Dynamic surface properties

J. Lucassen, F. Hollway, and J.H. Buckingham Surface Properties of Mixed Solutions of Poly-Lysine and Sodium Dodecyl Sulfate. 2. Dynamic Surface Properties. J. Colloid Interface Sci. 67, 432 (1978). 

Prins, A., van Kalsbeek, H.K.A.I. (1998). Foaming behaviour and dynamic surface properties of liquids. Current Opinion in Colloid and Interface Science, 3, 639-642. 

Similar to the theoretical model of Palm et al. [54], based on dynamic surface properties for the prediction of drug absorption into human intestinal Caco-2 cell lines the authors used their molecular dynamics/GRID approach to correlate the absorption of the same set of six P-adrenoceptor antagonists with the coefficients obtained by the water probe at contour level -2 and -3 kcal/mol. 

Tnteractions at surfaces have long been at the center of interest in the study of surfactant monolayers and have been thought to influence both static and dynamic surface properties considerably (1,2). Although the theoretical interpretation and even the definition of surface interactions may be controversial, the experimental method has not been in doubt. Invariably, the equilibrium surface pressure vs. molar area relationship has been used as a criterion for assessing interactions in mono-layers since interactions, no matter what their precise definition, must appear in the measurable quantity of surface tension (y) or surface pressure (7r = y° — y) at a given surface concentration (r) or molar... 

It contains little information on surfactants and very little on dynamic surface properties. An excellent monograph is... 

The determination of some dynamic surface properties is treated by... 

Finally, making an emulsion or a foam is a highly complex process, involving several different, though mutually dependent, phenomena in the realm of hydrodynamics and dynamic surface properties. The quantitative relations depend on the composition of the system, the construction of the apparatus used, and the energy input level. We will not consider all these aspects but merely outline the most important principles involved. 

The situation changes for non-equilibrium systems. The dynamic surface properties of micellar solutions depend strongly on the concentration in a broad range of surface life time and/or of the frequency of surface compression and dilation. First of all this is related to the fact that the adsorption rate of surfactants increases with concentration for both sub-micellar and micellar solutions. As an example, dynamic surface tensions of SDS in 0.1 M NaCl measured by Fainerman and Lylyk [77] are shown in Fig. 7. As one can see entirely different values of the dynamic surface tension and of the adsorption can correspond to the same surface age at c > CMC. 

The kinetic model, which can explain the origin of these two relaxation processes and can describe the dependence of the corresponding relaxation times on the concentration, has been proposed by Aniansson and Wall [114, 115, 119]. This model allows to explain the main experimental facts and is generally accepted nowadays. At the same time, subsequent studies allowed for the determination of the application limits of this theory [116-118, 128]. Because the model of Aniansson and Wall is frequently used also for the analysis of dynamic surface properties of micellar solutions [93, 96-103, 133-138], it will be considered below in details. 

The region of the CMC (n (c +o c ) c l) requires special consideration. Substitution of of t2 and ti from Eq. (5.264) into (5.272), and the transition to the limit c -> 0 leads to the dynamic surface elasticity of sub-micellar solutions [165] and thus to a rather obvious conclusion if a solution contains mainly monomers, micelles do not influence the dynamic surface properties. Therefore, even for low frequencies (diffusion controlled adsorption kinetics) there is a concentration range close to the CMC where the surface elasticity is almost constant and begins to increase gradually only at further increasing concentration. Finally the surface elasticity takes values given by relations (5.275) - (5.278). This concentration dependence was observed in experiments with nonionic surfactants [95]. The oscillating barrier... 

Note that relation (5.281) does not contain any kinetic characteristics of micellisation. If the micellar concentration is low and the formation (disintegration) of micelles is sufficiently fast (tj o), the adsorption rate and, consequently, the dynamic surface elasticity depend only on the efficiency of the surfactant transfer by micelles from the bulk to the surface, and, therefore, on the diffusion coefficient of micelles and the mean aggregation number. This means that the micellar size can be determined from dynamic surface properties. Really, if approximation (5.231) for the diffusion coefficient of micelles is used, it follows from Eq. (5.281)... 

T. Maekawa, S. Kamata, M. Matsuo, The relationship between structures and dynamic surface properties of perfluoroalkyl containing polymers, J. Fluorine Chem. 54 (1) (1991) 84. 

Prins, A., Dynamic surface properties and foaming behaviour of aqueous surfactant solutions, in Foams (Akers, R.J., ed.). Academic Press, London, 1977, p45. 

There is also a difference in dynamic surface properties between methyl ester ethoxylates and alcohol ethoxylates. As shown in Fig. 12 for pure 7-mol homologs, the methyl ester ethoxylate maintains a lower surface tension than its alcohol ethoxylate counterpart as measurements become more dynamic (bubble rate of bubble tensiometer is increased). This suggests that methyl ester ethoxylate is more effective in lowering surface tension (can achieve the same surface tension reduction with a lower surfactant concentration at the interface) and/or it diffuses through aqueous solution at a faster rate. 

A thorough theoretical investigation was made of the dynamic surface properties of mixed anionic-cationic surfactant solutions [50]. This study is more general than previous studies [51], which considered both ionic species to diffuse as an electroneutral combination, because it includes cases where the anionic and cationic surfactants can diffuse to the surface at different rates and will affect the diffusion of their inorganic counterions as well. 

There is also a difference in dynamic surface properties between methyl ester ethoxylates and alcohol ethoxylates. As shown in Figure 12 for pure 7-mole homologs, the methyl ester ethoxylate maintains a lower surface tension than its al-... 

II. DYNAMIC SURFACE PROPERTIES OF FOAM FILM A. Dynamic Surface Tension... 

---