Kinetics of surfactant adsorption

D. O. Johnson, and K. J. Stebe, Oscillating bubble tensiometry A method for precisely measuring the kinetics of surfactant adsorptive-desorptive exchange. J. Colloid Int. Sci. 168 526-538 ( 994). 

Kinetics of Surfactant Adsorption in a Transient Foam Body... 

Thermodynamics of Surfactant Adsorption Kinetics of Surfactant Adsorption Dynamic Surface Tension of Solutions Drop and Bubble Shape Experiments Adsorption Behaviour of Mixed Systems... 

Abstract We review a new theoretical approach to the kinetics of surfactant adsorption at fluid-fluid interfaces. It yields a more complete description of the kinetics both in the aqueous solution and at the interface, deriving all equations from a free-energy functional. It also provides a general method to calculate dynamic surface tensions. For non-ionic surfactants, the results coincide with previous models. Non-ionic surfactants are shown to usually undergo diffusion-limited adsorption, in agreement with the experiments. Strong electrostatic interactions in salt-free ionic surfactant solutions are found to... 

The kinetics of surfactant adsorption is a fundamental problem of interfacial science playing a key role in various processes and phenomena, such as wetting, foaming and stabilization of liquid films. Since the pioneering theoretical work of Ward and Tordai in the 1940s [1], it has been the object of thorough experimental and theoretical research [2]. 

As already mentioned, the surfactants are used to stabilize the liquid films in foams, in emulsions, on solid surfaces, and so forth. We will first consider the equilibrium and kinetic properties of surfactant adsorption monolayers. Various two-dimensional equations of state are discussed. The kinetics of surfactant adsorption is described in the cases of dijfusion and barrier control. Special attention is paid to the process of adsorption from ionic surfactant solutions. Theoretical models of the adsorption from micellar surfactant solutions are also presented. The rheological properties of the surfactant adsorption mono-layers, such as dilatational and shear surface viscosity and suiface elasticity, are introduced. The specificity of the proteins as high-molecular-weight surfactants is also discussed. 

As mentioned earlier, below we focus om attention on the kinetics of surfactant adsorption. First, we introduce the basic equations. Next, we consider the two alternative cases of surfactant adsorption under diffusion and barrier control. Special attention is paid to the adsorption of ionic surfactants, whose molecules are involved in long-range electrostatic interactions. Finally, we consider the adsorption from micellar surfactant solutions, which is accompanied by micelle diffusion, assembly, or disintegration. 

At concentrations above the critical micellization concentration (CMC), the kinetics of surfactant adsorption is strongly affected (accelerated) by the presence of micelles. In general, the micelles exist in equilibrium with the surfactant monomers in the bulk of solution (Fig. 5). A dilatation of the surfactant adsorption monolayer leads to a transfer of monomers from the subsurface to the surface, which causes a transient decrease of the subsurface concentration of monomers. The latter is compensated by disintegration of part of the micelles in the subsurface layer (Fig. 5). These processes are accompanied by diffusion transport of monomers and micelles driven by the concentration gradients. 

The kinetics of surfactant adsorption mainly depend on two factors the diffusion coefficient and the concentration of the surfactant or lipid. It is worth mentioning, that the interfacial tension was already significantly lowered at the starting point of the measurement. The interfacial tension experiments began 10 s after drop formation (Fig. 3). Compared to 53 mNfin, which represents the interfacial tension of the pure isooctane/water interface, the adsorption for the Span 80 concentrations of 1 and 10 mmol/1 was almost complete. These results demonstrated that it was necessary to work at high surfactant concentrations in order to form stable monolayers at short time scales. The interfacial tension of Brij 72, for example, decreased much slower. This might be the main reason for the lower stability of... 

For practical purposes, it is not only important to know how much of a fluorinated surfactant is needed to reduce surface tension to a desired value. The time required to decrease surface tension is also highly significant. Many industrial processes do not allow sufficient time for a surfactant to attain equilibrium and depend on the kinetics of surfactant adsorption. 

The kinetics of surfactant adsorption depend on the surfactant structure. Fluorination of the hydrophobe increases the rate (dyldt) of surface tension decrease, but the time needed to attain equilibrium may not be affected considerably. The surface tensions of sodium perfluorooctanesulfonate and its hydrocarbon-... 

