Measurement of a Langmuir monomolecular film pressure-area compression isotherm using laser light diffracted from surface capillary waves

Small periodic mechanical disturbances of a liquid-air interface will give rise to ripples propagating along the interface. The spatial separation of consecutive ripples is dependent on the frequency of disturbance and the surface tension of the interface. A laser beam striking these surface capillary waves will experience diffraction. Measurement of the resulting diffraction pattern allows the calculation of the surface tension of the interface. The introduction of a Langmuir monomolecular layer to the water-air interface will, under suitable compression, perturb the surface tension and, therefore, the diffraction pattern. Analysis of the diffraction pattern perturbation leads to the quantification of the surface tension change i.e. the 'surface pressure'. This method has been employed to monitor the surface pressure of a Langmuir film of eicosanoic acid under increasing compression and hence to record its pressure-area compression isotherm. The isotherm obtained agrees favourably with those recorded using a conventional Wilhelmy plate apparatus. If sufficiently refined, this method may offer an alternative, non-invasive, real time monitor for surface pressure.