Adsorption of neutral molecules

The adsorption of neutral molecules is discussed in Chapter 13. Here we show electrocapillary curves obtained in the presence of n-butanol and compare them to an 

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The exponential term in eqn. (173) reflects the effect of the electrode potential on the free energy of adsorption of a charged species or adsorption of neutral molecules with charge transfer at the interface. 

Adsorption of neutral molecules from nonaqueous media has only been studied in isolated cases (Payne, 1970). As far as one can judge... 

Comparison of the values of E for the various pesticides and the neutral and anionic species of the simple nitrophenol indicates that a much higher activation energy is associated with adsorption of neutral molecules (parathion and the neutral nitrophenol) than with adsorption of anions (2,4-D, DNOSBP, and the anionic nitrophenol). This observation suggests the possibility of two different rate-limiting steps in the intraparticle transport mechanism. Current studies are being directed toward more detailed exploration of the observed thermokinetic phenomena. 

Because of the short lifetime of ions in gaseous atmospheres, even at low pressure, gas-phase IR measurements are limited to adsorption of neutral molecules. Electrochemical applications of the IR method offer the interesting possibility of providing data on the adsorption properties of charged particles (Secs. 8 and 9). In the electrochemical environment the applied potential allows ionic adsorbates to be studied under energetically controllable conditions. Otherwise the electrochemical double layer offers exceptional conditions to investigate the Stark effect on vibrational transitions by setting tunable electric fields of the order of 10 V cm at the interface. This phenomenon will be discussed in Sec. 10. 

Adsorption of neutral molecules as a function of electrode potential generally passes through a maximum near the potential of zero charge. The potential of zero charge for platinum acetonitrile interface has been determined to be 0.3V NHE (40). The concentration of adsorbed CO will decrease on either side of potential of zero charge, which is consistent with the present experimental results. 

The relative concentration of CH CN on the surface of platinum decreases with increasing cathodic potentials, because adsorption of neutral molecule decreases away from the potential of zero charge. The acetonitrile concentration will be further decreased at high cathodic potentials by increasing Since C0 and CH CN are... 

In general, adsorption of neutral molecules at the electrode surface depends on the electrode potential, and the amount adsorbed shows a so-called bell-type dependence on the potential having a maximum at pzc. The adsorption of adenine is regarded as an example of such a case. Contrary to this, deoxyadenosine and dAMP give no simple bell-type A/ /i o — relations (see Fig. 18B and C) that is, the decrease in A7 /Iio is not as significant as for adenine at potentials more positive than —0.1 V. 

The molecular approach to the adsorption of neutral molecules is represented below by the theory of Bockris et ai. 11), who considered the competition with the solvent molecules as the primary factor determining the potential dependence of organic adsorption. 

An apphcation of the thermodynamic theory of Pt hydrogen electrode to the reversible adsorption of organic substances leads to a conclusion [87] that the maximum adsorption of neutral molecules should be observed in the potential region, where the adsorption of both hydrogen and oxygen is low (so[Pg.344]

The adsorption of neutral molecules at electrified interfaces is characterized by a smooth variation of y, q, C and Tj as a function of electrode potential or temperature (dotted and dashed curve in Fig. 29) [449, 518]. The relation between surface excess Pj (or coverage 9i = Pi/Pim with rim as the maximum surface excess... 

The adsorption of neutral molecules on smectites is driven by various chemical interactions hydrogen bonds, ion-dipole interactions, coordination bonds, acid-base reactions, charge transfer, and van der Waals forces. Several polar molecules, such as alcohols, amines, and acids, form intercalation complexes with montmorillonites. The intercalation can be performed from the vapor, liquid, or even solid state. In intercalation from solution, solvent molecules are generally coadsorbed in the interlayer space. Guest molecules may be intercalated in dried clay minerals or may displace the water molecules of hydrated montmorillonite. 

The adsorption of organic cations is also dependent upon solution pH. At a given pH, the concentration of cations in solution relative to the concentration of uncharged molecules is dependent on the pK value of the base. If the pH is adjusted to be equal to the pK value, then the ratio of cation to free base is equal to unity, and cation exchange may be accompanied by adsorption of neutral molecules of the same organic species. For cations to be the dominant species in solution, the pH should be at least one or two units lower than the pK. If too acidic, the adsorption may be hindered due to competition with H+ ions or with metal cations released from the silicate lattice by acid attack. Adsorption will depend, too, on the solubility of the base in water that, in turn, may be pH dependent (22). 

The voltage dependence of the adsorption of neutral molecules is obviously connected with the interaction studied by A. N. Frumkin between the adsorption energy of a surface-active molecule and the value of the electrode potential determined by the surface eharge. The zero charge potential in... 

The difference in adsorption mechanism of multiprotic acids from aqueous solutions on the one hand and from alcoholic solutions on the other is that in aqueous solutimi, pre-existing ions are adsorbed, and the conductance is depressed. In alcoholic solution, adsorption of neutral molecules produces ions in solution, e.g.. 

Most studies of adsorption of neutral molecules are concerned with organic molecules (any substance which adsorbs at a surface is called surface active substance). These are usually present in only a small concentration... 

In this section we saw that adsorption of neutral molecules behaves very differently from that of charged species. The greatest difference being the quadratic dependence of the free energy of adsorption on potential for neutral molecules, while for ions no simple relationship between B and potential exists. 

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