Relaxation times adsorbed surface layer

The measured NMR signal amplitude is directly proportional to the mass of adsorbate present, and the NMR signal versus pressure (measured at a fixed temperature) is then equivalent to the adsorption isotherm (mass of adsorbate versus pressure) [24-25]. As in conventional BET measurements, this assumes that the proportion of fluid in the adsorbed phase is significantly higher than the gaseous phase. It is therefore possible to correlate each relaxation time measurement with the calculated number of molecular layers of adsorbate, N (where N = 1 is monolayer coverage), also known as fractional surface coverage. 

The relaxation time of the end-to-end vector correlation for the adsorbed chains depends on the number of contacts. Chains with one or two contacts have most of their segments free and thus due to their bulk like dynamics the end-to-end vector can rapidly relax. On the other hand, for the chains with most of their segments adsorbed this process becomes very slow as the segment dynamics are very sluggish inside the solid oligomer interface (Fig. 15). For strong wall attractions ( w=2 or 3) the chains with more than three contacts relax with almost the same time constant. This insensitivity shows that the slowdown of the dynamics is caused by the densification inside the first layer rather than the magnitude of the surface-fluid interactions [38a,d]. 

The amount of chain units in the adsorption layer can be estimated from the weight fraction of the relaxation time. As shown in Fig. 4b, the fraction of adsorbed chain units decreases from 55 % at 148 K to 20 % at 400 K for highly filled PDMS. The amount of adsorbed chain units increases proportionally to the total surface area of Aerosil available for the adsorption [7, 8]. The weight fraction of adsorbed chain units is equal to only 3 % at 246 K for filled PDMS with the weight ratio PDMS/Aerosil (380 m g" ) equal to 100 25 [21]. 

Relaxation time studies of the filling process of porous silica with water and cyclohexane have been used to establish whether the adsorption is homogeneous. It was found that water initially collected in small puddles at interstices in the structure, and then formed a surface layer over the silica surface before the remaining pore volume was filled. On the other hand, cyclohexane appeared to fill the smaller pores completely before spreading to the larger pores. A similar effect was observed for water adsorbing in a silica that had been chemically treated to make the surface hydrophobic. Thus, the fluid location in mesoporous materials at low loadings depends critically on the wettability of the surface. 

In [31] kinetics of the surface tension decrease was described using the model accounting for diffusion-controlled adsorption of protein molecule and for conformational changes of adsorbed molecule. The model corresponds to one proposed by Serrien [32] and describes diffusion toward a/w surface and subsequent reorientation and other changes in adsorption layer, which usually one gives a sence of conformational changes the adsorbed protein. The model yields the diffusion relaxation time (t) and (kc) - the rate constant of conformational changes. 

Here (m ) is the mass adsorbed on the sorbent mass (m ), m is the gas mass between the sorbent pellets moved along with the sorbent and Am is the equivalent mass of gas in a boundary layer on the surface of the rotator. In Eq. (5.48) the various relaxation times refer to relaxational motions of the... 

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