Desorption-adsorption kinetics interface

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. 

To appreciate the impact of SECM on the study of phase transfer kinetics, it is useful to briefly review the basic steps in reactions at solid/liquid interfaces. Processes of dissolution (growth) or desorption (adsorption), which are of interest herein, may be described in terms of some, or all, of the series of events shown in Figure 1. Although somewhat simplistic, this schematic identifies the essential elements in addressing the kinetics of interfacial processes. In one limit, when any of the surface processes in Figure 1 (e.g., the detachment of ions or molecules from an active site, surface diffusion of a species across the surface, or desorption) are slow compared to the mass transport step between the bulk solution and the interface, the reaction is kinetically surface-controlled. In the other limit, if the surface events are fast compared to mass transport, the overall process is in a mass transport-controlled regime. 

Further models of adsorption kinetics were discussed in the literature by many authors. These models consider a specific mechanism of molecule transfer from the subsurface to the interface, and in the case of desorption in the opposite direction ((Doss 1939, Ross 1945, Blair 1948, Hansen Wallace 1959, Baret 1968a, b, 1969, Miller Kretzschmar 1980, Adamczyk 1987, Ravera et al. 1994). If only the transfer mechanism is assumed to be the rate limiting process these models are called kinetic-controlled. More advanced models consider the transport by diffusion in the bulk and the transfer of molecules from the solute to the adsorbed state and vice versa. Such mixed adsorption models are ceilled diffusion-kinetic-controlled The mostly advanced transfer models, combined with a diffusional transport in the bulk, were derived by Baret (1969). These dififiision-kinetic controlled adsorption models combine Eq. (4.1) with a transfer mechanism of any kind. Probably the most frequently used transfer mechanism is the rate equation of the Langmuir mechanism, which reads in its general form (cf. Section 2.5.),... 

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. 

It is necessary to develop the theory of DAL for extending liquid interlayers. When the t.p.c. line moves, a transfer of surfactant fi-om the liquid/gas to the solid/liquid interface and vice versa is possible. Thus, there are changes in the interfacial energy and surface tension of the liquid in the region of the moving liquid meniscus which depend on the diffiision rate of surfactant molecules (Schulze 1992). Consequently, the movement of the liquid meniscus can also depend on the kinetics of the surfactant desorption-adsorption. Some additional remarks will be given in Section 12. 

Bockris Reddy (1970) describes the Butler-Volmer-equation as the "central equation of electrode kinetics . In equilibrium the adsorption and desorption fluxes of charges at the interface are equal. There are common principles for the kinetics of charge exchange at the polarisable mercury/water interface and the adsorption kinetics of charged surfactants at the liquid/fluid interface. Theoretical considerations about the electrostatic retardation for the adsorption kinetics of ions were first introduced by Dukhin et al. (1973). 

As mentioned above, the adsorption kinetics for a kinetic-controlled mechanism is given by the balance of surfactant adsorption and desorption fluxes to and from the interface and for the Langmuir kinetics this balance has the form of Eq. (4.15). The rate constants kad and kdes are functions of the activation energies adsorption and desorption and can be specified on the basis of the molecular kinetic [9, 120] or transition state theory [121]. Eq. (4.15) was applied to adsorption kinetics data of surfactants at the water/air interface by many authors, for example in [24, 39, 83, 97, 122, 123, 124, 125, 126, 127]. In these works, it was shown that the values of kad and kdes are not constant hut depend on the surfactant bulk, the degree of adsorption layer saturation, or its lifetime. To obtain better correspondence with the experimental data, some authors had assumed that the adsorption and desorption activation energies depend on the degree of adsorption layer saturation. These rather complicated kinetic equation are more or less empirical, although they transforms into a valid adsorption isotherm at equilibrium... 

Most surfactants adsorb diffusion controlled at liquid interfaces. It was discussed above that exceptions observed in the literature and interpreted in terms of adsorption and desorption barriers have been understood later by the pure diffusion model when the respective experimental conditions were considered properly. One of the most important points in this respect was the systematic analysis of impurity effects on the adsorption kinetics of surfactants. This point was for example discussed in detail in the book by Dukhin et al. [2]. Another reason for the observation of an adsorption process slower than expected from diffusion is the... 

