Adsorption isotherms surfaces

Adsorption at solid-gas surfaces, derivation of adsorption isotherms, surface area measurement, and structural analysis of surfaces and adsorbed layers using low-energy electron diffraction (Chapter 9)... 

Keywords silica, organosilane, metal adsorption, isotherms, surface modification 1. Introduction... 

Figure 1.35. Schematic monolayer adsorption isotherms (surface coverage a versus P/Psat) for the two cases in Table 1.2. At P/ Psat = 1, the solid surface is wet in case (1) and nearly dry in case (2).

Cerofolini GE (1975) A model which allows for the Freundlich and the Dubinin-Radushkevich adsorption isotherms. Surface Sci 51 333-335... 

We should clarify here that the above cited studies are largely exploratory and the role of each parameter in reaction specificity is currently unclear. They show, however, the need for a fundamental understanding of molecular and electronic surface interactions that determine electrocatalytic as well as catalytic specificity. Thus, adsorption isotherms, surface states, molecular configurations, electronic distributions, dipole formation, and bond hybridization should be explored for well-characterized catalysts and model reactions in the presence and in the absence of an electric field. 

GC methods have been used to measure the adsorption of non-swelling vapours on the surface of dry (. -8.) and moist (9.-12.) cellulose. Adsorption isotherms, surface areas, isosteric heats and entropies of adsorption have been measured for a range of hydrocarbons and organic vapours. Perhaps the most useful aspect of the method is that the effect of relative humidity on surface properties may be investigated. The validity of the method as applied to surface area measurements is described in some detail below. Other applications of GC to cellulose surfaces are then summarized briefly. 

FIGURE 10.7 Adsorption isotherms. Surface excess T over T00 (i.e., r at surface saturation) versus concentration in solution c over c0.s (i.e., c for r/ro0 =0.5). Langmuir isotherm and calculated isotherm for Na-stearate at the A-W surface. 

The amorphous silica used for LC packings is highly porous. The specific surface area of a silica sample is equal to the sum of its internal (pores) and external areas [4]. The specific surface area. A, of a porous silica sample is expressed in mVg. A is due to internal area at more than 99% [1]. It is easily measured by the BET method that uses gas adsorption isotherms. Surface area and porosity are linked. 

Consideration is now given to the experimental variables relating to the adsorption of surfactant mixtures. The adsorption isotherm of component 1 in a ternary system is again defined by Equation (1). However, the isotherm is now three-dimensional and for a consistent two-dimensional presentation, a constant ratio of X2/X2 seems to be a logical choice (components 1 and 2 are surfactants, component 3 is water or brine). Figure 4 represents a ternary adsorption isotherm surface. 

Since concentrations xj, X2, X3 represent equilibrium values (i.e. concentrations in the bulk phase after adsorption takes place) it is impossible to prepare the original samples of ternary solutions in such a way that the X2/X3 ratio stays constant without prior knowledge of the adsorption isotherm. This is the reason that adsorption isotherms seem to depend on the solid/liquid ratio in the system. An increase in the amount of the solid phase increases the total amount of surfactants adsorbed, which results in a change of X2/X3 ratio and a shift of the experimental point on the adsorption isotherm surface. Obviously, this effect is more pronounced in systems with large differences in individual surfactant adsorption characteristics. 

The adsorption-energy distribution fi om the equilibrium adsorption isotherm (surface coverage B versus pressure p) of heterogeneous adsorbents. 

Adsorption isotherms, surface tension-concentration equations and two-dimensional equations of the surface pressure against area are all concepts that are unified or linked via the Gibbs adsorption equation. Equation 4.7. The interrelations are presented in Table 7.1 (fcg is the Boltzmann eonstant). 

Figure 14.8 Modelling of the adsorption isotherms (surface excess) of liquid solutions on activated carbon using MPTA. (a) Ethyl acetate + cyclohexane atT = 303.15 K (b) ethanol + cyclohexane atT = 303.15 K. Reprinted from Monsalvo and Shapiro (2007b), with permission from Elsevier...

In the adsorption isotherm, surface phase transition (layering or prewetting) should appear as a sharp vertical step at some pressure of a water vapor below the saturated value (Fig. 10). At this particular pressure, two water phases coexist on the surface, and relative fraction of these phases depends on the average surface coverage. Experimental studies of water adsorption on various surfaces give information about the occurrence of the surface phase transitions. In some cases, the corresponding step in the... 

Influence of temperature on the Ni loading on titania Anatase A. A. was selected for this study due to its higher SSA (table 1). In figure 1 it is shown a representative adsorption isotherm (surface Ni concentration vs initial Ni concentration) at two temperatures of the impregnation suspension. 

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