Adsorption and Reorientation

Consider the co-adsorption process of two neutral solutes A and B on an electrode surface. If r = AbIAa, the whole adsorption process may 

We observe that the surface composition of an adsorbed layer composed of solvent S and two solute molecules A and B in terms of A () can be calculated from Eqs. (28)-(31) and in combination with Eqs. 

One is the reorientation of a solute on the electrode surface and another the effect of the specific adsorption of anions on the adsorption features of a neutral solute. 

The reorientation of the solute molecules on an electrode surface from the flat position (B) to the normal one (A) may be treated as co- 

We observe that the model gives an excellent semi-quantitative description of the adsorption features of 4-methylpyridine on Hg. 

We observe that the surface composition of an adsorbed layer composed of solvent S and two solute molecules A and B in terms of A can be calculated fromEqs. (28)-(31) and in combination with Eqs. (16) and (21) the properties of the adsorbed layer can be expressed as a function of either or 3. Comparisons of model predictions with experimental data when m = 1, A = A and A = 0 show that this simple model can provide a satisfactory description of the coadsorption of two organic adsorbates, without using additional adjustable parameters, but from the known properties of their single adsorption.  

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Screening by the ions of the diffuse layer, decreasing the mutual repulsion of the dipoles and leading to an increase in adsorption (e.g., the change in adsorption and reorientation ofcoumarin ). 

There are also several papers describing adsorption of quinoline. Sawamoto [143] have studied adsorption and reorientation of quinoline molecules at Hg electrodes by recording differential capacity-potential and differential capacity-time plots using the flow-injection method. Adsorption of quinoline was found reversible at any potential, with the possibility of reorientation of the molecules at the interface. Ozeld etal. [144] have studied adsorption, condensation, orientation, and reduction of quinoline molecules at pure Hg electrode from neutral and alkaline solutions, applying electrochemistry and Raman microprobe spectroscopy. The adsorbed quinoline molecules changed their orientation from the flat at —0.1 V > E > —0.3 V, to the upright at < —0.5 V. At potentials —0.3 V > > —0.5 V, both orientations were observed. Later, Ozeld et al. [145] have extended the studies on reorientation of quinolinium ions at the Hglacidic aqueous solution interface. For these conditions, the specific adsorption of quinoline was not observed. 

The interactions between electrode metals and solvents are reflected in the adsorption and catalytic properties. The adsorption of other solution components (ions and neutral molecules other than the solvent) is attended by a displacement of adsorbed solvent molecules or their reorientation. Therefore, a metal s adsorptive power is low under conditions where its energy of interaction with the solvent is high. 

Jones MA, Bohn PW (2001) Total internal reflection fluorescence and electrocapillary investigations of adsorption at the water-dichloroethane electrochemical interface. 2. Fluorescence-detected linear dichroism investigation of adsorption-driven reorientation of di-N-butylaminonaphthylethenylpyridiniumpropylsulfonate. J Phys Chem B 105(11) 2197-2204... 

Electrochemical and subtractively normalized interfacial FTIR studies of 4-cyanopyridine adsorption on Au(lll) electrode [245] have shown that this compound is totally desorbed at potentials lower than —0.7 V versus SCE. At less negative potentials, the molecules were flatly oriented n bonded) on the surface and reoriented to the vertical position, when potential approached OV. At potentials higher than 0.05 V, adsorption of 4-cyanopyridine becomes dissociative and the compound is partially hydrolyzed to isonicotinamide. 

Formation of 2D phase accompanying electrochemical reduction of 4,4 -pyridine on mercury in the presence of iodide ions occurred via adsorption-nucleation and reorientation-nucleation mechanisms [139]. The first reduction step of Bpy on Hg in the presence of iodide as counterion in acidic medium at 15 involved the BpyH2 " /BpyH couple and led to the formation of a 2D phase. The increased contribution of the reorientation term in the formation of the condensed phase was consistent with the increased adsorption strength of the anion to the electrode surface. 

Therefore, data of Fig. 6 show the change of the reorientational-vibrational relaxation time of acetonitrile molecules confined in mesopores upon adsorption and desorption. Before the capillary condensation, the relaxation time is smaller than that of bulk liquid, whereas it is greater than that of the bulk liquid after condensation. The difference of molecular motion between precondensation and postcondensation states is not significant, but this work can show clearly the presence of such a difference. If vibrational and reorientational relaxation processes are dominated by molecular collisions, the molecular reorientation is more rapid before condensation and it becomes slower than that of the bulk liquid with the progress of the capillary condensation, which indicates the formation of a weakly organized molecular assembly structure in mesopores. Even the mesopore can affect the state of the condensates through a weak molecular potential. The organized state should be stable in mesopores, because the relaxation time is almost constant above the condensation PIP,. 

To understand the principles of shape-selective adsorption and catalysis, a detailed knowledge of the microdynamics of the molecules inside the zeolitic pore system is required. Garcia and Weisz 4) pointed out the relevance of NMR methods to yield a unifying picture for the phenomena, mechanisms, and magnitudes of diffusivities. In the past few years, a deeper insight into molecular motions in zeolites has been achieved, especially by combining investigations of molecular translation with studies of reorientation processes on different time scales 5-14). 

Fig. 3.14. Dependence of the adsorption equilibrium constant b on nc for C DMPO as calculated from the Frumkin ( ) and reorientation (O) models, ( ) - the data for fatty acids reproduced from Fig. 3.10.

The adsorption behaviour of polyethylene glycol octylphenyl ethers (Tritons X-45, X-100, X-165 and X-305) is similar to that of the oxyethylated alcohols. The experimental data [58-62] and the results of the calculations according to the Frumkin and reorientation models are presented in Figs. 3.36, 3.37 and Tables 3.14 and 3.15. 

The experimental study of the adsorption of organic compounds on electrodes began in the first decade of the previous century with Gouy s electrocapillary work. Since then it has attracted considerable attention, mainly because it affects the mechanism of most of the processes occurring on electrodes. The first attempts to present a theoretical description of the effect of the electric field on adsorption appeared in 1925 and 1926 by Frumkin, who formulated the macroscopic model of condensers in parallel. The interpretation of the electrosorption of organic compounds at a molecular level was initiated by Butler" in 1929, but it was the work of Bockris and co-workers in 1963 that put the bases of the contemporary microscopic modelling. The main contribution by Bockris et al. was the introduction of the concept of the competition between solvent and adsorbate molecules for adsorption and the reorientation of the adsorbed molecules on the electrode upon the variation of the electric field. ... 

In search of a basis for shp, one observes that in the immediate neighborhood of the contact line region the continuum approach breaks down. Consequently the molecnlar activity there, like adsorption, relaxation, reorientation, etc., conld be important However, if these molecular effects are to be used to predict the shp velocity, they should not yield a length scale comparable to molecnlar dimensions because such dunensions arc not admissible under the continunm treatment Neogi and Miller (1982a) and Ruckenstein and Dunn (1977) considered the possibility that molecules at the solid-liquid interface can actnally move in the tangential direction. Thdr resnlts can be eventually expressed as a shp velocity that is depmdmt on surface (hflusivity. 

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