Adsorption effectiveness

To this point, we have seen that the efficiency of surfactant adsorption at the solution-vapor interface is dominated by the nature of the hydrophobic group and is relatively httle affected by the hydrophilic head group, ft is often found that the second characteristic of the adsorption process, the so-called adsorption effectiveness, is much more sensitive to other factors and quite often does not parallel the trends found for adsorption efficiency. 

The choice of 20 mN m as a standard value of surface tension lowering for the definition of adsorption efficiency is convenient, but, as mentioned, somewhat arbitrary. When one discusses the effectiveness of adsorption, as defined as the maximum lowering of surface tension regardless of surfactant concentration, the value of o-min is determined only by the system itself and represents a more firmly fixed point of reference. The value of oinm for a given surfactant will be determined by one of two factors (1) the solubility limit or Krafft temperature (Tk) of the compound, or (2) the critical micelle concentration (cmc). In either case, the maximum amount of surfactant adsorbed is reached, for all practical purposes, at the maximum bulk concentration of free surfactant. 

Because the activity of surfactants used below cannot reach their theoretical maximum as determined by the thermodynamics of surfactant aggregation (see also Chapter 15), they will also be unable to achieve their maximum degree of adsorption at the solution-vapor interface. It is therefore important to know the value of Tk for a given system before considering its application. Most surfactants, however, are employed well above their Krafft temperature, so that the controlling factor for the determination of their effectiveness will be the cmc. 

When one examines the shape of the surface tension-ln C curve for a surfactant, it can be seen that the curve becomes approximately horizontal at some concentration below the cmc. It can be shown that the effectiveness of the adsorption of a surfactant, Ao-omo, can be quantitatively related to the concentration of surfactant at which the Gibbs equation becomes linear, Q, the surface tension attained at Ci, o-i, and the cmc. The relationship has the general form 

It is often found that the efficiency and effectiveness of surfactants do not run parallel in fact, it is commonly observed that materials that produce significant lowering of the surface tension at low concentrations (i.e., are more efficient) are less effective (i.e., have a smaller T ). This follows from the complex relationship between adsorption at the interface and micelle formation in the solution. 

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Equation V-64 is that of a parabola, and electrocapillary curves are indeed approximately parabolic in shape. Because E ax tmd 7 max very nearly the same for certain electrolytes, such as sodium sulfate and sodium carbonate, it is generally assumed that specific adsorption effects are absent, and Emax is taken as a constant (-0.480 V) characteristic of the mercury-water interface. For most other electrolytes there is a shift in the maximum voltage, and is then taken to be Emax 0.480. Some values for the quantities are given in Table V-5 [113]. Much information of this type is due to Gouy [125], although additional results are to be found in most of the other references cited in this section. 

The basis of the classification is that each of the size ranges corresponds to characteristic adsorption effects as manifested in the isotherm. In micropores, the interaction potential is significantly higher than in wider pores owing to the proximity of the walls, and the amount adsorbed (at a given relative pressure) is correspondingly enhanced. In mesopores, capillary condensation, with its characteristic hysteresis loop, takes place. In the macropore range the pores are so wide that it is virtually impossible to map out the isotherm in detail because the relative pressures are so close to unity. 

In general there are two factors capable of bringing about the reduction in chemical potential of the adsorbate, which is responsible for capillary condensation the proximity of the solid surface on the one hand (adsorption effect) and the curvature of the liquid meniscus on the other (Kelvin effect). From considerations advanced in Chapter 1 the adsorption effect should be limited to a distance of a few molecular diameters from the surface of the solid. Only at distances in excess of this would the film acquire the completely liquid-like properties which would enable its angle of contact with the bulk liquid to become zero thinner films would differ in structure from the bulk liquid and should therefore display a finite angle of contact with it. 

Table 15. Granular Activated Carbon (GAG) Adsorption Effectively Removes Organics From a Variety of Industrial Wastewaters...