In general one can say that the thermodynamic description of an adsorption layer at a liquid interface provides the basis for the dynamic and mechanical understanding. As it is the final state of a process, it controls also the mechanism of its formation, the adsorption kinetics (sf. Fig. 1). The response to small or large deformations of a hquid interface is governed by the adsorption mechanism and hence the thermodynamic characteristics. After a compression, the surface concentration F reaches values larger than the respective equihbrium adsorption Fq and a desorption process sets in. Both, adsorption and desorption induced by interfacial perturbations are processes governed by the thermodynamic and kinetic characteristics. Thus, the surface rheological behaviour seems to be most sensitive to the specificity of adsorbed surfactants. 

Most theories for polymer adsorption kinetics start from (combinations of) the models discussed earlier. Other theories, often proposed for (bio)polymer adsorption, are based on the random sequential adsorption (RSA) model. According to this model, the adsorbate molecules arrive randomly at the interface and they stick where they hit. It implies that both desorption and tangential motion of the adsorbate at the interface are absent. Because the center of a newly arriving spherical molecule cannot be accommodated within the shaded areas enclosed by the dashed circles shown in Figure 15.6, only the unshaded fraction < ) of the surface is available for adsorption. It is obvious that ( ) is a function of 0, the fraction of the surface that is covered by the adsorbate. For sphere-like molecules, 0 = niUi (R being the radius of the molecule). The following expressions for the available surface function < )(0) can be derived from the RSA theory ... 

Abstract Spread monolayers of /(-lactoglobulin and bovine serum albumin and adsorbed films of lysozyme and /i-lactoglobulin were studied at the oil (n-tetradecane) -water (O-W) and air-water (A-W) interfaces. In general, spread monolayers were more expanded at the O-W interface than at the A-W interface. Desorption rates from monolayers increased greatly with increasing interfacial pressure, n, but were still quite low until typical equilibrium adsorption tc were exceeded. Desorption rates as a function of the energy barrier to desorption were similar at both types of interface. Adsorption kinetics of lysozyme at the A-W interface were... 

The above experiments on monolayers illustrate the strong dependence of desorption rates on n. In real systems stabilised by proteins, n for the film on average does not exceed a particular maximum value at which the rate of adsorption from solution is balanced by the rate of desorption. On perturbation from the equilibrium state of the film, such as a transient (local) expansion or compression a knowledge of both rates is important. Unfortunately, measurements of adsorption rates are not so straightforward since the surface concentration of protein, r, must be monitored with time and is not predetermined as in the spread monolayers. There is often disagreement between adsorption kinetic results obtained via different techniques - see below, for example. Relatively few measurements have been made of the adsorption kinetics of S-lactoglobulin at the A-W interface and for all proteins, because of experimental difficulties, there seem to be almost no direct measurements of r t) at O-W interfaces. 

Protein adsorption at sohd-hquid interfaces, in general, is characterized by a monolayer or a submonolayer surface coverage [33-41], and pure desorption, in a strict sense, constitutes a very unlikely event [11,16]. Because, as pointed out earher, it was found that k has a little control on the adsorption kinetics, in particular at low 6, the special cases [Eq. (33) and (36)] of the general kinetic model [Eq. (23)] will be considered for prediction of the experiment-based kinetic data. 

Until recently, the fast rate at which a surfactant layer forms at the solid-liquid interface has prevented accurate investigation of the adsorption process. As a result, the mechanism of surfactant adsorption has been inferred from thermodynamic data. Such explanations have been further confused by misinterpretation of the equilibrium morphology of the adsorbed surfactant as either monolayers or bilayers, rather than the discrete surface aggregates that form in many surfactant-substrate systems.2 However, the recent development of techniques with high temporal resolution has made possible studies of the adsorption, desorption,25>38,4i,48-6o exchange rates of surfactants. In this section, we describe the adsorption kinetics of C ,TAB surfactants at the silica-aqueous solution interface, elucidated by optical reflectometry in a wall-jet flow cell. The adsorption of C jTAB surfactants to silica is the most widely studied system - and hence the adsorption kinetics can be related to the adsorption process with great clarity. For a more thorough review of adsorptions isotherms, the t5q)es of surfactant structures that form at the solid-liquid interface, and the influence of these factors on adsorption, the reader is directed to Reference 24. 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

The role of various surfactant association structures such as micelles and lyotropic liquid crystals (372), adsorption-desorption kinetics at liquid-gas interfaces (373) and interfacial rheology (373) and capillary pressure (374) on foam lamellae stability has been studied. Microvisual studies in model porous media indicate... 

