensity is reflected. In the general case it can be said that the reflected light can have a different from the incident light polarisation state. The reflected light can easily be "captured" by a detector like a CCD camera and in that way the film can be visualised.

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2. The apparatus

A Brewster angle microscope (BAM) is comprised of a light source (laser), a set of one or two polarising filters of which the first is responsible for the polarisation of the beam prior to its reflection and the second is analysing the polarisation state of the reflected part of the beam and a light detector (a CCD camera).

The BAM that has been used in this project is a BAM2plus (Nanofilm, Germany), which makes possible the flexible imaging of many different systems. In this case and for the scope of the current project, the microscope has been mount above a home made, rhombic-shaped, PTFE trough. The entire configuration is resting on a vibration-free construction which includes an Halcyonics anti-vibration base.

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3. The project

Sugar surfactants are a relatively new class of surfactants that have a carbohydrate as their hydrophilic moiety. On their main advantages there are their very low toxicity, their biodiversity and the good functional properties that they generally exhibit. Milk proteins, on the other hand, are of great importance to the food industry because of their nutritional value and their functional properties.

The aim of the project is - at a first stage - to apply BAM as a complimentary technique for the investigation of surfactant monolayers and - at a later stage - to use it for the study of mixed monolayers of protein and surfactant in an attempt to clarify the nature of the interactions between the two species. It is believed that the information obtained will be useful for the understanding of other protein - surfactant systems.

The financial support for this project from EU (FAIR985016) and BBSRC (24/D11184) is gratefully acknowledged. Note: For a list of BBSRC bioimaging grants please visit the BBSRC Bioimaging Initiative webpage.

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4. ...and some images!

Low molecular weight surfactants such as long carbon chain organic acids give, in general, optically anisotropic monolayers. This can be easily demonstrated by spreading arachidic acid (n-eicosanoic acid) on water. Even at a relatively low surface pressure the water surface is covered by the surfactant. The following images correspond to such a monolayer of arachidic acid. They both show the same area of the monolayer (the dust particle in the white circle can be used as a reference point) but in the image on the right the analyser has been rotated so that new surfactant domains were revealed.

Brewster angle microscopy can also be applied in systems of monoglycerides. In the following set of images a monolayer of monopalmitin has been slowly compressed from 0 to ca. 35 mN m^-1 and images have been taken at 10, 15, 20, 25, 30 mN m^-1. The higher the compression the higher the intensity of the reflected light beam. As the film approaches the collapse point the contrast decreases indicating the loss of a well defined structure.

Sugar surfactants have a significantly different structure than the two previously mentioned surfactants. The interaction between their hydrophobic parts has to overcome the effect of their strongly hydrated, bulky headgroups. As a result, domains of high optical anisotropy, as seen previously, cannot be observed. The following images correspond to a sucrose stearate film. It is interesting to note that surfactant domains can only be seen at ca. 0 mN m^-1. The first image on the left corresponds to the film after the evaporation of the spreading solvent. Following onto the right the same film is shown at 15 and 25 mN m^-1 and then again at 0 mN m^-1. The "homogenising" effect of a single compression/expansion cycle is obvious!

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5. Links

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6. Contacts

For any additional information you can contact:

Gorgias Garofalakis

Brent S. Murray
Dept. of Food Science, Univ. of Leeds
Tel.: +44-113-2332962
Fax.: +44-113-2332982

IMPORTANT NOTE: The copyright of the present is property of the UNIVERSITY of LEEDS, the AUTHOR of this document (G. Garofalakis) and the INVESTIGATORS involved in this project. The information presented here is provided on an "as is" basis. No guarantee for the accuracy, quality or suitability for particular applications of this information is expressed or implied by any of the copyright holders or the author in particular.

Created by G. Garofalakis on 30.06.1999. Last updated on 26.07.2002.