Adsorption, isotherms effects

The adsorption isotherm corresponding to Eq. X-51 is of the shape shown in Fig. X-1, that is, it cannot explain contact angle phenomena. The ability of a liquid him to coexist with bulk liquid in a contact angle situation suggests that the him structure has been modihed by the solid and is different from that of the liquid, and in an enmirical way, this modihed structure corresponds to an effective vapor pressure F , F representing the vapor pressure that bulk liquid would have were its structure that of the... 

There is a number of very pleasing and instructive relationships between adsorption from a binary solution at the solid-solution interface and that at the solution-vapor and the solid-vapor interfaces. The subject is sufficiently specialized, however, that the reader is referred to the general references and, in particular, to Ref. 153. Finally, some studies on the effect of high pressure (up to several thousand atmospheres) on binary adsorption isotherms have been reported [154]. Quite appreciable effects were found, indicating that significant partial molal volume changes may occur on adsorption. 

The first term on the right is the common inverse cube law, the second is taken to be the empirically more important form for moderate film thickness (and also conforms to the polarization model, Section XVII-7C), and the last term allows for structural perturbation in the adsorbed film relative to bulk liquid adsorbate. In effect, the vapor pressure of a thin multilayer film is taken to be P and to relax toward P as the film thickens. The equation has been useful in relating adsorption isotherms to contact angle behavior (see Section X-7). Roy and Halsey [73] have used a similar equation earlier, Halsey [74] allowed for surface heterogeneity by assuming a distribution of Uq values in Eq. XVII-79. Dubinin s equation (Eq. XVII-75) has been mentioned another variant has been used by Bonnetain and co-workers [7S]. 

Fig. 2.27 Effect of mesoporosily on the adsorption isotherm and the t- (or a,-) plot, (a) (A) is the isotherm on a nonporous sample of the adsorbent (B) is the isotherm on the same solid when mesopores have been introduced into it, (i) being the adsorption, and (ii) the desorption branch. (b) I- (or a,-) plots corresponding to the isotherms in (a) (Schematic only.)...

The effect of these factors on the adsorption isotherm may be elucidated by reference to specific examples. In the case of the isotherm of Fig. 5.17(a), the nonporous silica had not been re-heated after preparation, but had been exposed to near-saturated water vapour to ensure complete hydroxylation. The isotherm is of Type II and is completely reversible. On the sample outgassed at 1000°C (Fig. 5.17(h)) the isotherm is quite different the adsorption branch is very close to Type III, and there is extrensive hysteresis extending over the whole isotherm, with considerable retention of adsorbate on outgassing at 25°C at the end of the run. 

Vapor Pressures and Adsorption Isotherms. The key variables affecting the rate of destmction of soHd wastes are temperature, time, and gas—sohd contacting. The effect of temperature on hydrocarbon vaporization rates is readily understood in terms of its effect on Hquid and adsorbed hydrocarbon vapor pressures. For Hquids, the Clausius-Clapeyron equation yields... 

Adsorption. Adsorption (qv) is an effective means of lowering the concentration of dissolved organics in effluent. Activated carbon is the most widely used and effective adsorbent for dyes (4) and, it has been extensively studied in the waste treatment of the different classes of dyes, ie, acid, direct, basic, reactive, disperse, etc (5—22). Commercial activated carbon can be prepared from lignite and bituminous coal, wood, pulp mill residue, coconut shell, and blood and have a surface area ranging from 500—1400 m /g (23). The feasibiUty of adsorption on carbon for the removal of dissolved organic pollutants has been demonstrated by adsorption isotherms (24) (see Carbon, activated carbon). Several pilot-plant and commercial-scale systems using activated carbon adsorption columns have been developed (25—27). 

Volume overload results from too large a volume of sample being placed on the column, and this effect will be discussed later. It will be seen that volume overload does not, in itself, produce asymmetric peaks unless accompanied by mass overload, but it does broaden the peak. Mass overload, however, frequently results in a nonlinear adsorption isotherm. However, the isotherm is quite different from the Langmuir isotherm and is caused by an entirely different phenomenon. 

The Langmuir adsorption isotherm for radioligand binding [A ] to a receptor to form a radioligand-receptor complex [A R] can be rewritten in terms of one where it is not assumed that receptor binding produces a negligible effect on the free concentration of ligand ([A free]) ... 

Current use of statistical thermodynamics implies that the adsorption system can be effectively separated into the gas phase and the adsorbed phase, which means that the partition function of motions normal to the surface can be represented with sufficient accuracy by that of oscillators confined to the surface. This becomes less valid, the shorter is the mean adsorption time of adatoms, i.e. the higher is the desorption temperature. Thus, near the end of the desorption experiment, especially with high heating rates, another treatment of equilibria should be used, dealing with the whole system as a single phase, the adsorbent being a boundary. This is the approach of the gas-surface virial expansion of adsorption isotherms (51, 53) or of some more general treatment of this kind. 

Typical adsorption isotherms are shown in Figs. 16 and 17. Despite the large experimental scatter, a steep increase in adsorption can be seen at low concentrations, followed by a plateau at concentrations exceeding the CMC. Similar behavior has been observed before with model surfactants [49-54] and has also been predicted by modem theories of adsorption [54]. According to Fig. 16, adsorption increases modestly with salinity provided that the calcium ion concentration remains low. The calcium influence, shown in Fig. 17, cannot be explained by ionic strength effects alone but may be due to calcium-kaolinite interactions. 

Such a model should be as simple as possible, without however missing any of the underlying thermodynamic and physicochemical factors which cause electrochemical promotion. In particular it will be shown that even the use of Langmuir-type adsorption isotherms, appropriately modified due to the application of potential (or equivalently by the presense of promoters) suffice to describe all the experimentally observed rules G1 to G7 as well as practically all other observations regarding electrochemical promotion including the effect of potential on heats of adsorption as well as on kinetics and reaction orders. 

The reason for enhancement of adsorption performance of PA/AC was considered to be due to combination effect of increase of BET surface area and chemical modification by the treatment with PA. Consequently, lwt%-PA/AC was determined to be a best candidate as an adsorbent for removing benzene, toluene, p-xylene, methanol, ethanol, and iso-propanol. Therefore, lwt%-PA/AC was used as the adsorbent to investigate the adsorption isotherm, adsorption and desorption performance. 

Before an antibacterial agent can exert its effect on a cell it must eombine with that eell. This process often follows the pattern of an adsorption isotherm. Clearly, faetors whieh affect the state of the cell surface, as the pH of the eelTs environment must do, must affect, to some extent, the adsorption process. An inerease in the external pH renders the cell surface more negatively charged. Bioeidal agents that are eationie in nature thus bind more strongly to the cell surface with a eonsequent inerease in aetivity. 

The SH signal directly scales as the square of the surface concentration of the optically active compounds, as deduced from Eqs. (3), (4), and (9). Hence, the SHG technique can be used as a determination of the surface coverage. Unfortunately, it is very difficult to obtain an absolute calibration of the SH intensity and therefore to determine the absolute number for the surface density of molecules at the interface. This determination also entails the separate measurement of the hyperpolarizability tensor jS,-, another difficult task because of local fields effects as the coverage increases [53]. However, with a proper normalization of the SH intensity with the one obtained at full monolayer coverage, the adsorption isotherm can still be extracted through the square root of the SH intensity. Such a procedure has been followed at the polarized water-DCE interface, for example, see Fig. 3 in the case of 2-( -octadecylamino)-naphthalene-6-sulfonate (ONS) [54]. The surface coverage 6 takes the form ... 

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