Adsorption. Many studies have been made of the adsorption of soaps and synthetic surfactants on fibers in an attempt to relate detergency behavior to adsorption effects. Relatively fewer studies have been made of the adsorption of surfactants by soils (57). Plots of the adsorption of sodium soaps by a series of carbon blacks and charcoals show that the fatty acid and the alkaU are adsorbed independently, within limits, although the presence of excess aLkaU reduces the sorption of total fatty acids (58). No straightforward relationship was noted between detergency and adsorption. 

Adsorption of bath components is a necessary and possibly the most important and fundamental detergency effect. Adsorption (qv) is the mechanism whereby the interfacial free energy values between the bath and the soHd components (sofld soil and substrate) of the system are lowered, thereby increasing the tendency of the bath to separate the soHd components from one another. Furthermore, the soHd components acquire electrical charges that tend to keep them separated, or acquire a layer of strongly solvated radicals that have the same effect. If it were possible to foUow the adsorption effects in a detersive system, in all their complex ramifications and interactions, the molecular picture of soil removal would be greatly clarified. 

Nontrace isothermal systems give the adsorption effect (i.e., significant change in fluid velocity because of loss or gain of solute). Criteria for the existence of simple waves, contact discontinmties, and shocks are changed somewhat [Peterson and Helfferich, J. Phy.s. Chem., 69, 1283 (1965) LeVan et al., AIChE J., 34, 996 (1988) Frey, AJChE J., 38, 1649(1992)]. 

Second, most membrane materials adsorb proteins. Worse, the adsorption is membrane-material specific and is dependent on concentration, pH, ionic strength, temperature, and so on. Adsorption has two consequences it changes the membrane pore size because solutes are adsorbed near and in membrane pores and it removes protein from the permeate by adsorption in addition to that removed by sieving. Porter (op. cit., p. 160) gives an illustrative table for adsorption of Cytochrome C on materials used for UF membranes, with values ranging from 1 to 25 percent. Because of the adsorption effects, membranes are characterized only when clean. Fouling has a dramatic effect on membrane retention, as is explained in its own section below. 

The complexity of the system increases with the number of solvents used and, of course, their relative concentrations. The process can be simplified considerably by pre-conditioning the plate with solvent vapor from the mobile phase before the separation is started. Unfortunately, this only partly reduces the adsorption effect, as the equilibrium between the solvent vapor and the adsorbent surface will not be the... 

If there is no data available on your particular solvent, it is possible to estimate the adsorption effectiveness by comparing your solvent to solvents that are similar in chemical structure. 

Zorbax PSM packings are produced in three forms unmodified, trimethyl-silane modified, and diol modified. Modified Zorbax PSM packings are produced by chemically bonding a layer on the silica surface through siloxane bonds (Table 3.1). Silanized Zorbax PSM packings suppress adsorption effects and are the preferred choice when the mobile phase contains organic solvents. Unsilanized and diol modified Zorbax PSM packings should be used when the mobile phase consists of aqueous solvents. 

TSK-GEL PW type columns are commonly used for the separation of synthetic water-soluble polymers because they exhibit a much larger separation range, better linearity of calibration curves, and much lower adsorption effects than TSK-GEL SW columns (10). While TSK-GEL SW columns are suitable for separating monodisperse biopolymers, such as proteins, TSK-GEL PW columns are recommended for separating polydisperse compounds, such as polysaccharides and synthetic polymers. 

The Jordi polyethyleneimine phase has been developed in an attempt to block adsorptive effects of the PDVB surfaces toward cationic polymers such as chitosan and quaternary water-soluble flocculants (Fig. 13.17). 

Current ongoing research at Jordi Associates is investigating new ways of bonding hydrophillic groups directly to the aromatic centers of PDVB gels, again in the hope of minimizing adsorptive effects. 

Electrostatic and adsorption effects conspire to make aqueous GPC more likely to be nonideal than organic solvent GPC. Thus, universal calibration is often not obeyed in aqueous systems. Elence, it is much more critical that the standard chosen for calibration share with the polymer being analyzed chemical characteristics that affect these interactions. Because standards that meet this criterion are often not available, it is prudent to include in each analysis set a sample of a secondary standard of the same composition and molecular weight as the sample. Thus, changes in the chromatography of the analyte relative to the standards will be detected. 

