Displacement model hydrogen-bonding systems

Thomas et al. [72] used a modified force microscope in which a compensatory force was applied to the probe to keep its displacement at zero. With this system they studied interactions between organomercaptan molecules with CH3, NH2, or COOH end groups. All measurements were performed in dry nitrogen. From SEM-measured tip radii and pull-off force they calculated the work of adhesion using the DMT model. They found that the work of adhesion values qualitatively scaled as expected for van der Waals, hydrogen bonding,... 

In contrast, the Scott-Kucera model considers a solvent system composed of an apolar solvent A and a polar solvent B (Scott and Kucera, 1975). When this mixture is pumped through a column, a monolayer of the most polar solvent B is formed by adsorption of B on the adsorbent. Sample molecules are adsorbed on this monolayer instead of on the adsorbent surface. In other words, there is no displacement of adsorbed solvent molecules, and interaction between the molecules of the monolayer and the sample molecules determines the retention of the component. This theory has been adapted by saying that the model is only valid for medium polar mobile phases and solutes with a polarity lower than the most polar solvent in the eluent. These medium polar solvents are called hydrogen-bonding solvents (esters, ethers, ketones). A monolayer of these solvents behaves as a hydrogen-bonding phase. Inter-... 

The distance between the two interacting atoms will be denoted by r, and fg will denote the average distance (or the equilibrium distance in a hypothetical vibrationless state) of the unperturbed system. (The distance will be independent of the isotopic mass.) A displacement from the average distance will be denoted by a = r—rg. The two-oscillator model subdivides the displacement into two components a corresponds to the motion of the isotopic hydrogen relative to the carbon atom to which it is attached, and corresponds to the motion of the remainder of the molecular framework joining the two non-bonded atoms. The motion of the isotopic hydrogen relative to the carbon will be sensitive to the isotopic mass, whereas the framework motion will not. The two kinds of displacement are assumed to be governed by the probabihty distribution functions... 

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