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A method for analysing thin films using a dual-waveguide interferometric technique is described. Alternate dual polarization addressing of the interferometer sensor using a ferroelectric liquid crystal polarization switch allowed the opto-geometrical properties (density and thickness) of adsorbed layers at a solid–liquid interface to be determined. Differences in the waveguide mode dispersion between the transverse electric and transverse magnetic modes allowed unique combinations of layer thickness and refractive index to be determined at all stages of the layer formation process. The technique has been verified by comparing the analysis of the surface adsorption of surfactants with data obtained using neutron scattering techniques, observing their behaviour on trimethylsilane coated silicon oxynitride surfaces. The data obtained were found to be in excellent agreement with analogous neutron scattering experiments and the precision of the measurements taken to be of the order of 40 pm with respect to adsorbed layer thicknesses. The study was extended to a series of surfactants whose layer morphology could be correlated with their hydrophilicity/lipophilicity balance. Those in the series with longer alkyl chains were observed to form thinner, denser layers at the hydrophobic solid/aqueous liquid interface and the degree of order attained at sub-critical micelle concentrations to be correlated with molecular fluidity.The technique is expected to find utility with those interested in thin film analysis. An important and growing area of application is within the life sciences, especially in the field of protein structure and function.

Lysozyme adsorption at the silica/water interface has been studied using a new analytical technique called dual polarization interferometry. This laboratory-based technique allows the build up or removal of molecular layers adsorbing or reacting on a lightly doped silicon dioxide (silica) surface to be measured in terms of thickness and refractive index changes with time. Lysozyme adsorption was studied at a range of concentrations from 0.03 to 4.0 g dm(-3) and at both pH 4 and pH 7. Adsorbed layers ranging from 14 to 43 +/- 1 A in thickness and 0.21 to 2.36 +/- 0.05 mg m(-2) in mass coverage were observed at pH 4 with increasing lysozyme concentration, indicating a strong deformation of the monolayer over the low concentration range and the formation of an almost complete sideways-on bilayer toward the high concentration of 4 g dm(-3). At pH 7, the thickness of adsorbed layers varied from 16 to 54 +/- 1 A with significantly higher surface coverage (0.74 to 3.29 +/- 0.05 mg m(-2)), again indicating structural deformation during the initial monolayer formation, followed by a gradual transition to bilayer adsorption over the high concentration end. The pH recycling performed at a fixed lysozyme concentration of 1.0 g dm(-3) indicated a broadly reversible adsorption regardless of whether the pH was cycled from pH 7 to pH 4 and back again or vice versa. These observations are in good agreement with earlier studies undertaken using neutron reflection although the fine details of molecular orientations in the layers differ subtly.

A novel method for the analysis of thin biological films, called dual polarization interferometry (DPI), is described. This high resolution (<1 Å), laboratory-based technique allows the thickness and refractive index (density) of biological molecules adsorbing or reacting at the solid–liquid interface to be measured in real time (up to 10 measurements per second). Results from the adsorption of bovine serum albumin (BSA) on to a silicon oxynitride chip surface are presented to demonstrate how time dependent molecular behaviour can be examined using DPI. Mechanistic and structural information relating to the adsorption process is obtained as a function of the solution pH.