The Adsorption of Polar Molecules

Molecules that have dipoles, such as organic halides, ethers, ketones, nitrocompounds, etc., will be attracted by the electrostatic field emanating from the surface of an ionic crystal. The contribution toward the adsorption energy is given by 

Calculations of the contributions toward the adsorption energy by these forces reveal them to be quite appreciable and, indeed, a strong and oriented adsorption will always be found. These calculations result in somewhat too low values when the dipole moment n is used, because of the short distances between the dipoles and the negatively charged attracting centers. In many cases the actual contribution may be 10% higher than the calculated one (39). 

A dipole that is not situated near the periphery of the adsorbed molecule makes a far smaller contribution toward the adsorption energy than do peripheral dipoles. Such nonperipheral dipoles may, however, cause enough difference in adsorption energy to effect a preferential adsorption between molecules that otherwise would show equal adsorption energies. They also may lead to a fixed and an oriented position of the adsorbed molecule instead of an adsorption of more or less freely moving and rotating molecules. 

A similar behavior may be found when the charge distribution in the molecules is more complex. In carbon dioxide the charge distribution is of the character of a quadrupole. Lenel (40) calculated the influence of the interaction of this quadrupole with the surface of an alkali halide crystal and reached the conclusion that a substantial contribution of roughly 3 kcal./mole is to be expected from this polar interaction. Recently Drain (41) succeeded in approaching the remarkable fact that the heat of adsorption of N2 on ionic crystals is often appreciably higher than that of 02 and A, which is not the case when these gases are adsorbed on nonionic surfaces. He shows that the quadrupole of N2 may be responsible. We shall return to this problem in Sec. VI,2. 

When a dipole molecule is adsorbed on the surface of a metal or on other conducting surfaces (charcoal), the attraction may be described by the image force. The energy of interaction is given by 

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The adsorption of polar molecules on surfaces of ionic crystals (Sec. V,5) is influenced by active spots of the same kind as influence the action of Coulomb forces. The effect of these active spots is, quantitatively, less for dipole-containing molecules than for ions. The effect of dipoles on metal surfaces is small (Sec. V,5), and active spots are not expected to give appreciably higher contributions. 

Another Russian scientist who played a leading role in the advancement of the understanding of adsorption mechanisms was A.V. Kiselev. With the help of a large team of co-workers and by making a systematic investigation of various well-defined adsorbents (notably oxides, carbons and zeolites), Kiselev was able to demonstrate that certain specific interactions were involved in the adsorption of polar molecules on polar or ionic surfaces. At the same time, in the UK the specificity of physisorption was under investigation by Barrer - especially in the context of his pioneering work on the properties of the molecular sieve zeolites. 

Electrostatic interaction forces (3) can contribute to the adsorption of polar molecules on polar adsorbents. The energy of interaction Eg of an adsorbate dipole of moment in a surface field of intensity F is given as... 

Luminescence measurements had shown that in addition to Wan der Waals interactions, the adsorption of polar molecules on the modified cellulose fibres involves polar and hydrogen bonding with the cellulose AGUs [34, 35]. 

Surface reconstruction resulting from the adsorption of polar molecules on proteins... 

It is not necessary that new chemical bonds are formed to have surface reconstruction the adsorption of polar molecules on biomolecules like proteins or nucleic acids is a familiar example. 

At the beginning, the spectrum is nearly a flat line and does not show any significant structure. Seemingly all the dipoles of the active atomic groups of the AC have been neutralized by the adsorption of polar molecules from the ambient air like (H2O, CO, H2S, SO2, NO etc.). Then the reactivation process was started. It consisted of three steps ... 

In order to evaluate the behavior of the pellets towards the adsorption of polar molecules, the adsorption isotherms for water vapor in air were determined at 23 °C (Figure 5.49), and show the different chemical nature of sepiolite and activated carbon. Thus, granular sepiolite S exhibits a hydrophilic behavior, whereas the type V isotherm for the activated carbon is typical of a small interaction between the carbon surface and the water molecules at low relative humidity (RH). The water adsorption isotherms, on pellets SP30 and SC30, have an intermediate shape between samples S and P or C. Furthermore, the uptake at each RH is the additive of the humidity adsorption by the individual two components, taking into account the proportion of each one in the mixture. 

The surface of an activated carbon adsorbent is essentially non-polar but surface oxidation may cause some slight polarity to occur. Surface oxidation can be created, if required, by heating in air at around 300°C or by chemical treatment with nitric acid or hydrogen peroxide. This can create some hydrophilic character which can be used to advantage in the adsorption of polar molecules but can cause difficulties in other applications such as the... 

In this seetion, we intend to present some simple and straightforward aspects of adsorption of nonelectrolytes from binary liquid solutions on to mineral surfaces. Owing to the vast literature on this subject, we limit the presentation to a description of some frequently observed adsorption isotherms applicable to experimental data represented by the adsorption of polar molecules on to mineral oxide surfaces from dilute nonaqueous solutions. 

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