Thickness adsorption time

The thickness, t, of the adsorbed layer for NaPSS-3 is plotted against adsorption time as shown in Figure 2. Hie t also increases with increasing adsorption time and becomes a constant value. Similar time dependence was obtained in the other experiments. Therefore, both Ap and t determined after 1.5 x 103 minutes were taken as the equilibrium values. 

Figure 2. Thickness, t, of the adsorbed layer for NaPSS-3 as a function of adsorption time. Symbols are the same as in Figure 1.

Fig. 2. Upper panel acJsorption kinetics for a 0.02 mg/ml PLL- -PEG solution in HEPES 2 as measured by OWLS (adsorbed mass vs time, line) and VASE (layer thickness vs time. ). 17 min is needed to reach a plateau value in the adsorption curve. Lower panel direct comparison of layer thickness from VASE and EG monomer surface daisity derived from OWLS measurem ts for different adsorption times. A good correlation was found betweai both techniques, allowing for a convCTsion of adsorbed thickness into EG monomer surface density rec-...

Langmuir (1916), whp put forward the fir quantitative theory of the adsorption of a gaS, assumed that a gas molecule condensing from the gas phase-would adhere to the surface fora short time before evaporating and that the condensed layer was only one atom or molecule thick. If 0 is the fraction of the surface area covered by adsorbed molecules at any time, the rate of desorption is proportional to 0 and equal to k 0 where is a constant at constant temperature. Similarly the rate of adsorption will be proportional to the area of bare surface and to the rate at which the molecules strike the surface (proportional to the gas pressurep). At equilibrium the rate of desorption equals the rate of adsorption... 

Figure 4 illustrates the dependence of on Aq for the case when r = 1 at several different values of [Fig. 4(a)] and when = 0.5 and at several different values of r [Fig. 4(b)]. From Fig. 4(a), one can see that takes a maximum around y = 0, i.e., Aq The volume ratio affects strongly the value of as shown in Fig. 4(b), which is ascribed to the dependence of the equilibrium concentration on r through Eq. (25). This simple example illustrates the necessity of taking into account the variation of the phase-boundary potential, and hence the adsorption of i, when one tries to measure the adsorption properties of a certain ionic species in the oil-water two-phase systems by changing the concentration of i in one of the phases. A similar situation exists also in voltammetric measurements of the transfer of surface-active ions across the polarized O/W interface. In this case, the time-varying thickness of the diffusion layers plays the role of the fixed volume in the above partition example. The adsorption of surface-active ions is hence expected to reach a maximum around the half-wave potential of the ion transfer. 

The effect of temperature on distribution ratios has already been mentioned on page 91. Although the separation proceeds more quickly at elevated temperatures, resolution suffers because of increased rates of diffusion. However, in adsorption TLC only small increases in Rt values are observed even with a 20°C rise. Strict temperature control is not necessary if samples and standards are run at the same time, although large fluctuations should be avoided. The quality of the thin-layer materials, and in particular the presence of impurities in them, determine the extent to which partition, adsorption, ion-exchange and exclusion participate in the sorption process. These factors affect Rr values in an unpredictable manner. Thin layers should be of uniform thickness, between 0.2 and 0.3 mm with thinner layers, local variations in thickness can result in appreciable variations in Rf values. 

In seawater the thickness" of the double layer as given by k1 (Eq. 3.9) is a few Angstroms, equal approximately to a hydrated ion. In other words, the double layer is compressed and hydrophobic colloids, unless stabilized by specific adsorption or by polymers, should coagulate. Some of this coagulation is observed in the estuaries where river water becomes progressively enriched with electrolytes (Fig. 7.14a). That these colloids exist in seawater for reasonable time periods is caused... 

According to this model, and in the absence of specific adsorption, the adsorbed solvent molecules are located in the inner Helmholtz plane, the thickness of which is determined by the radius of the molecule. At the same time, solvated ions define the location of the outer Helmholtz plane. Other ions, charged oppositely to the surface charge, are smeared out in the diffuse layer. 

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