Water has, in fact, a small but finite solubility in n-heptane, and n-heptane has a small but finite solubility in water. Although water-water interactions and hydrocarbon-hydrocarbon interactions are much stronger than water-hydrocarbon interactions, the latter does exist and is sufficiently strong to allow some slight mutual solubility.

The term "hydrophilic force", literally meaning "love of water" force, appears to merely be the complement to "hydrophobic". It is equivalent to the term polar, and polar solvents are hydrophilic solvents because they interact strongly with water or other polar solvents.

The reasons for the introduction of the terms "lyophobic" (meaning fear of lye) and "lyophilic" (meaning love of lye) are a little more obscure. The terms originated in the early days of the soap industry when soap was prepared by boiling a vegetable oil with an alkaline solution obtained from leaching 'wood ash' with water. The alkaline product from the wood ash was a crude solution of sodium and potassium carbonates called "lye". On boiling the vegetable oil with the lye, the soap (sodium and potassium salts of long-chained fatty acids) separated from the lye due to the dispersive interactions between the fatty acid alkane chains and were, thus, called "lyophobic". It follows that "lyophobic", from a physical chemical point of view, would be the same as "hydrophobic", and interactions between hydrophobic and lyophobic materials are dominantly dispersive. The other product of the soap-making industry was glycerol, which remained in the lye and was consequently termed "lyophilic". Thus, glycerol mixes with water because of its many hydroxyl groups and is very polar and hence is a "hydrophilic" or "lyophilic" substance.

Hydrophobic and hydrophilic terms are extensively employed in biotechnology to describe the interactive character of the molecule as a whole. The use of a more general term to describe the interactive property of a bio-molecule can be understood if one considers the character of a bio-polymer, for example a polypeptide. The peptide will contain a large number of different types of amino acids, each having different interactive groups. All will exhibit polar interactions with the carbonyl and amide groups but each amino acid will contribute its own unique interactive character to the peptide. Thus, the terms hydrophilic and hydrophobic are more often used to describe the overall interactive character of a large molecule as opposed to the individual group interactions. Nevertheless they are basically alternative terms that have been adopted to describe polar and dispersive interactions respectively.

Molecular Forces and Chromatographic Selectivity

To choose a suitable stationary phase for a particular separation it is necessary to select a substance with which the solutes will interact relatively strongly. If the solutes to be separated are predominantly dispersive, then a hydrocarbon-like stationary phase would be appropriate, which, in GC, might be a high molecular weight hydrocarbon such as squalane. The operating temperature would be chosen so that the kinetic energy of the dissolved solutes molecules was sufficiently high to provide adequate partial vapor pressure for each and, thus, permit elution in a reasonable time.

Interactions in the mobile phase are extremely weak in GC, (5) and are not employed to influence selectivity. In LC, an appropriate dispersive stationary phase might be a bonded phase with a long aliphatic chain. To ensure that the selectivity resided predominantly in the stationary phase, a complementary polar and weakly dispersive mobile phase would be used. In LC, it is usual to allow one type of interaction to dominate in the stationary phase while a different type of interaction remains controlling in the mobile phase.