Measurements adsorption kinetics

For laboratory-scale modification, distinction has to be made between static and dynamic adsorption procedures. In a static procedure, the substrate is contacted with a known volume of gas at a well-defined pressure. The modifying gas may be stationary or circulating in a closed loop. Modification in a static gas adsorption apparatus allows the careful control of all reaction parameters. Temperature and pressure can be controlled and easily measured. Adsorption kinetics may be determined by following the pressure as a function of the reaction time. Figure 8.13 displays a volumetric adsorption apparatus, in which mercury is used, as a means to change the internal volume and for pressure measurement. 

Microbalances have been used to measure adsorption kinetics under a wide variety of conditions. Several authorshave studied adsorption on single crystal surfaces of semiconductors under ultra high vacuum conditions and utilising ion bombardment to obtain a clean surface. Gulbransen et al and Walker et have made a special study of microbalance operation at high temper-... 

Beside the very frequently used methods of dynamic surface and interfacial tension measurements, adsorption kinetics processes at liquid interfaces can also be studied by other methods, such as dynamic surface potentials, ellipsometry and other light scattering and reflection methods, X-ray techniques, neutron scattering, radiotracer techniques. These methods yield more or less relative information on the change of adsorption with time at different time resolutions. 

This is the most useful technique for measuring adsorption kinetics at short times, particularly if correction for the sodead time , ra, is made. The dead time is simply the time required to detach the bubble after it has reached its hemispherical shape. Figure 11.23 gives a scheme of the principle of maximum bubble pressure, showing the evolution of a bubble at the tip of a capillary as well as the variation of pressure p in the bubble with time. 

If Langmuir adsorption occurs, then a plot of 9 versus p for a particular isothenn will display the fonn of equation (Al.7.3). Measurements of isothenns are routinely employed in this manner in order to detennine adsorption kinetics. 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

Adsorption Kinetics. In zeoHte adsorption processes the adsorbates migrate into the zeoHte crystals. First, transport must occur between crystals contained in a compact or peUet, and second, diffusion must occur within the crystals. Diffusion coefficients are measured by various methods, including the measurement of adsorption rates and the deterniination of jump times as derived from nmr results. Factors affecting kinetics and diffusion include channel geometry and dimensions molecular size, shape, and polarity zeoHte cation distribution and charge temperature adsorbate concentration impurity molecules and crystal-surface defects. 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

D. Axelrod, R. M. Fulbright, and E. H. Hellen, Adsorption kinetics on biological membranes Measurement by total internal reflection fluorescence, in Applications of Fluorescence in the Biomedical Sciences (D. L. Taylor, A. S. Waggoner, F. Lanni, R. F. Murphy, and R. Birge, eds.), pp. 461-467, Alan R. Liss, New York (1986). 

X ray photoelectron spectroscopy (XPS) is powerful in identifying species present at the surface/interface and atoms or functional groups involved in acid-base interactions [116]. Since XPS measures the kinetic energy of photoelectrons emitted from the core levels of surface atoms upon X ray irradiation of the uppermost atomic layers, it can be used to characterize surface acid sites, in combination with base probe molecules adsorption. 

Kumar N, Couzis A, Maldarelli C (2003) Measurement of kinetic rate constants for the adsorption of superspreading trisiloxanes to an air/aqueous interface and the rele-... 

Che and Naccache (199) have studied the kinetics of 02 formed on slightly reduced anatase using EPR. They found that the adsorption could be explained on the basis of different formation rates for 02 adsorbed at different sites, with zero- and first-order kinetics for the oxygen and Ti3+ concentrations, respectively. Using the same approach, Hauser (200) has extended this work and proposed different models to explain the kinetics based on the formation of 02, O2-, and 02 ions for which activation energies around 1 kcal/mol were obtained. Nikisha et al. (201) have studied the oxygen adsorption kinetics using EPR, conductivity, and volumetric measurements. 

Figure D3.5.6 Adsorption kinetics of a small molecule surfactant. Surface tension of polyoxyethylene (10) lauryl ether (Brij) at the air-water interface decreases as time of adsorption increases. Brij concentration is 0.1 g/liter, as measured by the drop volume technique and the maximum bubble pressure method (UNITD3.6).

It has recently been shown that the inverted setup is better suited to measurement of the adsorption kinetics of protein samples at, e.g., the oil/water interface since it prevents reservoir depletion. Reservoir depletion can occur if the concentration of surfactant in the solvent phase is low. If the protein is present in the drop-forming phase, then the concentration within the drop itself may decrease during the adsorption process. This in turn would affect the measured rate of adsorption. In this case, it is preferable to form an inverted oil droplet in the protein solvent. 

