Thickness of adsorbed layer

Technique of thin-layer chromatography. Preparation of the plate. In thin-layer chromatography a variety of coating materials is available, but silica gel is most frequently used. A slurry of the adsorbent (silica gel, cellulose powder, etc.) is spread uniformly over the plate by means of one of the commercial forms of spreader, the recommended thickness of adsorbent layer being 150-250 m. After air-drying overnight, or oven-drying at 80-90 °C for about 30 minutes, it is ready for use. 

Ifi fluorescence intensity k proportionality factor lo intensity of irradiating hght e molar absorption coefficient a amount of substance applied d thickness of adsorbent layer... 

Solution Equation (4.41) gives the Einstein relationship between [r/] and , the volume fraction occupied by the dispersed spheres. The volume fraction that should be used in this relationship is the value that describes the particles as they actually exist in the dispersion. In this case this includes the volume of the adsorbed layer. For spherical particles of radius R covered by a layer of thickness 8R, the total volume of the particles is (4/3) + 4ttR2 8R. Factoring out the volume of the dry particle gives Vdfy(1 + 38RJRS), which shows by the second term how the volume is increased above the core volume by the adsorbed layer. Since it is the dry volume fraction that is used to describe the concentration of the dispersion and hence to evaluate [77], the Einstein coefficient is increased above 2.5 by the factor (1 + 36/Vfts) by the adsorbed layer. The thickness of adsorbed layers can be extracted from experimental [77] values by this formula. ... 

The major advantage of protein adsorption studies on high surface area materials is that changes of some extensive properties which accompany the process of adsorption are large enough to be directly measured heat of adsorption through microcalorimetry 141), uptake or release of small ions by a combination of electrokinetic methods and titration 142), thickness of adsorbed layer or an increase of the volume fraction of solid phase by a hydrodynamic method like viscometry 143). Chromatographiclike analysis can also be applied to protein adsorption 144). 

Table 43—Adsorption Capacities of Different Soils and Vapors and the Thickness of Adsorbed Layer. (Temperature 28 Deg C)...

In addition to the molecular weight of the free polymer, there axe other variables, such as the nature of the solvent, particle size, temperature, and thickness of adsorbed layer which have a major influence on the amount of polymer required to cause destabilization in mixtures of sterically stabilized dispersions and free polymer in solution. Using the second-order perturbation theory and a simple model for the pair potential, phase diagrams relat mg the compositions of the disordered (dilute) and ordered (concentrated) phases to the concentration of the free polymer in solution have been presented which can be used for dilute as well as concentrated dispersions. Qualitative arguments show that, if the adsorbed and free polymer are chemically different, it is advisable to have a solvent which is good for the adsorbed polymer but is poor for the free polymer, for increased stability of such dispersions. Larger particles, higher temperatures, thinner steric layers and better solvents for the free polymer are shown to lead to decreased stability, i.e. require smaller amounts of free polymer for the onset of phase separation. These trends are in accordance with the experimental observations. 

If the change in m is measured in two different solvents (see the note of caution above), the mass or thickness of adsorbed layers on particles can be determined [185,186]. Before adsorption, only the effective particle mass is determined. After adsorption, the measured gain Am corresponds to the adsorbed layer. Eqs. (33) and (34) give for Am ... 

Stokes radius of a solute (X) rejection coefficient wall surface area of cores (cm /g) thickness of adsorbed layer (X) volume of pore cores (cm /g) volume of cylindrical pores (cm /g)... 

Historically, however, considerable attention has been given to corrections to the Kelvin equation arising from the thickness of adsorbed layer and the dependence of surface tension on curvature of interface. The first problem was initially considered as monolayen by Foster [48] and more recently as a function of equHibrium pressure of the system by Cohan [49], Degaguin [50], Foster [51], and Brockhoff and de Boer [52, 53]. The initial approaches of Foster and Cohan... 

The thickness of adsorbed layers depends on the extent of molecular interactions, but usually films of one molecule thickness, which are called monolayers, form at the liquid-vapor and liquid-liquid interfaces upon adsorption. In gas adsorption on solids, several molecular layers, which are called multilayers, form at high pressures, and mono-layers can only be formed if the gas pressure is sufficiently low. If van der Waals forces are operative during the adsorption process, it is called physical adsorption or physiosorption (see Section 8.3.1), whereas if chemical bonds are formed during the adsorption process, then it is called chemical adsorption or more preferably chemisorption (see Section 8.3.2). 

Fig. 1. Graph of thickness of adsorbed layer vs. relative pressure for several non-porous materials. G, precipitated silver (6) A, 200-mesh glass spheres (7) X, tungsten powder (8) H, zinc oxide (9) < >. -M glass spheres (10) +, glass spheres. Average 3 m (ff) zinc oxide. Sample K1602 (IS) , zinc oxide. Sample F1601 (IS) ...

In addition to yielding particle size and size distribution, particle volume, particle mass and particle density (Le. the properties on which sqtarations are based), SFFF can so provide information on the thickness of adsorbed layers such as surfactants, information on complex multiphase particles such as in core-shell latexes (especially when coupled with other diagnostic techniques) and can also address the important problem of agglomerates. 

