Dynamic adsorption of surfactants

T. Austad, S. Ekrann, I. Fjelde, and K. Taugbol. Chemical flooding of oil reservoirs Pt 9 Dynamic adsorption of surfactant onto sandstone cores from injection water with and without polymer present. Colloids Surfaces, Sect A, 127(l-3) 69-82, 1997. 

A. 1 Equilibrium and Dynamic Adsorption of Surfactants at the Air-Water Interface.523... 

A.1 EQUILIBRIUM AND DYNAMIC ADSORPTION OF SURFACTANTS AT THE AIR-WATER INTERFACE... 

Trogus, F., Sophany, T., Schechter, R.S., Wade, W.H. "Static and Dynamic Adsorption of Anionic and Nontonic Surfactants," SPE paper 6004, 1976 SPE Annual Technical Conference and Exhibition, New Orleans, October 3-6. 

Adsorption of surfactant on reservoir rock can be determined by static tests (batch equilibrium tests on crushed core grains) and dynamic tests (core flood) in the laboratory. The units of surfactant adsorption in the laboratory can be mass of surfactant adsorbed per unit mass of rock (mg/g rock), mass per unit pore volume (mg/mL PV), moles per unit surface area (peq/m ), and moles per unit mass of rock (peq/g rock). The units used in field applications could be volume of surfactant adsorbed per unit pore volume (mL/mL PV) or mass per unit pore volume (mg/mL PV). Some unit conversions follow ... 

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. 

This section is dedicated to some selected examples of the very broad spectrum of surface phenomena directly influenced by the dynamics of adsorption of surfactants. There are books and monographs which show these and other examples in much more detail (Adamson 1990, Dorfler 1994, Hunter 1992, Ivanov 1988, Krugljakov Exerowa 1990, Lyklema 1991, Schulze 1984). However, to make the reader acquainted with some of the huge variety of applications, we inserted this section into the book. 

This book deals mainly with dynamic properties of amphiphiles at liquid/air and liquid/liquid interfaces rather than at solid/liquid interfaces. However static and dynamic contact angles are discussed in Appendix 3B as these phenomena are determined by the kinetics of adsorption of surfactants also at the fluid interface. Some specific aspects of lateral transport phenomena studied by many authors are briefly review in Appendix 3D. 

The aim of this chapter is to present the fundamentals of adsorption at liquid interfaces and a selection of techniques, for their experimental investigation. The chapter will summarise the theoretical models that describe the dynamics of adsorption of surfactants, surfactant mixtures, polymers and polymer/surfactant mixtures. Besides analytical solutions, which are in part very complex and difficult to apply, approximate and asymptotic solutions are given and their range of application is demonstrated. For methods like the dynamic drop volume method, the maximum bubble pressure method, and harmonic or transient relaxation methods, specific initial and boundary conditions have to be considered in the theories. The chapter will end with the description of the background of several experimental technique and the discussion of data obtained with different methods. 

Later in Chapter 6 a large variety of technologies based on adsorption effects will be described. It will also be shown that in general, these technologies work under dynamic conditions and an improvement of the surfactant s efficiency, made in the by past trial and error or thanks personal experience, is now more and more based on a systematic analysis of the entire technology and the particular impact of the surfactants used. The optimisation of surfactants and their mixtures requires specific knowledge of their dynamic adsorption behaviour [1]. The most frequently used parameter to characterise the dynamic properties of liquid adsorption layers is the dynamic interfacial tension. Many techniques exist to measure dynamic tensions of liquid interfaces having different time windows from milliseconds to hours and days. As direct measurements of the time process of adsorption of surfactants at liquid interfaces are rare... 

The aim of this chapter is to present the fundamentals of adsorption kinetics of surfactants at liquid interfaces. Theoretical models will be summarised to describe the process of adsorption of surfactants and surfactant mixtures. As analytical solutions are either scarcely available or very complex and difficult to apply, also approximate and asymptotic solutions are given and their ranges of application demonstrated. For particular experimental methods specific initial and boundary conditions have to be considered in these theories. In particular for relaxation theories the experimental conditions have to be met in order to quantitatively understand the obtained results. In respect to micellar solutions and the impact of micelles on the adsorption layer dynamics a detailed description on the theoretical basis as well as a selection of representative experiments will follow in Chapter 5. 

