Desorption, dynamic processes

The SHG and SFG teclmiques are also suitable for studying dynamical processes occurring on slower time scales. Indeed, many valuable studies of adsorption, desorption, difhision and other surface processes have been perfomied on time scales of milliseconds to seconds. 

Most of the sorption/desorption transformation processes of various solid phases are time-dependent. To understand the dynamic interactions of organic pollutants with solid phases and to predict their fate with time, knowledge of the kinetics of these processes is important [20,23]. 

Contrary to carbon-black-filled conventional rubbers, which form a semi-rigid interface at the carbon black surface, PDMS chain units at the silica surface are not rigidly linked to the silica surface. Two types of dynamic processes are thought to occur at the interface relatively fast anisotropic reorientation of chain units in the interfacial layer and slow adsorption-desorption of chain units (Figure 10.13) [108, 113]. 

As lithogenic and biogenic particles are delivered to the coastal zone (via rivers and estuaries), they are exposed to many dynamic processes (e.g., aggregation, flocculation, desorption) that result in steep biogeochemical gradients on the continental shelf. 

Several theoretical studies have been carried out on a phenomenological basis to justify the idea that a dynamic slip can indeed occur [43-45]. New experimental techniques and theoretical treatments must be developed to reveal the molecular origin of dynamic slip and characteristic time scales governing the dynamic processes in the melt/wall interfacial region. Our own experiments on polyethylenes [27-29] have persistently indicated that slip, either through chain desorption on weakly interacting surfaces or interfacial chain disentanglement... 

Gas adsorption is a veiy dynamic process in the sense that the rates of adsorption and desorption are very high, even at low pressures. Contamination of pristine surfaces after exposure to gases is also very rapid. 

Among five types of species formed by CO2 adsorption on MgO, dynamic behavior of unidentate-type carbonate (species-1) upon desorption was investigated. Surface diffusion, O-exchange, and molecular substitution appreciably take place prior its desorption even below the room temperature. Since these dynamic processes necessitate bond rupture in species, MgO may be useful in CO2 conversion at low temperatures. 

In the case of desorption the processes have the opposite direction.) Such interfacial expansions are typical for foam generation and emulsification. The rate of adsorption relaxation determines whether or not the formed bubbles/drops will coalesce upon collision and, in final reckoning how large the foam volume and the emulsion drop-size will be. - Below, we focus on the relaxation time of surface tension, X , which characterizes the interfacial dynamics. 

At least for simple molecular systems, there is by now strong evidence that at low laser fluences, a thermal desorption/evaporation process operates, distinctly different from the unselective volume material ejection that is characteristic of ablation [34-35]. Specifically, both molecular dynamics simulations [34] and experimental work [35] strongly suggest that a fluence range can be delineated in which the ejection signals of compounds incorporated in a substrate correlate with their binding energy to the substrate (i.e.,... 

Direct etching of polymers in vacuum with SR which was mainly x-rays between 0.2 and 3 nm has been studied. This is a process involving only photons, like laser ablation, but it is different from laser ablation both in desorption dynamics and dissociation mechanisms. 

Langmuir was the first to provide of a theory of the interrelation between thermodynamics and macro-kinetics. It introduces the balance of adsorption and desorption fluxes into the adsorption theory and defines the equality as the equilibrium state of the adsorption layer. A disturbance of the adsorption equilibrium leads to a net adsorption or net desorption flux. This idea serves as a bridge from thermodynamics to macro-kinetics and allows a deeper understanding of the equilibrium state of adsorption as a dynamic process as demonstrated by de Boer in his monograph "Dynamic Character of Adsorption". 

In considering the dynamic behaviour of amphiphiles at interfaces, we have to include several dynamic processes. There are not only "simple" adsorption/desorption processes but also time-dependent orientations and lateral transport phenomena. Each of these processes is connected to a characteristic time, denoted as relaxation time. Two extreme cases exist. 

Transport in the solution bulk is controlled by diffusion of adsorbing molecules if any liquid flow is absent. The transfer of molecules from the liquid layer adjacent to the interface, the so-called subsurface, to the interface itself is assumed to happen without transport. This process is determined by molecular movements, such as rotations or flip-flops. As pointed out in Chapter 2 adsorption of surface active molecules at an interface is a dynamic process. In equilibrium the two fluxes, the adsorption and desorption fluxes, are in balance. If the actual surface concentration is smaller than the equilibrium one, T < F, the adsorption flux to the interface predominates, if F > F , the actual amount adsorbed at the interface is higher than the equilibrium value F , and the desorption flux prevails. 

This is an oversimplification of the adsorption process. A more acceptable view is to have CO adsorb into a precursor state and move around on the surface until it finds a suitable adsorption site, S. As long as the precursor state is in equilibrium with the gas phase, the Langmuir description of the adsorption process represented by Reaction 5.131 gives the same result as is obtained by a more accurate description of the adsorption and desorption dynamics. Since experimental results are often consistent with the precursor state in equilibnum with the gas phase, we present the Langmuir model. 

