In terms of structural basis, alkanes are insoluble or immiscible in water because of such presence of C–H...O interactions that are in regime of van der Waals interaction, which tends to be regarded as dispersion contact rather than as hydrogen bond. Benzenes and other planar molecules can make stronger O–H...π interactions that render small solubility of the planar molecules.
Then, I found these statements from a lecture note (sadly I can not find the proper sources in the lecture notes and I can not find any good result with varying keywords in Google).
There are several steps of solubilization of a molecule:
- Solutes must overcome solute:solute interactions in order to separate and hydrate solute molecules from one another.
- Solvents must overcome solvent:solvent interactions in order to create a cavity within which a solute molecule can fit.
- Both solutes and solvents must overcome solute:solvent interactions in order to reorganize its ice formation in the solvent shell.
From those above statements, it can be clearly seen that solubility of molecules is not suffice to be explained solely by looking only at single thermodynamic state of solute:solvent interaction.
Here are my questions:
To be honest, I have not found any paper mentioning crystal structure showing water orientation among larger planar molecules. Based on my intuition from above, the water molecules around planar molecules should orient to the planar surfaces because of presence of O–H...π interactions in hydrogen bonding regime. On similar reasonings, the water molecules around cyclic 3D (cyclopropanes, bicyclopropanes, etc.) should orient away from these cyclic molecules because of presence of C–H...O interactions in van der Waals regime. Are my reasonings correct?
If such solute:solvent interactions could make an ice network (or a cluster of water), does it mean thermodynamically solute:solvent interactions are trapped by themselves and can not proceed to make further solute:solute and solvent:solvent interactions? Can this particular solute:solvent interaction alone rationalize physical observation of very small solubility of larger planar molecules?
I find that this is a bit paradox for this case. Some drug discovery campaigns begin to design new chemotypes that have high sp3 characters because planar sp2 molecules have poor solubility and metabolization, although they have to put more efforts on making molecules with quaternary stereocenters. In this view, alkanes are seen to be more soluble than planar molecules although both of them should be insoluble in water. I feel that it is too confusing to say alkanes are both soluble and insoluble at same time. How should I make such words so that I do not make this confusion while keeping both chemical bonding and physical phenomenons are agree to each other.