H. Fruhner

Relationship between surface dilational properties and foam stability.

Abstract The knowledge of the stability of foams and emulsions is very important for the control of a large number of technological processes. However, in spite of much intense research, the mechanism of foam stability is still not completely clear. Foam stability depends on many parameters but the type of the added surfactant and the surface rheological properties of the adsorption layers play the most important role. Because of the lack of reliable surface rheological data a comprehensive investigation of this problem was not yet possible. However, we can now present measurements of surface dilational properties of soluble adsorption layers in a frequency range of 1–500 Hz using a new version of the oscillating bubble method. The results are compared with measurements of foam stability. This indicates that the surface dilational viscosity plays an important role in the stability of foam films. A direct relation between surface dilational elasticity values and foam stability could not be detected for the examined systems. Pure elastic adsorption layers were not able to stabilize foam lamellas.

A new oscillating bubble technique for measuring surface dilational properties

Abstract A new oscillating bubble device has been developed and used for measuring the surface dilational properties of surfactant, protein and surfactant/protein adsorption layers in the frequency range 1–300 Hz. Within a closed measuring chamber, a small hemispherical bubble is produced at the tip of a capillary. A piezoelectric driver generates sinusoidal oscillations of the bubble volume, and consequently produces changes in the surface area and the radius. The piezoelectric driver can also produce arbitrary changes of area. This method allows the measurement of the pressure difference across the curved bubble surface, due to changes in the radius and surface tension. The pressure amplitudes are monitored by a sensitive pressure transducer which is mounted at the bottom of the chamber. One obtains information on the deformation from the pumped volume, on the change in surface tension from the pressure amplitudes, on the rate of deformation from the frequency and on the phase angle Φ between deformation and surface tension. This novel technique can also be used for adsorption kinetics studies at the air/water and oil/water interfaces. Experimental results on the frequency dependence of the dilational elasticity of cetyltrimethylammonium bromide solutions are in good agreement with a diffusional exchange of matter theory. Gelatin behaves as an almost insoluble layer; no frequency dependence was observed in the studied frequency interval. A mixed gelatin/anionic surfactant adsorption layer shows an almost linear dependence of Δγ(f) in the frequency range 50–300 Hz. This increase in the stress amplitude (Δγ) with the deformation rate is a characteristic feature of an intrinsic viscosity.

Surface dilatational properties of mixed sodium dodecyl sulfate/dodecanol solutions

Abstract The surface dilatational properties of aqueous solutions of sodium dodecyl sulfate (SDS) and n -dodecanol are investigated in the frequency range 1≤f≤500 Hz using the oscillating bubble method. The results demonstrate that a pure dodecanol solution has an elastic surface without viscous effect whereas the surface of a SDS solution without added dodecanol exhibits a strong viscoelastic behavior. Mixtures show graduated properties. The time behavior of their surface dilatational moduli demonstrates that dodecanol molecules drive the SDS molecules slowly out of the surface. Therefore, the known one-component model describing the surface dilatational modulus can be used also for these mixtures. A simple theoretical consideration explains this effect.

Surface elasticity of oscillating spherical interfaces.

Abstract The general framework to study the effects of of the kinetics adsorption on the surface elasticity of oscillating spherical interfaces is proposed. It is based on the balance of adsorption and desorption fluxes of surfactant molecules at the oscillating interface. We applied the developed formalism to explain the experimental results for the surface elasticity of dimethyldecyl phosphine oxide and n -octanol obtained using the oscillating bubble technique. The effect of high amplitude of oscillation as well as the influence of the convective transport of surfactant molecules is also considered. We demonstrate that for the surface area oscillations exceeding 10% of the average value the higher harmonics of the basic frequency of the oscillations start to play a role. It is shown that for moderate amplitude of the bubble oscillations (up to 10% of the average size) the analytical formula derived for low amplitude and without convection taken into account can be used to describe the experimental data for the surface elasticity.

