Hydrophilic surface sites

Dr. Turov and Professor Leboda combine their expertise in nuclear magnetic resonance and adsorption phenomena to propose a new tool for a more incisive analysis of adsorbate-adsorbent interactions. Such an analysis is of critical importance in so many applications where it is becoming increasingly clear that adsorbate-carbon interactions are governed by both pore size and surface chemistry effects. These range from the ubiquitous water adsorption to the design of carbon-coated silicas with tailored ratios of hydrophobic to hydrophilic surface sites. 

Since oxygen functionalities are believed to act as primary adsorption sites for water molecules [304], the enthalpies of immersion of carbon materials into water have been successMly correlated to the specific and non-specific interactions between the liquid and the carbon surface [206, 299302, 304]. From basic and acidic heats of neutralization measurements on a series of porous carbons, it was found that the acidic heat values increase rapidly with the total oxygen content, while the basic heat values decreased, regardless the actual structure to which the oxygen atom is incorporated. This was explained by the fact that the hydrophilic surface sites are oxygen-based while basic sites were associated with the carbon itself [300]. In other words, the oxygen surface complexes are predominantly of acidic character. 

A. J. Groszek, S. Partyka, Measurements of hydrophobic and hydrophilic surface sites by flow microcalorimetry. Langmuir 9(10), 2721-2725 (1993)... 

It might also be expected that the aromatic portion of the PAH alkylating agent will tend to lie in the interior of the protein, as far as possible from the hydrophilic surface and its aqueous environment. Thus the result of alkylation can be extensive and can lead to a loss of any function, such as catalytic activity (if the protein is an enzyme) and/or loss of ability to recognize an attachment site or another macromolecule. 

We should note that, in general, solid surfaces must be expected to exhibit chemical and morphological heterogeneities. In this case different surface sites would have to be described by different surface parameter values. However, the adsorbed water film that is always found at ambient conditions on hydrophilic surfaces levels out these heterogeneities. Hence, the minerals and salt surfaces in Table 11.1 can be characterized by single values just like homogeneous surfaces. 

The problem associated with zeolites as nitration catalysts will be a reversible deactivation by coke deposition, and an irreversible deactivation by framework A1 removal (acid leaching). Optimization of zeolite activity, selectivity and life will be controlled by density of acid sites, crystalline size and hydrophobic/hydrophilic surface properties. 

Water-soluble polymers coat hydrophobic solid surfaces with multilayers and thus render the solid hydrophilic (i.e., wetting). The number of adsorbed chains (or the amount of polymer adsorbed) per surface site (or unit weight of adsorbent) is related to the volume fraction of segments in each layer. As the length of the chains increases,... 

This distribution suggests that positions of water molecules located in the hydrophobic cavity are more densely populated and better ordered than those in the interstices bounded by the hydrophilic surface of the cyclodextrin molecules. In the interstices, the occurrence of many potential hydrogen-bonding donor and acceptor sites appears to provide several different alternative hydrogen-bonding patterns of comparable energy and hence more opportunity for disorder of the water molecules. 

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