Molecular layering method, adsorbent

Keywords nanotechnology, nanomaterials, molecular layering method, adsorbents, catalysts, nanoceramic, nanocomposition materials, pigments, nanofillers, polymers, retardants of combustibility... 

From the isotherms in Figure 11.8 a range of information can be obtained. The first important piece of information is the total surface area of the porous material, which can be calculated using the Brunauer-Emmett-Teller (BET) method. This derives a relationship between the volume and relative pressure, based on how the molecular layers of adsorbate build up on the surface of the porous material [36]. The BET equation is as follows ... 

Effect of PVA Molecular Weight on Adsorbed Layer Thickness. Figure 4 shows the variation of reduced viscosity with volume fraction for the bare and PVA-covered 190nm-size PS latex particles. For the bare particles, nre(j/ is independent of and the value of the Einstein coefficient is ca. 3.0. For the covered particles, rired/ t increases linearly with tp. Table IV gives the adsorbed layer thicknesses calculated from the differences in the intercepts for the bare and covered particles and determined by photon correlation spectroscopy, as well as the root-mean-square radii of gyration of the free polymer coil in solution. The agreement of the adsorbed layer thicknesses determined by two independent methods is remarkable. The increase in adsorbed layer thickness follows the same dependence on molecular weight as the adsorption density, i.e., for the fully hydrolyzed PVA s and... 

Special techniques in surface science to study molecular motion in adsorbed layers are a problem that has found little attention in surface science, even though it is of utmost importance. Electron spin resonance is one such method. We discuss diffusion of NO2, rotational motion of ((CH3)3C)2NO and melting of self-assembled layers of spin-labeled fatty acids adsorbed on Al203(0001). 

The dynamic characteristics of adsorbed molecules can be determined in terms of temperature dependences of relaxation times [14-16] and by measurements of self-diffusion coefficients applying the pulsed-gradient spin-echo method [ 17-20]. Both methods enable one to estimate the mobility of molecules in adsorbent pores and the rotational mobility of separate molecular groups. The methods are based on the fact that the nuclear spin relaxation time of a molecule depends on the feasibility for adsorbed molecules to move in adsorbent pores. The lower the molecule s mobility, the more effective is the interaction between nuclear magnetic dipoles of adsorbed molecules and the shorter is the nuclear spin relaxation time. The results of measuring relaxation times at various temperatures may form the basis for calculations of activation characteristics of molecular motions of adsorbed molecules in an adsorption layer. These characteristics are of utmost importance for application of adsorbents as catalyst carriers. They determine the diffusion of reagent molecules towards the active sites of a catalyst and the rate of removal of reaction products. Sometimes the data on the temperature dependence of a diffusion coefficient allow one to ascertain subtle mechanisms of filling of micropores in activated carbons [17]. 

Recently we have proposed a thermodynamic method, which allows the experimental evaluation of n [6-8]. The prerequisites needed for the application of the method are the following the adsorbed layer should be composed of solvent and constant orientated adsorbate molecules and its thickness should be equal to one molecular diameter of adsorbate molecules. These conditions can be safely detected experimentally and if they are fulfilled, n may be obtained from surface pressure data by means of the following equation [8] ... 

In the case of metallic adsorbates (metal deposits, underpotentially deposited upd-layers, catalytically active metal deposits), the type of coordination to surface sites (one-, two- or three-fold) and the distance to these sites may be of interest. Vice versa the same type of data may be of importance in the case of adsorbed ions on metal electrodes or about the atomic environment of a given atom/ion in an interphase. Analysis of the fine structure of X-ray absorption (EXAFS, XANES) close to the X-ray absorption edge of the species (atom) of interest will yield this data provided the sample can be prepared in a very thin layer in order to exclude unwanted bulk interference. Otherwise the experiment can be done in reflection (SEXAFS). Information about the distance between the atom of interest and its first and sometimes even second shell of surrounding species can be derived from the spectra [95]. Availability of a suitable light source, generally a synchrotron (for details see p. 15), is an experimental prerequisite. The method has been applied in studies of passive and corrosion layers on various metals [96-102] and of molecular and ionic adsorbates on single crystal surfaces [103]. 