Theoretical description of surface elasticity of ionic surfactants

Abstract We propose a new theoretical description of surface elasticity of ionic surfactant solutions. Assuming small periodic disturbances of the solution surface we show that in the range of frequencies available for most experimental techniques used to study viscoelastic properties of interfaces, the analytical expression for the surface dilatational elasticity can be obtained. Our analytical approach is based on the separate solution of the diffusion equation in two regions, inside and outside the electric double layer formed at the interface due to adsorption of ionic surfactant. Then, two solutions are matched together. The obtained results depend strongly on the theoretical model used to describe adsorption of ionic surfactant at the interface. We also present a comparison between theoretical predictions and preliminary experimental results obtained for both anionic and cationic surfactants using the oscillating bubble technique.

Dynamic surface tension and surface area elasticity of adsorbed pulmonary surfactant layers

The mechanical surface properties of adsorbed artificially (bronchoalveolar lavages) as well as naturally produced (breathing condensate) lung surface-active materials were investigated by measuring dynamic surface tension and surface dilational elasticity. Surface tension was determined by applying the modified ring and Wilhelmy plate method. The lowest surface tension values observed were not lower than 25 mN m -1 . Values like these are characteristic of classical surfactants. The transport process of the lung surfactants to the surface seems to obey a mechanism which can be described by two different time constants. Surface elasticity was measured by three different methods : the oscillating bubble, the oscillating barrier and the oscillating meniscus technique. The results of the surface elasticity obtained by the first two methods are rather high whereas those obtained by the third are distinctly lower. The role of surface elasticity in lung function in comparison with static surface tension is discussed. As the effect of surface elasticity on the capillary pressure of an alveolus is opposite to that of the static surface tension, it is concluded that it is high surface elasticity rather than very low surface tension which is decisive for normal lung function. The surface properties of the lung surfactants obtained from the three different sources do not differ from each other in principle. Emphasis is also put on the necessity of taking into account more carefully the boundary conditions of the measuring techniques applied.

Adsorption-catalysed hydrolysis of sodium n-dodecyl sulphate at solid/liquid interfaces

Abstract Sodium n -alkyl sulphates ( n -decyl, n -dodecyl, n -tetradecyl) have been found to be degraded when they are contained in aqueous suspensions of acidic alumina. The surface tension and surface potential of aqueous “surface chemically pure” sodium n -dodecyl sulphate (SDDS) solutions containing aluminium oxide have been studied as a function of adsorption time, surfactant concentration, and the type and amount of alumina. The decomposition of the dodecyl sulphate molecule in aqueous suspensions was followed from the dynamic surface tension and surface potential behaviour. Decomposition depends on the concentration and on the length of time during which the solutions are in contact with acidic alumina. The parent n -dodecanol could be detected in the corresponding SDDS by thin layer chromatography. A small but significant increase in the solution pH was observed when neutral SDDS was added to aqueous suspensions of acidic alumina. This shift of pH, together with the corresponding point of zero charge (PZC) of the acidic alumina, supports an anionic exchange mechanism at the alumina surface. From these findings it is concluded that the labile COS bond of the alkyl sulphates is hydrolysed catalytically by polarization during adsorption. The hydrolysis rates observed in the acidic alumina suspensions exceed those in homogeneous solutions by orders of magnitude. Hydrolysis of sodium n -alkyl sulphates has not been observed in basic alumina suspensions.

The influence of molecular exchanges on surface dilational properties of surfactant solutions

Molecular exchange processes between fluid surfaces and the adjacent bulk phases were investigated in the frequency range 1 Hz ≤ f ≤ 500 Hz using a new version of the oscillating bubble method. The measuring data result in a complex dilational modulus of the fluid surface which depends on elastic, viscous and transfer properties of the surface. Three models of the surface exchange mechanism are considered to explain the frequency behavior of the modulus. It was possible to approximate all chosen measurements by one of these models. The parameters of the models could be determined by a fit-procedure. Using our measurements we have demonstrated the influence of an intrinsic surface dilational viscosity for special solutions which is independent of bulk diffusion effects. This can be interpreted as a consequence of the molecular exchange dynamics near the surface in a nonequilibrium state. An appropriate theoretical model allows estimation of the molecular exchange rate and the dissipative loss.