Modem drop volume tensiometers are connected to a computer with sophisticated software that can be used to automatically record the surface tension as a function of the true interfacial age. Adsorption kinetics experiments with the drop volume technique can be conducted using either the constant drop formation method or the quasistatic method (for details, see Commentary). The choice of the dynamic measurement method depends primarily on the time range over which the adsorption kinetics needs to be measured. 

While the quasistatic method is quite accurate, it requires a long time to determine a complete adsorption kinetics curve. This is because a new drop has to be formed at the tip of the capillary to determine one single measurement point. For example, if ten dynamic interfacial tension values are to be determined over a period of 30 min, -180 min will be required to conduct the entire measurement. On the other hand, the constant drop formation method is often limited because a large number of droplets have to be formed without interruption, which may rapidly empty the syringe. Furthermore, the critical volume required to cause a detachment of droplets depends on the density difference between the phases. If the density difference decreases, the critical volume will subsequently increase, which may exacerbate the problem of not having enough sample liquid for a complete run. 

An almost overwhelmingly large number of different techniques for measuring dynamic and static interfacial tension at liquid interfaces is available. Since many of the commercially available instruments are fairly expensive to purchase (see Internet Resources), the appropriate selection of a suitable technique for the desired application is essential. Dukhin et al. (1995) provides a comprehensive overview of currently available measurement methods (also see Table D3.6.1). An important aspect to consider is the time range over which the adsorption kinetics of surface-active substances can be measured (Fig. D3.6.5). For applications in which small surfactant molecules are primarily used, the maximum bubble pressure (MBP) method is ideally suited, since it is the only... 

As described below, two different measurement variations exist to determine adsorption kinetics of surface-active substances continuous drop formation and quasistatic measurements. 

Figure D3.6.8 Drop growth at the tip of a capillary and subsequent drop detachment during adsorption kinetics measurements using the drop volume technique (DVT).

The continuous formation of drops, however, can lead to substantial errors in obtained adsorption kinetic data. For short drop formation times, hydrodynamic effects have to be taken into account. At large flow rates, the measured drop volume at the moment of detachment must be corrected. This is because a finite time is required for the drop meniscus to be disrupted and the drop to detach. Even though the volume has already reached its critical value, fluid may still flow from the reservoir into the drop. The volume of the drop is thus larger than its measured value, which leads to larger calculated interfacial tension values. The shorter the drop formation time is, the larger the error w i 11 be. K1 oubek et al. (1976) were the first to quantify this effect by introducing a corrected critical drop volume, Vc ... 

The adsorption and desorption kinetics of surfactants, such as food emulsifiers, can be measured by the stress relaxation method [4]. In this, a "clean" interface, devoid of surfactants, is first formed by rapidly expanding a new drop to the desired size and, then, this size is maintained and the capillary pressure is monitored. Figure 2 shows experimental relaxation data for a dodecane/ aq. Brij 58 surfactant solution interface, at a concentration below the CMC. An initial rapid relaxation process is followed by a slower relaxation prior to achieving the equilibrium IFT. Initially, the IFT is high, - close to the IFT between the pure solvents. Then, the tension decreases because surfactants diffuse to the interface and adsorb, eventually reaching the equilibrium value. The data provide key information about the diffusion and adsorption kinetics of the surfactants, such as emulsifiers or proteins. 

Attempts were made to measure the kinetics of adsorption on the other oxides. In the case of MgO, the uptake was very small and very approximate kinetic measurements at pressures of 10-2 mm. and below gave activation energies of the order of 10-15 kcal./mole, whereas with ZnO the uptake was complicated by oxidation of Zn atoms exposed during the outgassing process an approximate analysis gave an activation energy of 15 6 kcal./mole for what appeared to be true chemisorption at low pressures ( 5 mm.) near saturation. 

The above studies have clearly shown that p-jump relaxation measures chemical kinetics and thus one derives the actual rate constants. The implications these types of measurements have for ascertaining mechanisms of ion exchange and of catalytic reactions on soil constituents is tremendous. The application of p-jump relaxation to studying ion exchange kinetics of NH on zeolite (Ikeda et al., 1984b) and the adsorption/ desorption of Pb2+ on y-Al203 (Hachiya et al., 1979) is presented below,... 

For the measuring of adsorption kinetics two criteria have to be met. The amount of compound adsorbed has to be quantified, and analyses have to be performed with short time intervals. Both these criteria are difficult to meet for reactions in solution. Various researchers have set up analysis procedures for this type of measurement. 

The adsorption kinetics from time resolved ellipsometry measurements showed two regimes (a) a diffusion controlled process at the initial stages and (b) at longer times, an exponential behavior where the arriving chain must penetrate the barrier formed by the already adsorbed chains. The experimental data indicated that the stars penetrate this barrier faster than the linear chains. 

---