The r-plot is involved in most of the methods for the reason that on a relatively flat surface in the absence of pores, adsorption occurs and the adsorbed film becomes several molecular layers thick before the vapor pressure reaches p/p l.O for the bulk liquid. Obviously, in the multilayer film the properties of the nitrogen are not the same as in the bulk. As already pointed out, determination of pore size requires not only the Kelvin equation to calculate the size of pores that fill with bulk liquid- nitrogen but also the thickness of the adsorbed film on the inner surface of pores that are not filled. 

The method proposed by Harkins et al. (1944) which they called the absolute method , included the previous coverage of the outgassed sample with an adsorbed film (five to seven molecular layers) of the immersion liquid. During the immersion experiment, the liquid sees a surface with an extent equivalent to that of the solid, but with a chemical nature corresponding to that of the bulk liquid. An improvement of this method was later proposed by Partyka et al. (1979) who deduced that, for a number of non-porous solids, the coverage with just 1.5 molecular layers was enough to screen the solid surface without reducing the available surface area. In this modified Harkins-Jura technique, water was used as the immersion liquid for solids with hydrophilic surfaces and pentanol for solids with hydrophobic surfaces. 

To have insight into the structure of adsorbed layer and chain conformations, Killmann used the method based on isotherms of layer thickness, polymer concentration in adsorption layer and adsorbed amount relative to solution concentration, and the same isotherms at saturation versus the molecidar mass. The data can be obtained for the dependence of the thickness on the molecular mass, solution concentration, etc. It was established that, at rather high concentration of solution, the macromolecules are adsorbed in the form of coils. 

Although certain adsorption techniques have been in use for over fifty years for determining the surface area of powders and porous materials, the interpretation of the adsorption data is still under discussion. Most procedures depend on the evaluation of the monolayer capacity, (i.e. the amount of gas or solute required to cover the surface with a complete single layer of adsorbed molecules). To obtain the surface area. A, from n it is necessary to adopt a standardised value for the effective molecular area, a, of the ad.sorbate in the completed monolayer. Many attempts have been made to check the validity of this approach and to establish the conditions under which a particular adsorption method may be used with confidence. Unfortunately, it is often difficult to find a truly independent method which is capable of providing... 

A remarkable attempt was made by Harkins and Jura nearly fifty years ago to overcome some of these difficulties. Their idea was to cover a non-porous adsorbent with a multilayer thick enough to have a liquid-like surface. The pre-coated adsorbent should therefore exhibit a surface enthalpy identical to that of the liquid and immersion in the same liquid should liberate an amount of energy equivalent to the removal of this surface. In principle, therefore, it would seem a fairly simple matter to calculate the surface area from the heat of immersion of the coated solid. The measurements Harkins and Jura appeared to indicate that 5-7 molecular layers were required to overcome the influence of the solid surface and reduce the surface energy to that of the liquid. Since this layer thickness would correspond to a very high relative pressure, it would be difficult to avoid some capillary condensation -even with non-porous powdered materials-and therefore the method appeared to have very limited applicability. 

A more recent investigation has revealed that this problem can be overcome by using water as the liquid since two molecular layers are sufficient to effectively screen the underlying surface of many adsorbents. These results have led to a modification of the original Harkins-Jura "absolute method for surface area determination and they make it possible to apply the technique to mesoporous solids (by avoiding the complication of capillary condensation). Obviously, the approach cannot be used in isolation to study micropore filling, activated entry or molecular sieving, but it becomes a powerful tool when combined with gas adsorption. 

Rheological methods of measuring the interphase thickness have become very popular in science [50, 62-71]. Usually they use the viscosity versus concentration relationships in the form proposed by Einstein for the purpose [62-66], The factor K0 in Einstein s equation typical of particles of a given shape is evaluated from measurements of dispersion of the filler in question in a low-molecular liquid [61, 62], e.g., in transformer oil [61], Then the viscosity of a suspension of the same filler in a polymer melt or solution is determined, the value of Keff is obtained, and the adsorbed layer thickness is calculated by this formula [61,63,64] ... 

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