Klaus Lunkenheimer

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 criterion for judging the purity of adsorbed surfactant layers

Abstract A criterion for judging the quality of surfactant solutions with regard to surface-chemical contaminations is suggested. This criterion is based on the determination of the surface tension (shift) of a surfactant solution dependent upon the relative content of impurity. It need not make allowances for theoretical assumptions on the surfactants sorption kinetics and can be applied under equilibrium and nonequilibrium conditions.

Novel results on the adsorption of ionic surfactants at the air/water interface — sodium-n-alkyl sulphates

Abstract Equilibrium surface tension vs. concentration isotherms of aqueous, surface-chemically pure solutions of the homologous series of sodium n-alkyl sulphates (from C7 to C14) have been evaluated using a new surface equation of state approach. All adsorption parameters (standard free energy of adsorption, interaction parameter, saturation adsorption) and the CMC reveal a pronounced even/odd effect. Surprisingly, it was found that the cross-sectional areas of the short-chain homologues decrease with increasing chain length, reaching a minimum value for C10 and C11. However the cross-sectional areas of the longer-chain homologues (Cn ≥ C12) increase again drastically with increasing chain length. This novel effect is attributed to increasing electrostatic repulsion within the adsorption layer caused by increasing Debye lengths of the stronger surface-active homologues. The good agreement between the data for the cross-sectional area of sodium-n-dodecyl sulphate, as obtained from the new surface equation of state approach, from the Gibbs equation and from crystal structure analysis, make us suggest that it is unreasonable to apply an additional factor in the Gibbs equation for ionic amphiphiles as long as the counterion is very small in comparison with the oppositely charged amphiphilic molecule.

Ein Modell für die diffusionskontrollierte Adsorption von Tensidgemischen an fluiden Phasengrenzen

SummaryA system of integral equations and a possibility for its numerical solution for the case of the diffusion-controlled adsorption from solutions of surfactant mixtures is proposed. Using a generalized Langmuir-isotherm the process of adsorption for a solution containing two surfactants is proposed.ZusammenfassungFür den Fall der diffusionskontrollierten Adsorption von Tensidgemischen wird ein Integralgleichungssystem als Ergebnis und eine Möglichkeit für dessen numerische Lösung angegeben. Anhand von numerischen Ergebnissen für den Fall, daß eine verallgemeinerte Langmuir-Isotherme das Adsorptionsgleichgewicht beschreibt, wird der Vorgang der Gemischadsorption von zwei in der Lösung befindlichen Tensiden diskutiert.

A relation between dynamic foam stability and surface elasticity

Abstract In our previous papers, we described a method for measuring the retention time which is a measure of the foam stability. In addition, the dependence of this retention time on surface elasticity was considered. On the basis of these results a simple mathematical formula for this dependence has now been derived and is tested by a fitting procedure. The formula is not very exact, however, but it can be used to explain the main influence of the adsorption properties on the stability of a wet foam.

Untersuchungen über das Desorptionsverhalten einiger löslichern-Alkansäuren an der Wasser-Luft-Grenzfläche

ZusammenfassungDie Adsorptionsschichten der salzsauren wäßrigen Lösungen vonn-Pentan-,n-Hexan-,n-Heptan-,n-Octan- undn-Decansäure wurden nach Erreichen des Adsorptionsgleichgewichtes komprimiert. Dabei zeigte sich, daß in der überwiegenden Zahl der Fälle die komprimierte Oberfläche nach einer bestimmten Zeit einen konstanten Oberflächenspannungwert erreicht, der unterhalb des Wertes für das Adsorptionsgleichgewicht liegt. Diese Differenz kann einige dyn/cm ausmachen und hängt von dem Tensid, seiner Konzentration und dem Kompressionsverhältnis ab. Für die untersuchten Systeme werden nach verschiedenen theoretischen Modellen die Zeiten für die Einstellung des Desorptionsgleichgewichts berechnet. Spezielle experimentelle Versuche und theoretische Betrachtungen deuten auf die Anwesenheit geringer Mengen stärker grenzflächenaktiver Verunreinigungen als Ursache der Oberflächenspannungsdifferenzen. Das Problem der Charakterisierung der spezifischen Wirkung des Tensids im Zusammenhang mit seiner grenzflächenchemischen Reinheit wird diskutiert.AbstractAdsorption layers of aqueous hydrochloric solutions of normal pentanoic, hexanoic, heptanoic, octanoic and decanoic acid were compressed after having reached adsorption equilibrium. In most cases after a certain time the compressed surface layer reached a constant surface tension value which lies below the value for the adsorption equilibrium.The difference between both values can amount to some dynes/cm and depends on the kind and concentration of the surfactant and the compression ratio. The time intervals necessary for establishing desorption equlibrium for various systems have been calculated using several theoretical models. Special experiments and theoretical considerations give evidence for the presence of small amounts of strongly surface active impurities causing the surface tension discrepancies. The problem of characterizing the specific effect of a surfactant is discussed in connection with purity.

Novel method and parameters for testing and characterization of foam stability.

The paper presents novel parameters which can be used for a swift characterization of all kinds of liquid foams. The procedure of the automated method developed consists of introducing a predefined volume of gas into the test solution contained in a cylindrical glass column at constant flow rate. The levels of the foam and of the solution are recorded simultaneously in dependence on time using a photosensor system. Two novel parameters, called time of deviation and time of transition, have been derived on the basis of simultaneous measurements of the changes in the foam volume (DeltaV(F)) and the corresponding volume of the drained solution (DeltaV(S)). These parameters enable one to distinguish three different stages of the foam decay, and on their basis the foam stability can be predicted, irrespective of whether constituting an unstable (wet) or a (meta)stable (dry) foam system. The validity of the method elaborated is demonstrated by applying various unstable and stable foam systems, including biological surfactants such as sugar and lung surfactants.

Investigations on the possibility of purifying surfactant solutions by adsorption on solids

Originating from a known purification procedure for the fluid-gaseous boundary a technique for a definite purification of surfactant solution is presented using solid adsorbents. The capability of the procedure is demonstrated using various surfactants and different adsorbents. It is applicable at concentrations below as well as above the CMC.

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.

Study of the modulus of elasticity by linear compression of an adsorbed layer

Abstract It is shown that for an adsorbed monolayer, the modulus of elasticity can be obtained from linear compression of the surface. A theory has been developed using the diffusion penetration theory, in which convection due to the surface compression is neglected. The method, which is comparable with that of J. Lucassen and M. Van den Tempel (J. Colloid Interface Sci., 41 (1972) 491, and Chem. Eng. Sci., 17 (1972) 1283), is illustrated for adsorbed decanoic acid monolayers.