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Rate constants monolayers

As in a monolayer adsorption process, we consider that the rate of filling of sites by TCP molecules follows first-order kinetics. If No represents the total number of free sites per unit area at time t = 0, and N(t) is the number of sites available at time t, then dN(t)ldt = -kN(t), where k is the rate constant of the adsorption process. Therefore, N(t) decreases as No exp(-kt), and the number of sites occupied by TCP molecules at t becomes [No -N(t)], a quantity that determines directly the parameter (t) in Eq. (25). So Wo(t) can be written as... [Pg.301]

In general, a preparation of mixed monolayer can be realized by either a kinetic control or a thermodynamic control (Figure 1, left). Kinetic control is based on a suggestion that for an initial deposition step the desorption rate is ignorable in comparison with the adsorption rate. In this case, the concentration ratio of the adsorbed species A and B on the surface corresponds to the ratio of products of their adsorption rate constant ( a or b) and concentration (Ca or Cb) A aCa/A bC b. The validity of the initial assumption on low desorption rate means that the total surface coverage obtained under kinetic control is essentially lower than 100%. This non-complete coverage does not disturb most of optical applications of the... [Pg.321]

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. [Pg.321]

FIG. 28 Normalized steady-state diffusion-limited current vs. UME-interface separation for the reduction of oxygen at an UME approaching an air-water interface with 1-octadecanol monolayer coverage (O)- From top to bottom, the curves correspond to an uncompressed monolayer and surface pressures of 5, 10, 20, 30, 40, and 50 mN m . The solid lines represent the theoretical behavior for reversible transfer in an aerated atmosphere, with zero-order rate constants for oxygen transfer from air to water, h / Q mol cm s of 6.7, 3.7, 3.3, 2.5, 1.8, 1.7, and 1.3. (Reprinted from Ref. 19. Copyright 1998 American Chemical Society.)... [Pg.326]

Stress-Jump Experiments. The results of stress-jump experiments for HM-HEC monolayers with various compositions are shown in Table II, where the relaxation rate constants, ctRT, were calcu-... [Pg.194]

The segmental mobility of the polymer in the monolayer is enhanced by the solvation of the hydrophobes with toluene (9) the relaxation rate constant at the toluene/aqueous interface was three times that at the air/aqueous interface, as shown by Experiments Numbers 1 and 2 in Table II. [Pg.194]

The dynamic behavior of HM-HEC monolayers depends on the concentration in bulk solution (10, 11, 12) the monolayer obtained from dilute solution, having a higher relaxation rate constant, is more flexible and presumably thinner (Experiments Numbers 1 and 3). [Pg.194]

An increase in the amount of hydrophobic modification restricts segmental mobility by an increase of viscosity within the monolayer for the same molecular weight (300,000) and hydrophobe chain length (C g), the polymer monolayer with the higher amount of hydrophobe has a smaller relaxation rate constant (Experiments Numbers 3 and 7). [Pg.194]

Exp. No. Monolayer Interface Concentrat ion of Bulk Solution Degree of Compression vtl/Ao Relaxation Rate Constant aRT/A1 (sec-1) Corrected Relaxation Rate Constant aRT/AQ (sec-1)... [Pg.195]

Monolayer Experimental ir-A Curve Speed of Compression/cm min- Corrected Relaxation Rate Constant/ sec-3 9... [Pg.199]

Figure 8.3 Rate constants for the reduction of [Mo(CN)g]3 (upper curve) and [W(CN)g]3 (lower curve) on gold electrodes derivatized with a monolayer of HO(CH2)i6SH. The electrode potential is given with respect to a Ag/AgCl electrode in saturated KC1. Data taken from Ref. 2. Figure 8.3 Rate constants for the reduction of [Mo(CN)g]3 (upper curve) and [W(CN)g]3 (lower curve) on gold electrodes derivatized with a monolayer of HO(CH2)i6SH. The electrode potential is given with respect to a Ag/AgCl electrode in saturated KC1. Data taken from Ref. 2.
Here FMON and - mon represent the available vacant sites and surface sites occupied by B, respectively, of the first monolayer on a solid absorbate. The equilibrium constant KB for the reaction is given by the ratio of the rate constant for k.d for adsorption and k for desorption... [Pg.192]

FIGURE 4.1 3. a RDEV response of a monolayer catalytic coating for the reaction scheme in Figure 4.10 with a slow P/Q electron transfer. Kinetic parameter [equation (4.5)] kr°8/DA = 5. The same electrode transfer MHL law as in Figure 1.18. Dotted line Nemstian limiting case. Solid lines from left to right, e (5r0DAC = 1, 0.1, 0.01. h Derivation of the catalytic rate constant, c Derivation of the kinetic law. [Pg.274]

One obtains the data listed in Table P.2 for an electrochemical reaction involving a chemisorbed species on the electrode surface. Calculate the amount of charge required to form a monolayer of adsorbed intermediates and the rate constant for the formation of the intermediate on the surface under highly irreversible conditions. (Kim)... [Pg.731]

Thus, the rate constant for desorption can be expressed as a function of the standard free energy of desorption, A, and the change in free energy of the monolayer resulting from interactions within the film, , when the film is compressed. It is noteworthy that Equation 6 is similar to one developed by Davies (6). [Pg.124]

Figure 1. Schematic of apparatus for measuring rates of monolayer desorption at constant surface pressure... Figure 1. Schematic of apparatus for measuring rates of monolayer desorption at constant surface pressure...
Measurement of the free energies of monolayer desorption from the rates of desorption depends on whether equilibrium exists between the monolayer and a thin region of solution immediately beneath the film. The relation which tests this condition (Equation 8) must correctly predict the dependence of the rate constant for desorption, k8, on 7r. For the sulfate, phosphonate, and carboxyl films in this study Equation 8 is obeyed within the range of experimental error (2 to 5%). Therefore, it is reasonable to assume that the necessary equilibrium condition does exist. The cohesive forces in the monolayer follow directly from the evaluation of the free energies of desorption. [Pg.132]


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See also in sourсe #XX -- [ Pg.104 , Pg.105 , Pg.106 ]




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