Figure 7 Surface load of casemate on droplets in emulsions containing 20% soya oil and different amormts of caseinate. Solid line and left-hand axis surface load of caseinate dotted line and right-hand axis thickness of adsorbed layer of casein as measured hy photon-correlation spectroscopy, showing the tendency for the layer to be thin or extended depending on the amormt of protein adsorbed.

The forces acting between two molecularly smooth surfaces, normally mica or modified mica, can be measured as a function of their absolute separation with the interferometric SFA (Fig. 4) (27). This provides a convenient way to measure not only long-range forces but also the thickness of adsorbed layers. The absolute separation is determined inter-ferometrically to within 0.1 -0.2 nm by using fringes of equal chromatie order. The surfaces are glued on to opti-... 

Ae is the excess of the evaporation heat due to the interference of the layering on the opposite wall of pores (determined as a varied parameter using local isotherm approximation [LIA]) tip, / p) is the statistical thickness of adsorbed layer flni is the BET monolayer capacity... 

Scattering Techniques. The thickness of adsorbed layers can be determined using light. X-ray, and neutron scattering techniques (3). Quasi-elastic light scattering measurements on dispersion of particles measure the diffusion coefficients of particles in the dispersion. The size of these particles is related to the diffusion coefficient by the Stokes-Einstein equation. This measurement is performed on the dispersion with and without the adsorbed polymer. The difference in the radii in the two measurements is the thickness of the adsorbed layer. In these measurements it should be ensured that the particles in the dispersion are monodisperse and that there is no flocculation. In X-ray and neutron... 

Surface Force Measurements. Another method to measure the thickness of adsorbed layers is by the surface force apparatus (SFA) (17). In this method two freshly cleaved mica sheets are glued to the surfaces of two crossed cylindrical lenses. Polymer chains are then allowed to adsorb on the mica sheets. In order to measure the thickness of the adsorbed layers the two cylinders are brought in contact and the force between them is measured as a fimction of separation. The onset of the repulsive force associated with compression of the adsorbed layer can be related to the thickness of the adsorbed layer. On the other hand, in the event of bridging between the adsorbed layers the force will be attractive. Recent advances in the instrument have made it possible to probe the effect adsorption has on the flow of fluid past a surface (18). 

The thickness of adsorbed layers at liquid interfaces has been experimentally studied using neutron reflectivity (54). The structure of the adsorbed polymer at the liquid-liquid interface depends on the nature of the polymer and the interaction of the segments with the two phases. For example, if a diblock copolymer adsorbs at an oil water interface, the chain will partition itself in the two phases such that the block that is preferentially solvated by water will be in the aqueous phase, while the block that is preferentially solvated by oil remains in the organic phase. This results in the chains stretching at the interface, and the adsorbed layer becomes thick. On the other hand, if a random copolymer having the same composition as the diblock copolymer were to be adsorbed at the same interface, the interfacial thickness would be smaller because the unfavorable interactions between the parts of the copolymer and the two phases would localize the chain at the interface. 

The interactions between proteins and low-molecular-weight surfactants at interfaces have crucial effects on physical states of interfaces, such as interfacial energy, interfacial rheological properties, C potential, and thickness of adsorbed layer. The competitive displacement of globular proteins by surfactants at liquid interfaces (normally, oil-water interfaces) has been extensively... 

When nitrogen adsorption is carried out at liquid nitrogen temperature (—19S.8°C=77.34 K), nitrogen adsorption on the surface and capillary condensation of nitrogen in the pores take place (Fig. 2.12). The thickness of adsorbed layer on the surface, t, and the size of the pore where condensation happens, ri, depend on the partial pressure of nitrogen. Thus adsorption isotherm can be converted to the pore size distribution by assuming proper relations between both / and r and the partial pressure, p. 

The question as to why, in the case of bovine albumin films, the amount of adsorbed antibodies increases with the number of underlying monolayers is of importance. First, it seems probable that the antibody molecules are piled up on top of each other in the thick layers of antibodies adsorbed on four double layers of bovine albumin. If this is true, the same process might take place in other cases. It has often been reported that undiluted immune sera gave much thicker layers of specifically adsorbed material than diluted sera. Bateman, Calkins and Chambers (19) found increments in thickness of 200 A and 60 A with undiluted and diluted serum, respectively. These variations have usually been assumed to result from a different orientation of the adsorbed antibody molecule which in the case of the rabbit antibody has approximately the dimensions of 40 X 270 A. Great variations in the thickness of adsorbed layers of antibodies have also been observed by electron microscopy. For instance, Anderson and Stanley (24) reported an adsorbed thickness of 225 A of rabbit antibodies on the surface of tobacco mosaic virus molecules but they also have observed much smaller increments. 

New possibilities to evaluate the thickness of adsorbed layer were opened by using electrochemical methods. The methods are based on the measurement of the -potential of colloid particles covered with adsorbed polymer. Decrease in the value of -potential with increase of pol3rmer adsorption is determined by a shift of the sliding plane into the solution bulk as a resrdt of the formation of the adsorption layer. From measurements of -potential, the distance of the sliding plane from the surface may be calculated, which is taken as a measure of the layer thickness. In such a way the dependence of the layer thickness on the value of adsorption may be determined. 

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