Pure component adsorption

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

Ideal Adsorbed Solution Theory. Perhaps the most successful approach to the prediction of multicomponent equiUbria from single-component isotherm data is ideal adsorbed solution theory (14). In essence, the theory is based on the assumption that the adsorbed phase is thermodynamically ideal in the sense that the equiUbrium pressure for each component is simply the product of its mole fraction in the adsorbed phase and the equihbrium pressure for the pure component at the same spreadingpressure. The theoretical basis for this assumption and the details of the calculations required to predict the mixture isotherm are given in standard texts on adsorption (7) as well as in the original paper (14). Whereas the theory has been shown to work well for several systems, notably for mixtures of hydrocarbons on carbon adsorbents, there are a number of systems which do not obey this model. Azeotrope formation and selectivity reversal, which are observed quite commonly in real systems, ate not consistent with an ideal adsorbed... 

Many simple systems that could be expected to form ideal Hquid mixtures are reasonably predicted by extending pure-species adsorption equiUbrium data to a multicomponent equation. The potential theory has been extended to binary mixtures of several hydrocarbons on activated carbon by assuming an ideal mixture (99) and to hydrocarbons on activated carbon and carbon molecular sieves, and to O2 and N2 on 5A and lOX zeoHtes (100). Mixture isotherms predicted by lAST agree with experimental data for methane + ethane and for ethylene + CO2 on activated carbon, and for CO + O2 and for propane + propylene on siUca gel (36). A statistical thermodynamic model has been successfully appHed to equiUbrium isotherms of several nonpolar species on 5A zeoHte, to predict multicomponent sorption equiUbria from the Henry constants for the pure components (26). A set of equations that incorporate surface heterogeneity into the lAST model provides a means for predicting multicomponent equiUbria, but the agreement is only good up to 50% surface saturation (9). 

Note that knowledge of the initial slopes of the adsorption isotherms gives some constraint to be fullfilled between parameters X, N, and K. In order to fit the adsorption isotherms, frontal analysis has performed with the pure components at 1, 25, 50, 75 and 100 g on the analytical column at 1 ml min k... 

Pure Component Adsorption and Specificity with Respect to Zeolite Topology... 

The interaction of adsorbed reactants (phenol and methanol adsorbed separately and coadsorbed) and possible reaction products of phenol methylation with the Cul-xCoxFe204 system has been studied at temperatures between lOOoC and 350oC and probed by in situ FTIR spectroscopy. The spectra of adsorbed methanol, phenol and methylated products on catalyst surface, at lOOoC, did not possess much changes compared to the spectra of pure components that indicated the molecular adsorption of species on catalyst surface. The remarkable changes in the spectra occur, above 100°C due to the chemisorption of substrates, were observed and correlated with the observed reaction trend. 

When two similarly structured anionic surfactants adsorb on minerals, the mixed admicelle approximately obeys ideal solution theory (jUL - Below the CMC, the total adsorption at any total surfactant concentration is intermediate between the pure component adsorption levels. Adsorption of each surfactant component in these systems can be easily predicted from pure component adsorption isotherms by combining ideal solution theory with an empirical correspond ng states theory approach (Z3). ... 

Scamehorn et. al. (20) also presented a simple, semi—empirical method based on ideal solution theory and the concept of reduced adsorption isotherms to predict the mixed adsorption isotherm and admicellar composition from the pure component isotherms. In this work, we present a more general theory, based only on ideal solution theory, and present detailed mixed system data for a binary mixed surfactant system (two members of a homologous series) and use it to test this model. The thermodynamics of admicelle formation is also compared to that of micelle formation for this same system. 

Consider the pure surfactant adsorption isotherms shown in Figure 1. At concentrations between the CAC and the CMC, there is a unique concentration level correspond ng to each adsorption level for each pure component. Since this concentration corresponds to formation of admicelles on specific patchs on the surface, for component i, we call the concentration CACi" the variable CAC (no superscript) will be reserved to refer to the concentration which corresponds to admicelle formation on the most energetic patch on the surface. We will only consider binary mixtures of surfactants, so the subscript i can refer to either component A or B. For a surfactant mixture, the total surfactant concentration required to reach a specified adsorption level is defined as CACm. ... 