These experiments show that it is possible to achieve positive results using EOR after a thorough investigation of the nature of mineral rock constituents of the oil reservoir and the choice of the surfactant delivery method. The dynamic interfacial tension is crucial in EOR. Using a model acidic oil, alkali solutions and surfactants at an optimum ratio, ionised water and surfactant adsorb simultaneous onto the interface, resulting in low dynamic interfacial tension [229]. Combined adsorption of surfactant (alkyl propoxyethoxy sulphate) and polymer (xanthan) was studied in [230]. 

The third paper in this subject that we were able to retrieve is due to Biswas et al. [145]. In their introduction to the paper they said that dynamic and mechanistic aspects of adsorption of surfactants at the solid-liquid interface, particularly silica surface, were rare and quoted six papers. The most recent among them was due to Tiberg [146] in 1996. Adsorption kinetics was studied by Biswas et al. [145] using classical batch experiments. They found that the adsorption follows a two-step first-order rate equation. From the calculated rate constants they obtained the activation energies and entropies concluding that both processes are entropy controlled. 

The adsorption of surfactants at interfaces is a time process. After the creation of a new surface the adsorption is zero and increases with time until reaching the equilibrium state. The main mechanism controlling this process is the diffusion of surfactants in the solution bulk. In this lesson the basic models will be discussed and the main physical parameters analysed. In particular, the type of adsorption isotherm plays an important role. On the basis of dynamic surface tensions the application of the theoretical models will be demonstrated in the subsequent paragraph. Besides complete solutions of the diffusion model, also approximate solutions exist. These models... 

Figure 15. Dynamic adsorption of ethoxylated surfactant mixture onto Berea vs. time. At 250 PV 0.4 wt% PEG-4000 was added (T = 80 °C brine = synthetic seawater pH = 6.9-7.1). (Reproduced with permission from reference 62, copyright 1992 Elsevier.)...

The role of the surfactant as the essential ingredient in the formation and persistence of an emulsion is associated with several nonequilibrium proces.ses. When the drops are formed by effect of mechanical shearing the presence of surfactant reduces the interfaciai tension, so that breaking is favored. On the other hand, the adsorption of surfactant onto the freshly produced interface results in a repulsion that prevents the iiuiiiediaie coalescence of neighboring Jioplets. Thus, the surfactant plays a role in both the breaking and coalescence steps, i.e.. the dynamic balance that determines the drop size. 

The equilibrium and dynamic aspects of surface tension and adsorption of surfactants at the air-water interface are important factors in foam film stability [82]. Dynamic adsorption models with the diffusion-controlled and mixed-kinetic mechanisms are discussed in some surfactant solution litera-... 

For two other capillaries r = 5.2 and 5.5 pm) close values, 10 and 12.4 mN/m, were obtained. Because of adsorption of surfactant on oil-free hydrophobic surface, the interface tension becomes higher than yo- Therefore, the dynamic retreating angle of silieon oil may range from 0R = Oatyi2 lO mN/m to 0R =75°, when the meniseus surface beeomes free from surfaetant and yi2 is equal to 41 mN/m, as in the bulk water-oil interface. 

Flow of surfaetant solutions is drastically influenced by adsorption of surfactant on capillary surface. This results in a change of surface or interface tension of moving menisci. In this coimection, investigation of physicochemical mechanisms of dynamic tension and dynamic contact angles was possible only when the solution is previously equilibrated with the capillary. 

Figure 6.12 illustrates the various processes that occur during emulsification Break up of droplets, adsorption of surfactants and droplet collision (which may or may not lead to coalescence) [8]. Each of these processes occurs numerous times during emulsification and the time scale of each process is very short, typically a microsecond. This shows that the emulsification is a dynamic process and events that occur in a microsecond range could be very important. 