If the supply of surfactant to and from the interface is very fast compared to surface convection, then adsorption equilibrium is attained along the entire bubble. In this case the bubble achieves a constant surface tension, and the formal results of Bretherton apply, only now for a bubble with an equilibrium surface excess concentration of surfactant. The net mass-transfer rate of surfactant to the interface is controlled by the slower of the adsorption-desorption kinetics and the diffusion of surfactant from the bulk solution. The characteris-... 

The finite kinetics of the adsorption/desorption steps at the interface have been extensively studied by Hudson and Morel [13,15]. A wealth of literature is available on dealing with such interfacial processes [94-96] and its inclusion in the biouptake model should be implemented when experimental evidence of its necessity arises. 

Interface and colloid science has a very wide scope and depends on many branches of the physical sciences, including thermodynamics, kinetics, electrolyte and electrochemistry, and solid state chemistry. Throughout, this book explores one fundamental mechanism, the interaction of solutes with solid surfaces (adsorption and desorption). This interaction is characterized in terms of the chemical and physical properties of water, the solute, and the sorbent. Two basic processes in the reaction of solutes with natural surfaces are 1) the formation of coordinative bonds (surface complexation), and 2) hydrophobic adsorption, driven by the incompatibility of the nonpolar compounds with water (and not by the attraction of the compounds to the particulate surface). Both processes need to be understood to explain many processes in natural systems and to derive rate laws for geochemical processes. 

Due to the fast kinetics of adsorption/desorption reactions of inorganic ions at the oxide/aqueous interface, few mechanistic studies have been completed that allow a description of the elementary processes occurring (half lives < 1 sec). Over the past five years, relaxation techniques have been utilized in studying fast reactions taking place at electrified interfaces (1-7). In this paper we illustrate the type of information that can be obtained by the pressure-jump method, using as an example a study of Pb2+ adsorption/desorption at the goethite/water interface. 

Having chosen a particular model for the electrical properties of the interface, e.g., the TIM, it is necessary to incorporate the same model into the kinetic analysis. Just as electrical double layer (EDL) properties influence equilibrium partitioning between solid and liquid phases, they can also be expected to affect the rates of elementary reaction steps. An illustration of the effect of the EDL on adsorption/desorption reaction steps is shown schematically in Figure 7. In the case of lead ion adsorption onto a positively charged surface, the rate of adsorption is diminished and the rate of desorption enhanced relative to the case where there are no EDL effects. 

Adsorption-Desorption Kinetics at the Metal-Oxide-Solution Interface Studied by Relaxation Methods... 

Chemical relaxation methods can be used to determine mechanisms of reactions of ions at the mineral/water interface. In this paper, a review of chemical relaxation studies of adsorption/desorption kinetics of inorganic ions at the metal oxide/aqueous interface is presented. Plausible mechanisms based on the triple layer surface complexation model are discussed. Relaxation kinetic studies of the intercalation/ deintercalation of organic and inorganic ions in layered, cage-structured, and channel-structured minerals are also reviewed. In the intercalation studies, plausible mechanisms based on ion-exchange and adsorption/desorption reactions are presented steric and chemical properties of the solute and interlayered compounds are shown to influence the reaction rates. We also discuss the elementary reaction steps which are important in the stereoselective and reactive properties of interlayered compounds. 

The fast reactions of ions between aqueous and mineral phases have been studied extensively in a variety of fields including colloidal chemistry, geochemistry, environmental engineering, soil science, and catalysis (1-6). Various experimental approaches and techniques have been utilized to address the questions of interest in any given field as this volume exemplifies. Recently, chemical relaxation techniques have been applied to study the kinetics of interaction of ions with minerals in aqueous suspension (2). These methods allow mechanistic information to be obtained for elementary processes which occur rapidly, e.g., for processes which occur within seconds to as fast as nanoseconds (j0. Many important phenomena can be studied including adsorption/desorption reactions of ions at electri fied interfaces and intercalation/deintercalation of ions with minerals having unique interlayer structure. 

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