Adsorption specific nonspecific

The adsorption of organic ligands onto metal oxides and the parameters that have the greatest effect on adsorption were also studied (Stone et al., 1993). The extent of adsorption was measured by determining the loss of the compound of interest from solution. The physical and chemical forces that control adsorption into two general categories were classified as either specific or nonspecific adsorptions. Specific adsorption involves the physical and chemical interaction of the adsorbent and adsorbate. Under specific adsorption, the chemical nature of the sites influences the adsorptive capacity. Nonspecific adsorption does not depend on the chemical nature of the sites but on characteristics such as surface charge density (Stone et al., 1993). The interactions of specific adsorption can be explained in two ways. The first approach uses activity coefficients to relate the electrochemical activity at the oxide/water interface to its electrochemical activity in bulk solution (Stone et al., 1993). This approach is useful in situations... 

Thus, the determination of Qi and Ci values on the basis of measurements carried out at ordinary temperatures in these cases becomes difii-cult. In such cases, however, one can approximately estimate the additional contribution of the energy of specific interaction arising from TT-bonds or the functional groups at a somewhat higher a—e.g., when values of a correspond to 1 molecule per cavity. AQspecific is determined by subtracting the heat of adsorption of the reference molecule (incapable to specific interaction) from the heat of adsorption of the specifically adsorbed molecule 10, 28S0). Both molecules must have similar geometry, close values of polarizability and of heat of adsorption on nonspecific adsorbent (6). 

Two types of adsorptions differ in several ways. The most important difference between the two kinds of adsorption is the magnitude of the enthalpy of adsorption. In physical adsorption the enthalpy of adsorption is of the same order as the heat of liquefaction and does not usually exceed 10 to 20 KJ per mol, whereas in chanisorption the enthalpy change is generally of the order of 40 to 400 KJ per mol. Htysical adsorption is nonspecific and occurs between any adsorbate-adsorbent systans, but chemisorption is specific. Another important point of difference between physisorption and chemisorption is the thickness of the adsorbed phase. Although it is multimolecular in physisorption, the thickness is unimolecular in chanisorption. The type of adsorption that takes place in a given adsorbate-adsorbent systan depends on the nature of the adsorbate, the nature of the adsorbent, the reactivity of the surface, the surface area of the adsorbate, and the temperature and pressure of adsorption. 

Quartz crystal microbalance (QCM) is a well-established technique for monitoring surface deposition as the natural frequency of quartz decreases when mass is deposited onto the chip. This frequency change can be directly related to film formation and can be monitored in real time. The QCM experiment in Figure 15b depicts this phenomenon and indicates that polymer is rapidly adsorbed onto SAMs with the selectively expected from the DAP-Thy recognition dyad. These data also provide a method for determining the varying contributions of adsorption from nonspecific or specific interactions. Any frequency change in the case of A -methylated SAM 17 is due to nonspecific adsorption and can be subtracted from the response observed with SAM 16 to isolate the contribution of specific adsorption processes. 

On the other hand, the changes caused by specific adsorption of anions in the solution side of the double layer can also be followed through the measurement of TCO4 adsorption. Specifically adsorbed anions depending on the extent of their adsorption compensate the positive charge of a protonated surface therefore, the negative charge on the solution side, i.e., the surface excess concentration of nonspecifically adsorbed anions, should decrease. This effect is well demonstrated by the curve presented in Fig. 9. 

Some authors have suggested the use of fluorene polymers for this kind of chromatography. Fluorinated polymers have attracted attention due to their unique adsorption properties. Polytetrafluoroethylene (PTFE) is antiadhesive, thus adsorption of hydrophobic as well as hydrophilic molecules is low. Such adsorbents possess extremely low adsorption activity and nonspecific sorption towards many compounds [109 111]. Fluorene polymers as sorbents were first suggested by Hjerten [112] in 1978 and were tested by desalting and concentration of tRN A [113]. Recently Williams et al. [114] presented a new fluorocarbon sorbent (Poly F Column, Du Pont, USA) for reversed-phase HPLC of peptides and proteins. The sorbent has 20 pm in diameter particles (pore size 30 nm, specific surface area 5 m2/g) and withstands pressure of eluent up to 135 bar. There is no limitation of pH range, however, low specific area and capacity (1.1 mg tRNA/g) and relatively low limits of working pressure do not allow the use of this sorbent for preparative chromatography. 

Generally, for specific and nonspecific adsorption, the following equation is derived. 

Finally, it should be remarked that, as long as the interfacial region is extended sufficiently to include all structural and electronic deviations from the reservoirs, (5.18) and (5.19) are valid for any type of connection between a metallic electrode and an electrolyte. They also include the cases of nonspecific and specific adsorption on the electrode. 

Exchangeable ions (EXC), sometimes including ions nonspecifically adsorbed and specifically absorbed on the surface of various soil components, such as carbonate, organic matter, Fe, Mn, Si, and Al oxides, and clay minerals. This part is controlled by adsorption-desorption processes. 

A major disadvantage is that the direct sensor detection cannot distinguish between the sensor response to the specific analyte binding from the response to a possible nonspecific adsorption of other compounds. The nonspecific fouling from blood or blood serum seems to be one of the main barriers for practical application of immunosensors in medical diagnostics. 

Uchida K, Otsuka H, Kaneko M, Kataoka K, Nagasaki Y (2005) A reactive poly(ethylene glycol) layer to achieve specific surface plasmon resonance sensing with a high S/N ratio the substantial role of a short underbrushed PEG layer in minimizing nonspecific adsorption. Anal Chem 77 1075-1080... 

Basic molecules such as pyridine and NH3 have been the popular choice as the basic probe molecules since they are stable and one can differentiate and quantify the Bronsted and Lewis sites. Their main drawback is that they are very strong bases and hence adsorb nonspecifically even on the weakest acid sites. Therefore, weaker bases such as CO, NO, and acetonitrile have been used as probe molecules for solid acid catalysts. Adsorption of CO at low temperatures (77 K) is commonly used because CO is a weak base, has a small molecular size, a very intense vc=0 band that is quite sensitive to perturbations, is unreactive at low temperature, and interacts specifically with hydroxyl groups and metal cationic Lewis acid sites.26... 

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