Surface Dilatational Modulus Research Articles

Dynamic and static properties of proteins adsorbed at the air water interface.

The adsorption of β- and κ-casein at the air/water interface from solutions containing concentrations ranging from 10–5 to 10–1 weight % has been monitored using both radiotracer and ellipsometric techniques. The surface pressure and dilatational modulus have also been measured both during the adsorption process and at steady state conditions. Differences between the surface properties of β- and κ-casein are related to the structures of the molecules. The adsorption data cannot be described by Gibbs' adsorption equation. The adsorption isotherms and film thicknesses are discussed in terms of the distribution of amino-acid residues between trains and loops. At low surface coverage where loop formation is not pronounced, theories based on polymer statistics describe the relationships between film pressure, dilatational modulus and surface coverage satisfactorily. At low surface concentrations of protein the dilatational modulus is purely elastic whereas at higher surface concentrations it has both elastic and viscous components.

Surface tension relaxation in a surface containing surfactant particles

At the surface of aqueous systems containing slightly soluble surfactants, slow relaxation processes occur that are accompanied by high values of the surface dilational modulus. A detailed investigation into these phenomena was carried out in systems containing dodecanol, tetradecanol and hexadecanol, using longitudinal surface waves and stress-relaxation experiments. The state of the surfaces involved resembles a monolayer after collapse: particles are present which do not consist of the thermodynamically stable bulk phase of the surfactant. A quantitative interpretation of the surface tension relaxation is given in terms of two processes: surfactant exchange in the surface between the monolayer and the particles, and diffusional exchange of surfactant between monolayer and bulk solution. The parameters characterizing these processes were measured; the values obtained are in satisfactory agreement with the results of other experiments.

Dynamic measurements of dilational properties of a liquid interface

A new method is described for the measurement of dilational surface properties of solutions containing surface-active materials. The surface dilational modulus is obtained from a surface-tension variation, caused by a small-amplitude, sinosoidal-area variation. The method is limited to frequencies below about 0·1 Hertz because of higher frequencies wave-propagation effects play a disturbing role. Simultaneous measurement of the phase difference between area changes and surface-tension variations provides information about possible relaxation processes occurring on the surface. The use of the method is illustrated by means of experiments on dilute solutions of deconoic acid. For this system, the relaxation process consists of diffusional exchange of solute between bulk and surface. A quantitative analysis of this phenomenon shows satisfactory agreement with the measured behaviour of the modulus as a function of frequency and concentration.

Propagation of surface tension changes over a surface with limited area

Surface tension changes resulting from a periodic local compression and expansion of a surface propagate over the available surface area. Whether or not the extent of deformation is uniform over the whole area is determined by the value of a dimensionless parameter, the wave propagation number. Uniformity increases with increasingly elastic surface behaviour and with decreasing frequency, area of the surface, and bulk density and viscosity.A number of numerical examples is given. The significance of surface uniformity for the determination of the surface dilational modulus and for stability of thin films is discussed.