Solvents substrate, adsorptivity

Many studies have revealed that the adsorption behavior of macromolecules is very specific for individual polymer-solvent-substrate systems. Hence, the formulation of a general theory of polymer adsorption might be extremely difficult. Nevertheless, we believe that more experimental studies with well characterized polymers and surfaces are imperative of this difficulty to be overcome. 

To unravel the detailed mechanism, substrate adsorption, quenching, inhibition and kinetic studies were conducted for the ZnS-catalyzed photodehydrodimeriza-tion of 2,5-DHF [107, 148]. A plot of the amount of 2,5-DHF adsorbed ( eq) against the residual concentration in solution (cgq) exhibits saturation plateaus at eq(max) of 2.8 X 10 and 65 X 10 mol g . The first plateau is due to the formation of a mixed solvent-solute surface monolayer and the second corresponds to multilayer adsorption. Assuming that the formation of the monolayer can be described by competitive adsorption between water and 2,5-DHF, the data can be analyzed according to Hiemenz (Eq. 30) [149] ... 

Substrate adsorption studies as conducted with ZnS in aqueous solution were performed also with CdS, CdS--Si02 and SiOi in methanol, the solvent employed in... 

It is difficult to predict a priori the direction of the reaction, since its actual course may be affected by many parameters. Among these are the nature of the catalyst, the geometry of the substrate adsorption on the surface, the density of the adsorbed H atoms etc. One of the reactions mentioned above (equation 136) which was performed in MeOH can serve as an example of the effect of the conditions on product distribution. A change in catalyst and solvent inflicted a drastic change in the ratio of the products (equation 140). 

The most dramatic effect of the solvent is that it co-adsorbs with solute molecules on the substrate forming a regular multicomponent 2D crystal (see Sect. 5). In most cases though, solvent co-adsorption was not anticipated and was observed by serendipity. However, its effect could be rationalized in most cases in a straightforward way. 

SF calculations for adsorption from a selective solvent were carried out by van Lent and Scheutjens [39]. Of course, the calculations had to deal with the self-assembly process as well. For the case of a rather poorly soluble block connected to a soluble block and not too extreme block lengths the critical micellization concentration (CMC) is extremely low. For any practical situation it then can be safely assumed that adsorption takes place from a solution of almost constant chemical potential, so in equilibrium there is no significant effect of concentration. The longer the A blocks (and hence the shorter the B blocks), the thicker the A film and the higher the density of the brush of B blocks. These calculations make clear that it is favorable to use a strongly insoluble block as anchor, and that the wettability of the solvent-substrate pair by A is important. The case of a heterogeneous layer of hemimicelles requires a more elaborate two-dimensional SF scheme, which was not considered by Van Lent. 

The ultimate purpose of surface modification by graft polymerization, as diseussed previously, is to alter surface properties of the substrate so as to manipulate its physicochemical interactions with the surrounding fluid medium. Knowledge of surface properties such as molecular weight, surface density, and chemical stmcture is essential to predicting solute-substrate and solvent-substrate interactions, which in turn govern the macroscopic behavior (adsorption, wetting) of the modified surface. 

Usually," " more polymer is adsorbed from poorer solvents. Variation in solvent-substrate interactions can complicate this simple finding, which was predicted theoretically by Scheutjens and Fleer. In good solvents, repulsion between segments e.g. in tails) of adsorbed polymers will be greater than in poor solvents, leading to lower adsorption in the former. The lower adsorption in good solvents has been wrongly attributed to less-extended conformations. 

Table 3-1 illustrates the influence of solvent polarity on the amount of 6W-product e.g. 10) formed from hydrogenation of octalone (9), testo-sterone and cholestenone. The product composition is determined by the amount of 1,2 and 1,4 adsorption of the substrate. 

In cases when the two surfaces are non-equivalent (e.g., an attractive substrate on one side, an air on the other side), similar to the problem of a semi-infinite system in contact with a wall, wetting can also occur (the term dewetting appHes if the homogeneous film breaks up upon cooHng into droplets). We consider adsorption of chains only in the case where all monomers experience the same interaction energy with the surface. An important alternative case occurs for chains that are end-grafted at the walls polymer brushes which may also undergo collapse transition when the solvent quality deteriorates. Simulation of polymer brushes has been reviewed recently [9,29] and will not be considered here. 

Adsorption on solid matrices, which improves (at optimal protein/support ratios) enzyme dispersion, reduces diffusion limitations and favors substrate access to individual enzyme molecules. Immobilized lipases with excellent activity and stability were obtained by entrapping the enzymes in hydrophobic sol-gel materials [20]. Finally, in order to minimize substrate diffusion limitations and maximize enzyme dispersion, various approaches have been attempted to solubilize the biocatalysts in organic solvents. The most widespread method is the one based on the covalent linking of the amphiphilic polymer polyethylene glycol (PEG) to enzyme molecules [21]. 

Lakes are prepared by adsorption or precipitation of a soluble dye on an insoluble substrate (e.g., alumina). They are useful in fatty products that have insufficient moisture to dissolve dyes (coated tablets, cake mixes, hard candies, chewing gum). Lakes are insoluble in most solvents including water, have high opacity, are easily incorporated in dry media, and show higher stability to light and heat. They are effective colorants for candies, pills, fats, and oils. The main characteristics and differences between lakes and dyes are well documented. ... 

Sulfiir-anchored SAMs and thin films, mostly from organosulfiir precursors, have been discussed at length by a number of authors [10, 181]. SAMs of organosulfiir compounds (thiols, disulfides, sulfides) form on gold substrates by spontaneous adsorption from either the liquid or the vapor phase. A number of experimental factors can affect the formation and structure of SAMs such as choice of solvent, temperature, concentration, immersion time, purity of adsorbate, oxygen concentration in solution, cleanliness, and structure of the adsorbate. Interestingly, the... 

In 1958 Sarda and Desnuelle [79] discovered the lipase activation at the interfaces. They observed that porcine pancreatic lipase in aqueous solution was activated some 10-fold at hydrophobic interfaces which were created by poorly water-soluble substrates. An artificial interface created in the presence of organic solvent can also increase the activity of the lipase. This interfacial activation was hypothesized to be due to a dehydration of the ester substrate at the interface [80], or enzyme conformational change resulting from the adsorption of the lipase onto a hydrophobic interface [42,81,82]. 

Then we extended the 2D-model to a 3D one [21]. We considered crystallization of a single polymer chain C500 from a vapor phase onto a solid substrate, taking into account detailed interactions between the chain and the substrate. Though the polymer molecule in a vacuum was collapsed, like in a very poor solvent, under the influence of bare van der Waals interactions between atoms, the molecule was found to show quick adsorption and crystallization into a rather neat chain folded lamella. 

Real polymer processes involved in polymer crystallization are those at the crystal-melt or crystal-solution interfaces and inevitably 3D in nature. Before attacking our final target, the simulation of polymer crystallization from the melt, we studied crystallization of a single chain in a vacuum adsorption and folding at the growth front. The polymer molecule we considered was the same as described above a completely flexible chain composed of 500 or 1000 CH2 beads. We consider crystallization in a vacuum or in an extremely poor solvent condition. Here we took the detailed interaction between the chain molecule and the substrate atoms through Eqs. 8-10. 

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