Adsorption surface resistance

A porous adsorbent in contact with a fluid phase offers at least two and often three distinct resistances to mass transfer external film resistance and intraparticle diffusional resistance. When the pore size distribution has a well-defined bimodal form, the latter may be divided into macropore and micropore diffusional resistances, Depending on the particular system and the conditions, any one of these resistances may be dominant, or the overall rate of mass transfer may be determined by the combined effects of more than one resistance. The magnitude of the intraparlicle diffusional resistances, or any surface resistance to mass transfer, can be conveniently determined by measuring the adsorption or desorption rate, under controlled conditions, in a batch system. 

The sensor is based on high affinity of gold to mercury and on chemo-resistive properties of ultrathin gold layers adsorption of mercury leads to increase in the surface resistance [1,2]. However, this effect is not selective similar changes are caused by adsorption of water vapor and sulfuric compounds. The use of monomolecular layer of alkylthiols as a filter excludes this interference completely [3]. Sensor calibration is performed by thermoinjection of nanogram-amount of mercury quantitatively deposited by electrochemical reduction [4],... 

When a polymer is treated with enzymes for surface modification, some of the undesired protein tends to adsorb on the polymer surface, which subsequently creates problems in the surface analysis and causes a slow down in the rate of catalysis. Adsorbed proteins can be removed from the surfaces by washing with large volumes of 1.5% Na2C03 and water (Eischer-Colbrie et al., 2006) as part of a preparation for surface analysis. Protein-resistant molecules such as polyethylene glycol can be used to prevent the nonspecific protein adsorption. Surfaces can be precoated with an inert protein such as bovine serum albumin (Salisbury et al., 2002) for increasing the rate of catalysis. 

Bhatia SK, Teixeira JL, Anderson M et al (1993) Fabrication of surfaces resistant to protein adsorption and application to 2-dimensional protein patterning. Anal Biochem 208 197-205 Liu XH, Wang HK, Herron JN et al (2000) Photopatteming of antibodies on biosensors. Bioconj Chem 11 755-761... 

In many industrial reactions, the overall rate of reaction is limited by the rate of mass transfer of reactants and products between the bulk fluid and the catalytic surface. In the rate laws and cztalytic reaction steps (i.e., dilfusion, adsorption, surface reaction, desorption, and diffusion) presented in Chapter 10, we neglected the effects of mass transfer on the overall rate of reaction. In this chapter and the next we discuss the effects of diffusion (mass transfer) resistance on the overall reaction rate in processes that include both chemical reaction and mass transfer. The two types of diffusion resistance on which we focus attention are (1) external resistance diffusion of the reactants or products between the bulk fluid and the external smface of the catalyst, and (2) internal resistance diffusion of the reactants or products from the external pellet sm-face (pore mouth) to the interior of the pellet. In this chapter we focus on external resistance and in Chapter 12 we describe models for internal diffusional resistance with chemical reaction. After a brief presentation of the fundamentals of diffusion, including Pick s first law, we discuss representative correlations of mass transfer rates in terms of mass transfer coefficients for catalyst beds in which the external resistance is limiting. Qualitative observations will bd made about the effects of fluid flow rate, pellet size, and pressure drop on reactor performance. 

To be suitable for long-term wear, a contact lens material has to satisfy several criteria. It must be hydrophilic enough to maintain a stable, continuous tear film on its surface, resist fouling by tear components, not irritate the eye, and be comfortable to wear. It should also be realized that the metabolism of the cornea is highly dependent on dissolution and adsorption of atmospheric oxygen and therefore any lens material must have sufficient oxygen permeability to maintain this, else corneal anoxia will set in. 

Changes in resistivity are restricted to thin films or thin wires [194] in which the surface layer contributes a measureable fraction of the overall conductivity and such measurements are often run alongside measurements of heats of adsorption [195]. Resistance changes due to adsorption have been attributed to a demetallisation of the surface layer of metals [196] and to diffuse scattering of metal conduction electrons from virtual... 

Fig. 17 Transient concentration profiles in y-direction (i.e., along 8-ring channels) measured by interference microscopy for a adsorption and b desorption of methanol in a large crystal of ferrierite for pressure steps 5 -> 10 and 10 5 mbar. The form of the profiles shows that both surface resistance and internal diffusion (along the 8-ring chan-...

Ammonia, which possesses a large dipole moment, has been used extensively as a probe molecule for the characterisation of both Lewis and Bronsted acidic sites. Figure 22 shows the significant difference in the FR data between ammonia in zeohte crystals and in pellets. The FR spectra of ammonia in zeolite crystals demonstrated that the rate of the ammonia adsorption on different acidic sites in the crystals controls the overall dynamics of the processes occurring in the systems, hi the case of pellets, the rate-controlhng step was found to be macropore diffusion with (Fig. 22a,2,b,2) or without (Fig. 22c,2) surface resistances [77]. 

The sorption (adsorption or desorption) rate is measured with a sorption balance (spring or electrical) whereas the solid sample is kept in a controlled environment. Assuming negligible surface resistance to mass transfer, the method is based on Pick s diffusion equation. 

Chemical sensors are generally used in a liquid or a gas medium. In either case their interaction with the sample, and the generation of the primary signal, can take place at the surface (by adsorption), in the bulk (by absorption), or by both mechanisms. This seemingly trivial fact has some important implications with respect to the use of the selective layer in the sensor. For example, an electropolymerized layer can be used in a quartz crystal microbalance in a simple and predictable way with the response originating in the bulk. The use of the same material in a chemi-resistor may depend mostly on the modulation of the surface resistance by adsorption. 

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