Colloids and Surfaces A: Physicochemical and Engineering Aspects
Relationship between surface dilational properties and foam stability
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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