The value of the critical micelle concentration (CMC) is an important parameter in a wide variety of industrial applications involving adsorption of surfactant molecules at interfaces, such as foams, froths, emulsions, suspensions, and surface coatings. It is probably the simplest means of characterizing the colloid and surface behaviour of a surfactant solute, which in turn determines its industrial usefulness. Many industrial processes are also dynamic processes in that they involve a rapid increase in interfacial area, such as foaming, wetting, emulsification and solubilization. First, the available monomers adsorb on to the freshly created interface. Then, additional monomers must be provided by the breakup of micelles. Especially when the free monomer concentration (i.e. CMC) is low, the micellar breakup time or diffusion of monomers to the newly created interface can be rate-limiting steps in the supply of monomers, which is the case for many nonionic surfactant solutions (3). 

Several interfacial aspects must be considered when dealing with agrochemical formulations (i) Both equilibrium and dynamic aspects of adsorption of surfactants at the air/liquid interface. These aspects determine spray formation (spray droplet spectrum), impaction and adhesion of droplets on leaf surfaces as well as the various wetting and spreading phenomena, (ii) Adsorption of surfactants at the oil/water interface which determines emulsion formation and their stability. This subject is also important when dealing with microemulsions, (ill) Adsorption of surfactants and polymers at the solid/liquid interface. This is important when dealing with dispersion of agrochemical powders in liquids, preparation of suspension concentrates and their stabilization. 

This section will deal with the above interfacial aspects starting with the equilibrium aspects of surfactant adsorption at the air/water and oil/water interfaces. Due to the equilibrium aspects of adsorption (rate of adsorption is equal to the rate of desorption) one can apply the second law of thermodynamics as analyzed by Gibbs (see below). This is followed by a section on dynamic aspects of surfactant adsorption, particularly the concept of dynamic surface tension and the techniques that can be applied in its measurement. The adsorption of surfactants both on hydrophobic surfaces (which represent the case of most agrochemical solids) as well as on hydrophilic surfaces (such as oxides) will be analyzed using the Langmuir adsorption isotherms. The structure of surfactant layers on solid surfaces will be described. The subject of polymeric surfactant adsorption will be dealt with separately due to its complex nature, namely irreversibility of adsorption and conformation of the polymer at the solid/liquid interface. 

As mentioned above, the most important adjuvants are surface active agents of the anionic, nonionic or zwitterionic type. In some cases polymers are added as stickers or antidrift agents. The production of spray droplets (from a spray nozzle) is determined by the adsorption of surfactants under dynamic conditions (with time in the region of 1 ms). The droplet adhesion to the target surface and its wetting and spreading is also determined by the dynamic contact angle which is also determined by the rate of surfactant adsorption to the surfeice. Above the critical micelle concentration (cmc), the supply of monomers is determined by the relaxation time of micelle formation and its breakdown. The dynamics of surfactant adsorption is determined by the monomer concentration and the diffusion coefftcient of the surfactant molecules to the interface. 

The book first discusses. self-assembling processes taking place in aqueous surfactant solutions and the dynamic character of surfactant self-assemblies. The next chapter reviews methods that permit the. study of the dynamics of self-assemblies. The dynamics of micelles of surfactants and block copolymers,. solubilized systems, microemulsions, vesicles, and lyotropic liquid crystals/mesophases are reviewed. successively. The authors point out the similarities and differences in the behavior of the.se different self-as.semblies. Much emphasis is put on the processes of surfactant exchange and of micelle formation/breakdown that determine the surfactant residence time in micelles, and the micelle lifetime. The la.st three chapters cover topics for which the dynamics of. surfactant self-assemblies can be important for a better understanding of observed behaviors dynamics of surfactant adsorption on surfaces, rheology of viscoelastic surfactant solutions, and kinetics of chemical reactions performed in surfactant self-assemblies used as microreactors. 

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