Degree of coverage

The heat of adsorption is an important experimental quantity. The heat evolution with each of successive admissions of adsorbate vapor may be measured directly by means of a calorimeter described by Beebe and co-workers [31]. Alternatively, the heat of immersion in liquid adsorbate of adsorbent having various amounts preadsorbed on it may be determined. The difference between any two values is related to the integral heat of adsorption (see Section X-3A) between the two degrees of coverage. See Refs. 32 and 33 for experimental papers in this area. 

In a separate study using the JKR technique, Chaudhury and Owen [48,49] attempted to understand the correlation between the contact adhesion hysteresis and the phase state of the monolayers films. In these studies, Chaudhury and Owen prepared self-assembled layers of hydrolyzed hexadecyltrichlorosilane (HTS) on oxidized PDMS surfaces at varying degrees of coverage by vapor phase adsorption. The phase state of the monolayers changes from crystalline (solidlike) to amoiphous (liquid-like) as the surface coverage (0s) decreases. It was found that contact adhesion hysteresis was the highest for the most closely packed... 

Depending on the degree of coverage wifli oxygen the rate constant varied between Uq range between 4.6-10 2-10 cm s. ... 

If the degree of coverage of the ruthenium by the copper is very high, the copper atoms should be coordinated extensively to ruthenium atoms. It is emphasized that the ruthenium-copper clusters are of such a size (average diameter of 32A by electron microscopy (33)) that the surface metal atoms constitute almost half of the total. Hence for a Cu/Ru atomic ratio of one, the number of copper atoms would correspond roughly to that required to form a monolayer on the ruthenium. 

In Eq. (28), the methanol residue is mainly adsorbed CO. As diseussed earlier, other kinds of adsorbed speeies ean also be present on the eleetrode surfaee. The different species and their degree of coverage on the Pt surface depend on the electrode potential and also on the nature and the structure of the electrode surface. For the sake of simplicity, if only two types of adsorbed species, COads and CHOads, are considered, Eq. (28) becomes ... 

It is not a trivial point that 0fj vs. E curves are practically linear. In a reversible system the electrode potential can be linked to the activities (concentrations) of the potential-determining substances. In the system being discussed, this substance is atomic hydrogen. According to the Nemst equation we have E = const - (RTIF) X In Cjj. It follows that the degree of coverage, 0, is linearly related to the logarithm of concentration c in the solution ... 

According to these results methanol adsorbate seems to consist of a mixture of (C, O) and (C, O, H) particles, the actual ratio depending on the concentration and total degree of coverage. This is in good agreement with coulometric determinations of the charge ratio for methanol adsorption, Qad, (see Eqs. 2.1 to 2.3) and adsorbate oxidation, Qox (see Eqs. 2.4 to 2.6) [14,47], These results will be discussed in Section 2.1.4. 

The ratio QaJQ0X varies between 1 and 2 depending on methanol concentration and degree of coverage. In principle this can be interpreted in terms of a mixture of particles of composition (C, O) and (C, O, H). The nature of the Pt surface seems to influence also the charge ratio. In principle, the following configurations could be expected ... 

A COH species requires three adjacent adsorption sites. Under conditions of large degrees of coverage, the site requirement should be an important factor in determining the kind of adsorbate formed. 

The degree of coverage, however, seems to influence the adsorbate composition at low methanol concentrations also. In particular, on smooth platinum, the dependence of surface composition on 6 is observed at concentrations as low as 5x 10-3 M (Fig. 2.7). In this case it could be possible that COH can be formed as long as adjacent sites are available. At high coverage (by all species involved in the adsorption process), the formation of COH should be geometerically prevented. 

A voltammogram after exchange of the solution with base electrolyte is shown in Fig. 4.3. The degree of coverage by tin, 9Sn, calculated from the charge relations in the H-region is shown in Fig. 4.4a. 

Figure 2.18 Cyclic vollammograms of a Pt working electrode immersed in aqueous perchloric acid (in the absence of CO) showing the oxidation peaks of adsorbed CO for different degrees of coverage 0. The scan rate was 50 mV s 1. The adsorption was effected by exposing the platinum working electrode to CO-saturated electrolyte for a sufficient length of time to give the coverage required. From Bcdcn et al. (1985).

Connors and Jozwiakowski have used diffuse reflectance spectroscopy to study the adsorption of spiropyrans onto pharmaceutically relevant solids [12]. The particular adsorbants studied were interesting in that the spectral characteristics of the binary system depended strongly on the amount of material bound. As an example of this behavior, selected reflectance spectra obtained for the adsorption of indolinonaphthospiropyran onto silica gel are shown in Fig. 1. At low concentrations, the pyran sorbant exhibited its main absorption band around 550 nm. As the degree of coverage was increased the 550 nm band was still observed, but a much more intense absorption band at 470 nm became prominent. This secondary effect is most likely due to the presence of pyran-pyran interactions, which become more important as the concentration of sorbant is increased. 

If, on the timescale of observation, the degree of coverage of any site type becomes appreciable, the precise nature of the relationship between Fi, F2 and has to be taken into account. For the case of a Langmuirian isotherm (implying sufficiently fast kinetics of adsorption/desorption) this means that equations in (2) are applicable. Two particular cases are described here ... 

The Langmuir and BET models incorporate an assumption that the energy of adsorption is the same for all surface sites and not dependent on degree of coverage. Since in reality the energy of adsorption may vary because real surfaces are heterogeneous, the Freundlich adsorption model (see Chap. 2) [37] attempts to account for this ... 

Gritti, R and Guiochon, G, Influence of the degree of coverage of C-18-honded stationary phases on the mass transfer mechanism and its kinetics, J. Chromatogr. A, 1128,45,2006. 

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