Relationship between surface dilational properties and foam stability

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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.

Introduction

Several technological steps in the detergent industry, flotation, brewing industry, and sewage disposal are based on the formation of foams of a definite lifetime. The knowledge of their physicochemical properties, particularly the factors which influence their stability, are very important for the process control. Up until now many aspects of this problem are not clear. Foam stability depends on various parameters. In particular, the added surfactants play the most important role in their stability. However, the effect of surfactants cannot be explained just by a decrease in surface tension. The stability or instability of foams and emulsions is to a great extent determined by the surface rheological properties of adsorption layers at the liquid interfaces.

In this paper we present measurements of surface dilational properties of several surfactant solutions by using a newly constructed oscillating bubble method.

The purpose of this paper is to demonstrate the influence of surface dilational properties, in particular the dilational viscosity of soluble adsorption layers on the foam stability.

Section snippets

Surface rheological measurements

Surface rheology, like its three-dimensional counterpart, investigates the two-dimensional functional relationships between stress and deformation as well as between stress and the rate of deformation.

The complete surface rheological characterization of soluble or insoluble layers at liquid interfaces requires the knowledge of the shear viscosity, shear elasticity, dilational elasticity, dilational viscosity, and transport effects. The surface dilational modulus ε of a layer at a liquid

Experimental results

For the comparison of the surface dilational properties with the foam lamella stability we used a few examples of the different types of surfactants and surface active substances. Anionics: sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate, tetra ethylammonium perfluorooctane sulfonate. As an example of cationics cetyltrimethylammonium bromide (CTAB) was chosen. Nonionics: Triton-X-100, decyldimethyl phosphine oxide, nonanol, dodecanol, and decanoic acid in 0.005 M HCl. All chemicals were

Conclusions

The present level of knowledge in foam science enables the solution of many problems relating to kinetic processes such as film thinning, drainage, and rupture. Unfortunately, all these processes cannot satisfactorily explain why a foam is stable or not. There obviously exist additional factors in the stabilizing action of surfactants which are responsible for the large differences in the foaming ability of surfactants. The experiments presented have demonstrated that the surface rheological

Acknowledgements

This research was supported by the Federal Ministry of Education, Science, Research and Technology (Projekt No. 13 N 7283-0). The authors thank the Max Planck Gesellschaft and Professor H. Möhwald for their support of the work.

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