Cooperative adsorption/desorption

Figure 19. Schematic representation of the cooperative adsorption and desorption of DOPC molecules between an air/water interface and a sublayer.

SAMs consisting of a mixture of non-fimctionalised aUcanethiols and co-functionalised alkanethiols provide the chance to study isolated functionalised sites in SAMs. In these mixed SAMs the non-functionalised alkanethiols serve as dummy molecules to dilute the fimctionalised compounds and to isolate the active sites in order to minimise their interactions. These mixed SAMs play an important role, e.g. in the development of biorecognition surfaces [137-139). One obstacle in producing mixed monolayers is the different rate of adsorption of the non-functionalised and the end-fimctionalised thiols. Consequently, the final composition of the mixed SAMs not only depends on the initial ratio of the two thiols but also on their individual as well as cooperative adsorption and desorption kinetics [140]. Another problem is the tendency for similar molecules to aggregate into islands on the surface (Fig. 15) [141,142[. Therefore, the final SAM composition caimot be predicted easily. 

It is obvious from works devoted to the investigation of adsorption-desorption dynamics of chains on attractive surfaces that chains attached to the surface, even by strong specific bonding, are able to detach completely due to thei-mally activated processes [62-65]. Thus, the chain once attached to the surface can leave the surface aftei- a period correlated to the time characteristic of cooperative multiple adsorption-desorption steps. For instance, Grauick and co-workers [62,64] have shown that the time needed for complete desorption of PS chains from the oxygen-plasma treated silicon surface in cyclohexane solution may reach the value 1(1 s and higher (in dependence on molecular-weight). 

With the availabihty of computers, the transfer matrix method [14] emerged as an alternative and powerful technique for the study of cooperative phenomena of adsorbates resulting from interactions [15-17]. Quantities are calculated exactly on a semi-infinite lattice. Coupled with finite-size scaling towards the infinite lattice, the technique has proved popular for the determination of phase diagrams and critical-point properties of adsorbates [18-23] and magnetic spin systems [24—26], and further references therein. Application to other aspects of adsorbates, e.g., the calculation of desorption rates and heats of adsorption, has been more recent [27-30]. Sufficient accuracy can usually be obtained for the latter without scaling and essentially exact results are possible. In the following, we summarize the elementary but important aspects of the method to emphasize the ease of application. Further details can be found in the above references. 

Thus, the cooperative character of the conformational rearrangement of hypercrosshnked networks causes a certain time lag in the expansion and shrinkage of the bead and explains the imusually strong hysteresis in the sorption—desorption kinetics. Generally, hypercros-slinked polystyrene sorbents display superb adsorption capacity, combined with high and steadily growing adsorption rates and very high desorption rates. 

Although ATR has been used to quantify the variation in composition at the surface in TPEs (Sung and Hu, 1980), a related utility is its ability to monitor in situ processes such as reaction injection molding (RIM) (Ishida and Scott, 1986) and protein adsorption onto a polyurethane substrate (Pitt and Cooper, 1986). In the latter, the effect of shear rate on the kinetics of protein adsorption and desorption from phosphate-buffered saline (PBS) was studied in a specially designed flow cell. A very thin film of the commercial MDI-ED-PTMO polyurethane Biomer was cast from solution onto a Ge ATR prism. The thickness of the film was less than the penetration depth so the protein concentration could be monitored after the infrared absorption of the polymer... 

In Canada, the Liric model was developed (Wren et al., 1991) and is still undergoing improvement. Liric is a comprehensive model of an essentially mechanistic nature, showing similarities in the reaction sets used for the Inspect code described above. Liric was developed to predict the time-dependent behavior of iodine in the containment under a variety of reactor accident conditions. Out of the large number of reactions involving physical and chemical processes, aqueous thermal reactions of iodine are considered, as well as water radiolysis processes and the interaction of the radiolysis products with aqueous iodine species. In addition, radiolytic decomposition of organic substances in the aqueous phase, formation and decomposition of organoiodine compounds, iodine reactions with aqueous impurities such as buffers and metal ions, mass transfer between the aqueous and gas phases and, finally, adsorption of gaseous I2 onto surfaces and its desorption from them are included in the model. The Liric model has been developed in close cooperation with the experimental work carried out in the Radioiodine Test Facility. 

Stuart L. Cooper (University of Wisconsin) Have you considered desorption of polymer from surfaces into pure solvent, as opposed to a solution of polymer and solvent Studies of protein adsorption show the process to be somewhat irreversible, especially when the protein solution is replaced by pure solvent (water). For example, it has also been shown using radio-labelled protein that adsorbed protein will exchange with protein in solution, but that very little will desorb into pure solvent. 

For a type III, the slope of the adsorption branch, within a great extension of the hysteresis loop, is higher than that corresponding to the desorption branch. Here cooperative phenomena during adsorption are more intense than during desorption. This has not been mentioned by previous authors. 

In real materials the pores are connected to one another and form a three-dimensional network. The interconnection of pores accounts for the cooperative character of adsorption phenomena. In capillary condensation the effect of the initiation of condensation in the wide pores appears after condensation in the narrow pores adjacent to them. The delay in desorption from the wider pores is stipulated by its blocking by the narrower ones. These cooperative effects cannot be allowed for by a model of unrelated pores the requirements for filling or emptying of a given pore depend not only on its own characteristics but on the characteristics of adjacent pores as well. 

Both these effects displace the isotherms of adsorption and desorption towards smaller relative pressure as compared with the system of unrelated pores. Hence it follows l) the pore size distribution, calculated in the frameworks of unrelated pores model, gives the decreased values of pore radii 2) the distribution obtained on the bases of adsorption isotherm, differs from the distribution obtained on the bases of desorption isotherm. Cooperative effects can be taken into account by means of network models, reflecting the special features of the pore structure more fully, than a system of unrelated pores. 

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