A previously proposed theory to describe mixed adsorption in these systems (20) depended not only on ideal solution theory, but also on the correspond ng states theory to apply to surfactant mixtures. In that model, it was assumed that the adsorption isotherms for the pure components coincided when plotted against a reduced concentration. This occurs when the ratio CACB E/CACrt is the same at any adsorption level. When true, this simplifies the prediction of mixed adsorption isotherms somewhat, but that model is really a special case of the model presented here. 

Pure component loadings for CO2, N2 and O2 on commercial pelleted forms of Linde type 4A, 5A and 13X molecular sieve zeolites were derived from various gravimetric and volumetric measurements. The range of pressures and temperatures over which these measurements were made were at least as broad as those encountered in the breakthrough experiments described here, to permit accurate estimations of heats of adsorption in the manner described by equation (6) above. As mentioned above, the pure component data were correlated to the LRC model, and the CO2 loadings predicted by the multicomponent LRC model compared to actual loadings in the breakthrough runs at bed saturation. 

From a true catalytic point of view, what is looked for are so-called synergetic effects , i.e., a reciprocal influence between two or more components so as to obtain a material whose activity exceeds that of the pure components [74]. This usually involves intimate electronic interaction between the various components so that their electronic structures become profoundly modified. It is well possible that a metal deprived of part of its valence electrons may behave as the element on its left in the Periodic Table [75]. However, the theory of synergetic effects is still in its infancy in electrochemistry. Predictions for a bimetallic catalyst with two non-interacting sites obtained by combining two metals with different adsorption energies are that... 

For selective acetylene adsorption from other hydrocarbons (e.g., ethylene and ethane), NiCl2 supported on alumina or silica can form reversible jr-complexation bonds with acetylene but not olefins. Pure component acetylene-ethylene ratios of up to 3 were obtained (Kodde et al., 2000). The bonding between acetylene and NiCl2 is reasonably understood (Huang and Yang, 1999). 

Integration of this dependence through the whole concentration range actually allows the calculation of the column void volume, or the total volume of the liquid phase in the column. Since excess adsorption of pure component is equal to 0 (F(0) = r(lOO) = 0), then... 

Theoretical predictions of ideal mixture adsorption can be made on an a priori basis if the pure component adsorption isotherms are known. 

Scamehorn et. al. ( ) also developed a reduced adsorption equation to describe the adsorption of mixtures of anionic surfactants, which are members of homologous series. The equations were semi-empirical and were based on ideal solution theory and the theory of corresponding states. To apply these equations, a critical concentration for each pure component in the mixture is chosen, so that when the equilibrium concentrations of the pure component adsorption isotherms are divided by their critical concentrations, the adsorption isotherms would coincide. The advantage of... 

The adsorption of binary mixtures of anionic surfactants in the bilayer region has also been modeled by using just the pure component adsorption isotherms and ideal solution theory to describe the formation of mixed admicelles (3 ). Positive deviation from ideality in the mixed admicelle phase was reported, and the non-ideality was attributed to the planar shape of the admicelle. However, a computational error was made in comparison of the ideal solution theory equations to the experimental data, even though the theoretical equations presented were correct. Thus, the positive deviation from ideal mixed admicelle formation was in error. 

Q from the pure component adsorption isotherms by reading... 

The only information needed to predict the mixture surfactant concentration to attain a specified adsorption level is the pure component adsorption isotherms measured at the same experimental conditions as the mixture isotherms. These isotherms are needed to obtain the pure component standard states. 

The method of predicting the mixture adsorption isotherms is to first select the feed mole fractions of interest and to pick an adsorption level within Region II. The pure component standard states are determined from the total equilibrium concentration that occurs at that set level of adsorption for the pure surfactant component adsorption isotherms. The total equilibrium mixture concentration corresponding to the selected adsorption level is then calculated from Equation 8. This procedure is repeated at different levels of adsorption until enough points are collected to completely descibe the mixture adsorption isotherm curve. 

---