It is based on equilibrium properties and is directly related to the Gibbs elasticity (17.). In the present context a gauges how strongly the surface tension depends on the surfactant distribution along the bubble interface. Second, captures the kinetics of the adsorption process and is defined by... 

The adsorption and desorption kinetics of surfactants, such as food emulsifiers, can be measured by the stress relaxation method [4]. In this, a "clean" interface, devoid of surfactants, is first formed by rapidly expanding a new drop to the desired size and, then, this size is maintained and the capillary pressure is monitored. Figure 2 shows experimental relaxation data for a dodecane/ aq. Brij 58 surfactant solution interface, at a concentration below the CMC. An initial rapid relaxation process is followed by a slower relaxation prior to achieving the equilibrium IFT. Initially, the IFT is high, - close to the IFT between the pure solvents. Then, the tension decreases because surfactants diffuse to the interface and adsorb, eventually reaching the equilibrium value. The data provide key information about the diffusion and adsorption kinetics of the surfactants, such as emulsifiers or proteins. 

While microemulsions are thermodynamically stable, and the stability of emulsions has a kinetic origin, in both cases the adsorption of the dispersant upon the interface of the globules is responsible for stability. For this reason it appears natural to attempt to explain the above equality between the two inversion temperatures on the basis of surfactant adsorption. In addition, both the micro and macro-emulsions obey in many cases the Bancroft rule [8,9], which indicates that the phase in which a larger amount of dispersant is present becomes the continuous phase there are, however, some violations of this rule which will be discussed later in the paper. 

The special properties of thin liquid films, in particular of foam films, involve studying various colloid-chemical aspects, such as kinetics of thinning and rupture of films, transition from CBF to NBF, isotherms of disjoining pressure, thermodynamic (equilibrium) properties, determination of the electrical parameters of surfactant adsorption layer at the liquid/gas... 

Vogler 31) developed a mathematical model to derive semiquantitative kinetic parameters interpreted in terms of transport and adsorption of surfactants at the interface. The model was fitted to experimental time-dependent interfacial tension, and empirical models of concentration-de-pendent interfacial tension were compared to theoretical expressions for time-dependent surfactant concentration. Adamczyk (32) theoretically related the mechanical properties of the interface to the adsorption kinetics of surfactants by introducing the compositional surface elasticity, which was defined as the proportionality coefficient between arbitrary surface deformations and the resulting surface concentrations. Although the expressions to describe the adsorption process differed from one another, it was demonstrated that the time-dependent interfacial tensions mirrored the change of surface-active substances at the interface. 

G. Kretzschmar, R. Miller, Dynamic Properties of Adsorption Layers of Amphiphilic Substances at Fluid Interfaces, Adv. Colloid Interface Set 36 (1991) 66-124. (Review, 227 refs. Background reading of sec. 4.5. This review is not very different from R. Miller, G. Kretzschmar, Adsorption Kinetics of Surfactants at Fluid Interfaces, Adv. Colloid Interface ScL 37 (1991) 97-121.)... 

The overall rate of surfactant adsorption is controlled by the slowest stage. If it is stage (1), we deal with diffusion control, whereas if stage (2) is slower, the adsorption occurs under barrier (kinetic) control. The next four subsections are dedicated to processes under diffusion control (which are the most frequently observed), whereas in Section 5.2.2.5 we consider adsorption under barrier control. 

Rate of Surfactant Adsorption for Different Kinetic Models... 

We must point out that if the adsorption layer contacts with a sufficiently deep liquid, then the diffusion relaxation time can be comparable with the adsorption relaxation time. In this case, the kinetics of the adsorption layer filling, which is determined by Eqs. (7.3.3) and (7.3.4), can be diffusion-controllable for small volume concentrations of surfactants in the solution or be governed by a diffusion-kinetic mechanism for higher concentrations [274]. A pure kinetic region of the adsorption layer filling is possible only in thin layers of surfactant solutions, for example, in liquid elements of foam structures. 

Theoretical modelling of surfactant adsorption uses as a starting point the theoretical isotherms derived from statistical and kinetic data for the gas-solid interface. The most common model is the one based on the Langmuir adsorption isotherm (see Chapter 11,2) ... 

