A method is described for measuring the specular optical reflectance R($) of the interface between a fluid and a glass prism, the incident and reflected light being in the glass, and the angle of incidence 4 being varied about the critical angle. Transparent, absorbing and some turbid fluids give R y ) close to theoretical predictions, and experimental reflectance data are analysed to give the real (n') and imaginary (n") refractive indices of the fluid. Other turbid fluids gave R(4) data that differed strongly from theoretical expectation. This is attributed to heterogeneity of the fluid on the optical wavelength scale, and problems of refractometly for such fluids are discussed.

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The combined use of small-angle X-ray scattering (SAXS) and photon correlation spectroscopy (PCS) to characterise adsorbed layers of /?-casein at the solid/liquid interface is reported. The protein was adsorbed to polystyrene latex particles at room temperature, low ionic strength and neutral pH and adsorption densities assessed by a solution-depletion technique which showed a plateau in the adsorption. Results from the SAXS experiments were analysed to provide electron-density profiles. These were backed up with results from PCS which provided hydrodynamic thicknesses over the range of the adsorption isotherm. This information, together with calculated hydrophobicity and charge profiles for the protein, yielded a molecular model for the adsorbed layer. Although B-casein in solution has a largely random coil conformation, it appears to adopt a much more compact form when it is adsorbed on polystyrene latex. Most of the protein lies close to the surface, leaving part of the chain extended into the aqueous phase. The most likely candidate for the extended chain is part of the highly charged sequence of 40 or so amino acids at the N terminus of the protein. The hydrodynamic thickness of the protein layers increases with adsorbed concentration of protein. The thicknesses reached are substantially greater than those predicted by theories of self-avoiding walks of the extended chain with volume exclusion interactions included and it is suggested that long-range electrostatic repulsive forces are involved.

The authors describe a modified Abbe refractometer for measuring the optical transmittance across a plane interface from a diffusely scattering turbid sample into a prism. When the prism refractive index exceeds that of the sample, a critical angle is present even when the sample contains particles which are wavelength sized or larger. Hence, the sample refractive index may be measured. However, measurements of transmittance near the critical angle show a poor definition of the critical angle for some samples, with a consequent loss of experimental precision in the refractive index. The method is applied to several polymer latex dispersions and some glass ballotini. For all lattices the measured refractive index varies linearly with latex volume fraction. For all dispersions except one the dependence of refractive index on volume fraction is in good agreement with the theory of refractive index based on Mie scattering by spherical particles.

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