Iodides can also be determined by this method, and in this case too there is no need to filter off the silver halide, since silver iodide is very much less soluble than silver thiocyanate. In this determination the iodide solution must be very dilute in order to reduce adsorption effects. The dilute iodide solution (ca 300 mL), acidified with dilute nitric acid, is treated very slowly and with vigorous stirring or shaking with standard 0.1 M silver nitrate until the yellow precipitate coagulates and the supernatant liquid appears colourless. Silver nitrate is then present in excess. One millilitre of iron(III) indicator solution is added, and the residual silver nitrate is titrated with standard 0.1M ammonium or potassium thiocyanate. 

Determination of beryllium by precipitation with ammonia solution and subsequent ignition to beryllium oxide Discussion. Beryllium may be determined by precipitation with aqueous ammonia solution in the presence of ammonium chloride or nitrate, and subsequently igniting and weighing as the oxide BeO. The method is not entirely satisfactory owing to the gelatinous nature of the precipitate, its tendency to adhere to the sides of the vessel, and the possibility of adsorption effects. 

For Yiv > YPv> where y v and Ypv are the surface tensions of liquid and protein, respectively, AFads increases with increasing ysv, predicting decreasing polymer adsorption. An example of this is phosphate buffer saline where y]v = 72.9 mJ/m2 and Ypv is usually between 65 and 70mJ/m2 for most proteins [5]. Therefore, supports for gel-permeation and affinity chromatography should be as hydrophilic as possible in order to minimize undesirable adsorption effects. 

Figure 6.14. CO chemisorption on a transition metal. Molecular orbitals and density of states before (a,b) and after (c and d) adsorption. Effect of varying 0 and EF on electron backdonation (c) and donation (d). Based on Fig. 4 of ref. 98. See text for discussion. Reprinted with permission from Elsevier Science.

The functions of the solution environment will be considered under four sub-headings which are basic requirements, the environment as a reactant, pH effects and double layer and adsorption effects. 

Perhaps the best known example of adsorption effects in electrosynthetic reactions is the beneficial role of tetraalkylammonium ions in the hydrodimerization of acrylonitrile... 

In addition, it is of interest to note that investigations of the microscopic processes leading to nucleation, growth, oriented growth by the surfactant monolayer, and growth inhibition of nanoparticles in reversed micelles and of confinement and adsorption effects on such phenomena represent an intriguing and quite unexplored research field [218]. 

Singh P, Singh R, Gale R, Rajeshwar K, DuBow J (1980) Surface charge and specific ion adsorption effects in photoelectrochemical devices. J Appl Phys 51 6286-6291 Bard AJ, Bocarsly AB, Pan ERF, Walton EG, Wrighton MS (1980) The concept of Fermi level pinning at semiconductor/liquid junctions. Consequences for energy conversion efficiency and selection of useful solution redox couples in solar devices. J Am Chem Soc 102 3671-3677... 

The effect of pH in the range studied is completely eliminated by addition of 300,000 MW polyethylene oxide with "collapse" of the data onto a single calibration curve. This competitive adsorption effect will be discussed more fully in section 4. 

Figures 6, 7 and 9 show calibration curves using two multi-column combinations and illustrate the degree of "optimization obtained in this system. The mobile phases for Figures 6 and 7 contained 0.025 g polyethylene oxide and ion exclusion and adsorption effects should therefore be largely eliminated. Figure 6 shows that reasonably good resolution can be obtained with a combination of five columns but does exhibit some loss of peak separation at the low cuid high MW ends. In Figure 7 the effect of adding a sixth column of small pore size is illustrated and it is seen that resolution at the low MW end is thereby somewhat improved. This calibration curve is effectively linear with a change of slope at 500,000 MW. It should provide a useful aqueous GPC system for MW and MWD determination of nonionic polyacrylamides.

Wang JX, Markovic NM, Adzic RR. 2004. Kinetic analysis of oxygen reduction on Pt(ll 1) in acid solutions Intrinsic kinetic parameters and anion adsorption effects. J Phys Chem B 108 ... 

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