Substrate - adsorbate interface

In a lipase-catalyzed reaction of a water-insoluble oily substrate, the enzyme reversibly adsorbs at the substrate-water interface, and the enzymatic reaction takes place at that interface. The interfacial area strongly depends upon some physical parameters of emulsion, such as the size of the emulsion droplets as well as the amount of the substrate present. However, the importance of the... 

In sec. 5.2 we emphasised the fact that polymers have a large number of interned degrees of freedom. Adsorption must imply changes in this number. The usual description of conformations at an adsorbing Interface, first proposed by Jenkel and Rumbach and depicted schematically in fig. 5.1, is in terms of three types of subchains trains, which have all their segments In contact with the substrate, loops, which have no contacts with the surface and connect two trains, tind tails which are non-adsorbed chain ends. 

Leo Vroman and co-workers, who present additional data in this volume, have made major contributions to our understanding of the fundamentals of this adsorption process. They have shown that events at the substrate/blood interface are not static after the first layer of protein is adsorbed, but that the protein layer is continuously remodeled, reacted with, or converted by other surface-active components in intact plasma (37). 

A foreign substrate, S, is a solid with a different composition from that of Me, and is considered as electrochemically inactive in a certain potential range which is considered in this book. A 2D Me phase is considered as a specific Me adsorbate located in the inner Helmholtz plane of the electrochemical double layer existing at a substrate/electrol3de interface. A 3D Me phase can be either a bulk phase or a small atomic cluster of Me, where bulk has the meaning of infinitely large. 

The formation of 2D Meads phases on a foreign substrate, S, in the underpotential range can be well described considering the substrate-electrolyte interface as an ideally polarizable electrode as shown in Section 8.2. In this case, only sorption processes of electrolyte constituents, but no Faradaic redox reactions or Me-S alloy formation processes are allowed to occur, The electrochemical double layer at the interface can be thermodynamically considered as a separate interphase [3.54, 3.212, 3.213]. This interphase comprises regions of the substrate and of the electrolyte with gradients of intensive system parameters such as chemical potentials of ions and electrons, electric potentials, etc., and contains all adsorbates and all surface energy. Furthermore, it is assumed that the chemical potential //Meads is a definite function of the Meads surface concentration, F, and the electrode potential, E, at constant temperature and pressure Meads (7", ). Such a model system can only be... 

When the adsorption/desorption kinetics are slow compared to the rate of diffusional mass transfer through the tip/substrate gap, the system responds sluggishly to depletion of the solution component of the adsorbate close to the interface and the current-time characteristics tend towards those predicted for an inert substrate. As the kinetics increase, the response to the perturbation in the interfacial equilibrium is more rapid and, at short to moderate times, the additional source of protons from the induced-desorption process increases the current compared to that for an inert surface. This occurs up to a limit where the interfacial kinetics are sufficiently fast that the adsorption/desorption process is essentially always at equilibrium on the time scale of SECM measurements. For the case shown in Figure 3 this is effectively reached when Ka = Kd= 1000. In the absence of surface diffusion, at times sufficiently long for the system to attain a true steady state, the UME currents for all kinetic cases approach the value for an inert substrate. In this situation, the adsorption/desorption process reaches a new equilibrium (governed by the local solution concentration of the target species adjacent to the substrate/solution interface) and the tip current depends only on the rate of (hindered) diffusion through solution. 

The effect of slippage at a substrate-film interface can also be described in terms of sbp time [39]. To understand the physical meaning of the slip time, one can consider an adsorbate film on a substrate, moving at constant velocity. If the substrate stops, the velocity and momentum of the film decay exponentially, and the time constant of this process is the slip time. If this process is very rapid, i.e., we have a rigidly adsorbed film, the time constant will be close to zero, and there will be no noticeable shp. The shp time is related to the interfacial friction coefficient through the equation [39] ... 

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 general principle of surfactant action to remove soils from a surface is illustrated for oily soil in Fig. 3. When the soil is detached from the substrate, the surface between soil and substrate is destroyed, but two new surfaces are created—that between soil and bath and between substrate and bath. Detachment will be thermodynamically favorable when the sum of the interfacial tensions of the two new surfaces is less than that of the destroyed surface. A primary function of surfactants is to adsorb at the soil/bath and substrate/bath interfaces and reduce these interfacial tensions to make detachment more thermodynamically favorable. As surfactants adsorb and modify interfacial tensions, the contact angle (8 in Fig. 3) increases, and the contact area between soil and substrate decreases. As shown in Fig. 4, this results in the... 

The combination of classical electrochemical measurements with ex situ transfer experiments into UHV [242], and in situ structure-sensitive studies such as electroreflectance [25], Raman and infrared (IR)-spectroscopies [29, 243], and more recently STM and SXS [39] provided detailed knowledge on energetic, electronic and structural aspects of (ordered) anion adsorption and phase formation. These experimental studies have been complemented by various theoretical approaches (1) quantum model calculations to explore substrate-adsorbate interactions [244-246] (2) computer simulation techniques to analyze the ion and solvent distribution near the interface [247] (3) statistical models [67] and (4) MC simulations [38] to describe phase transitions in anionic adlayers. 

Recent studies have started to broaden the choice of substrates to include surfaces that possess intriguing properties that could be used to interface with the adsorbed supramolecular structure or to influence the self-assembly process via substrate-adsorbate interactions. Examples of such surfaces include two surfaces that exhibit a moire superstructure, graphene, and boron-nitride monolayers. ... 

The addition of detergent increases the contact angle at the dirt/substrate/water interface so that the dirt rolls up and off the substrate. Surfactants that adsorb both at the substrate/water and the dirt/water interfaces are the most effective. If the surfactant adsorbs only at the dirt/water interface and lowers the interfacial tension between the oil and substrate dirt removal is more difficult. Nonionic surfactants are the most effective in liquid dirt removal since they reduce the oil/ water interfadal tension without reducing the oil/substrate tension. 

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