In order to describe the evolution of the surface pressure during the adsorption process, the three involved dynamic processes have to be considered, i.e. the diffusion process in the bulk, the adsorption-desorption exchange between the surface and the subsurface, and the change in the orientation of the adsorbed molecules. In the present model, adsorption is considered to proceed in the following way. The molecules, which are randomly oriented in the bulk, adsorb either in the state 1 or 2, with the respective probabilities x and 1-x. The diffusion sets in when there is a concentration gradient established in the bulk. As the distribution of the freshly adsorbed molecules between the two states is out of equilibrium, a re-orientation process is induced. The time evolution of the partial adsorptions T1 and T2 is described then by... 

By applying an appropriate perturbation to a relevant parameter of a system under equilibrium, various frequency modulation methods have been used to obtain kinetic parameters of chemical reactions, adsorption-desorption constants on surfaces, effective diffusivities and heat transfer within porous solid materials, etc., in continuous flow or batch systems [1-24]. In principle, it is possible to use the FR technique to discriminate between all of the kinetic mechanisms and to estimate the kinetic parameters of the dynamic processes occurring concurrently in heterogeneous catalytic systems as long as a wide enough frequency range of the perturbation can be accessed experimentally and the theoretical descriptions which properly account for the coupling of all of the dynamic processes can be derived. 

The liquid-solid interface supports the growth of 2D crystals on surfaces too. The liquid phase acts as a reservoir of dissolved species which can diffuse towards the substrate, adsorb, diffuse laterally and desorb. These dynamic processes favor the repair of defects. Under equihbrium conditions, relatively large domains of well-ordered patterns are formed. Large domains grow at the expense of small domains via a process which is called Ostwald ripening. Furthermore, the solvent plays a significant role in the network formation. The choice of solvent affects the mobility of molecules, especially, the adsorption-desorption dynamics via the solvation energy and possibly also via solvent viscosity. 

The equihbration curve, that is, a curve in which the F values are plotted as a function of time is shown in Fig. 4.9 (for all cations, the initial concentration was 1.0xl0 moldm ). The observed sequence is Niconcentration value at which the adsorption and desorption rates are equal, is reached firsdy by Ni and Zn. So, it can be concluded that the kinetic and thermodynamic features of the adsorption of cations are very different, since the cation with maximum adsorbed amount (Cu) exhibits practically the same equihbration time as the cation with minimum adsorbed amount (Co). Such facts must be carefully taken into account if the hybrid matrix is to be used for metal adsorption purposes in a dynamic process (as a filter, for example). 

Many of the dynamic processes occurring on the electrode surface take place without the accompanying electron transfer process. Such processes can be represented by adsorption-desorption or change of electric dipole orientation. We need to gain sensitive access to the non-faradaic processes to track the non-fara-daic dynamics of molecular assembhes on electrode surfaces. The following non-faradaic processes can be the targets of the ER measurements. 

Sub-millisecond time-resolved SEIRAS has also been applied for other dynamic processes such as the redox reactions of heptylviologen on Ag [12], adsorption/desorption of fumaric acid [13] on Au and sulfate on Ag [16], and phase transitions in uracil adlayers on Au [15]. In the latter two studies, discrepancies between the time-resolved IR and chronoamperometry were suggested. 

Nie Baisheng 1998. Coal particle gas desorption diffusion dynamic process of the experimental research, Taiyuan university of science and technology mining system (Taiyuan), 4 417-421 (in Chinese). 

Dobmskin [57] proposed a model for the adsorption equilibria of multicomponent vapor mixtures based on the concept of TVFM and an adsorbed phase model in which the adsorbate-adsorbent interactions predominate over the lateral interaction between adsorbed molecules. The proportions of the component in the adsorbed phase are determined by a statistical distribution based on Frenkel s [70] mechanism and kinetic gas theory [71,72]. In Dobruskin s study, the equilibrium is viewed as a dynamic process in which the average molecular residence time T is the reciprocal of the rate constant for desorption, k. For adsorption of a binary mixture in an elementary volume dW, the ratio of the average times between two components is... 

Most previous theoretical studies of dynamic operating reactors used kinetic expressions obtained under steady-state operation. These models do not account for the detailed dynamics of the adsorption and desorption rate processes, and they may lead to erroneous predictions in periodic operation of the reactor. Thus, simulations of periodic processes may require use of kinetic models that are much more detailed than those used for predicting steady-state operation. These dynamic models also need to account for the rate of adsorption, desorption, and adsorption capacity of the catalyst. As mentioned above, the hot-temperature zone in a cooled RFR may exhibit complex dynamic features under... 

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