Hydrogen bonded systems, solvation molecule

The reverse micelle and the associated structural properties that were investigated in this section, require the detailed level of description to describe phenomena such as ion-exchange or the hydrophobic effect. Thus, water is treated explicitly in these systems to capture effects associated with hydrogen bonding and solvation. In what follows, we will illustrate phenomena in which this level of atomistic detail is not required and the solvent (water) is treated as a bead that represents a group of water molecules and hydrocarbon chains treated as a collection of beads to represent lipid molecules. These reduced descriptions, which form the basis for mesoscale descriptions, are able to probe phenomena on larger length and time scales compared with the fully atomistic descriptions. 

Not only it is difficult to distinguish between a co-crystal and a solvate (as a matter of fact all molecules are solid and form crystals at sufficiently low temperature) but it is also often difficult to distinguish between a crystal and a salt. As discussed above in the case of hydrogen-bonded systems between acids and bases the transition between a salt and a co-crystal may be a very semantic issue depending exclusively on the position of a proton along a N O supramolecular link [90]. 

For systems containing polar components, however, ly values can be larger and in the few cases where there is strong solvation, i.e. hydrogen bonding between the molecules of the different species, ly can assume negative values that are functions of temperature. Thus for the acetonitrile-acetaldehyde system, /i2(40 C) = -0.32 /i2(100°C) = -0.23 (see Problem 11.41). 

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