Since this book is dedicated to the dynamic properties of surfactant adsorption layers it would be useful to give a overview of their typical properties. Subsequent chapters will give a more detailed description of the structure of a surfactant adsorption layer and its formation, models and experiments of adsorption kinetics, the composition of the electrical double layer, and the effect of dynamic adsorption layers on different flow processes. We will show that the kinetics of adsorption/desorption is not only determined by the diffusion law, but in selected cases also by other mechanisms, electrostatic repulsion for example. This mechanism has been studied intensively by Dukhin (1980). Moreover, electrostatic retardation can effect hydrodynamic retardation of systems with moving bubbles and droplets carrying adsorption layers (Dukhin 1993). Before starting with the theoretical foundation of the complicated relationships of nonequilibrium adsorption layers, this introduction presents only the basic principles of the chemistry of surfactants and their actions on the properties of adsorption layers. 

The adsorption kinetics of surfactant molecules to a liquid interface is controlled by transport processes in the bulk and the transfer of molecules from a solution state into an adsorbed state or vice versa. In this paragraph qualitative and quantitative models are discussed. 

The indices "diff and "kin" refer to the diffusional and transfer steps, respectively. Recently, theoretical models of surfactant adsorption kinetics were developed mainly to take into account specific experimental conditions or surfactant properties, such as molecular charge (Dukhin et al. 1983, 1991, Borwankar Wasan 1986, Chang Franses, 1992, Miller et al. 1994a), micelle formation (Rakita Fainerman 1989, Dushkin Ivanov 1991, Fainerman 1992, Serrien et al 1992) or other specific effects (Lin et al. 1991, Ravera et al. 1993, 1994, Jiang et al. 1993). Some of these models will be discussed in sections or chapters below. 

Adsorption Kinetics of Surfactants at Liquid/Liquid Interfaces... 

Qualitative and quantitative models of adsorption kinetics of surfactants and polymers are described in this chapter. A comprehensive presentation of the most developed physical model, the difRision-controlled adsorption and the desorption model, is given and different methods of solving the resulting differential equations are discussed (Miller Kretzschmar 1991). A direct numerical integration enables us to consider any type of adsorption isotherm relating the surfactant bulk concentration with the adsorbed amount at the interface. 

Electrostatic retardation of adsorption kinetics of surfactant anions is expected in the case... 

It follows from (7.26) that at small Stem potentials at the bubble surface, the ratio K(vj7s,) / 6 is a quantity of the order of (1/(k8d), i.e., it is much less than unity. The new effect, the electrostatic retardation of kinetics of surfactant anions adsorption becomes visible when the dimensionless parameter exp(-v /sj) equals or exceeds (kSq). ... 

The rate of the exchange process of surfactant molecules between the surface of a bubble (drop) and the bulk solution is determined not only by convective diffusion but in the general case also by the kinetics of the adsorption step itself Details of the physical model of the adsorption process are given in chapter 2 and 4. A method which takes into account the effect of adsorption kinetics on the formation of the dynamic adsorption layer was developed by Levich (1962). Using this method, attempts were made to generalize the theory of the dynamic adsorption layer of bouyant bubbles (Dukhin 1965). 

The aim of this section is to consider the dynamic adsorption layer structure of ionic surfactant on the surface of rising bubbles. Results obtained in the previous section cannot be transferred directly to this case. The theory describing dynamic adsorption layers of ionic surfactant in general should take into accoimt the effect of electrostatic retardation of the adsorption kinetics of surfactant ions (Chapter 7). The structure of the dynamic adsorption layer of nonionic surfactants was analysed in the precedings section in the case when the adsorption process is kinetic controlled. In this case, it was assumed that the kinetic coefficients of adsorption and desorption do not depend on the surface coverage. On the other hand, the electrostatic barrier strongly depends on F , and therefore, the results of Section 9.1. cannot be used for the present case.. 

Now the condition under which, electrostatic retardation of adsorption kinetics of surfactant anions appears at the main part of the siuface, is determined. Substituting the estimate for T from Eq. (9.34) into (7.26) and (7.29) yields the following condition. 

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