Isotherms, adsorption, additive effect

Adsorption isotherms are an effective means to measure the quantity of enzyme adsorbed as a function of bulk concentration. In addition, the shape of the isotherm gives an indication of the affinity of the enzyme for the solid surface (1,2). By determining... 

The heat of adsorption of Ar was also measured for acidity evaluation. In the case of Ar-TPD, an effect of the probe molecule diffusion in micropores is observed with some samples, such as zeoUtes, at high temperature-programmed rates. The adsorption method is not influenced by diffusion of the adsorbed molecule because the Ar isotherm is measured at static equilibrium. It is also advantageous that the usual BET apparatus can be used to obtain the adsorption isotherm. In addition, the adsorption behavior of Ar is of the Henry type at temperatures around room temperature. 

The adsorption analysis can be performed using the experimental sensor effect data from gravimetric QCM measurements. The frequency shift, which is proportional to the partial pressure of the analyte, is correlated to the number of specific incorporation sites in the linear range of the isotherm slope. Additional evidence for specific interactions between the analyte and the polymer matrix can be demonstrated using infrared spectroscopic analysis [19]. NMR [4,7] can also provide information about imprinting effects. 

Obstacles to practical application of the Langmuir and Freundlich isotherm theories include the following (1) These isotherms do not effectively address adsorption versus degradation and competitive adsorption (2) the conclusions are not all inclusive i.e., adsorption constants and coefficients do not hold true in all cases within similar oil types let alone across different oil types (3) the process has so many variables that the additive variance is commonly too great to prove any subtle difference between clays other than a vastly different level of activity (this problem is especially true when using log vs. log plots with incremental changes on the order of 0.1%) and (4) the adsorption constants and coefficients have limited use for the refiner. 

The width of the instability window depends on three factors, the magnitude of Ay, due to the surfactant adsorption, the curvature of an electrocapillary curve, and the relative location of Ao f with respect to Aq pzc- The quantity Ay,- is a function of, i.e., AG, and the amount of ionic surfactant, and the form of the adsorption isotherm, in addition to r. The effect of the relative location of Aq pzc and on... 

Molybdates, as adsorbent additives, 175 Monolayer adsorption, 41 Monolayer capacity, determination of, 58 Montmorillonite, see Organo clays Multilayer adsorption, 41 effect on isotherm, 59 Multiple development, 30-34 Multistage separations, 352-353... 

Since the goal of preparative chromatography is the production of large quantities of purified compounds, a large sample volume or a large mass of sample or a combination of both is usually applied to the column. This leads to additional effects not commonly encounter in analytical chromatography, such as shifts in retention times and additional broadening of the peaks. In the discussions in the remainder of the book, we tacitly implied the ab ce of such overload effects. For example, we assumed that the concentration of the analytes remains within the linear portion of the adsorption isotherm, while... 

The exact shape and quantitative features of the voltammograms will depend on a variety of parameters, e.g. the adsorption isotherm followed, the surface concentration of the redox species, or the presence or absence of inter-molecular forces between the adsorbed molecules (see Fig. 4b for an example with attractive forces between product molecules) [31-33]. Additional effects such as slow transport of counterions between bulk and the surface layer may further change the characteristics of the voltammograms [34]. 

Fig.5 shows an additive effect on the observed texture. The isotherms are seen to be of type IV (BDDT classificadon) and so characteristic of well-developed mesoporous powders [21]. Compared with pure Zr02, the addidon of ceria improves isotherm hysteresis even if adsorption at p/p, above 0.5 was decreased in extent, and therefore the additive may promote the mesoporosity and increase or sustain surface area. The values of S et function of calcination temperature (see Fig.6) confirm that die additives (typically CeOj here) prevent or retard a decrease in the surface area of the aerogels on calcination. TTiis may be attributed to an improvement of the thermal stability of the tetragonal phase produced by the additives. 

It would be unreasonable to expect GSC to compete with GLC in overall simplicity and effectiveness, yet the practical problems of GSC (and thus the gap between GSC and GLC) have been reduced considerably in recent developments. With sensitive ionization detectors it appears that there is no major difficulty in operating GSC in the linear range of adsorption isotherm. In addition the recent work of Scott shows that GSC is no longer limited solely to low molecular weight compounds hydrocarbons up to 45 carbon atoms were separated with specially modified solids [12). The problem of obtaining reproducible adsorption surfaces is also found to be solved as more is understood about GSC . 

The effect of the precision of z values on the determination of the amount adsorbed is significant because an adsorption isotherm is measured by an accumulative manner in the course of pressure increments. In other words, we can only measure the increment of the amount adsorbed for a given increment of pressure. The equilibrium amount adsorbed rf at a pressure p is the sum of all the adsorption increments up to pressure p. Therefore, the effect of the precision of z values is not local, but accrnnulative, for an isotherm. In addition, a stable state is very difficult to recognize in the vicinity of the critical point Some slow periodic variation of pressure readings adds more xmcertainty to the measurement. This might be the cause of the different results reported in the hterature. 

The simplest mode of IGC is the infinite dilution mode , effected when the adsorbing species is present at very low concentration in a non-adsorbing carrier gas. Under such conditions, the adsorption may be assumed to be sub-monolayer, and if one assumes in addition that the surface is energetically homogeneous with respect to the adsorption (often an acceptable assumption for dispersion-force-only adsorbates), the isotherm will be linear (Henry s Law), i.e. the amount adsorbed will be linearly dependent on the partial saturation of the gas. The proportionality factor is the adsorption equilibrium constant, which is the ratio of the volume of gas adsorbed per unit area of solid to its relative saturation in the carrier. The quantity measured experimentally is the relative retention volume, Vn, for a gas sample injected into the column. It is the volume of carrier gas required to completely elute the sample, relative to the amount required to elute a non-adsorbing probe, i.e. 

Electrolytes are used to promote the exhaustion of direct or reactive dyes on cellulosic fibres they may also be similarly used with vat or sulphur dyes in their leuco forms. In the case of anionic dyes on wool or nylon, however, their role is different as they are used to facilitate levelling rather than exhaustion. In these cases, addition of electrolyte decreases dye uptake due to the competitive absorption of inorganic anions by the fibre and a decrease in ionic attraction between dye and fibre. In most discussions of the effect of electrolyte on dye sorption, attention is given only to the ionic aspects of interaction. In most cases, this does not create a problem and so most adsorption isotherms of water-soluble dyes are interpreted on the basis of Langmuir or Donnan ionic interactions only. There are, however, some observed cases of apparently anomalous behaviour of dyes with respect to electrolytes that cannot be explained by ionic interactions alone. 

In addition to the dependence of sorption on the organic fraction of the sorbent, and the KQw of the sorbate, Chiou et al. (13) cite the following observations as support for the hypothesis that the sorptive mechanism is hydrophobic partitioning into the organic (humic) fraction of the sediments (1) the linearity of the isotherms as the concentration approaches solubility, (2) the small effect of temperature on sorption, and (3) the lack of competition between sorbates for the sorbent. These arguments also illustrate the applicability of the approach for modeling sorption on hydro-phobic compounds an approach which has been criticized when used in the context of adsorption of trace metals onto oxides (17). 

Comparison of these adsorption isotherms with those obtained for the linear alkyl aryl sulfonates (Figure 6) reveals the behavior of the 2 ( ) HDBS to be close to that which would be expected for a 1 (t> HDBS and that of the 8 ( ) HDBS to be equivalent to that of a tridecyl benzene sulfonate. Development of a cguantitative model that can account for the effect of the position of the benzene group on the chain warrants additional data for a variety of surfactants with branched chains. 

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