Solvation forces, nonpolar liquids

A nonpolar liquid like heptane (C7H16) has intermolecular bonds with relatively weak London dispersion forces. Heptane is immiscible in water because the attraction that water molecules have for each other via hydrogen bonding is too strong. Unlike Na+ and CP ions, heptane molecules cannot break these bonds. Because bonds of similar strength must be broken and formed for solvation to occur, nonpolar substances tend to be soluble in nonpolar solvents, and ionic and polar substances are soluble in polar solvents like water. Polar molecules are often called hydrophilic and non-polar molecules are called hydrophobic. This observation is often stated as like dissolves like. Network solids (e.g., diamond) are soluble in neither polar nor nonpolar solvents because the covalent bonds within the solid are too strong for these solvents to break. 

Christensson, H. K. and Blom, C. E., Solvation forces and phase-separation of water in a thin-film of nonpolar liquid between mica surfaces, J. Chem. Phys., 86, 419-424 (1987). 

To date there is no conclusive experimental evidence that the solvation forces in nonpolar liquids are indeed crystallization processes. Gao and Landman suggest with their MD simulation that the molecular surface corrugations are "imprinted" into the nanoconfmed and highly pressured (MPa) liquid. Their simulation predicts that if the commensurability of the molecular surface corrugation of the two solid surfaces around the confined liquid of spherical shaped molecules is altered, e.g.,... 

Christenson, H. K., Interactions between hydrocarbon surfaces in a nonpolar liquid effect of surface properties on solvation forces, J. Phys. Chem., 90, 4-6 (1986). 

Polar compounds will dissolve in polar solvents because the latter will solvate the compound and thereby overcome the electrostatic forces which hold the crystal together. It is for this reason that polar compounds will not dissolve in nonpolar solvents. Similarly, nonpolar compounds -will not dissolve in polar solvents because the relatively strong intermolecular forces in the liquid would be decreased if a solution were formed. This makes the formation of a solution energetically unfavorable. 

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