Solvent-surface interaction

These equations imply that A132 will exceed A12 if A33 is larger than A13 + A23. This effect, termed lyophobic bonding, occurs if the solvent-surface interaction is weaker than that between the solvent molecules. More interestingly, the dispersion interaction will be repulsive (A 132 < 0) when An and/or A23 are sufficiently large. Israelachvili [1] tabulates a number of Am values Awhw Ahwh 0-4X 10 erg, Apwp 1 x 10" erg, and Aqwq = O.SX -IO erg, where W, H, P, and Q denote water, hydrocarbon, polystyrene and quartz respectively. 

Recent experimental and theoretical studies on crystal growth, especially in the presence of tailor-made inhibitors, provide a link between macroscopic and microscopic chirality. We shall discuss these principles in some detail for chiral molecules. Furthermore, we shall examine whether it is indeed feasible today to establish the absolute configuration of a chiral crystal from an analysis of solvent-surface interactions. Since these analyses are based on understanding the interactions between a growing crystal and inhibitors present in solution, we shall first illustrate the general mechanism of this effect in various chiral and nonchiral systems. 

Some of the difficulties encountered in establishing the effect of solvent on crystal growth may be circumvented by focusing on polar crystals. This is because the difference in the rates of growth of opposite faces (hid) and (hkl) along a polar direction must arise primarily from differences in their solvent-surface interactions. Thus, one generally does not have to be concerned with faces other than the hemihedral ones in question. We illustrate below an approach to understanding solvent-surface interactions in the polar crystals of resorcinol (102). 

This approach to studying solvent-surface interactions is encouraging. We intend to apply it to the crystals of enantiomers in order to assign their absolute configurations and so close the circle started by Waser. 

The polar crystalline properties of (/ ,S)-alanine and y-glycine have been used to probe the effect of solvent-surface interactions on crystal growth [33]. Both crystals grow unidirectionally at the CO end of the polar axis (see Figure 11.27) in aqueous solution. This was explained in terms of a preferred adsorption of H2O molecules at a subset of surface sites at the CO end of the crystal, leaving the other sites partially unsolvated, and so available for easy docking by solute molecules. 

The adsorption process in condensed media is a complex one involving several different kinds of interactions solvent-solute, solvent-adsorbent, and solute-adsorbent. In adsorption form solution, as said before, what happens actually is an exchange process between the solute and the solvent a requirement for the Langmuir isotherm to remain valid is that the solvent-surface interaction is weaker than the solute-surface (Hiemenz and Rajagopalan 1997). Also, if the solution is not diluted, the solvent concentration may be comparable to that of the solute and not constant, thus its chemical potential should be included in Equation 4.44, and its activity should show up in the isotherm expression (Bartell and Sloan 1929). 

The forces acting between surfaces across non-polar media are often dominated by structural forces that arise due to changes in the liquid density in the gap between the surfaces as they approach each other (59, 218). The appearance of a structural force is due to the geometrical constraints imposed by the confining walls and does not require any attractive or repulsive solvent-surface interaction. The structural forces are of rather short-range, below 10 molecular diameters, and vary in a periodical fashion from repulsive to... 

The opposite case of depletion occurs when the monomer-surface interaction is less favorable than the solvent-surface interaction. This is, e.g., the case for polystyrene in toluene, which is depleted from a mica substrate. The depletion layer is denned as the layer adjacent to the surface from which the polymers are depleted. Their concentration in the vicinity of the surface is lower than the bulk value, as shown schematically in Fig. 2b. 

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